Ramsey-bord ion frequency reference and methods of making and using the same

Abstract

In some variations, an interferometric frequency reference includes an atomic source configured to supply neutral atoms to be ionized, an ionizer configured to excite the neutral atoms to form ionized atoms, an ion collimator configured to form a collimated beam of the ionized atoms, a probe laser, and a Doppler laser configured to determine a ground state population of the ionized atoms, where the atomic source, the ionizer, and the ion collimator are arranged within a vacuum chamber. Other variations provide a method of generating a stable frequency reference, the method including forming ionized atoms from an atomic vapor, forming a collimated beam of the ionized atoms, illuminating the ionized atoms with first and second probe lasers, adjusting the frequencies of the first and second probe lasers to the S→D transition of the ionized atoms using Ramsey spectroscopy, and determining a ground state population of the ionized atoms with another laser.

Description

[0001] Priority data

[0002] This international patent application claims priority to U.S. Provisional Patent Application No. 63 / 159,167, filed March 10, 2021, and U.S. Patent Application No. 17 / 576,897, filed January 14, 2022, each of which is hereby incorporated herein by reference. Invention Field

[0003] This invention generally relates to an optical frequency reference. Background of the Invention

[0005] A frequency standard is a device used to generate or detect frequencies. An optical frequency standard refers to a stable optical frequency generated by an optical atomic clock and an optical cavity. Frequency standards are used in fiber optic communications, timing, radio frequency (RF) photonics, inertial sensing, and other technologies. Applications of ultra-precision optical frequency standards include high-precision laser spectroscopy, miniature atomic instruments (such as atomic clocks and gyroscopes), global positioning systems, precision laser sensing (e.g., remote nuclear explosion detection), and ultra-stable oscillators for high-speed analog-to-digital converters and radar systems.

[0006] Active optical frequency standards are laser sources (e.g., stable HeNe lasers) that emit light with very well-defined and known optical frequencies. Passive optical frequency standards are passive devices with well-defined frequency responses that can be used to construct active standards. Important examples are high-quality factor reference cavities and devices such as multi-channel gas cells for probing certain optical transitions.

[0007] Optical frequency standards are typically based on some optically detected electronic transitions (usually dipole-forbidden but tetrapole-allowed transitions) involving atoms (e.g., Ca, Rb, Sr, Yb, Mg, or H) and ions (e.g., Hg). + 、Sr + Yb + Ba + In + , or Al + A narrow bandwidth of a single-frequency laser, or a molecular (e.g., CH4 or I2), is used. This electronic transition is employed to stabilize the frequency of a single-frequency laser to the electronic transition frequencies of atoms, ions, or molecules. To reduce inhomogeneous broadening caused by thermal motion and collisions, these particles can be retained in traps within a vacuum chamber as the laser cools. This conventional setup allows for precise spectroscopic measurements of clock transitions.

[0008] Papers describing frequency benchmarks include McFerran et al., "Fractional frequency instability in the 10..." -14 range with a thermal beam optical frequency reference [at 10] -14 [Fractional frequency instability within the range], J. Opt. Soc. Am. B, 27, 277-285 (2010); Norcia et al., “Frequency Measurements of Superradiance from the Strontium Clock Transition”, Phys. Rev. X, 8, 021036 (2018); Davila-Rodriguez et al., “Compact, thermal-noise-limited reference cavity for ultra-low-noise microwave generation”, Opt. Lett. 42, 1277-1280 (2017); Matei et al., “1.5 μm Lasers with Sub-10 mHz” “A 1.5 μm laser with a linewidth below 10 mHz”, Phys. Rev. Lett. 118, 263202 (2017); Kessler et al., “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity”, Nature Photonics 6, 687-692 (2012); Cook et al., “Laser-Frequency Stabilization Based on Steady-State Spectral-Hole Burning in Eu 3+ :Y2SiO5[based on Eu 3+"Laser-frequency stabilization of steady-state spectral borehole burning in Y₂SiO₅", Phys. Rev. Lett. [Physical Review Letters] 114, 253902 (2015); and Olson et al., "Ramsey-Bordé Matter-Wave Interferometry for Laser Frequency Stabilization at 10 -16 Frequency Instability and Below [Ramsey-Borde matter wave interferometry used in 10] -16 [Frequency and lower instability laser frequency stabilization], Phys. Rev. Lett. [Physical Review Letters] 123, 073202 (2019), each of which is hereby incorporated by reference.

[0009] High-precision optical frequency references play a crucial role in many applications. For example, optical frequency references with performance superior to commercially available standards are desired to enable high-precision spectroscopy at multiple locations. Generally, known frequency references are extremely accurate at the cost of enormous size, weight, and power, or they sacrifice performance to reduce size, weight, and power. Large-scale optical cavity-based frequency references are currently the standard method for generating optical frequency references with short integration / averaging times. For longer averaging times, optical atomic clocks are used.

[0010] There remains a long-standing need for a compact, ultra-stable, atom-based frequency reference that operates on short (e.g., <1 second) timescales and exhibits high stability over long timescales. Invention Overview

[0012] The present invention addresses the aforementioned needs in the art, as will now be outlined and then described in detail below.

[0013] In some variations, interferometric frequency-reference apparatuses include:

[0014] Vacuum chamber;

[0015] An atomic source, configured to supply neutral atoms to be ionized;

[0016] An ionizer, configured to excite neutral atoms to form ionized atoms (such as, but not limited to, Ca). + or Sr + );

[0017] An ion collimator, which is configured to form a collimated beam of ionized atoms;

[0018] One or more probe lasers, whose frequencies are close to clock transitions (e.g., quadrupole transitions); and

[0019] A readout laser is configured to determine the ground-state population of ionized atoms, typically by measuring cyclic transitions of the atoms (such as the sp transition in Sr+).

[0020] The atomic source, ionizer, and ion collimator are arranged in a vacuum chamber.

[0021] In some embodiments, the atom source is a solid-state electrochemical atom source.

[0022] In some embodiments, the ionizer is disposed inside the ion collimator. In these embodiments, ionized atoms are formed within the ion collimator. In other embodiments, the ionizer is disposed outside the ion collimator. In these embodiments, ionized atoms are formed and then injected into the ion collimator. The ionizer may be one or more ionizing lasers, electron beam radiators, or the like.

[0023] In some embodiments, the ion collimator is a linear collimator. For example, a linear collimator may be selected from the group consisting of a linear quadrupole trap, a Penning trap, a surface ion trap, and a mass filter. In other embodiments, the ion collimator is a nonlinear collimator. A nonlinear collimator may be in a recirculating configuration, such as a racetrack configuration, a loop, or a curved loop.

[0024] The ion collimator can be configured such that the collimated beam of ionized atoms has a beam waist selected from about 10 nanometers to about 10 meters. Preferably, the beam waist is selected from about 50 nanometers to about 500 nanometers. The ion collimator can be configured such that the collimated beam of ionized atoms has a beam velocity selected from about 1 μm / s to about 0.99c, where c is the speed of light in vacuum. Preferably, the beam velocity is selected from about 1 m / s to about 20 m / s.

[0025] In some embodiments, one or more probe lasers are configured to perform Ramsey or Rabi spectroscopy on ionized atoms. In a preferred embodiment, all present probe lasers are configured to perform Ramsey spectroscopy on ionized atoms. The number of probe lasers can vary, but is preferably two or more, such as four probe lasers (e.g., see...). Figure 4 ).

[0026] In some embodiments, the one or more probe lasers are configured to detect quadrupole transitions or both dipole and quadrupole transitions of ionized atoms. The configuration includes selecting a suitable wavelength and ensuring a sufficiently narrow linewidth.

[0027] In some embodiments, the readout laser can be used for quantum transition measurements. Quantum transition measurements can be performed using the readout laser via fluorescence from the S→P cyclic transition after Ramsey probing.

[0028] In some embodiments, the interferometric frequency reference instrument further includes a cooled laser. The cooled laser is configured to cool ionized atoms in preparation for Ramsey spectroscopy. In some embodiments, the readout laser itself is configured for cooling, and a physically distinct cooled laser is not necessarily present.

[0029] The interferometric frequency reference instrument may further include an injection electrode. The injection electrode differs from an ionizer. In embodiments where atoms are ionized outside the ion collimator, the injection electrode can generate a potential field that guides the ionized atoms into the ion collimator. In a preferred embodiment, atoms are ionized within the ion collimator; therefore, an injection electrode is not necessary.

[0030] The interferometric frequency reference instrument may further include an ion sink configured to collect ionized atoms exiting the ion collimator. The ion sink is preferably located within a vacuum chamber.

[0031] The interferometric frequency reference instrument preferably further includes an imaging system configured to focus fluorescence from ionized atoms.

[0032] In some embodiments, the interferometric frequency reference provides an optical frequency reference. In some embodiments, the interferometric frequency reference provides a microwave frequency reference.

[0033] Other variations of the present invention provide a method for generating a stable frequency reference, the method comprising:

[0034] (a) Generates atomic vapor;

[0035] (b) Ionize at least some of the atoms in the atomic vapor to form ionized atoms;

[0036] (c) Collimate the ionized atoms in an ion collimator to form a collimated bundle of ionized atoms;

[0037] (d) Optionally, some of the ionized atoms are irradiated with a cooled laser;

[0038] (e) Irradiate at least some of the ionized atoms with a first probe laser at a first probe laser frequency;

[0039] (f) Irradiate at least some of the ionized atoms with a second probe laser at the second probe laser frequency;

[0040] (g) Using Ramsey spectroscopy, the frequencies of the first and second probe lasers are tuned to the S→D transitions of at least some of the ionized atoms; and

[0041] (h) Irradiate at least some of the ionized atoms with a readout laser to determine the ground state population of the ionized atoms.

[0042] In some methods, atomic vapors and / or ionized atoms are obtained from a solid-state electrochemical atomic source. In some embodiments, the ionized atoms may be Ca... + and / or Sr + .

[0043] The ionized atoms provided in step (b) can be formed within the ion collimator provided in step (c). Alternatively, or additionally, the ionized atoms provided in step (b) are formed and then injected into the ion collimator. Steps (b) and (c) can be integrated such that at least some of the ionization occurs within the ion collimator. In some preferred embodiments, steps (b) and (c) can be integrated such that all ionization occurs within the ion collimator.

[0044] In some methods, step (d) is performed to cool the ionized atoms in preparation for Ramsey spectroscopy. The cooling can be performed using the same laser as the readout laser (in this case, which is also the cooling laser), or it can be performed using a physically different laser.

[0045] In some methods, the ion collimator is a linear collimator. For example, the ion collimator can be a linear collimator selected from the group consisting of a linear quadrupole trap, a Penning trap, a surface ion trap, and a mass filter. In other methods, the ion collimator is a nonlinear collimator. For example, the ion collimator can be a nonlinear collimator selected from the group consisting of a racetrack configuration, a loop, a curved loop, or other recirculation configuration.

[0046] The collimated beam of ionized atoms can have a beam waist selected from about 10 nanometers to about 10 meters, such as from about 50 nanometers to about 500 nanometers. The collimated beam of ionized atoms can have a beam velocity selected from about 1 μm / s to about 0.99c, such as from about 1 m / s to about 20 m / s, where c is the speed of light in a vacuum.

[0047] In some embodiments, the method further includes irradiating at least some of the ionized atoms with a third probe laser. In some embodiments, the method further includes irradiating at least some of the ionized atoms with a fourth probe laser after irradiating at least some of the ionized atoms with the third probe laser.

