Microwave direction finding based on rydberg molecules

Abstract

The probe laser beam causes the molecules to transition from a ground state to an excited state. The control laser beam causes the molecules in the excited state to transition to a laser-induced Rydberg state. A microwave lens converts a microwave wavefront into a corresponding microwave beam. The microwave beam counter-propagates through the molecules, thereby creating a microwave interference pattern of alternating maxima and minima. The microwave interference pattern is imposed on the probe transmission pattern as a probe transmission pattern. The direction of propagation of the microwave wavefront can be determined from the translational position of the probe transmission pattern; the intensity of the microwave wavefront can be determined from the intensity difference between the minima and maxima of the probe transmission pattern.

Description

[0001] Cross-references to other applications This application claims priority to U.S. Provisional Patent Application No. 63 / 048,302, filed July 6, 2020, entitled “RYDBERG-ATOM-BASED MICROWAVE DIRECTION FINDING,” and U.S. Patent Application No. 17 / 021,033, filed September 15, 2020, entitled “RYDBERG-MOLECULE-BASED MICROWAVE DIRECTION FINDING,” both of which are incorporated herein by reference for all purposes. Background Technology

[0002] Microwaves have numerous applications, including point-to-point communication links, satellite and spacecraft communications, remote sensing, radio astronomy, radar, and medical imaging. The term "microwave" as used broadly in this paper encompasses electromagnetic radiation with wavelengths from one meter (corresponding to a frequency of 300 MHz) to 100 micrometers (corresponding to a frequency of three terahertz (THz); in other words, the term "microwave" as used in this paper covers the frequency ranges defined by the International Telecommunication Union (ITU): UHF, SHF, EHF (also known as "millimeter waves"), and THF.

[0003] In many microwave applications, determining the propagation direction and electric field strength of a received microwave wavefront can be important. In this paper, "microwave wavefront" refers to a propagating microwave field or field component, which can be characterized by a combination of: 1) the propagation direction corresponding to the orientation of the wavefront; and 2) the electric field strength corresponding to the intensity of the wavefront. If the microwave wavefront carries information, it qualifies as a microwave signal. For example, the direction and intensity of the information-carrying microwave signal can be used to locate its transmitter, such as to orient the receiver's antenna or for geolocation purposes. While microwave sensors have been implemented using various techniques, their performance has been limited by insufficient sensitivity. What is needed is a microwave sensing method that provides higher sensitivity in both direction and intensity measurements. Attached Figure Description

[0004] Figure 1 This is a schematic diagram of a microwave direction finder based on Rydberg atoms.

[0005] Figure 2 It is applicable Figure 1 A diagram showing the atomic excitation level of a microwave orientation measuring instrument.

[0006] Figure 3 It is a graph of probe transmission intensity as a function of probe detuning when microwave radiation is turned on versus off.

[0007] Figure 4 yes Figure 1 A more detailed schematic diagram of the microwave orientation measuring instrument based on Rydberg atoms.

[0008] Figure 5 This is a flowchart of the microwave direction determination process. Detailed Implementation

[0009] This invention provides a microwave orientation finder (MDF) based on Rydberg molecules, which employs passive correlative interferometry to achieve high sensitivity, high angular resolution, wide tuning bandwidth, and in-band and out-of-band selective filtering. In this document, "molecule" refers to the smallest particle in a substance that retains all the properties of that substance and is composed of one or more atoms; this definition—which is based on… Merriam-Webster ( Merriam Webster Dictionary The definition described in [reference needed] covers both monatomic (single-atom) and polyatomic molecules. Therefore, gas-phase alkali atoms (e.g., potassium, rubidium, and cesium) qualify as molecules under this definition. [The following is not used in this text:] IUPAC Gold Book The alternative and more restrictive definition presented in the text is: "an electrically neutral entity consisting of more than one atom".

[0010] A probe laser causes molecules in the ground state to transition to an excited state, and the laser is controlled to cause molecules in the excited state to transition to a laser-induced Rydberg state. A microwave lens converts a microwave wavefront into a corresponding microwave beam. The microwave beam propagates counter-propagate through the molecules, causing them to interfere and establish a microwave interference pattern with alternating maximum and minimum microwave intensities. When the microwave wavefront has the correct frequency to cause molecules to transition from laser-induced Rydberg states to microwave-induced Rydberg states, the microwave interference pattern results in a corresponding probe transmission pattern, which can be captured by a camera. The position of the probe transmission pattern indicates the direction of the received microwave wavefront, while the minimum and maximum probe transmission intensities can be used to determine the microwave wavefront intensity.

