High-temperature quantum sensor

Abstract

A vapor cell includes a cell body and a cell window anodically bonded to the cell body. The cell window and the cell body define a vapor cavity configured to contain a vapor comprising a plurality of alkali atoms.

Description

HIGH-TEMPERATURE QUANTUM SENSOR

[0001] This application claims the benefit of U.S. Provisional Patent Application 63 / 536,153, filed September 1, 2023, the entire content of which is incorporated herein by reference.GOVERNMENT RIGHTS

[0002] This invention was made with Government support under contract number HR00112390115, awarded by the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in the invention.TECHNICAL FIELD

[0003] This disclosure generally relates to quantum sensors.BACKGROUND

[0004] Quantum sensors use quantum phenomena such as superposition, entanglement, and quantum tunneling to measure physical quantities with very high precision. Quantum sensors can detect minute changes in environmental conditions, including magnetic fields, electric fields, temperature, pressure, and gravitational fields. Quantum sensors may be used in atomic clocks, gyroscopes, magnetometers, interferometers, and other devices that precisely measure relatively small changes in parameters, such as time and acceleration. In some instances, quantum sensors use atomic spectroscopy and light-atom interactions to detect changes in properties of gas phase atoms and obtain precise measurements for parameters acting on the gas phase atoms. To dissociate alkali atoms into gas phase atoms and maintain thermal equilibrium of the gas phase atoms, atomic sensors may operate under high temperatures, and may consume large amounts of power to maintain these high temperatures.SUMMARY

[0005] In general, systems and techniques are described for quantum sensors that may operate at high temperatures, e.g., temperatures greater than or equal to 150 degrees Celsius (°C), or temperatures greater than about 150 °C. A quantum sensor includes a vapor cell configured to withstand high temperatures, e.g., mechanical survivability of the vapor cell, and that is configured to control the density of alkali atoms vaporized within the vapor cell. In some examples, a quantum sensor includes a vapor cell including materials that are non- reactive with alkali atoms and that are anodically bonded, e.g., for mechanical survivability.In some examples, a quantum sensor includes a vapor cell including an intermetallic alloy comprising a metal and a plurality of alkali atoms, e.g., for controlling the density of alkali atoms vaporized within the vapor cell.

[0006] In one example, this disclosure describes a vapor cell including: a cell body; and a cell window anodically bonded to the cell body, wherein the cell window and the cell body define a vapor cavity configured to contain a vapor comprising a plurality of alkali atoms.

[0007] In another example, this disclosure describes a sensor including: a vapor cell including: a cell body; a cell window, wherein the cell window and the cell body define a vapor cavity and are configured to contain a vapor comprising a plurality of alkali atoms; and an intermetallic alloy comprising a metal and the plurality of alkali atoms; and a detector configured to receive a probe light transmitted through the vapor cell.

[0008] In another example, this disclosure describes a method of making a vapor cell, the method including: disposing at least one of silicon or germanium on at least one of a cell window or a cell body, wherein the cell window and the cell body define a vapor cavity; and anodically bonding the cell window to the cell body via the at least one of silicon or germanium.

[0009] The details of one or more examples of the disclosure are set forth in the accompanying drawings, and in the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.BRIEF DESCRIPTION OF DRAWINGS

[0010] FIG. l is a block diagram illustrating an example sensing system, in accordance with the techniques of the disclosure.

[0011] FIG. 2A is a cross-sectional diagram of an example vapor cell of a quantum sensor, in accordance with the techniques of the disclosure.

[0012] FIG. 2B is a cross-sectional diagram of the example vapor cell of FIG. 2A in a different configuration of a quantum sensor, in accordance with the techniques of the disclosure.

[0013] FIG. 3 is a cross-sectional diagram of another example vapor cell of a quantum sensor, in accordance with the techniques of the disclosure.

[0014] FIG. 4 is a flowchart of an example method of fabricating a vapor cell, in accordance with the techniques of the disclosure.DETAILED DESCRIPTION

[0015] In general, systems and techniques are described for quantum sensors that may operate at high temperatures, e.g., temperatures greater than about 150 degrees Celsius (°C). In some examples, a quantum sensor may operate at temperatures greater than or equal to 500 °C, e.g., greater than about 500 °C. Quantum sensors may include a magnetometer (scalar or vector), an atomic clock, a Rydberg electromagnetic (EM) radiation sensor (e.g., EM waves and / or EM energy), or any suitable quantum sensor.

[0016] Conventional quantum sensors may degrade at substantially high temperatures. For example, fluxgate magnetic sensors may be subject to breakdown of the magnetic-wire insulator coatings at high temperatures, issues related to differences in the coefficients of thermal expansion (CTE) of constituent parts, and temperature impact on microcircuitry closely mounted to the fluxgate sensors. Typical vapor cell magnetometers operate include alkali atoms that are maintained at relatively low temperatures, such as about 100 °C or less. At higher temperatures, the alkali atoms used to sense the magnetic field react with the walls of the vapor cell, which are usually made from borosilicate glass, which depletes the alkali atoms and “darkens” the cell, e.g., reducing optical transmission of the interrogating light, e.g., excitation and / or probe laser light. In other words, the material of the vapor cell may react with the vaporized alkali atoms, and the inner surfaces of the vapor cell may have reduced optical transmission and / or may become substantially opaque to excitation and / or probe radiation (e.g., light) in addition to depleting the vapor of alkali atoms, reducing the sensing capability of the quantum sensor.