[0048] In a preferred embodiment, the method is continuous, utilizing a continuously moving atomic beam (rather than fixed, trapped atoms) and a continuous-wave laser array with continuous interrogation. As the atomic beam passes through a series of laser beams, the atoms are continuously referenced, allowing for direct and rapid locking of atomic transitions. Optical transitions produce a higher resonant quality factor than RF transitions. The result of this continuous method is continuous and rapid readout of optical transitions, which is a significant benefit for next-generation timing solutions.

[0049] In some methods, the stable frequency reference is an optical frequency reference. In other methods, the stable frequency reference is a microwave frequency reference.

[0050] Some methods utilize an interferometric frequency reference instrument comprising: a vacuum chamber; an atomic source configured to supply neutral atoms to be ionized; an ionizer configured to excite the neutral atoms to form ionized atoms; an ion collimator configured to form a collimated beam of ionized atoms; one or more probe lasers; and a readout laser configured to determine the ground-state population of the ionized atoms, wherein the atomic source, ionizer, and ion collimator are arranged within the vacuum chamber. Attached Figure Description

[0051] Figure 1A This is a schematic cross-sectional view of a linear quadrupole well with a rod.

[0052] Figure 1B This is a schematic cross-sectional view of a linear quadrupole well with blades.

[0053] Figure 2 This is a top view of a linear ion collimator in some embodiments.

[0054] Figure 3 This is a top view of a nonlinear ion collimator in some embodiments.

[0055] Figure 4 This is a schematic diagram of an interferometric frequency reference instrument in some variations.

[0056] Figure 5 This is a method flowchart in some embodiments. Detailed Implementation

[0057] The instruments, methods, and systems of the present invention will be described in detail with reference to various non-limiting embodiments.

[0058] This description will enable those skilled in the art to make and use the invention, and describes several embodiments, modifications, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the invention will become more apparent to those skilled in the art when taken in conjunction with the following detailed description of the invention in conjunction with the accompanying drawings.

[0059] As used in this specification and the appended claims, unless the context clearly indicates otherwise, the singular forms “a / an” and “the” include plural indicators. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0060] Unless otherwise indicated, all numerical values ​​used in the specification and claims to indicate conditions, concentrations, dimensions, etc., should be understood to be modified by the term "about" in all cases. Accordingly, unless otherwise indicated, the numerical parameters set forth in the following specification and appended claims are approximations, which may vary at least depending on the specific analytical technique.

[0061] The term "comprising," synonymous with "including," "containing," or "characterized in," is inclusive or open-ended and does not exclude additional, unlisted elements or method steps. "Comprising" is a specialized term used in claims language to mean that the specified claim element is essential, but other claim elements may be added and still constitute a concept within the scope of the claims.

[0062] As used herein, the phrase “consisting of” excludes any element, step, or component not specified in the claims. When the phrase “consisting of” (or variations thereof) appears in a clause of the body of a claim rather than immediately following the preamble, the phrase limits only the element set forth in that clause; other elements are not excluded from the claims as a whole. As used herein, the phrase “substantially constitutes” limits the scope of the claims to the specified elements or method steps, plus those that do not substantially affect the basis and one or more novel features of the claimed subject matter.

[0063] Regarding the terms “comprising,” “consisting of,” and “substantially consisting of,” when one of these three terms is used herein, the currently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly enumerated, any instance of “comprising / including” may be replaced by “consisting of” or alternatively by “substantially consisting of.”

[0064] In some variations, this invention provides an ion-based frequency standard that offers an optical frequency standard with extremely high levels of stability and fast, continuous readout in a compact package. This invention is at least in part based on a Ramsey-Bordé interferometer. A Ramsey-Bordé interferometer involves a linear flow of atoms probed by a well-defined sequence of oscillating fields. In this invention, instead of using thermally neutral atoms, a quasi-DC current of cooled, guided ions is employed. The ions can be guided by a specially designed radio frequency (RF) POW trap or a mass filter precisely designed for this purpose. By employing a cooled, controlled ion flow, a combination of features unattainable by any other frequency standard can be achieved: (i) very low fractional frequency instability, (ii) fast, continuous readout, (iii) compact size due to reduced atomic velocities, and / or (iv) reduced vacuum requirements due to deep trap depth.

[0065] To the knowledge of the inventors of this invention, Ramsey-Borde interferometry over cooled ion streams has not yet been achieved. As described herein, Ramsey-Borde interferometry is performed on cooled ion streams with a high signal-to-noise ratio using an ion collimator (such as a modified RF-Paul trap / mass filter) that collimates the cooled ions and confines them in a one-dimensional stream. In this disclosure, "trap / filter" is synonymous with "ion collimator." The trap / filter is configured to optimize fractional frequency instability and the size, weight, and power of the device. The trap / filter controls the ion propagation velocity, thereby significantly reducing size requirements, Doppler error, and transit time error. The trap / filter also provides a deep trap potential to relax vacuum requirements.

[0066] In this disclosure, “frequency reference” is synonymous with “interferometric frequency reference instrument” or similar terms.

[0067] The stability and limiting size of conventional Ramsey-Borde interferometers are constrained by the Doppler contributions to errors (first and second order) and the broadening of the transit time of the atomic species with respect to the laser beam. Historically, Ramsey-Borde spectroscopy used a neutral atom beam transiting an array of four lasers. According to the present invention, the neutral atom beam used in historical implementations is replaced with a collimated ion beam. The applied electric field of the trap / filter allows for precise velocity control, as well as controlled ion density and spacing. This novel use of the ion beam significantly reduces the Doppler broadening and Ramsey-Borde transit time broadening suffered by conventional neutral atom methods. The controlled ion beam velocity also enables long Ramsey-Borde transit times without sacrificing size and power requirements.

[0068] To date, atomic frequency references (both RF and optical) employ trapped atoms whose transitions are detected via low duty cycle (on the order of Hz) long integration time spectroscopy. Each pulse sequence can only guarantee one reference to the atom, resulting in very slow atomic feedback, on the order of seconds. In contrast, this invention preferably utilizes a continuously moving beam of atoms (instead of stationary, trapped atoms) and a continuous-wave laser array with continuous interrogation. As the atomic beam passes through a series of laser beams, the atoms are continuously referenced, allowing for direct and rapid locking of atomic transitions. Optical transitions produce a higher resonance quality factor than RF transitions. The result of this continuous approach is continuous and rapid readout of optical transitions, a significant benefit for next-generation timing solutions.

[0069] The frequency reference disclosed in this paper is achieved by generating an ultra-narrow linewidth laser. The frequency reference can be used to lock onto a laser source for precision sensing / timing applications. It can also be used as an ultra-stable optical frequency reference, to which an external laser can be locked via a standard beat-note lock for use by other optical, quantum, metrology, or communication instruments.

[0070] In some variations, the present invention provides a fieldable (e.g., approximately 10 cm long) interferometric frequency reference instrument that has comparable stability to conventional atomic clocks, can operate at shorter averaging times, and has greater drift impedance than prior art cryogenic optical cavities.

[0071] The disclosed device (interferometric frequency reference instrument) can fill the gap left by traditional stable oscillators, and it also has other applications enabled by realizing ultra-stable lasers. One application is precision sensing. An example of precision sensing is legal seismology using submarine fiber optic cables for long-distance explosion detection, where traditional seismographs are unavailable or impractical.

[0072] In some variations, this invention provides a compact frequency reference with low fractional frequency instability (low Allan deviation) and fast continuous readout. Allan variance is the most common statistical function used to characterize and classify frequency fluctuations in a frequency reference. See Allan, Statistics of Atomic Frequency Standards, PROCEEDINGS OF THE IEEE, Vol. 54, No. 2, 1966, which is incorporated herein by reference. Allan deviation (“ADEV”) is the square root of Allan variance. ADEV is used to characterize random deviations associated with noise in the frequency. While other ways exist to define fractional frequency instability, including modified Allan deviation, in this disclosure, “fractional frequency instability” means the unitless Allan deviation, ADEV, as defined above.

[0073] In various embodiments, the present invention provides several advantages over prior art optical frequency standards. Unlike cavities, which are passive optical resonators, frequency standards are essentially limited only by atomic properties and are therefore capable of long-term averaging. Thus, the limit of fractional frequency stability is approximately 10⁻⁶, as defined by quantum physics. -18 The fundamental stability limit of (ADEV).

[0074] The frequency references disclosed in this article do not require cryogenic operation or any cryogenic components.

[0075] Unlike conventional optical clocks, frequency references have a fast readout and require no system calibration.

[0076] When atoms are cooled, the propagation length of the atomic beam can be reduced proportionally when the beam size is set by the velocity of the atoms relative to the intrinsic atom quadrupole linewidth.

[0077] By using trapped ions instead of neutral atoms, a greater degree of control over the atomic velocity distribution can be achieved. Exemplary ion trap / filter geometries include linear filters and racetrack trap geometries. Linear filters are conceptually simpler, while racetrack trap geometries can eliminate residual Doppler broadening and can be used to shut down the atomic source for ion recycling.

[0078] The conventional frequency references with the lowest fractional frequency instability to date are optical atomic clocks. They maintain a fractional frequency instability of 10-1. -18The world record for optical atomic clocks is on the order of magnitude. However, their extreme complexity and enormous size / weight / power requirements disqualify them from practical deployment. Furthermore, optical atomic clocks require their lasers to be locked into large (approximately 40 cm long) or cryogenic optical cavities. Laser locking narrows the laser linewidth and provides the electromagnetic source (laser) as a stable optical oscillator. However, optical cavities are large, operate at their fundamental limits, and cannot achieve the long-term stability sufficient for independent operation without an optical atomic clock. Some demonstrations of atom-based frequency references exist, but they are physically large because their size is determined by atomic velocity distributions and / or cryogenic operation. Any attempt to miniaturize laboratory-scale optical atomic clocks must address the requirements of optical frequency references for operation.

[0079] Most field-compatible frequency references are high-quality factor RF cavities—for example, oven-controlled crystal oscillators (OCXOs)—that interface with standard electronics. While attractive in terms of size / weight / power requirements, such frequency references are based on materials like quartz, which experience long-term drift and cannot meet the requirements for long-term stability (ADEV ≈ 10). -10 -10 -13 Used in the following situations.

[0080] For applications requiring timing and long-term stability superior to OCXOs while maintaining deployability (e.g., GPS applications), warm-RF atomic clocks are used. However, these clocks remain complex, requiring their own separate precision oscillators for readout, which limits them to very long timescales. Subsequently, chip-scale atomic clocks are almost always paired with their own highly stable frequency references. However, their performance on short timescales remains within ADEV≈10. -13 The magnitude is [missing information], and improvements are only seen on long timescales such as those exceeding 100 seconds.

[0081] In a preferred embodiment, the significant difference between the disclosed frequency references and those of prior art Ramsey-Borde interferometers or atomic clocks is that thermally neutral atoms in the interferometer or cold-trapped atoms in the clock are replaced by a continuously moving ion beam.

[0082] Another significant difference from conventional techniques is that, in a preferred embodiment, the disclosed frequency reference replaces the pulsed laser (in the clock) with a series of spatially separated continuous-wave lasers. By implementing a Ramsey-Borde interferometer in the guided ion (collimation) system, a continuous beam of atoms is used for rapid readout, while avoiding the velocity distribution problem of neutral atoms.

[0083] Another difference from conventional techniques is that, in the preferred embodiment, the disclosed frequency reference significantly relaxes the requirements for complex cooling procedures. The preferred frequency reference employs minimal laser cooling and then traps / guides ions in a deep radio frequency (RF) potential field. Deeper traps can be fabricated using RF-Paul traps compared to typical magnetic field traps required for neutral atoms. Furthermore, deep RF-Paul traps allow for controlled propagation speeds. The preferred method utilizes only RF and DC fields, thus requiring fewer lasers. The disclosed frequency reference is compatible with integrated solid-state paths and is more easily miniaturized.