[0011] Compared to orientation systems that use antennas to convert incident microwaves into electrical signals—based on which direction determination is performed—the Rydberg molecule-based MDF offers: (1) high sensitivity; (2) selective in-band and out-of-band filtering (due to the narrow bandwidth associated with the Rydberg-Rydberg transition); (3) high angular resolution; and (4) a very wide microwave tuning bandwidth. Examples provide a frequency range of ~1-1000 GHz, including between 10 GHz and 100 GHz, for example, for engagement and fire control radar.

[0012] like Figure 1 As shown, the MDF 100 includes a microwave lens system 102, an ultra-high vacuum (UHV) unit 104 containing atoms 106, a laser system 108, a controller 110, and an analysis system 112. An alternative embodiment uses polyatomic molecules instead of atoms. The control laser 113 of the laser system 108 outputs a control beam 114, and the probe laser 115 of the laser system 108 outputs a probe beam 116. The control beam 114—also referred to as a “pump” beam or a “coupled” beam—can be tuned to select different microwave frequencies for measurement in its application direction. The probe beam 116 is used to capture an image of the interference pattern associated with the selected microwave frequency.

[0013] Laser system 108 guides beams 114 and 116 through vacuum unit 104, which maintains atoms 106 at a temperature below 10°C. -9 At the pressure of Torr, the probe beam 116 causes atoms in the ground state to transition to an excited state. For example... Figure 2 As shown in Figure 200, it has a wavelength λ p A probe beam of 780 nanometers (nm) caused rubidium-87 atoms to move from their ground state |1>5S 1 / 2 202 jumps to 5P 3 / 2 The excited state |2> 204. The transition to the excited state is associated with the absorption of the probe beam, thus when the probe beam leaves the atom 106 ( Figure 1 When the beam is detected, an absorption peak (or transmission valley) is generated in the spectrum of the probe beam.

[0014] Control beam 114 ( Figure 1 ) makes it in the excited state 204 ( Figure 2 The atom transitions to the laser-induced Rydberg state 206. It has a wavelength λ. c A controlled beam of 480 nm caused atoms in the excited state |2>204 to transition to 28D. 5 / 2 The laser-induced Rydberg state |3>206. It has a frequency Ω MWMicrowave radiation of 104.7 GHz caused atoms in the laser-induced Rydberg state 206 to transition to the microwave-induced Rydberg state |4>29P 3 / 2 208. This transition to the laser-induced Rydberg state is associated with a phenomenon known as "electromagnetic induced transparency (EIT)," which is described as the transition of the probe beam away from the atom 106 ( Figure 1 The transmission peak in the beam spectrum is detected during the process. To perform direction determination for different microwave frequencies, different control wavelengths can be used to select different laser-induced Rydberg states, which can have different microwave-induced Rydberg states associated with different desired microwave frequencies.

[0015] like Figure 1 As shown, the microwave lens system 102 includes microwave lenses 124 and 126 with corresponding optical axes 128 and 130, arranged parallel to each other. In an alternative embodiment, the microwave lens system includes more than two microwave lenses. Wavefronts having propagation directions parallel to the optical axes 128 and 130 arrive at microwave lenses 124 and 126 simultaneously. In an alternative embodiment, wavefronts having propagation directions parallel to the corresponding optical axes arrive at microwave lenses at different times. Wavefront 132, arriving at an angle α relative to the optical axes 128 and 130, arrives at microwave lenses 124 and 126 at different times, thereby producing a phase difference θ corresponding to the propagation direction represented by angle α.

[0016] Microwave lenses 124 and 126 convert the incident microwave wavefront 132 into corresponding microwave beams 134 and 136. The microwave lens system 102 guides the microwave beams 134 and 136 so that they propagate in reverse through the atoms 106 in the vacuum unit 104 (i.e., propagating in opposite directions along the same path). The reverse-propagating microwave beams 134 and 136 produce a microwave interference pattern 140 within the vacuum unit 104.