[0017] The amount of alkali atoms within the vapor cell may need to be carefully controlled. For example, changes in alkali atom density may cause variation in pressure within the vapor cell or increase collision between alkali atoms and a buffer gas. Typically, a vapor cell may be operated at a constant temperature with a liquid droplet reservoir of alkali atoms within the cell that functions to balance losses of alkali atoms to atom sinks (e.g., reactions with other materials, such as the cell body, cell window, or other materials) and to maintain a substantially constant density of vaporized alkali atoms. Such a configuration may not be used at a wide range of temperatures because the optical density of the vapor may vary considerably, and the density may increase such that the vapor itself becomes substantially opaque to probe light and / or excitation (e.g., “pump”) radiation.

[0018] In accordance with the techniques, systems, devices, and / or sensors disclosed herein, a quantum sensor includes a vapor cell configured to withstand high temperatures, e.g., mechanical survivability of the vapor cell, and that is configured to control the density ofalkali atoms vaporized within the vapor cell. The alkali, or vaporized alkali atoms, may be lithium, sodium, potassium, rubidium, cesium, or any suitable alkali. In some examples, a quantum sensor includes a vapor cell including materials that are non-reactive with alkali atoms and that are anodically bonded, e.g., for mechanical survivability. For example, a vapor cell may include a cell body comprising a material that is non-reactive (e.g., reaction free, or not chemically reactive) with alkali atoms or an alkali vapor, e.g., materials such as sapphire, titanium, germanium, or any material that is non-reactive with vaporized alkali atoms.

[0019] The vapor cell may also include a cell window for probing contents of the vapor cell. The cell window is configured to form a seal with the cell body in order to retain a vapor including a plurality of alkali atoms within a volume (e.g., the cell volume) defined by the cell body and the cell window, e.g., the cell body may be substantially hollow and open at opposing ends that are hermetically sealed via cell windows on both ends. For example, the cell window may comprise a material that is non-reactive with alkali atoms, such as sapphire, titanium, germanium, or any material that is non-reactive with vaporized alkali atoms, and that is configured to be anodically bonded with the cell body. The window may also be substantially optically transparent to at least certain light frequencies, e.g., excitation radiation and probe light. For example, a vapor cell may include a sapphire cell window anodically bonded to a sapphire cell body with amorphous silicon.

[0020] Rather than use a homogeneous source of alkali atoms, vapor cells described herein may include a heterogeneous source of alkali atoms that is chemically configured to release the alkali atoms at a reduced rate at high temperatures. The vapor cell may include an intermetallic alloy comprising a metal and a plurality of alkali atoms. The intermetallic alloy may be configured for temperature-independent control of the density of vaporized alkali atoms within the vapor cell. In some examples, the intermetallic alloy may be configured to retain the alkali atoms at substantially high temperatures, e.g., greater than about 150 °C, such that the alkali atoms do not further vaporize and increase the density of vaporized alkali atoms. For example, a vapor cell may be sealed with the intermetallic alloy within the cell and heated to a first high temperature which may be equal to or greater than a phase transition temperature of the intermetallic alloy. The first temperature may be significantly higher than an operating temperature of the vapor cell, e.g., greater than 500 °C, and alkali atoms may vaporize from the intermetallic alloy. The intermetallic alloy may be heated to a first temperature for a first amount of time configured to result in a vapor of alkali atoms with a particular, desired density. The intermetallic alloy may then be cooled, or allowed to cool,and the vapor cell may keep a relatively consistent density of vaporized alkali atoms as a function of temperature even for temperatures at or above 150 °C. For example, the intermetallic alloy may comprise an alkali-gold alloy that may have a 495 °C melting point, or a 490 °C melting point, or a melting point of about 500 °C. The alkali-gold alloy may be heated to above its melting point while monitoring the absorption of a probe beam (by vaporized alkali atoms) to release a known density of alkali atoms to the vapor.

[0021] The sensors and techniques described herein may provide a quantum sensor with improved temperature stability and increased operational temperature range. The sensors and techniques described herein may provide improved control of the density of vaporized alkali atoms within the vapor cell, and the ability to change the density of alkali atoms within the vapor cell, e.g., via heating an included intermetallic alloy. For example, a quantum sensor that includes a vapor cell comprising inert, or non-reactive, materials for the vapor cell body and window, e.g., to eliminate alkali sinks, and uses a controllable source, e.g., an intermetallic alloy, may control the number and / or density of alkali atoms to be substantially constant across a large range of temperatures, including temperatures at or above 150 °C. The sensor may include a vapor cell that improves sensitivity, at normal or at relatively high (e.g., at or above 150 °C) temperatures. For example, the vapor cell may prevent and / or reduce darkening of the vapor cell, e.g., via deposition of materials such as alkali atoms on a cell window and / or via an increase in the density of vaporized alkali atoms that increases the optical density of the cell to probe light and / or excitation radiation that degrades a sensor signal, signal strength, or signal-to-noise ration. The sensors and techniques described herein may provide improved sensor survivability. For example, vapor cells disclosed herein may provide improved survivability of bonding and / or sealing of the window to the cell body (e.g., via anodic bonding and / or sealing, such as a sapphire-to-sapphire bond or seal) at relatively higher temperatures, e.g., reduced bond / seal degradation, leaking, incursion of materials into the vapor cell, or the like, at temperatures at or above 150 °C, or at or above 500 °C, or at or above any suitable relatively high temperature.

[0022] FIG. 1 is a block diagram illustrating an example quantum sensing system 100, in accordance with the techniques of the disclosure. In the example shown, quantum sensing system 100 includes a sensor 112 and a computing device 106. Sensor 112 includes a vapor cell 102, a detection system 104, and a preparation system 108. Although sensor 112 is described herein as a magnetometer, quantum sensing system 100 and / or sensor 112 may be configured to sense magnetic fields, incident EM radiation, and / or any other suitable stimulus.