[0084] In some variations, interferometric frequency-reference apparatuses include:

[0085] Vacuum chamber;

[0086] An atomic source, configured to supply neutral atoms to be ionized;

[0087] An ionizer, configured to excite neutral atoms to form ionized atoms (such as, but not limited to, Ca). + or Sr + );

[0088] An ion collimator, which is configured to form a collimated beam of ionized atoms;

[0089] One or more probe lasers; and

[0090] A readout laser is configured to determine the ground-state population of ionized atoms.

[0091] The atomic source, ionizer, and ion collimator are arranged in a vacuum chamber.

[0092] In some embodiments, the atomic source is a solid-state electrochemical atomic source. Integration with a solid-state electrochemical atomic beam source can reduce size, weight, and power because the frequency reference is smaller, more power-efficient, and provides a collimated beam (compared to conventional atomic furnaces).

[0093] In some embodiments, the ionizer is disposed inside the ion collimator. In these embodiments, ionized atoms are formed within the ion collimator. In other embodiments, the ionizer is disposed outside the ion collimator. In these embodiments, ionized atoms are formed and then injected into the ion collimator.

[0094] In some embodiments, the ion collimator is a linear collimator. For example, the linear collimator may be selected from the group consisting of a linear quadrupole trap, a Penning trap, a surface ion trap, and a mass filter. Figure 1A This is a schematic cross-sectional view of a linear quadrupole well with a rod. Figure 1BThis is a schematic cross-sectional view of a linear quadrupole well with blades. Figure 2 This is a top view of a linear ion collimator in some embodiments.

[0095] In other embodiments, the ion collimator is a nonlinear collimator. The nonlinear collimator can be in a recirculating configuration, such as a racetrack configuration (e.g., see...). Figure 3 ), loops, or curved circuits. Figure 3 This is a top view of a nonlinear ion collimator in some embodiments.

[0096] The ion collimator can be configured such that the collimated beam of ionized atoms has a beam waist selected from about 10 nanometers to about 10 meters. Preferably, the beam waist is selected from about 50 nanometers to about 500 nanometers. The ion collimator can be configured such that the collimated beam of ionized atoms has a beam velocity selected from about 1 μm / s to about 0.99c, where c is the speed of light in vacuum. Preferably, the beam velocity is selected from about 1 m / s to about 20 m / s.

[0097] In some embodiments, one or more probe lasers are configured to perform Ramsey spectroscopy on ionized atoms. In a preferred embodiment, all present probe lasers are configured to perform Ramsey spectroscopy on ionized atoms. The number of probe lasers can vary, but is preferably two or more, such as four probe lasers (e.g., see [link to relevant documentation]). Figure 4 ).

[0098] In some embodiments, the one or more probe lasers are configured to detect quadrupole transitions or both dipole and quadrupole transitions of ionized atoms. The configuration includes selecting a suitable wavelength and ensuring a sufficiently narrow linewidth.

[0099] In some embodiments, the readout laser is a Doppler laser configured to determine the ground state population by performing quantum transition measurements via fluorescence from the S→P cyclic transition after Ramsey interrogation.

[0100] In some embodiments, the interferometric frequency reference instrument further includes a cooled laser. The cooled laser is configured to cool ionized atoms in preparation for Ramsey spectroscopy. In some embodiments, the readout laser itself is configured for cooling, and a physically distinct cooled laser is not necessarily present.

[0101] The interferometric frequency reference instrument may further include an injection electrode. The injection electrode differs from an ionizer. In embodiments where atoms are ionized outside the ion collimator, the injection electrode can generate a potential field that guides the ionized atoms into the ion collimator. In a preferred embodiment, atoms are ionized within the ion collimator; therefore, an injection electrode is not necessary.

[0102] The interferometric frequency reference instrument may further include an ion sink configured to collect ionized atoms exiting the ion collimator. The ion sink (if present) is preferably arranged in a vacuum chamber.

[0103] The interferometric frequency reference instrument preferably further includes an imaging system configured to focus fluorescence from ionized atoms. The imaging system may include optical lenses or a system of lenses, a camera, a photomultiplier tube, and / or a photon barrel detector.

[0104] Figure 4 This is a schematic diagram of an interferometric frequency reference instrument in some variations. Figure 4 The frequency reference shown includes a vacuum chamber, an atomic source, an ionizer, an ion collimator, an injection electrode, an ion sink, four probe lasers, a Doppler laser (typically a readout laser), and an imaging system. The injection electrode and imaging system are optional components.

[0105] In some embodiments, an interferometric frequency reference precursor instrument without a vacuum chamber is provided. The precursor instrument is then placed within a vacuum chamber at a later time. Prior to being placed within the vacuum chamber, the interferometric frequency reference precursor instrument comprises:

[0106] An atomic source, configured to supply neutral atoms to be ionized;

[0107] An ionizer, configured to excite neutral atoms to form ionized atoms (such as, but not limited to, Ca). + or Sr + );

[0108] An ion collimator configured to form a collimated beam of ionized atoms; one or more probe lasers; and

[0109] The readout laser is configured to determine the ground-state population of ionized atoms.

[0110] In some embodiments, the interferometric frequency reference provides an optical frequency reference. In some embodiments, the interferometric frequency reference provides a microwave frequency reference.

[0111] Other variations of the present invention provide a method for generating a stable frequency reference, the method comprising:

[0112] (a) Generates atomic vapor;

[0113] (b) Ionize at least some of the atoms in the atomic vapor to form ionized atoms;

[0114] (c) Collimate the ionized atoms in an ion collimator to form a collimated bundle of ionized atoms;

[0115] (d) Optionally, some of the ionized atoms are irradiated with a cooled laser;

[0116] (e) Irradiate at least some of the ionized atoms with a first probe laser at a first probe laser frequency;

[0117] (f) Irradiate at least some of the ionized atoms with a second probe laser at the second probe laser frequency;

[0118] (g) Using Ramsey spectroscopy, the frequencies of the first and second probe lasers are tuned to the S→D transitions of at least some of the ionized atoms; and

[0119] (h) Irradiate at least some of the ionized atoms with a readout laser to determine the ground state population of the ionized atoms.

[0120] Figure 5 A process flowchart is depicted in some embodiments. The steps of irradiating ions with a cooled laser, irradiating ions with a third probe laser, and irradiating ions with a fourth probe laser are optional. The readout laser may be a Doppler laser. The step of irradiating ions with a Doppler laser (or other readout laser) may be performed before or after the step of adjusting the frequency of the probe laser using Ramsey spectroscopy.

[0121] In some methods, atomic vapors and / or ionized atoms are obtained from a solid-state electrochemical atomic source. In some embodiments, the ionized atoms may be Ca... + and / or Sr + Solid-state electrochemical atomic sources will be described in more detail later in this description.

[0122] The ionized atoms provided in step (b) can be formed within the ion collimator provided in step (c). Alternatively, or additionally, the ionized atoms provided in step (b) are formed and then injected into the ion collimator. Steps (b) and (c) can be integrated such that at least some of the ionization occurs within the ion collimator. In some preferred embodiments, steps (b) and (c) can be integrated such that all ionization occurs within the ion collimator.

[0123] In some methods, step (d) is performed to cool the ionized atoms in preparation for Ramsey spectroscopy. The cooling can be performed using the same laser as the readout laser (in this case, which is also the cooling laser), or it can be performed using a physically different laser.

[0124] In some methods, the ion collimator is a linear collimator. For example, the ion collimator can be a linear collimator selected from the group consisting of a linear quadrupole trap, a Penning trap, a surface ion trap, and a mass filter. In other methods, the ion collimator is a nonlinear collimator. For example, the ion collimator can be a nonlinear collimator selected from the group consisting of a racetrack configuration, a loop, a curved loop, or other recirculation configuration.

[0125] The collimated beam of ionized atoms can have a beam waist selected from about 10 nanometers to about 10 meters, such as from about 50 nanometers to about 500 nanometers. The collimated beam of ionized atoms can have a beam velocity selected from about 1 μm / s to about 0.99c, such as from about 1 m / s to about 20 m / s, where c is the speed of light in a vacuum.

[0126] In some embodiments, the method further includes irradiating at least some of the ionized atoms with a third probe laser. In some embodiments, the method further includes irradiating at least some of the ionized atoms with a fourth probe laser after irradiating at least some of the ionized atoms with the third probe laser.

[0127] In some methods, the stable frequency reference is an optical frequency reference. In other methods, the stable frequency reference is a microwave frequency reference.

[0128] Some methods utilize an interferometric frequency reference instrument comprising: a vacuum chamber; an atomic source configured to supply neutral atoms to be ionized; an ionizer configured to excite the neutral atoms to form ionized atoms; an ion collimator configured to form a collimated beam of ionized atoms; one or more probe lasers; and a readout laser configured to determine the ground-state population of the ionized atoms, wherein the atomic source, ionizer, and ion collimator are arranged within the vacuum chamber.

[0129] Some variations will now be described in more detail. It should be understood that such description is non-limiting and the invention is not limited by any assumptions or theories.

[0130] An interferometric frequency reference instrument is a device used by Ramsey-probing collimated ion beams to directly and rapidly feedback lock onto ultra-stable lasers. (Reference) Figure 4 It shows the components and optional components.

[0131] The vacuum chamber houses the atomic source, ion sink, ionizer, and ion collimator, such as an improved RF ion Porpoise filter / trap. Because the ions are collimated, rather than trapped themselves, the requirements for ultra-high vacuum (to reduce background collisions) are relaxed compared to conventional ion trapping. The vacuum level can be moderate, such as around 10⁻⁶. -5 To about 10 -8The chamber pressure is within a certain range. Laser arrays and imaging systems can be placed inside or outside the vacuum chamber, depending on the compactness of these components. The vacuum chamber can be a standard off-the-shelf (e.g., stainless steel) chamber, or it can be a custom-made vacuum chamber, for example, from stainless steel, aluminum, borosilicate glass, aluminosilicate glass, sapphire, or a combination thereof.

[0132] An atomic source supplies a stream of neutral atoms to be ionized. This stream of neutral atoms is in a gas phase containing atoms. The atomic source can be located at or near the entrance of the ion collimator. For example, the atomic source can be a single chamber with conduction-limited apertures close to the ionizer, a SAES dispenser, a pill dispenser, an alpha source dispenser, a liquid or solid phase of atoms (possibly mixed with other species), a LIAD (photoinduced atom desorption) source, a graphite (or other) intercalated compound of atoms, or an electrochemical solid-state source. For examples of electrochemical solid-state atomic sources that can be used in the frequency references described herein, see the section titled "Solid-State Electrochemical Atom Sources" later in this specification. The neutral atoms to be ionized can be selected, for example, from the group consisting of Ca, Sr, Yb, Li, Na, K, Rb, Cs, Hg, and combinations thereof. In some embodiments, the atoms are Ca and / or Sr, because the single-electron ionization of calcium or strontium produces hydrogen ions with broad cyclic dipole transitions and narrow quadrupole transitions (both readily addressable by commercial lasers). Atomic species can be isotopically enriched relative to their natural isotopic abundance.