[0023] Vapor cell 102 may include a vapor of atoms, for example, alkali atoms, that are suitable for probing and measuring optical and magnetic properties due to their well-defined electronic structure and the precision with which their resonant frequencies and energy levels can be manipulated and detected. In some examples, vapor cell 102 may be configured to be a transducer to convert physical phenomena 110 into electrical information via detection system 104. Although described below as magnetic field 110 for brevity, physical phenomena 110 may include magnetic fields (e.g., strength, direction, and / or relative changes of magnetic fields), incident EM radiation (e.g., EM energy, light, radio waves, or the like), or other physical phenomena. For example, vapor cell 102 may be configured to convert at least a portion of the energy of a scalar or vector magnetic field 110 and / or EM radiation 110 (e.g., having frequencies in a first frequency range such as MHz, THz, or GHz radio waves), to a signal, which may be a light signal (e.g., EM radiation having frequencies in a second range such as infrared or visible light) detectable by a photodiode, or to an ionization state of the atoms in the vapor cell, or to any suitable, detectable signal.

[0024] Detection system 104 may include one or more detectors, circuits, meters, and the like, configured to detect a response of the alkali atoms to magnetic field 110 after the alkali atoms are prepared via preparation system 108, e.g., after a plurality of alkali atoms are prepared to be in a Rydberg state. For example, detection system 104 may include an optical detector configured to detect an amount of probe light from preparation system 108 passing through vapor cell 102 and capture an absorption spectrum of the alkali atoms as a function of detuning of the frequency of the probe light and indicating / quantifying electromagnetic induced transparency (EIT) of the vapor of alkali atoms. Detection system 104 may be configured to detect a response of the alkali atoms, when prepared in a Rydberg state, to magnetic field 110 and convert the detected response to one or more signals, e.g., analog and / or digital signals.

[0025] Computing device 106 may be configured to receive analog and / or digital signals from detection system 104. For example, computing device 106 may be configured to process and record and / or store received signals from detection system 104, and may be configured to store and / or output raw and / or processed data indicative of incident EM radiation 110, e.g., an amount and / or spectral content of EM radiation 110. Computing device 106 may include one or more processors, memory, and interface components.

[0026] For example, the one or more processors of computing device 106 may include any one or more of processing circuitry, a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array(FPGA), or equivalent discrete or integrated logic circuitry. The functions attributed to processors described herein may be provided by processing circuitry of a hardware device, e.g., as supported by software and / or firmware.

[0027] In some examples, memory of computing device 106 may include any volatile or nonvolatile media, such as a random-access memory (RAM), read only memory (ROM), nonvolatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, and the like. The memory may be a storage device or other non-transitory medium and may be used by processing circuitry to, for example, store information related to sensing system 100, such as information relating to vapor cell 102, detection system 104, preparation system 108, and incident EM radiation 110. In some examples, the memory may store information or previously received data from detection system 104 for later retrieval. In some examples, the memory may store settings, determined values, and / or calculated values for later retrieval.

[0028] In some examples, interface components of computing device 106 may include output devices, such as a display, sound card, video graphics adapter card, speaker, presencesensitive screen, one or more USB interfaces, video and / or audio output interfaces, or any other type of device capable of generating tactile, audio, video, or other output. A display device may use technologies including liquid crystal displays (LCD), quantum dot display, dot matrix displays, light emitting diode (LED) displays, organic light-emitting diode (OLED) displays, cathode ray tube (CRT) displays, e-ink, or monochrome, color, or any other type of display capable of generating tactile, audio, and / or visual output.

[0029] In some examples, computing device 106 may be integrated with sensing system 100, e.g., integrated with one or more of vapor cell 102, detection system 104, and preparation system 108. In other examples, computing device 106 may be an external device, e.g., a computing device separate from sensing system 100 and configured to communicate with sensing system 100.

[0030] Preparation system 108 may be configured to emit one or more EM radiation frequencies that are configured to, or may be used to, prepare alkali atoms within vapor cell 102 to a second, higher quantum energy state, e.g., a Rydberg state, from a first quantum energy state, which may be a ground state of the vapor of alkali atoms. In some examples, preparation system 108 may include any hardware suitable for preparing the atoms to be in the Rydberg state. For example, preparation system 108 may include a plurality of EM radiation sources and physical / optical elements configured to direct EM radiation from the one or more EM radiation sources. In some examples, preparation system 108 may be all-optical, e.g., including EM radiation sources may include optical sources such as lasers, light emitting diodes, incandescent sources, fluorescent sources, or the like. Physical / optical elements may include lenses, mirrors, diffraction gratings, windows, filters, waveguides, fibers, or any physical / optical element for directing and / or shaping (e.g., beam shaping) EM radiation from a radiation source to be incident on the alkali atoms within vapor cell 102.

[0031] In some examples, preparation system 108 may configured to emit a plurality of EM radiation frequencies arranged to be incident on alkali atoms within vapor cell 102. For example, preparation system 108 may be configured to emit a plurality of EM radiation “beams” each having one or more frequencies (e.g., wavelength(s)) and directed into vapor cell 102. In some examples preparation system 108 may include three EM frequencies configured to excite alkali atoms to a Rydberg state (e.g., a “three-photon” system). In some examples, preparation system 108 may include more or fewer than three EM radiation frequencies, e.g., one EM radiation frequency, two EM radiation frequencies or four or more EM radiation frequencies. In some examples, the plurality of EM radiation frequencies of preparation system 108 may be configured to prepare alkali atoms within vapor cell 102 to be in a Rydberg state via one or more intermediate quantum energy states.