[0133] An ionizer excites neutral atoms, promoting valence electrons from the atom to the continuum region and leaving behind ions, such as hydrogen ions. The ionizer can employ photoionization, i.e., ionization produced by the action of electromagnetic radiation (e.g., optical radiation). Some embodiments employ two-photon optical ionization, where an ECDL (external cavity diode laser) is tuned to the electric dipole transition (S→P) of a neutral atom, exciting valence electrons from the S ground state to the P excited state. From there, a second ECDL promotes P-state electrons to the continuum region. The two laser beams are preferably focused near the entrance of an ion collimator. Optical ionization allows for isotope selection: the first ECDL can be frequency-tuned to selectively ionize one of several naturally occurring isotopes in the atomic sample (typically large isotope shifts, on the order of GHz – easily addressable by MHz-wide lasers). Frequency tuning can be accomplished by loosely and easily modified DAVLLs (dichroic atomic vapor laser locks) locked to a vapor cell filled with neutral atoms. The second ECDL does not require frequency stabilization at all. The drawback of this method is that it requires a dedicated ionization laser (the first S→P laser). Secondly, a P→continuous region laser can also be used as a readout laser.

[0134] Alternatively, or additionally, the ionizer can employ electroionization. In electroionization, small current-carrying filaments can be placed near the neutral atom beam departing from the atomic source to directly excite valence electrons into the continuous region. While this technique eliminates the need for a dedicated ionization laser, it lacks selectivity for certain isotopes. Therefore, if a frequency reference is desired using isotopically enriched atoms, an isotopically enriched atomic source will be required. Electroionization is often the preferred ionization technique when size, weight, and power requirements are given priority.

[0135] An ion collimator (trap) is used to collimate ionized atoms into a one-dimensional, velocity-controlled beam. The collimated beam travels along the length of the trap, passing through a series of spatially separated lasers for cooling, Ramsey probing, and readout. The ion collimator can be an improved RF ion filter. In some embodiments, the ion collimator contains an ionizer within the ion collimator. In other embodiments, the ion collimator receives ions generated separately by an ionizer physically located outside the ion collimator.

[0136] Ion collimators of many geometries can be used. A linear collimator can be one with four parallel (e.g., tungsten) rods. Figure 1A ) or leaf ( Figure 1B This is an improved RF ion filter. Two of these rods / blades support MHz RF, such as that supplied by an amplified RF function generator, while the other two rods / blades remain in DC and connected to a digital-to-analog converter. This standard configuration provides longitudinal trapping (x, y), ensuring the ion beam remains collimated to a one-dimensional axial crystal (z). The DC rods can be segmented (with different DC voltages) to provide axial propagation control of the ion beam. In this way, the ion beam can be continuously propagated along the collimator at a well-controlled speed distribution. This speed control is particularly beneficial when the frequency reference is an ultra-narrow frequency reference.

[0137] Unlike conventional RF ion traps, linear collimators preferably do not include a DC end cap that would cause harmonic axial trapping. Instead, the ion collimator produces a slowly moving ion beam that is not trapped in any of the three dimensions. The linear design of the ion collimator can be similar to that of an ion-based mass filter, which typically operates in high vacuum ranges. However, since a long trap lifetime is not required, the vacuum level can be relaxed compared to a typical ion trap.

[0138] The rods / blades of the ion collimator can be machined and aligned in a support structure (e.g., Macor), such as by... The structure is manufactured using machinable glass-ceramic (Corning Incorporated, Corning, New York, USA). The rods / blades can be made from metal deposited in strips on the walls of a hollowed-out, machined (e.g., laser-machined) part (such as a fused silica part). Typical dimensions of the Macor structure are approximately 30cm × 10cm × 10cm, and typical dimensions of a laser-machined system are approximately 10cm × 3cm × 1mm. Smaller laser-machined designs are preferred if stability and simplicity are priorities.

[0139] Ion collimators can be nonlinear collimators, such as racetrack collimators. (See racetrack collimator for more details.) Figure 3 The linear collimator is essentially bent and its ends are joined together to form a racetrack shape. In this configuration, the remaining Doppler shift can be canceled out by interrogating two ion beams with opposite momentum. The racetrack collimator, or other recirculation configuration, can utilize commercial ion pumps to maintain the vacuum. Ion collimators with recirculation configurations provide long atomic source lifetimes.

[0140] Ions exiting the ion collimator can be collected by an ion sink for recycling. The ion sink is an optional component, which can be particularly beneficial in the case of a linear collimator. The ion sink can be positioned at the output port of the ion collimator to collect the ion beam after use. The ion sink can be an electrochemical solid-state atomic sink, which can be similar to an electrochemical solid-state atomic source but designed to act as an atomic sink. In some embodiments, the atomic source is reused as an ion sink, which can significantly increase the lifetime of the device. For example, when the atomic source is depleted, the ion sink can be reused as a new source, guiding the atomic beam in the opposite direction along the ion collimator. The ion beam can then be collected by the original atomic source (which is now itself reused as an ion sink).

[0141] Other implementations of atomic sinks include, but are not limited to, graphite intercalation compounds, mixtures of atomic species with alkali or alkaline earth metals, or cold surfaces. Once an atomic sink contains a high amount of absorbing, adsorbed, or intercalated atoms, it can be heated to release the atomic species of interest. An initial atomic source can be cooled to transform it into an atomic sink, thereby reversing the flow of atoms and ions, allowing for the reuse of atoms, and resulting in longer device life and / or smaller atomic source size.

[0142] The laser array includes an optional cooled laser, one or more probe lasers, and a Doppler laser that acts as a readout laser. The laser array is preferably arranged orthogonally to the direction of ion propagation. The laser array is configured to perform a Ramsey interrogation sequence on the ion beam as the ions pass through an ion collimator.

[0143] A Doppler laser, or other cooled laser, can be used to initialize the ion beam to the S-state, preparing it for Ramsey interrogation. The Doppler laser cools the ion beam to the Doppler limit (e.g., a temperature of about 70 mK) and into a Lamb-Dicke state, where a first-order Doppler shift can be separated into moving sidebands, with a slight red detuning relative to the ion's S→P electric dipole transition (a wide-cycle transition with high scattering rate). The laser can be a small external cavity diode laser (ECDL) of medium power (a few mW), locked to prevent accidental blue detuning heating of the ions. For this purpose, a small commercial cavity with a Pound-Drever-Hall (PDH) lock can be used. Depending on the atomic species chosen, one or more red-wavelength re-pump lasers can also be used to keep the ions in a Doppler-cooled cycle. Such re-pump lasers have even more relaxed power and locking requirements; inexpensive distributed feedback (DFB) lasers can be used, locked only to a slow wavelength meter.

[0144] The probe laser was locked to a narrow transition in the ion using optical Ramsey spectroscopy. In Ca... + or Sr + In this case, the narrow transition is an electric quadrupole S→D transition with a linewidth of approximately Hz. Quadrupole transitions are clock transitions, do not emit light very well, have long coherence, and possess a high quality factor (Q). To perform spectroscopy with sufficient accuracy to resolve the Ramsey fringe pattern, the laser is preferably pre-stabilized and narrowed to approximately kHz laser linewidth via PDH lock to an ultra-low expansion (ULE) cavity. It should be noted that the cavity does not need to be narrowed to a laser linewidth of mHz. To scan the laser frequency, a small amount of tuning (100 kHz for 100 s) is achieved by guiding the probe laser beam (after locking) through an acousto-optic modulator (AOM).

[0145] Table 1 below lists the examples used for Sr + and Ca + Laser wavelengths for ion cooling / detection, quadrupole transitions, and re-pumping.

[0146] Cooling / Detection 421.7nm 396.8nm Repump 1 1091.5nm 866.2nm Repump 2 1033nm 729nm Four poles 674nm 854nm

[0147] Table 1: Laser wavelengths of strontium and calcium ions used in interferometric frequency reference instruments.

[0148] Both Ramsey and Ramsey-Borde spectral sequences can be used for device operation. A Ramsey sequence can be understood as consisting only of the first two pulses from a Ramsey-Borde pulse sequence. The Ramsey-Borde spectral sequence will now be described in detail, as seen by the ions in their own reference frame when they pass through a continuous-wave laser array. In the ions' reference frame, the sequence consists of four coherent π / 2 pulses, all preferably at the same frequency.

[0149] (1) The first π / 2 pulse promotes the coherent superposition of the ion from the S state to the S and D states. After this π / 2 pulse, the ion evolves freely for a period of time τ, which can be called the Ramsey interrogation time. If measured in a rotating reference frame during this time, the phase delay between the local oscillator (probe laser) and the rotation of the state vector in the Bloch sphere will develop if and only if the probe laser is detuned to the atomic transition.

[0150] (2) The second π / 2 pulse pulses at the end of the Ramsey interrogation time τ. Depending on the phase delay obtained during the interrogation time, the ion will either return to the ground state S with a well-defined probability or be excited to the D state.

[0151] (3) The third π / 2 pulse is a counter-propagating pulse that excites the ions with a small delay T after the first Ramsey interrogation time. This π / 2 pulse causes the atoms to revert to a coherent superposition of S and D states during time t. However, a negative phase is now obtained relative to the first interrogation time τ. This excites an echo, effectively eliminating low-frequency dephasing from unwanted noise sources. This is a well-known technique, commonly referred to as stimulated spin echo, which is an optical simulation of stimulated spin echo. Preferably, τ = t is ensured for all measurements.

[0152] Preferably, the first, second, and third π / 2 pulses maintain maximum phase coherence. Otherwise, phase jitter of the probe laser relative to the ion beam—due to the physical movement of the laser output by acoustic vibrations and potentially causing a Doppler shift—can significantly reduce Ramsey fringe contrast. Thus, in a preferred embodiment, the phase stability of the laser beam path is actively stabilized via a miniature Mach-Zehnder (MZ) beamsplitter interferometer.

[0153] (4) The fourth and final π / 2 pulse converts the ion to either the S or D state. The probability of becoming excited (D state) or unexcited (S state) is determined by the phase acquired during the Ramsey interrogation (τ,t) time. The population state is determined by measuring the fluorescence achieved by the cyclic / Doppler laser. Since the probability of being in the S or D state is partly a function of the detuning of the probe laser to the atomic transition, the probe laser can be locked to the Ramsey fringe (locking electronics discussed below) by scanning the probe laser frequency after locking via the AOM as described above, which leads to the appearance of the Ramsey fringe.

[0154] In some embodiments, the velocity distribution of the ion beam resembles a delta-function to ensure a constant Ramsey interrogation time τ from ion to ion, thereby minimizing the loss of Ramsey fringe contrast. It should also be noted that the fringe features have a width of 1 / τ, therefore τ is preferably as long as possible, limited by decoherence time and readout rate as well as lock-in bandwidth. The velocity control of the ion beam provided by the ion collimator is a substantial benefit.

[0155] A readout laser is used to perform quantum transition measurements to determine the ground-state population after Ramsey probing. The readout laser can be the same Doppler laser used via beam splitting and cooling, or it can be a different Doppler laser. Quantum transition measurements utilize fluorescence from S→P cyclic transitions. If the measurement collapses the ion's wavefunction to the S ground state, the ion will cycle through the S→P transition and will fluoresce. Otherwise, if the wavefunction collapses to the excited D state, the ion will be outside the cyclic transition and in a dark state. Repeated fluorescence measurements using an imaging system quickly provide a measure of the probability of a successful quantum transition.

[0156] The imaging system includes a photomultiplier tube (PMT) and lenses to focus ion fluorescence onto the PMT aperture. Readouts can be taken as the average value over time as ions pass through, or by using a field-programmable gate array (FPGA) to gate the photons arriving during ion passage one-by-one. Ultimately, ion velocity and spacing will set the readout time, along with the scattering rate of cyclic transitions. The estimated readout time can be achieved using the following equation, considering the time it takes for ions to pass through the ion filter. This is given by T. a1 =d RB / V 原子 Given, where d RB It is the well length and V 原子 That is the velocity of the ion beam. Then, it must be determined at time T. a2 =d RB / V 原子 +d 原子 / V 原子 Read the second ion immediately following the first ion, where d 原子 It is the interatomic distance. Therefore, the measurement update time is determined by T. m =d 原子 / V 原子 The setting is the time between consecutive ion-to-ion measurements. By considering the 7.62-cm-long ion trap and the velocity of 76 m / s, an upper limit for the measurement time / bandwidth can be found – this corresponds to T0. m = 65 nanoseconds, the rate is approximately 15 MHz (1 / T) m This will be the fastest rate at which the laser linewidth can be updated for readout. It should be noted that for Ca... + and Sr+ The cyclic transition linewidth is approximately 20 MHz. Ultimately, there is no benefit to having a faster ion-ion update rate exceeding approximately 20 MHz, since the fastest rate at which a single ion can be read out is essentially set by that rate.