[0032] Vapor cell 102 may include a cell body and one or more cell windows anodically bonded to the cell body. The cell body and cell windows may define a vapor cavity, e.g., a volume within vapor cell 102, and vapor cell 102 may be configured to contain a vapor comprising a plurality of alkali atoms, e.g., within the vapor cavity. The anodic bonding between the cell body and the cell window(s) may create a hermetic seal, which may enable vapor cell 102 to have a relatively low or zero vapor transmission rate, e.g., low or zero alkali vapor loss and / or low or zero gas or vapor ingress or leak into vapor cell 102, e.g., low or no water, oxygen, or helium ingress. In some examples, the cell body and cell window may have substantially the same CTE, e.g., a matched CTE, and the anodic bond may also have a CTE substantially matched with the cell body and / or cell window.

[0033] FIGS. 2A and 2B are cross-sectional diagrams of an example vapor cell 202 of a quantum sensor, in accordance with the techniques of the disclosure. FIG. 2A illustrates vapor cell 202 and a first configuration of probe light 208A and excitation radiation 208B, and FIG. 2B illustrates a second configuration of probe light 208A and excitation radiation 208B. Vapor cell 202 may be substantially similar to vapor cell 102 described above.

[0034] In the example shown, vapor cell 202 includes a cell body 204, a cell window 206a, and a cell window 206b on an opposing side of cell body 204 from cell window 206a. Cell body 204 may comprise sapphire or germanium. Cell windows 206a and 206b may beanodically bonded to cell body 204 via anodic bonds 222a and 222b, respectively. Cell windows 206a and 206b (collectively “cell windows 206”) may comprise sapphire and may have a high optical transparency for probe light 208A and / or excitation radiation 208B, e.g., cell windows 206 may be substantially optical transparent to probe light 208A and / or excitation radiation 208B. Cell body 204 and cell windows 206 may define vapor cell cavity 212 and may be configured to contain a vapor comprising a plurality of alkali atoms within vapor cell cavity 212.

[0035] Probe light 208A may be EM radiation having radio wave frequencies or optical frequencies detectable via detection system 104. Probe light 208 A and excitation radiation 208B may be arranged to propagate to and / or through vapor cell cavity 212, e.g., to and / or through the plurality of alkali atoms. In the example of FIG. 2A, both probe light 208 A and excitation radiation 208B are arranged to propagate through a vapor cell window, e.g., 206a, to and / or through vapor cell cavity 212, and at least a portion of probe light 208A (e.g., a portion not absorbed by the alkali vapor) is arranged to propagate out of vapor cell cavity 212 through cell window 206b, e.g., for detection by detection system 104. Namely, in FIG. 2A, probe light 208A and excitation radiation 208B propagate in the same direction. In the example shown in FIG. 2B, probe light 208A and 208B are arranged to propagate to and / or through vapor cell cavity 212 in different directions. For example, cell body 204 may also be substantially transparent to at least one of excitation radiation 208B or, as shown in the example, probe light 208A.

[0036] Probe light 208A and excitation radiation 208B may have frequencies that may excite at least a portion of the plurality of alkali atoms within the vapor cell cavity 212 to an excited quantum state, such as a Rydberg state. For example, probe light 208A may be infrared, visible, or ultraviolet laser light, such as 780 nanometer (nm) laser light. Excitation radiation 208B may be EM radiation having radio wave frequencies or optical frequencies, such as 480 nm laser light. A portion of probe light 208A may excite a portion of the plurality of alkali atoms within vapor cell 202 to a first quantum state, and excitation radiation 208B may excite the portion of the plurality of alkali atoms to a second, Rydberg quantum state. The quantum state of the alkali atoms excited to the Rydberg state may be perturbed by, e.g., sensitive to, magnetic fields, radio waves, or other physical phenomena, such that the alkali atoms may be “detuned” from the Rydberg state thereby changing the amplitude of probe light 208 A propagating through vapor cell 202 to detection system 104. In some examples, excitation radiation 208B may comprise multiple radiation frequencies (e.g., from multiple radiation sources) configured to excite alkali atoms, along with probelight 208 A, to a Rydberg state via one or more intermediate quantum states. For example, excitation radiation 208B may include 776 nanometer (nm) laser light and 1260 nm laser light.

[0037] Vapor cell 202 is configured to maintain the proportion and / or amount of alkali atoms excited to the Rydberg state for detection of magnetic fields or external EM radiation such as radio waves, even at relatively high temperatures, e.g., temperatures greater than about 150 °C, or greater than about 250 °C, or greater than about 490 °C, or greater than about 495 °C, or any suitable elevated temperature (e.g., elevated from room temperature). For example, vapor cell 202 may be configured to maintain a hermetic seal at temperatures greater than about 150 °C, or greater than about 250 °C, or greater than about 495 °C, maintain a density of the plurality of alkali atoms in the vapor cell cavity 212 that is substantially constant as a function of temperature for vapor cell 202 temperatures that are greater than about 150 °C, or greater than about 250 °C, or greater than about 490 °C, or greater than about 495 °C, and prevent vaporized alkali atoms from reacting with cell body 204 and / or cell windows 206 at temperatures greater than about 150 °C, or greater than about 250 °C, or greater than about 490 °C, or greater than about 495 °C.

[0038] For example, vapor cell 202 comprises anodic bonds 222a and 222b (collectively, “anodic bonds 222”), which may maintain a hermetic seal between cell body 204 and cell windows 206. Anodic bonds 222 may comprise silicon, germanium, and / or glass. In the example shown, anodic bond 222a comprises three layers, namely, two layers 218a and 220a of amorphous silicon adjacent to opposing sides of glass 216a, and similarly anodic bond 222b comprises three layers - two layers 218b and 220b of amorphous silicon adjacent to opposing sides of glass 216b. In some examples, the layers 218a, 218b (collectively, “layers 218”) may be silicon or germanium, and may be disposed on an inner surface of cell windows 206, a surface of glass 216a, or both. Similarly, layers 220a, 220b (collectively, “silicon 220”) may be silicon or germanium, and may be disposed on a surface of glass 216b, a surface of cell body 204, or both. In some examples, anodic bonds 222 may have a thickness (e.g., thickness 348 as shown in FIG. 3) of less than or equal to about 10 micrometers, e.g., in the x-direction as shown. For example, glass 216a and / or 216b may have a thickness of less than 10 micrometers, and layers 218a, 218b, 220a, 220b may each have a thickness of equal to or less than about 200 nanometers. Although shown as three layers, anodic bonds 222 may comprise more or fewer layers. For example, anodic bonds 222 may comprise a single layer 218 (or 220) of amorphous silicon or germanium between cell window 206 and cell body 204.

[0039] Vapor cell 202 may be inert in the presence of alkali metals, e.g., at elevated temperatures such as temperatures greater than about 150 °C, or greater than about 250 °C, or greater than about 490 °C, or greater than about 495 °C. For example, cell body 204 and cell windows 206 may be sapphire, which may remain substantially transparent to probe light 208A and / or excitation radiation 208B and not function as a sink for vaporized alkali atoms, e.g., not react with alkali atoms such that the atoms deposit on inner surfaces of cell body 204 and / or cell windows 206. Cell windows 206, cell body 204, and / or anodic bonds 222 may have a CTE of greater than about 6 parts per million per degree Celsius (ppm / °C). For example, cell windows 206 may have a CTE that is substantially matched with cell body 204 and / or anodic bonds 222. Glass 216a and glass 216b may comprise D-263 Schott glass, BK- 7 optical glass, soda lime glass, or any suitable glass, e.g., having a CTE of greater than about 6 (ppm / °C). Vapor cell 202 may be made of materials such as those listed in Table 1 below. In some examples, vapor cell 202 may comprise silicon having a CTE of less than about 6 (ppm / °C), however, the amount of silicon (e.g., a thin layer) for anodic bonding may be such that the CTE of the silicon does not substantially contribute to differential thermal expansion or contraction, e.g., the silicon may have such a small mass that it does not negate the CTE match of the materials of vapor cell 202.Table 1

[0040] Generally, vapor cell 202 may include and be assembled using materials and methods that can tolerate substantial temperature ranges. For example, the optics for alkali atom preparation and signal readout may be made of high-quality quartz and fused silica and cemented in place with high-temperature ceramic adhesives.

[0041] FIG. 3 is a cross-sectional diagram of an example vapor cell 302 of a quantum sensor, in accordance with the techniques of the disclosure. Vapor cell 302 may be substantially the same as vapor cell 102 of FIG. 1 and vapor cell 202 of FIGS. 2 A and 2B, except for the differences described herein. In the example shown, vapor cell 302 includes an alkali source 330 formed from an intermetallic alloy, and thermal control elements 332.

[0042] Alkali source 330 may comprise a metal and an alkali, e.g., an intermetallic alloy including a plurality of alkali atoms. For example, alkali source 330 may comprise gold and / or silver, and rubidium. Alkali source 330 may have a relatively high decomposition temperature, e.g., a decomposition temperature that is greater than about 150 °C, or greater than about 250 °C, or greater than about 490 °C, or greater than about 495 °C. At or above the decomposition temperature, the alkali atoms may be released from alkali source 330 and may vaporize. In some examples, alkali source 330 may be heated such that a desired density of alkali atoms are released from alkali source 330 and vaporized. In some examples, alkali source 330 may have a decomposition that is substantially higher than the operating temperature of vapor cell 302. For example, alkali source 330 may have a decomposition temperature of about 495 °C, and may be heated to a temperature equal to or greater than 495 °C for a period of time (or heated for a period of time according to a temperature profile as a function of time) in order to release alkali atoms from alkali source 330 such that a particular desired density of vaporized alkali atoms is achieved within vapor cell cavity 212. Subsequently, during use, vapor cell 302 may be operated at relatively high temperatures, e.g., equal to or greater than about 150 °C, but less than the decomposition temperature of alkali source 330 (e.g., less than about 495 °C) to thereby maintain the density of vaporized alkali atoms within vapor cell 302 even at relatively high temperatures. If the density of vaporized alkali atoms within vapor cell 302 changes over time, the desired density may be reached again via alkali source 330, e.g., which may act as a source or sink of alkali atoms to maintain the density of vaporized alkali atoms. For example, alkali source 330 may be heated to a temperature equal to or greater than its decomposition temperature, e.g., to increase the density of vaporized alkali atoms, or the temperature of alkali source 330 may be controlled to reduce the density of vaporized alkali atoms within vapor cell cavity 212.

[0043] In the example shown, alkali source 330 is disposed on an inner surface of cell body 204. In other examples, alkali source 330 may be of any form and be located anywhere within vapor cell cavity 212. For example, alkali source 330 may be a pellet disposed within vapor cell cavity 212.