[0157] Interferometric frequency reference instruments are preferably configured with locking electronics for frequency locking, utilizing PID-based proportional (P), integral (I), and derivative (D) feedback control. A conventional operational amplifier with a feedback loop can be used to keep the probe laser frequency locked to one side of a Ramsey fringe. The Ramsey fringe width is set by 1 / τ, and the narrower the fringe, the tighter the lock (and therefore the narrower the locked laser). However, the fringe must not be so narrow that the locking range is too narrow, which could easily lead to unlocking. The input to the feedback loop is likely an error, calculated as a reading minus a setpoint, where the reading is a measured, calibrated quantum transition probability determined by the readout laser, and the setpoint is determined in-circuit. The output of the feedback loop is likely P × error + I, where I is the integral of the error as a function of time. The feedback is fed to the acousto-optic modulator described above. In some embodiments, the feedback is also partially fed to the probe laser, depending on the type of laser.

[0158] Interferometric frequency references may include a residual amplitude modulation feedback system. Interferometric frequency references can be configured to stabilize or polarize the input light to an electro-optic modulator (EOM).

[0159] Interferometric frequency references are primarily described as independent optical frequency references. If the desired operation is for an ultra-stable microwave frequency source, a high-repetition-rate frequency comb (e.g., 10 GHz) is preferably locked to an ultra-stable narrow-linewidth laser using a PID lock similar to that disclosed above. Locking the comb line to the ultra-narrow linewidth transfers the stability of the narrow-linewidth laser to the frequency comb. In a fully self-referenced system, the rate of approximately 200 femtosecond pulses emitted from the laser (approximately 10 GHz) of the frequency comb is locked with the stability of the optical reference. Detection of these pulses on a photodiode generates an ultra-stable microwave frequency reference. For this purpose, frequency combs are commercially available. Alternatively, two or more devices (as disclosed herein) can be used for frequency transfer. In some embodiments, a microwave frequency reference is generated by striking two independent devices against each other. The resulting beat can have relative stability to the optical frequency reference.

[0160] This invention is applicable to portable atomic instruments, sensors, and lasers. Current electronic warfare systems will benefit from highly stable local oscillators, which will enable analog-to-digital converters to operate at higher frequencies and with more bits. Similarly, radar systems benefit from lower local oscillator noise, enabling the detection of slow-moving objects and the use of SAR at higher altitudes or in geostationary orbit. Additionally, there is a need for ultra-narrow lasers for long-range legal seismology (explosion detection) using underwater fiber optic cables.

[0161] Interferometric frequency reference instruments can be manufactured in a wide variety of sizes, such as, for example, from about 1 cm. 3 Approximately 100cm 3 The total device volume. In some embodiments, the size of the interferometric frequency reference instrument is less than 100 cm. 3 Less than 10cm 3 or less than 1cm 3 .

[0162] Interferometric frequency reference instruments can be operated according to the following examples, which are by no means limiting.

[0163] In one instance, an interferometric frequency reference instrument and its output are considered to act as a "black box" for an ultrastable laser. Ultrastable lasers have a variety of applications in timing, sensing, and spectroscopy.

[0164] In another example, an interferometric frequency reference is used to generate RF timing. An RF clock can be generated by striking the output lasers of two or more devices together. The beat tone, containing the relative stability of the two devices, is then down-converted to radio frequency.

[0165] Another method for generating an RF clock can be achieved by locking an optical frequency comb to an ultra-stable laser. In this case, the repetition rate of the frequency comb serves as the RF clock, where the stability of the ultra-stable laser translates into the stability of the comb repetition rate.

[0166] Interferometric frequency reference instruments can be used as stable clocks. On satellites, submarines, or other vehicles, stable clocks allow for secure communication over long periods (years), even in GPS-denied environments. This contrasts sharply with existing secure communication technologies that operate on timescales of only minutes.

[0167] For space-based radar, the disclosed interferometric frequency reference instrument—operating as a stabilizing clock—is capable of (i) identifying slowly moving targets, (ii) removing clutter from radar echo signals, (iii) having the capability of geosynchronous synthetic aperture radar (SAR) with millimeter resolution, and (iv) longer integration time for SAR satellites.

[0168] The disclosed interferometric frequency reference instrument could be an ultra-stable laser in an optical atomic clock. In optical atomic clocks, the ultra-stable laser is a key component for miniaturization. The ultra-stable laser replaces the cryogenic cavity used to generate stable laser light in existing optical clocks.

[0169] The disclosed interferometric frequency reference instrument can be an ultra-stable laser in spectroscopy. In spectroscopy, ultra-stable lasers provide a tool for studying various narrow radioactive elements (nuclear transitions) by transferring their stability to another laser. Therefore, the device disclosed herein can be used for experiments on the states of nuclear matter. Furthermore, in spectroscopy, an ultra-stable laser locked to an atomic reference can serve as an absolute frequency calibration source relevant to future astronomical experiments.

[0170] Solid-state electrochemical atomic source

[0171] Some variants utilize atomic beam source devices as solid-state electrochemical atomic sources, wherein the atomic beam source devices include:

[0172] First electrode;

[0173] The second electrode is electrically isolated from the first electrode; and

[0174] A first ion conductor is inserted between a first electrode and a second electrode, wherein the first ion conductor is capable of transporting metal ions, and wherein the first ion conductor is in contact with the first electrode and the second electrode.

[0175] The atoms emitted from the atomic beam source device (as atomic vapor) can be alkali metal atoms, alkaline earth metal atoms, rare earth metal atoms, mercury, or combinations thereof. For example, metal atoms can be selected from the group consisting of Rb, Cs, Ca, Na, K, Sr, Li, Yb, Hg, and combinations thereof. Other metal atoms can be emitted from the atomic beam source device, including, for example, Si, Ga, Al, In, As, Sb, Ge, Sn, Pb, Mg, Ba, Te, Au, Pt, Cr, and Cd.

[0176] A voltage can be applied to the source atoms for a given duration at two electrodes located opposite to the first ion conductor. The voltage polarity can be switched, turning the atomic beam source device into an atomic sink. The voltage amplitude is selected to control the atomic flux.

[0177] In different embodiments, the applied voltage between the two electrodes ranges from about 0.01V to about 100V, such as from about 0.1V to about 10V. The power input of the device used to obtain metal atoms is preferably less than about 500mW, more preferably less than about 200mW, and most preferably less than about 100mW.

[0178] An “electrode” is a region that is conductive or comprises one or more material phases that are inherently conductive. A first electrode allows the conduction of electrons and is in contact with a first ionic conductor (discussed below). The first electrode allows (a) conduction of the same ionic species as conducted by the first ionic conductor, (b) diffusion of the reduced form of the same ionic species as conducted by the first ionic conductor, or both (a) and (b).

[0179] In some embodiments, the first electrode is a porous conductive structure. In some embodiments, the first electrode is a selectively permeable conductive layer. For example, see U.S. Patent No. 10,545,461 to Roper et al., which is incorporated herein by reference. In that patent application, "selectively permeable" means the transport of metal atoms through the electrode (by diffusion or conduction). In some embodiments, the first electrode is a hybrid ion-electron conductor. For example, see U.S. Patent No. 10,828,618 to Roper et al., which is incorporated herein by reference.

[0180] The first electrode is preferably a porous conductive layer. The porous conductive layer is preferably a patterned metal layer directly on one surface of the first ion conductor. The metal layer is preferably thin, such as having a thickness of less than 1 micrometer, more preferably less than 200 nanometers or less than 100 nanometers. The patterning of the metal layer preferably results in small spacing between metal regions, such as less than 100-micrometer row spacing, more preferably less than 10-micrometer row spacing, and most preferably less than 2-micrometer row spacing. The metal layer can be patterned using photolithography, electron beam lithography, direct-write lithography, direct-write metal deposition (e.g., ion beam induced deposition), interference lithography, etc.

[0181] Exemplary electrode materials for porous conductive layers include Pt, Mo, W, Ni, Cu, Fe, Al, and combinations thereof. Porous conductive layers may also require more than one layer, such as a Ti adhesion layer and a Pt layer.

[0182] The porous conductive layer preferably does not chemically interact with ionic species conducted by the first ionic conductor. For example, the porous conductive layer preferably does not form an intermetallic phase and does not chemically react with ionic species except to enable electrochemical oxidation and reduction. Furthermore, apart from possible chemical bonding to adhere to the surface of the first ionic conductor, the porous conductive layer preferably does not chemically interact with the first ionic conductor itself. For example, portions of the porous conductive layer preferably do not form mobile ions transported to the first ionic conductor.

[0183] In some embodiments, the first electrode has a high diffusivity for the source metal atoms. The metal atoms, including atomic vapor, preferably have a diffusion rate of at least about 10 as measured at 25°C or the operating temperature. -10 cm 2 / s and more preferably at least about 10-6 cm 2 The diffusion rate in the first electrode is / s.

[0184] The first electrode is at least a uniform electrical conductor. The resistivity of the first electrode, measured at 25°C, is preferably less than 10 kΩ·cm, more preferably less than 1 kΩ·cm, and most preferably less than 1 Ω·cm.

[0185] In some embodiments, the first electrode includes an intercalation compound, which is a material capable of atomic intercalation by atomic vapor. In some embodiments, the intercalation compound is, for example, graphite, MoS2, TaS2, or a combination thereof. The intercalation compound can be disposed in a uniform layer consisting substantially of the intercalation compound and any intercalation atoms. The thickness of the intercalation compound layer is preferably less than 100 micrometers and more preferably less than 10 micrometers.

[0186] In some embodiments, the first electrode comprises particles of an intercalation compound in a matrix. The matrix is ​​preferably a polymer binder, such as (but not limited to) poly(vinylpyrrolidone), poly(methacrylate), poly(methyl methacrylate), poly(ethyl methacrylate), poly(2-hydroxyethyl methacrylate), fluorinated elastomers, cellulose resins, or combinations thereof. The polymer binder preferably has low degassing at the device operating temperature and is compatible with ultra-high vacuum. Matrix additives may be included to increase the conductivity of the first electrode. For example, small conductive carbon particles (e.g., ...) may be included. Carbon black).

[0187] The first electrode may also include regions and / or layers with high conductivity to minimize the sheet resistance of the first electrode. For example, the first electrode may consist of two layers: a layer that is essentially graphite and a layer that is a porous conductive layer (such as a thin platinum mesh). This layered configuration can be beneficial in ensuring that the potential does not change significantly on the electrode surface when applied (e.g., <0.1V), even if the intercalated material has moderate conductivity or even if the intercalated material is very thin. The highly conductive layer may include Pt, Mo, W, or combinations thereof. The highly conductive layer may also require more than one sublayer, such as a Ti adherent sublayer and a Pt sublayer. The highly conductive layer preferably does not form an intermetallic phase with ionic species or otherwise chemically react with ionic species. The highly conductive layer preferably does not chemically interact with the first ionic conductor.