[0044] In the example shown, vapor cell 302 includes thermal control elements 332.Thermal control elements 332 may be configured to heat and / or cool alkali source 330 within vapor cell cavity 212. For example, thermal control elements 332 may comprise a heater and / or heating elements configured to heat alkali source 330 to at least the alloy decomposition temperature. In the example shown, thermal control elements 332 are platinum heaters formed as coils within vapor cell body 204 and positioned close enough to alkali source 330 to heat or cool alkali source 330. In other examples, thermal control elements 332 may be disposed on an inner surface of cell body 204, or an outer surface of cell body 204. In some examples, vapor cell 302 may not include thermal control elements 332, but may be configured such that alkali source 330 may be thermally controlled. For example, vapor cell 302 may be substantially transparent to radiation that may be directed onto alkali source 330 to change the temperature of alkali source 330, or vapor cell 302 may be configured such that alkali source 330 may be inductively heated, e.g., via an induction coil. In some examples, thermal control elements 332 may function as induction coils.

[0045] In the example shown, vapor cell 302 may be substantially cylindrical with a circular cross-section. Vapor cell 302 may have a diameter 340, a cell body thickness 342, and a vapor cavity diameter 344, e.g., along the y-direction in the example shown. In some examples, diameter 340 may be about 3 millimeters (mm), or about 5 mm, or about 7 mm, or about 10 mm, or any suitable diameter. Cell body thickness 342 may be about 0.5 mm, or about 1 mm, or about 3 mm, or about 5 mm, or any suitable thickness. Vapor cavity diameter 344 may be about 2 mm, or about 5 mm, or about 7 mm, or any suitable diameter. Cell windows 206 may have a thickness 346 (e.g., along the x-direction as shown), which may about 0.1 mm, or about 0.5 mm, or about 1 mm, or any suitable thickness. Vapor cell 302 may have a total length 350 (e.g., along the x-direction as shown) of about 2 mm, or about 5 mm, or about 10 mm, or any suitable total length.

[0046] Vapor cell 302 may have sources and sinks of vaporized alkali atoms removed over the operating conditions, and the atomic number and / or density of vaporized alkali atoms within vapor cell cavity 212 may remain substantially constant, e.g., as a function of temperature. In some examples, vapor cell 302 may include glass layers 216a and 216b of anodic bonds 222a and 222b, which may be sinks for vaporized alkali atoms, however, the diffusion of alkali atoms into glass layers 216a and 216b may remove less than 1010alkali atoms from the cell (e.g., less than a picogram for rubidium atoms), and the diffusion rate of the alkali atoms may exponentially decay until the sink is effectively saturated. Because glass layers 216a and 216b are not in the optical access area (e.g., probe light 208A and / orexcitation radiation 208B do not propagate to or through glass layers 216a and 216b), any darkening of glass layers 216a and 216b associated with this diffusion will not impact the sensor signal using vapor cell 302.

[0047] Vapor cell 302 may be configured such that the density of alkali atoms within vapor cell cavity 212 is substantially constant as a function of temperature via alkali source 330. For example, the number density of alkali atoms may be comparable to the equilibrium amount of alkali atoms at 100 °C, which is 1.927 x 10'7amagat (amg) or equivalently 5.178 x 1018atoms / m3. Using the density of gold as the metal, an alloy volume of 0.175 cubic micrometers to fill a 1-cc vapor cell cavity 212 if the alkali source 330 is completely depleted. By depleting only 1% of the alkali source 330, the volume of alkali source 330 would only increase to 17.5 cubic micrometers to fill a 1-cc vapor cell. The alkali source 330 may be deposited so that it does not impact the clear aperture of vapor cell 302 and the sensor (e.g., magnetometer) during operation. To release 1% of the alkali atoms, e.g., for rubidium, the temperature of alkali source 330 may be increased about 25 °C above the melting point of alkali source 330 before alkali source 330 would resolidify, enabling significant control of the final number density of alkali atoms in vapor cell 302. In some examples, release of the alkali atoms from alkali source 330 may also be monitored in situ with Doppler-free saturated absorption spectroscopy to ensure the correct number density in vapor cell 302 before operation.

[0048] FIG. 4 is a flowchart of an example method of fabricating a vapor cell, in accordance with the techniques of the disclosure. Although the method is described with reference to sensing system 100 and vapor cell 102 of FIG. 1 and vapor cells 202 and 302 of FIGS. 2A-3, the methods discussed herein may include and / or utilize other systems and methods in other examples.

[0049] A manufacturer may dispose at least one of silicon or germanium 218, 220 on at least one of a cell window 206 or a cell body 204 (402). For example, the manufacturer may dispose silicon or germanium by coating, sputtering, depositing, evaporating (e.g., via atomic layer deposition) silicon or germanium layers 218, 220 directly onto a surface of cell windows 206 or cell body 204, or onto glass layers 216. In some examples, the manufacturer may dispose glass on the silicon or germanium layers 218, 220, e.g., that are already disposed on a surface of cell window 206 or a cell body 204.

[0050] The manufacturer may anodically bond the cell window 206 to the cell body 204 via the at least one of silicon or germanium 218, 220 (404). For example, the manufacturer may compress the silicon or germanium layers 218, 220, and optionally glass layers 216, betweencell window 206 and a cell body 204 while heating cell window 206 or a cell body 204 and / or applying a voltage or electrical current to silicon or germanium layers 218, 220 to form anodic bonds 222.

[0051] In some examples, the manufacturer may heat alkali source 330 within vapor cell cavity 212 cavity to an alloy decomposition temperature to release at least a portion of the plurality of alkali atoms from the alloy as a vapor within the vapor cell cavity 212. The manufacturer may heat alkali source 330 before anodically bonding cell window 206 to cell body 204, e.g., in a vacuum or inert gas environment where alkali source 330 may be within vapor cell cavity 212 or external to vapor cell cavity 212 but configured to vaporized alkali atoms into vapor cell cavity 212, or the manufacturer may heat alkali source 330 after anodically bonding cell window 206 to cell body 204.

[0052] The following examples may illustrate one or more aspects of the disclosure:

[0053] Example 1 : A vapor cell includes: a cell body; and a cell window anodically bonded to the cell body, wherein the cell window and the cell body define a vapor cavity configured to contain a vapor comprising a plurality of alkali atoms.

[0054] Example 2: The vapor cell of example 1, wherein the cell window is substantially optically transparent to a probe light.

[0055] Example 3 : The vapor cell of example 1 or example 2, wherein the cell window comprises sapphire, wherein the cell window has a coefficient of thermal expansion (CTE) of greater than about 6 parts per million per degree Celsius (ppm / °C).

[0056] Example 4: The vapor cell of any one of examples 1-3, wherein the anodic bond comprises at least one of silicon or germanium, and wherein the cell body comprises at least one of silicon or germanium.

[0057] Example 5: The vapor cell of any one of examples 1-4, wherein the cell body and cell window are non-reactive with the plurality of alkali atoms for vapor cell temperatures greater than about 150 degrees Celsius (°C).

[0058] Example 6: The vapor cell of any one of examples 1-5, wherein the vapor cell is configured to maintain a density of the plurality of alkali atoms within the vapor cavity that is substantially constant as a function of temperature for vapor cell temperatures greater than about 150 °C.

[0059] Example 7: The vapor cell of any one of examples 1-6, further comprising an intermetallic alloy comprising a metal and the plurality of alkali atoms.

[0060] Example 8: The vapor cell of examples 7, where in the metal comprises at least one of gold or silver.

[0061] Example 9: The vapor cell of example 7 or example 8, wherein the intermetallic alloy has an alloy decomposition temperature that is greater than about 150 °C.

[0062] Example 10: The vapor cell of any one of examples 7-9, further comprising a heater configured to heat the intermetallic alloy to at least the alloy decomposition temperature.

[0063] Example 11 : A sensor includes: a vapor cell including: a cell body; a cell window, wherein the cell window and the cell body define a vapor cavity and are configured to contain a vapor comprising a plurality of alkali atoms; and an intermetallic alloy comprising a metal and the plurality of alkali atoms; and a detector configured to receive a probe light transmitted through the vapor cell.

[0064] Example 12: The sensor of example 11, wherein the cell window is substantially optically transparent to the probe light.

[0065] Example 13: The sensor of example 11 or example 12, wherein the cell window comprises of sapphire, wherein the cell window has a coefficient of thermal expansion (CTE) of greater than about 6 parts per million per degree Celsius (ppm / °C).

[0066] Example 14: The sensor of any one of examples 11-13, wherein the cell window is anodically bonded to the cell body.

[0067] Example 15: The sensor of example 14, wherein the cell body comprises at least one of sapphire or germanium, and wherein the anodic bond comprises at least one of silicon or germanium.

[0068] Example 16: The sensor of any one of examples 11-15, wherein the cell body and cell window are non-reactive with the plurality of alkali atoms for vapor cell temperatures greater than about 150 degrees Celsius (°C).

[0069] Example 17: The sensor of any one of examples 11-16, wherein the vapor cell is configured to maintain a density of the plurality of alkali atoms within the vapor cavity that is substantially constant as a function of temperature for vapor cell temperatures greater than about 150 °C.

[0070] Example 18: The sensor of any one of examples 11-17, where in the metal comprises at least one of gold or silver.

[0071] Example 19: The sensor of any one of examples 11-18, wherein the intermetallic alloy has a decomposition temperature that is greater than about 150 °C.

[0072] Example 20: The sensor of any one of examples 11-19, further comprising a heater configured to heat the alloy to at least the alloy decomposition temperature.

[0073] Example 21 : A method of making a vapor cell, the method including: disposing at least one of silicon or germanium on at least one of a cell window or a cell body, wherein thecell window and the cell body define a vapor cavity; and anodically bonding the cell window to the cell body via the at least one of silicon or germanium.

[0074] Example 22: The method of example 21, further including: disposing glass on at least one of the silicon, the germanium, the cell window, or the cell body; and anodically bonding the cell window to the cell body via the glass.

[0075] Example 23: The method of example 21 or example 22, further including: heating an intermetallic alloy within the vapor cavity, the intermetallic alloy comprising a metal and a plurality of alkali atoms, to an alloy decomposition temperature to release at least a portion of the plurality of alkali atoms from the alloy as a vapor within the vapor cavity.

[0076] Example 24: A vapor cell including: a cell body; a cell window having a coefficient of thermal expansion (CTE) of greater than about 6 parts per million per degree Celsius (ppm / °C), wherein the cell window is anodically bonded to the cell body, wherein the cell window and the cell body define a vapor cavity configured to contain a vapor comprising a plurality of alkali atoms, wherein the anodic bond comprises at least one of silicon or germanium, wherein the cell body and cell window are non-reactive with the plurality of alkali atoms for vapor cell temperatures greater than about 150 degrees Celsius (°C); and an intermetallic alloy comprising a metal and the plurality of alkali atoms, wherein the intermetallic alloy has an alloy decomposition temperature that is greater than about 490 °C.