[0188] In some embodiments, the first electrode is a hybrid ion-electron conductor, meaning that the first electrode is both an ion conductor and an electronic conductor. The hybrid ion-electron conductor preferably has a sheet resistance of less than 10 MΩ / □ (10 million ohms / square), more preferably less than 100 kΩ / □, and most preferably less than 1 kΩ / □. The resistivity of the hybrid ion-electron conductor is preferably less than 100 kΩ·cm, more preferably less than 10 kΩ·cm, and most preferably less than 100 Ω·cm. The ionic conductivity of the hybrid ion-electron conductor is preferably at least 10. -12 Ω -1 ·cm -1 More preferably at least 10 -9 Ω -1 ·cm -1 And most preferably at least 10 -6 Ω -1 ·cm -1 The ionic conductivity of the mixed ion-electron conductor passing through the electrode thickness is preferably less than 10 kΩ, more preferably less than 1 kΩ, and most preferably less than 100 Ω.

[0189] Exemplary doped mixed ion-electronic conductors include, but are not limited to, Rb 1-2x M x AlO2 (x ranges from 0 to less than 0.5), where M = Pb, Cd, and / or Ca; Rb 2-2x Fe 2-x M x O4 (x ranges from 0 to 1), where M = P, V, Nb and / or Ta; Rb 2-2x Ga 2-x M x O4 (x ranges from 0 to 1), where M = P, V, Nb and / or Ta; Rb 2-2x Al 2-x M x O4, where M = P, V, Nb and / or Ta; and Rb 1- x Al 1-x M x O2 (x ranges from 0 to less than 1), where M = Si, Ti, and / or Ge.

[0190] In some embodiments, the mixed ion-electron conductor material may be selected from alkali metal pyrophosphates, such as Rb₄P₂O₇. The alkali metal pyrophosphate may optionally be doped with one or more atoms selected from, for example, Ca, Sr, Ba, Pb, Y, La, and / or Nd. Exemplary compounds of doped alkali metal pyrophosphates include, but are not limited to, Rb. 4-2x M xP₂O₇ (x ranges from 0 to less than 2), where M = Ca, Sr, Ba, and / or Pb; and Rb 3-3x M x PO4 (x ranges from 0 to less than 1), where M = Y, La, and / or Nd.

[0191] In some embodiments, the mixed ion-electron conductor is a uniform layer consisting essentially of mixed ion-electron conductors. The thickness of the mixed ion-electron conductor material is preferably about 500 micrometers or less, and more preferably about 100 micrometers or less.

[0192] In some embodiments employing a mixed ionic-electronic conductor, the first electrode includes a region or layer with high conductivity to minimize the sheet resistance of the first electrode. For example, the first electrode may include two layers: a layer for the mixed ionic-electronic conductor and a layer for a highly conductive layer (e.g., a thin Pt mesh). This layered configuration allows the potential to remain relatively constant on the electrode surface when applied (e.g., <0.1V), even if the mixed ionic-electronic conductor has moderate conductivity or is very thin. The highly conductive layer may include Pt, Mo, W, or combinations thereof. The highly conductive layer itself may include sublayers, such as a Ti-adhesive sublayer and a Pt sublayer. The highly conductive layer preferably does not chemically interact with the ionic species and preferably does not form an intermetallic phase with the ionic species. Additionally, the highly conductive layer preferably does not chemically interact with the first ionic conductor. For example, when the highly conductive layer contains Pt, Pt is preferred. 2+ Or other platinum ions will not become mobile ions within the first ionic conductor.

[0193] The second electrode is preferably in contact with the first ionic conductor. The second electrode is not in electrical contact with the first electrode. The second electrode contains at least a second electrode first phase for storing and transporting neutral atoms. The transport of neutral atoms is preferably via diffusion, and the storage of neutral atoms is preferably via intercalation.

[0194] The atomic species contained in the first phase of the second electrode are preferably reduced forms of the same ionic species as those in the first ionic conductor. Alternatively, or additionally, different atomic species may be contained in the reservoir. For example, when the device acquires atoms, Na may be contained in the second electrode / reservoir and may be oxidized, while Rb may be reduced at the first electrode.

[0195] The first phase of the second electrode is preferably graphite. The first phase of the second electrode may mainly include sp 2 - Bonded carbon. 2 Examples of carbon-bonded materials include, but are not limited to, graphite, monolayer graphene, few-layer graphene, graphene sheets, porous graphene (perforated graphene), carbon nanotubes, and fullerenes (e.g., C64). 60 C 70(etc.), polycyclic aromatic hydrocarbons (e.g., pentanebenzene, rubinene, hexabenzocaproic acid, coumarin, etc.), chemically vapor-deposited graphite carbon, pyrolyzed carbon-containing molecules or polymers (including pyrolyzed poly(p-xylene) (e.g., pyrolyzed poly(p-xylene) or its analogues)), or combinations thereof.

[0196] The first phase of the second electrode may alternatively or additionally comprise a metal dichalcogenide. In various embodiments, the first phase of the second electrode comprises a transition metal oxide (e.g., ZnO), a transition metal sulfide (e.g., MoS2 or TaS2), a transition metal selenide (e.g., TiSe2), or a transition metal telluride (e.g., TiTe2).

[0197] The first phase of the second electrode is preferably in the form of particles. Preferably, the particles have at least one relatively short dimension to reduce the diffusion length of neutral atoms, thereby increasing the transport rate. For example, the particles of the first phase of the second electrode may have a minimum dimension of less than 1000 micrometers, less than 500 micrometers, less than 100 micrometers, less than 50 micrometers, less than 10 micrometers, less than 5 micrometers, less than 1 micrometer, or less than 500 nanometers (e.g., the diameter of a sphere or rod). In a preferred embodiment, the particles of the first phase of the second electrode have a minimum dimension selected from about 100 nanometers to about 20 micrometers. Particle size can be measured using various techniques, including dynamic light scattering, laser diffraction, image analysis, or sieving.

[0198] The first phase of the second electrode is preferably a continuous phase or a semi-continuous phase. For example, the first phase of the second electrode may be or include carbon aerogel, carbonized polymer, or reticulated glassy carbon foam.

[0199] The second electrode is preferably conductive. In various embodiments, the resistivity of the second electrode, measured at 25°C, is preferably less than 10 kΩ·cm, more preferably less than 1 kΩ·cm, even more preferably less than 100 Ω·cm, and most preferably less than 10 Ω·cm.

[0200] The thickness of the second electrode can be selected from about 1 micrometer (or less) to about 100 micrometers (or more). Typically, the second electrode is thicker than the first or third electrode.

[0201] In addition to the first phase, the second electrode may also contain one or more other phases to form a composite electrode / reservoir. See, for example, U.S. Patent No. 10,545,461 to Roper et al., which is incorporated herein by reference. The additional phase may be an atomic transport phase for storing and transporting neutral atoms. Transport of neutral atoms is preferably via diffusion. At a fixed point in time, neutral atoms may be in the process of being transported into or out of the atomic transport phase, may be stored at a fixed location within the atomic transport phase, or may move within the atomic transport phase without crossing its phase boundaries and thus be stored within that phase. Transport of neutral atoms within and / or across its phase boundaries can occur via various diffusion mechanisms, such as (but not limited to) bulk solid-phase diffusion, porous diffusion, surface diffusion, grain boundary diffusion, permeation, dissolution diffusion, etc. Storage of neutral atoms is preferably via intercalation. When the diffusion rate of metal atoms is negligible (e.g., less than 10⁻⁶), the storage of neutral atoms is also possible. -10 cm 2 When / s), it also leads to the storage of neutral atoms.

[0202] In the atomic transport phase of the second electrode, the selected metal atoms may have at least about 10 atoms as measured at the device operating temperature (such as 25°C, 100°C, 150°C, or 200°C). -10 cm 2 / s、10 -9 cm 2 / s、10 -8 cm 2 / s、10 -7 cm 2 / s、10 -6 cm 2 / s, or 10 -5 cm 2 The diffusion coefficient is / s. The diffusion of metal atoms throughout the second electrode will depend on the bulk diffusion rate of the atomic transport phase, the volume fraction of the atomic transport phase, and the connectivity / torsion of the atomic transport phase.

[0203] The atomic species contained in the atomic transport phase are preferably reduced (neutral charged) forms of at least one ionic species contained in the first ionic conductor. Alternatively, or additionally, the atomic transport phase may contain atomic species different from those contained in the first ionic conductor. For example, when the device is configured to obtain atoms, Na may be included in the atomic transport phase, and Na may be oxidized to Na at the second electrode. + Rb + It can be reduced to Rb at the first electrode, and the first ionic conductor can simultaneously contain Na. + and Rb + .

[0204] The atom transport phase and / or the first phase of the second electrode preferably contain an intercalable compound capable of being intercalated by at least one element in ionic and / or neutral form. As used herein, an "intercalable compound" (or "intercalatable compound") is a host material capable of forming an intercalation compound with guest atoms, including atomic vapors whose density is controlled. In other words, the intercalable compound is intercalable (capable of intercalation) to at least some of the atoms in the atomic vapor. The intercalated guest atoms can be neutral atoms, ionic species, or combinations thereof. Typically, the guest atoms are intercalated as neutral atoms.

[0205] In some embodiments, the host material actually contains a guest species, resulting in a material that may be referred to as an "intercalation compound". It should be noted that, for the purposes of this patent application, any reference to an intercalation compound may be replaced by an intercalation compound, and vice versa, because an intercalation compound must be able to insert a guest species, but may or may not actually contain an inserted guest species.

[0206] The term "intercalation" as used herein is not limited to the reversible inclusion or insertion of atoms, ions, or molecules between layers present in a compound; it should be referred to herein as "layer intercalation." Intercalation also includes the absorption of neutral atomic or ionic species into the bulk phase of a compound, whether the phase is amorphous or crystalline; the adsorption of neutral atomic or ionic species onto the outer or inner surfaces (e.g., phase boundaries) present in the compound; and reversible chemical bonding between neutral atomic or ionic species and the compound.

[0207] Some embodiments of the invention utilize layer intercalation, wherein a guest species such as K expands the van der Waals gaps between sheets of a layered compound such as graphite. This layer expansion requires energy. Without theoretical limitations, the energy can be supplied by an electric current to induce charge transfer between the guest (e.g., K) and the host solid (e.g., graphite). In this example, potassium graphite compounds such as KC8 and KC can be formed. 24 These compounds are reversible, meaning that when an electric current is adjusted, the potassium graphite compound may shed the inserted atoms (K). For example, those previously inserted atoms can be released into the gas phase or into the first ionic conductor. Electrical energy can be supplied to induce a change in chemical potential at the interface with the inserted compound, which then causes the layer to expand.

[0208] In some embodiments, the insertable compound is a carbonaceous material, such as a material selected from the group consisting of: graphite, graphite oxide, graphene, graphene oxide, porous graphene, graphene sheets, carbon nanotubes, fullerenes, activated carbon, coke, pitch coke, petroleum coke, carbon black, amorphous carbon, glassy carbon, pyrolytic carbon-containing molecules, pyrolytic poly(p-xylene), polycyclic aromatic hydrocarbons, and combinations thereof.

[0209] The intercalable carbonaceous material may contain at least 50 wt% carbon, preferably at least 75 wt% carbon, more preferably at least 90 wt% carbon, and most preferably at least 95 wt% carbon. In some embodiments, the carbonaceous material is essentially pure carbon, excluding impurities. The carbonaceous material may include mesoporous carbon, microporous carbon, nanoporous carbon, or combinations thereof.