[0077] The techniques described in this disclosure may be implemented, at least in part, in hardware, software, firmware, or any combination thereof. For example, various aspects of the techniques may be implemented within one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic QRS circuitry, as well as any combinations of such components, embodied in external devices. The terms “processor” and “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry, and alone or in combination with other digital or analog circuitry.

[0078] For aspects implemented in software, at least some of the functionality ascribed to the systems and devices described in this disclosure may be embodied as instructions on a computer-readable storage medium such as RAM, DRAM, SRAM, magnetic discs, optical discs, flash memories, or forms of EPROM or EEPROM. The instructions may be executed to support one or more aspects of the functionality described in this disclosure.

[0079] In addition, in some respects, the functionality described herein may be provided within dedicated hardware and / or software modules. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarilyimply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components or integrated within common or separate hardware or software components. Also, the techniques may be fully implemented in one or more circuits or logic elements.

Claims

CLAIMSWhat is claimed is:

1. A vapor cell comprising: a cell body; and a cell window anodically bonded to the cell body, wherein the cell window and the cell body define a vapor cavity configured to contain a vapor comprising a plurality of alkali atoms.

2. The vapor cell of claim 1, wherein the cell window is substantially optically transparent to a probe light.

3. The vapor cell of claim 1 or claim 2, wherein the cell window comprises sapphire, wherein the cell window has a coefficient of thermal expansion (CTE) of greater than about 6 parts per million per degree Celsius (ppm / °C).

4. The vapor cell of any one of claims 1-3, wherein the anodic bond comprises at least one of silicon or germanium, and wherein the cell body comprises at least one of silicon or germanium.

5. The vapor cell of any one of claims 1-4, wherein the cell body and cell window are non-reactive with the plurality of alkali atoms for vapor cell temperatures greater than about 150 degrees Celsius (°C).

6. The vapor cell of any one of claims 1-5, wherein the vapor cell is configured to maintain a density of the plurality of alkali atoms within the vapor cavity that is substantially constant as a function of temperature for vapor cell temperatures greater than about 150 °C.

7. The vapor cell of any one of claims 1-6, further comprising an intermetallic alloy comprising a metal and the plurality of alkali atoms.

8. The vapor cell of claim 7, where in the metal comprises at least one of gold or silver.

9. The vapor cell of claim 7 or claim 8, wherein the intermetallic alloy has an alloy decomposition temperature that is greater than about 150 °C.

10. The vapor cell of any one of claims 7-9, further comprising a heater configured to heat the intermetallic alloy to at least the alloy decomposition temperature.

11. A sensor comprising: a vapor cell comprising: a cell body; a cell window, wherein the cell window and the cell body define a vapor cavity and are configured to contain a vapor comprising a plurality of alkali atoms; and an intermetallic alloy comprising a metal and the plurality of alkali atoms; and a detector configured to receive a probe light transmitted through the vapor cell.

12. The sensor of claim 11, wherein the cell window is substantially optically transparent to the probe light.

13. The sensor of claim 11 or claim 12, wherein the cell window comprises of sapphire, wherein the cell window has a coefficient of thermal expansion (CTE) of greater than about 6 parts per million per degree Celsius (ppm / °C).

14. The sensor of any one of claims 11-13, wherein the cell window is anodically bonded to the cell body.

15. The sensor of claim 14, wherein the cell body comprises at least one of sapphire or germanium, and wherein the anodic bond comprises at least one of silicon or germanium.

16. The sensor of any one of claims 11-15, wherein the cell body and cell window are non-reactive with the plurality of alkali atoms for vapor cell temperatures greater than about 150 degrees Celsius (°C).

17. The sensor of any one of claims 11-16, wherein the vapor cell is configured to maintain a density of the plurality of alkali atoms within the vapor cavity that is substantially constant as a function of temperature for vapor cell temperatures greater than about 150 °C.

18. The sensor of any one of claims 11-17, where in the metal comprises at least one of gold or silver.

19. The sensor of any one of claims 11-18, wherein the intermetallic alloy has a decomposition temperature that is greater than about 150 °C.

20. The sensor of any one of claims 11-19, further comprising a heater configured to heat the alloy to at least the alloy decomposition temperature.

21. A method of making a vapor cell, the method comprising: disposing at least one of silicon or germanium on at least one of a cell window or a cell body, wherein the cell window and the cell body define a vapor cavity; and anodically bonding the cell window to the cell body via the at least one of silicon or germanium.

22. The method of claim 21, further comprising: disposing glass on at least one of the silicon, the germanium, the cell window, or the cell body; and anodically bonding the cell window to the cell body via the glass.

23. The method of claim 21 or claim 22, further comprising: heating an intermetallic alloy within the vapor cavity, the intermetallic alloy comprising a metal and a plurality of alkali atoms, to an alloy decomposition temperature to release at least a portion of the plurality of alkali atoms from the alloy as a vapor within the vapor cavity.

24. A vapor cell comprising: a cell body;a cell window having a coefficient of thermal expansion (CTE) of greater than about 6 parts per million per degree Celsius (ppm / °C), wherein the cell window is anodically bonded to the cell body, wherein the cell window and the cell body define a vapor cavity configured to contain a vapor comprising a plurality of alkali atoms, wherein the anodic bond comprises at least one of silicon or germanium, wherein the cell body and cell window are non-reactive with the plurality of alkali atoms for vapor cell temperatures greater than about 150 degrees Celsius (°C); and an intermetallic alloy comprising a metal and the plurality of alkali atoms, wherein the intermetallic alloy has an alloy decomposition temperature that is greater than about 490 °C.

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