[0210] Intercalable carbonaceous materials can be primarily sp 2 The form of bonded carbon. sp 2 Examples of bonded carbon include, but are not limited to, graphite, graphene, carbon nanotubes, carbon fibers, and fullerenes (e.g., C64). 60 Or C 70 Pyrolytic carbon-containing molecules or polymers (such as pyrolytic parylene, e.g., parylene-N, parylene-C, or parylene-AF-4) and large polycyclic aromatic hydrocarbons (e.g., pentacene, rubrene, hexabenzo[a]methyl]acetate, or benzo[a]methyl]acetate. In the case of graphene (which is essentially a single layer of graphite), the graphene can be a single layer or a multilayer graphene. Graphene sheets (few-layer graphene) can be used. Some embodiments utilize single-layer porous graphene, multilayer porous graphene, or graphene sheets.

[0211] In some embodiments, the carbonaceous material includes graphite. Graphite consists of planar carbon sheets. Metal atoms, particularly alkali metal atoms, readily intercalate between these carbon sheets, resulting in high diffusivity of these atoms. Graphite electrodes enable rapid metal transport at low voltages and consume very little power per atom removed. Graphite conducts electricity not only through intercalation but also due to electron delocalization within the carbon layers. Valence electrons in carbon can move freely, thus conducting electricity through graphite.

[0212] Graphite can be natural graphite (e.g., mined graphite) or synthetic graphite produced by various technologies. For example, graphite can be obtained from chemical vapor deposition of graphite carbon, carbide-derived graphite, recycled graphite, waste from graphene manufacturing, etc. Crystalline flake graphite exists as isolated, flat, plate-like particles with hexagonal edges when undisturbed; when fractured, the edges may be irregular or angular. Amorphous graphite is very fine flake graphite. Massive graphite exists in fractured veins or fissures and manifests as large, plate-like aggregates of fibrous or needle-like crystals. Highly oriented pyrolytic graphite is graphite with an angular spread of less than 1° between graphite flakes.

[0213] Graphite can be crystalline, amorphous, or a combination thereof. For example, the crystallinity of graphite can range from about 10% to about 90%. A mixture of crystalline and amorphous graphite can be beneficial not only for intercalation between crystalline layers but also for intercalation between crystalline and amorphous regions of graphite. With excessively high crystallinity, the diffusivity becomes highly anisotropic. If highly crystalline (i.e., highly anisotropic) graphite is oriented with its low diffusivity axis perpendicular to the device surface (which is typical), it will result in reduced alkali metal flux and thus degraded performance.

[0214] In some embodiments, the intercalable compound of the atom transport phase is a transition metal oxide, a transition metal dichalcogenide, or a combination thereof. The intercalable compound can also be a mixture of carbonaceous materials and transition metal oxides, or a mixture of carbonaceous materials and transition metal dichalcogenides, or a mixture of all these materials. Specifically, the intercalable compound can be a metal dichalcogenide selected from MoS2, TaS2, TiTe2, or any other transition metal dioxide, dichalcogenide, diselenide, or ditelluride.

[0215] The second electrode is preferably encapsulated by the first ion conductor and one or more reservoir walls. For example, the encapsulation can be a single encapsulation (e.g., UHV epoxy resin) or an adhesive substrate using UHV epoxy resin or hot-press bonding of silicon, borosilicate glass, or alumina die.

[0216] For the selected ion species, the first ionic conductor preferably has a high ionic conductivity. The ionic conductivity measured at 25°C or at the device operating temperature is preferably at least 10. -7 S / cm, and more preferably at least 10 -5 S / cm. The ion species can be the ionized form of atoms of interest in atomic physics and atomic measurement instruments. In various embodiments, the ion species are selected from the group consisting of: Li + Na + K + 、Rb + Cs + 、Sr + 、Sr 2+ Ca + Ca 2+ Ba + Yb 2+ Yb 3+ Hg + Hg 2+ and combinations thereof (i.e., multiple ions can exist in the device).

[0217] The first ionic conductor preferably comprises a solid electrolyte. For example, the first ionic conductor may be predominantly (>50% by weight) β-alumina, β″-alumina, or a combination of β-alumina and β″-alumina. β-alumina and β″-alumina are good conductors of their mobile ions but allow for negligible nonionic (i.e., electronic) conductivity. β″-alumina is a hard polycrystalline or single-crystal ceramic material. β-alumina and / or β″-alumina are also referred to herein as "β-alumina". When prepared as a solid electrolyte, β-alumina is combined with mobile ions (such as Na+)... + K + Li + 、Rb + Cs + 、Sr 2+ , or Ca 2+ Complexation occurs, in which case the material becomes sodium-β-alumina, potassium-β-alumina, lithium-β-alumina, rubidium-β-alumina, cesium-β-alumina, strontium-β-alumina, or calcium-β-alumina, respectively.

[0218] Other possible solid electrolyte materials for the first ion conductor include yttrium-stabilized zirconium oxide, NASICON, LISICON, KSICON, their alkali metal ion-exchange forms, and combinations thereof. In these or other embodiments, chalcogenide glasses can be used as the solid electrolyte material for the first ion conductor. Exemplary chalcogenide glasses include, but are not limited to, RbI-GeSe2-Ga2Ge3 and CsI-GeSe2-Ga2Ge3.

[0219] An atomic beam source device may include an atomic reservoir different from the second electrode / reservoir. This additional atomic reservoir is preferably in contact with the second electrode. The atomic reservoir may be partially or entirely composed of graphite or graphitic carbon. Graphite or graphitic carbon provides electrical conductivity and also provides means of storing atoms, such as in graphite intercalation compounds.

[0220] The atomic reservoir may contain a metal in the vapor phase, and may also contain a metal in the solid and / or liquid phase. The atomic species contained within the atomic reservoir are preferably reduced forms of the same ionic species as those in the first ionic conductor. Alternatively, different atomic species may be contained within the atomic reservoir.

[0221] The atomic reservoir and / or the second electrode can be designed to accommodate any mechanical strain from changes in the reservoir volume due to the loss or introduction of atoms. For example, a gap can be located between the intercalation compound and the reservoir wall to allow the intercalation compound to expand without tautnessing the reservoir wall. The reservoir wall can be designed to deform elastically and / or plastically. This configuration can be achieved through material selection (e.g., metals, polymers, or combinations thereof). Alternatively, or additionally, this configuration can be achieved through reservoir design (e.g., bellows).

[0222] The atomic reservoir and / or the second electrode have walls that are preferably impermeable to the atomic species contained within the reservoir. The walls are preferably thin films and supported by a substrate (e.g., glass, Si, alumina, etc.). One or more sides of the reservoir wall facing the interior of the reservoir preferably do not chemically interact with the ionic species. For example, the reservoir wall does not form an intermetallic phase with the ionic species and does not chemically react with them. Exemplary reservoir wall materials include Pt, Mo, W, or combinations thereof for the walls facing the interior of the reservoir. When one or more sides of the reservoir wall contact the first ionic conductor, the reservoir wall preferably does not chemically interact with the first ionic conductor except for chemical bonding to adhere to it. Exemplary reservoir wall materials include Pt, Mo, W, or combinations thereof for the walls (if present) contacting the first ionic conductor.

[0223] A single device may contain multiple ionic conductors, each with its own electrode. Multiple first electrodes may or may not be electrically connected via leads or traces. Similarly, multiple second electrodes may or may not be electrically connected via leads or traces.

[0224] Multiple sets of first electrodes, ion conductors, and second electrodes can typically be present. In some embodiments, two or more first (front) electrodes are used. In these or other embodiments, two or more second (back) electrodes are used. In any of these or other embodiments, two or more ion conductors are used.

[0225] Each electrode is typically connected to an electrical lead made of a conductive material. A lead is an electrical connection consisting of a wire of a given length, a metal pad, a metal trace, or other conductive structure. Leads are used to transmit power and may also provide physical support and potentially a heat sink. In some embodiments, a device without such leads is provided, which can be added at a later time before use.

[0226] There are many options for the electrical connections to the first and second electrodes of the atomic beam source device. The electrical connections can be attached to bonding pads for connection to external circuitry. The electrical connections can include through-holes through the wafer, patterned conductive films, doped regions of a semiconductor, wire bonding, or combinations thereof. The patterned film can be parallel to the first electrode, such as when the first electrode is substantially flat. A portion of the patterned film can be at an angle to the first electrode. In some embodiments, the electrode connector extends beyond the plane of the electrode to which it is connected.

[0227] Atomic beam source equipment can be contained within a furnace. For example, the purpose of the furnace may be to control the temperature of the equipment at a temperature higher than ambient temperature. In principle, atomic beam source equipment can be included in any type of temperature control system used for heating or cooling equipment.

[0228] Atomic beam source devices can operate over a wide range of temperatures and pressures. In various embodiments, atomic beam source devices can operate at temperatures ranging from about -200°C to about 500°C, preferably from about -50°C to about 250°C, and more preferably from about 10°C to about 200°C. After atoms are emitted from the atomic beam source device, these atoms can be cooled to ultra-low temperatures (e.g., 10 °C) as required by some application or measurement. -7 K to 10 -3 K). In different embodiments, the atomic beam source device can range from about 7600 Torr (10 atm) to about 10 -14 Tor, preferably from about 10 -3 To about 10 -13 Tor, and more preferably from about 10 -7 To about 10 -12 Operate under the pressure of the torrent.

[0229] Atomic beam source devices may include an integrated heater. The integrated heater may be a resistance heater, such as a fine wire or patterned fine metal trace (e.g., Pt or nickel-chromium alloy). For example, the integrated heater may also be a radiation heater or a thermoelectric heater. The integrated heater preferably includes a temperature sensor, such as a thermocouple or a resistance temperature detector (e.g., Pt). Preferably, the heater is in good thermal communication with a region of the first ion conductor near the first electrode.

[0230] In some embodiments, the atomic beam source device is a chip-level device that is mounted or integrated on a microelectromechanical system (MEMS) heater stage to minimize heater power.

[0231] When an integrated heater is included in the device, the heater may further include one or more thermal insulation structures. The thermal insulation structures minimize heat transfer from the heated area of ​​the device to the cooler surrounding environment. The thermal insulation structures are configured to minimize heat loss from the heated area to the cold area by acting as insulation to retain heat within the heated area. The thermal insulation structures preferably have a high thermal resistance value, as explained further below.

[0232] In some embodiments, the insulating structure may be made of the same material and layers as the atomic reservoir wall. In these or other embodiments, the insulating structure may be made of the same material and layers as the first ion conductor. Although metals may also be used, the insulating structure is preferably a polymer, ceramic, or glass, or a combination of the foregoing.

[0233] In some embodiments, the thermal insulation structure is made of a material selected from the group consisting of β-alumina (e.g., Rb-β-alumina, Na-β-alumina, or Sr-β-alumina), β″-alumina (e.g., Rb-β″-alumina, Na-β″-alumina, or Sr-β″-alumina), α-alumina, silica, fused silica, quartz, borosilicate glass, silicon, silicon nitride, silicon carbide, and combinations thereof.

[0234] Thermal insulation structures can be designed to accommodate materials with any thermal conductivity. High thermal conductivity materials will benefit from long, high aspect ratio connections, while lower thermal conductivity materials can utilize shorter, thicker connections.

[0235] A crucial design parameter for insulation structures (when present) is thermal resistance. Thermal resistance is the temperature difference across the insulation structure when a unit of heat energy flows through it per unit time; or equivalently, the temperature difference between two defined surfaces of the insulation structure that cause a unit heat flow rate in steady state. Because low heat flow rates are desired, high temperature differences, i.e., high thermal resistance, are also desirable. Preferably, the thermal resistance of the insulation structure is at least 100 K / W, more preferably at least 1,000 K / W, and most preferably at least 10,000 K / W.

[0236] The thermal insulation structure can also be configured to provide mechanical strain relief, thereby preventing mechanical damage due to accumulated thermal strain when the first ionic conductor is heated to a temperature higher than the base substrate. In some embodiments, the thermal insulation structure is mechanically connected to the base substrate, for example, by a frame. Preferably, the thermal insulation structure is designed to reduce thermal stress or residual stress by at least 2×, preferably at least 10×, and more preferably at least 100× from one side of the thermal insulation structure to the other. The reduction in thermal stress or residual stress is not an inherent material property, but a function of the geometry of the thermal insulation structure and its material properties.

[0237] In some embodiments, the insulation structure is a suspension beam. Typically, multiple suspension beams are present to connect the heated area to the cold area. The heated area contacts the cold area only through the suspension beams. The suspension beams can be straight beams, folded beams, curved beams, circular beams, etc. The suspension beams can be fabricated in any one (or more) layers using planar processes such as surface or bulk micromachining. The remainder of the heated area can be surrounded by a vacuum or a vapor phase (e.g., containing an inert gas), either of which has high thermal resistance to the cold area. As an alternative, the vapor / vacuum area can include an insulating material such as an aerogel.

[0238] In some embodiments, the thermal insulation structure has a thin metal film patterned thereon for electrical interconnection. In some preferred embodiments, a resistive heater and a temperature sensor are patterned on (in contact with) the same layer as at least one thermal insulation structure. Preferably, electrical connections to the heater and temperature sensor are also patterned on one or more thermal insulation structures. Optionally, part or all of the heater may be patterned on one or more thermal insulation structures. In some embodiments, a thin-film resistive heater is patterned on one or more sides of the same layer as the thermal insulation structure. Where the first ion conductor is separated from the thermal insulation structure, the heater may be patterned on the same side or opposite side of the thermal insulation structure relative to the location of the first ion conductor. Where the first ion conductor is the same as the thermal insulation structure or its layer, the heater may be patterned on either side of the first ion conductor (i.e., the first electrode side and / or the second electrode side). See commonly owned U.S. Patent Application No. 16 / 573,684, filed September 17, 2019, which is hereby incorporated by reference.

[0239] Integrating the heater and insulation structure within the system achieves low system power input. The system power input for controlling the vapor density of metal atoms is preferably less than about 500 mW, more preferably less than about 200 mW, and most preferably less than about 100 mW. In various embodiments, the system power input for obtaining and / or sinking metal atoms is about 1000, 500, 400, 300, 200, 100, 50, 25, or 10 mW.

[0240] In some embodiments where a high vapor density is desired, the density of metal atoms can be at least 10. 9 atoms / cm 3 Preferably at least 10 10 pcs / cm 3 And more preferably at least 10 11 pcs / cm 3 In some embodiments where a low vapor density is desired, the density of metal atoms can be less than 10. 8 atoms / cm 3 Preferably, it is less than 107 atoms / cm 3 In different embodiments, the density of metal atoms is about, at least about, or at most about 10. 6 atoms / cm 3 10 7 atoms / cm 3 10 8 atoms / cm 3 10 9 atoms / cm 3 10 10 atoms / cm 3 10 11 atoms / cm 3 , or 10 12 atoms / cm 3 .

[0241] Atomic beam source devices can be fabricated on a variety of length scales. A length scale can be characterized as the square root of the area of ​​the first electrode. This length scale can vary from 10 m to 1 micrometer, with 1 m to 10 mm being typical for macroscopic-scale atomic timing and navigation systems, and 30 mm to 10 micrometers being typical for chip-scale atomic timing and navigation systems.

[0242] Chip-level devices are preferably constructed using microfabrication techniques, including some or all of photolithography, evaporation, shadow masking, sputtering, wafer bonding, die bonding, anodic bonding, glass frit bonding, metal-to-metal bonding, and etching.

[0243] This disclosure hereby incorporates, by reference, the following patents used to teach solid-state electrochemical atomic sources as atomic sources and / or atomic sinks in some embodiments: U.S. Patent No. 9,763,314, published September 12, 2017; U.S. Patent No. 9,837,177, published December 5, 2017; U.S. Patent No. 10,056,913, published August 21, 2018; U.S. Patent No. 10,545,461, published January 28, 2020; U.S. Patent No. 10,775,748, published September 15, 2020; U.S. Patent No. 10,828,618, published November 10, 2020; and U.S. Patent No. 11,101,809, published August 24, 2021. All publications, patents, and patent applications referenced in this specification are incorporated herein by reference in their entirety as if each publication, patent, or patent application had been expressly and separately set forth herein.

[0244] In this detailed description, reference has been made to various embodiments and accompanying drawings, wherein specific exemplary embodiments of the invention are illustrated by way of example. These embodiments have been described in sufficient detail to enable those skilled in the art to practice the invention, and it should be understood that modifications can be made to the various disclosed embodiments by those skilled in the art.

[0245] When the above methods and steps indicate that certain events occur in a certain order, those skilled in the art will recognize that the order of certain steps can be modified, and such modifications are made according to variations of the invention. Furthermore, certain steps can be performed simultaneously as a parallel process where possible, or sequentially.

[0246] The above embodiments, variations, and drawings should provide an indication of the practicality and versatility of the invention. Other embodiments not providing all the features and advantages set forth herein may be used without departing from the spirit and scope of the invention. Such modifications and variations are considered to fall within the scope of the invention as defined by the claims.

Claims

1. An interferometric frequency reference instrument, the interferometric frequency reference instrument comprising: Vacuum chamber; An atomic source, configured to supply neutral atoms to be ionized; An ionizer configured to excite the neutral atoms to form ionized atoms; An ion collimator, configured to form a collimated beam of the ionized atoms; Two or more probe lasers, wherein the two or more probe lasers are configured to perform Ramsey spectroscopy on the ionized atoms; as well as A readout laser is configured to determine the ground-state population of the ionized atoms. The atomic source, the ionizer, and the ion collimator are arranged within the vacuum chamber. The ion collimator includes a radio frequency (RF) ion trap or a radio frequency (RF) ion filter, which is configured to control the beam velocity of the collimated beam of ionized atoms by applying a radio frequency electric field. Furthermore, the interferometric frequency reference instrument is configured to utilize a continuously moving beam of ionized atoms and a continuous-wave laser array with continuous interrogation.

2. The interferometric frequency reference instrument as described in claim 1, wherein, The atomic source is a solid-state electrochemical atomic source.

3. The interferometric frequency reference instrument as described in claim 1, wherein, The ionizer is arranged inside the ion collimator.

4. The interferometric frequency reference instrument as described in claim 1, wherein, The ionizer is arranged outside the ion collimator.

5. The interferometric frequency reference instrument as described in claim 1, wherein, The ion collimator is a linear collimator.

6. The interferometric frequency reference instrument as described in claim 5, wherein, The linear collimator is selected from a group consisting of a linear quadrupole trap, a Penning trap, a surface ion trap, and a mass filter.

7. The interferometric frequency reference instrument as described in claim 1, wherein, The ion collimator is a nonlinear collimator.

8. The interferometric frequency reference instrument as described in claim 7, wherein, The nonlinear collimator is in a recirculating configuration.

9. The interferometric frequency reference instrument as described in claim 1, wherein, The ion collimator is configured such that the collimated beam of the ionized atoms has a waist selected from 10 nanometers to 10 meters.

10. The interferometric frequency reference instrument as described in claim 1, wherein, The ion collimator is configured such that the collimated beam of the ionized atoms has a velocity selected from 1 micrometer / second to 0.

99. c The beam velocity, where c It is the speed of light in a vacuum.

11. The interferometric frequency reference instrument as described in claim 1, wherein, The continuous wave laser array includes the two or more probe lasers and the readout laser.

12. The interferometric frequency reference instrument as described in claim 1, wherein, The two or more probe lasers are configured to detect either quadrupole transitions or both dipole and quadrupole transitions of the ionized atoms.

13. The interferometric frequency reference instrument as described in claim 1, wherein, The interferometric frequency reference instrument further includes a cooled laser.

14. The interferometric frequency reference instrument as described in claim 1, wherein, The readout laser is further configured for cooling.

15. The interferometric frequency reference instrument as described in claim 1, wherein, The interferometric frequency reference instrument further includes an injection electrode.

16. The interferometric frequency reference instrument as described in claim 1, wherein, The interferometric frequency reference instrument further includes an ion sink configured to collect the ionized atoms exiting the ion collimator, and wherein the ion sink is arranged in the vacuum chamber.

17. The interferometric frequency reference instrument as described in claim 1, wherein, The interferometric frequency reference instrument further includes an imaging system configured to focus fluorescence from the ionized atoms.

18. The interferometric frequency reference instrument as described in claim 1, wherein, The interferometric frequency reference instrument provides an optical frequency reference.

19. The interferometric frequency reference instrument as described in claim 1, wherein, The interferometric frequency reference instrument provides a microwave frequency reference.

20. A method for generating a stable frequency reference, the method comprising: (a) Generates atomic vapor; (b) Ionize at least some of the atoms in the atomic vapor to form ionized atoms; (c) The ionized atoms are collimated in an ion collimator to form a collimated beam of the ionized atoms, wherein the ion collimator includes a radio frequency ion trap or a radio frequency ion filter, the radio frequency ion trap or radio frequency ion filter being configured to control the beam velocity of the collimated beam of the ionized atoms by applying a radio frequency electric field. (d) Irradiate at least some of the ionized atoms with a first probe laser at a first probe laser frequency; (e) Irradiate at least some of the ionized atoms with a second probe laser at a second probe laser frequency; (f) Using Ramsey spectroscopy, the frequencies of the first and second probe lasers are tuned to the S → D transitions of at least some of the ionized atoms; and (g) Irradiate at least some of the ionized atoms with a readout laser to determine the ground-state population of the ionized atoms. The method is continuous, utilizing a continuously moving beam of ionized atoms and a continuous-wave laser array with continuous probing.

21. The method of claim 20, wherein, The atomic vapor and / or the ionized atoms are obtained from a solid-state electrochemical atomic source.

22. The method of claim 20, wherein, The ionized atom is Ca. + and / or Sr + .

23. The method of claim 20, wherein, The ionized atoms provided in step (b) are formed within the ion collimator provided in step (c).

24. The method of claim 20, wherein, The ionized atoms provided in step (b) are injected into the ion collimator.

25. The method of claim 20, wherein, The method further includes irradiating some of the ionized atoms with a cooling laser to cool the ionized atoms in preparation for the Ramsey spectroscopy.

26. The method of claim 20, wherein, The ion collimator is a linear collimator.

27. The method of claim 26, wherein, The linear collimator is selected from a group consisting of a linear quadrupole trap, a Penning trap, a surface ion trap, and a mass filter.

28. The method of claim 20, wherein, The ion collimator is a nonlinear collimator.

29. The method of claim 28, wherein, The nonlinear collimator is in a recirculating configuration.

30. The method of claim 20, wherein, The collimated beam of the ionized atoms has a waist selected from 10 nanometers to 10 meters.

31. The method of claim 20, wherein, The collimated beam of the ionized atoms has a speed selected from 1 micrometer / second to 0.

99. c The beam velocity, where c It is the speed of light in a vacuum.

32. The method of claim 20, wherein, The method further includes irradiating at least some of the ionized atoms with a third probe laser.

33. The method of claim 32, wherein, The method further includes irradiating at least some of the ionized atoms with a fourth probe laser after irradiating at least some of the ionized atoms with the third probe laser.

34. The method of claim 20, wherein, The continuous wave laser array includes the first probe laser, the second probe laser, and the readout laser.

35. The method of claim 20, wherein, The stable frequency reference is an optical frequency reference.

36. The method of claim 20, wherein, The stable frequency reference is a microwave frequency reference.

37. The method of claim 20, wherein, The method utilizes the interferometric frequency reference instrument according to claim 1.

Images