[0017] The microwave interference pattern 140 comprises a spatial distribution pattern with alternating maxima (peaks) and minima (valleys) of microwave intensity. Microwave radiation induces transitions from laser-induced Rydberg states to microwave-induced Rydberg states. This transition leads to compensation for the EIT induced by the transition from the excited state to the laser-induced Rydberg state. In other words, the probe transmission intensity is negatively correlated with the microwave intensity.

[0018] like Figure 3 As indicated in graph 300, and given this negative correlation, the microwave minimum corresponds to the maximum transmission intensity, while the microwave maximum corresponds to the minimum transmission intensity when the probe beam is zero-detuned. Therefore, the microwave interference pattern is applied to the probe beam 116 in the form of a spatial distribution pattern with minimum and maximum transmission intensities. Figure 1 The intensity pattern of the probe beam transmission is captured by camera 142 of analysis system 112.

[0019] Figure 4 The curve 400 shows the probe transmission intensity pattern 402 captured by the wavefront 122. Pattern 402 is shown as displaced from a reference pattern 404, which corresponds to when the microwave wavefront is parallel to the optical axes 128 and 130 of the microwave lens. Figure 1 The microwave wavefront 122 produces a probe transmission intensity pattern upon reaching the microwave lens. The amount of this displacement corresponds to the angle α between the wavefront and the optical axis. Therefore, the propagation direction of the microwave wavefront 122 can be determined based on the displacement of the pattern 402 from the reference pattern 404. The intensity of the microwave wavefront corresponds to the intensity difference between the maximum value 406 and the minimum value 408 of the captured probe transmission intensity pattern 402.

[0020] like Figure 4 As shown, microwave lenses 124 and 126 are Cassegrain lenses, each comprising a concave "dish" reflector 410 and a convex reflector 412. The disks have a diameter of 30 centimeters (cm), and their optical axes are 40 cm apart. Other embodiments use different spacings (e.g., between 20 cm and 120 cm), different sized disks (e.g., between 10 cm and 100 cm), and / or other types of microwave lenses, such as phased array lenses comprising separate receiving and transmitting antennas coupled to each other with spatially varying delay elements. A second pair of microwave lenses can be used to distinguish directions along orthogonal axes, for example, so that azimuth and altitude wavefront components can be resolved. Alternatively, one of these lenses can serve as part of a second pair of lenses, acting as a double-duty lens with a third lens to provide an additional dimension of orientation determination.

[0021] The microwave lens system 102 also includes microwave repeaters 414 and 416. Microwave repeater 414 includes microwave reflectors 420 and 422 that cooperate to guide microwave beam 134 into vacuum unit 104. Microwave repeater 416 includes microwave reflectors 424, 426, and 428 that cooperate to guide microwave beam 136 into vacuum unit 104, such that beams 134 and 136 propagate in opposite directions within vacuum unit 104. Typically, there is an angle-dependent "walk-off" from the microwave lens. In the illustrated configuration, the beam walk-off will be in the opposite direction as they leave the lens, thus reducing the interference pattern. By using an odd number of microwave mirrors in one repeater instead of an odd number in the other, the walk-offs in the beams are in the same direction as they leave the repeaters, resulting in a stronger interference pattern and thus a stronger signal-to-noise ratio for the orientation meter readout.

[0022] Laser System 108 ( Figure 1 ) including dichroic reflectors 450 and 452 ( Figure 2 Dichroic reflectors 450 and 452 are used to ensure that the probe beam 116 and the control beam 114 propagate orthogonally in the opposite direction to the microwave beams 134 and 136 through the vacuum unit 106. Dichroic mirror 450 allows the probe beam 116 to be transmitted straight through to the camera 142, and also reflects the control laser beam 114. As a result, when the control beam 114 enters the vacuum unit 104, it propagates in the opposite direction to the probe beam 116. Dichroic mirror 452 also allows the probe beam 116 to be transmitted straight through, while the control laser beam 114 is reflected out of the probe beam path after passing through the vacuum unit 104, and thus away from the probe laser 115. Figure 1 In an alternative embodiment, the probe beam and the control beam can propagate together into the Rydberg cell through the same wall, or they can intersect at right angles or other angles within the cell.

[0023] Using laser cooling, atoms 106 ( Figure 1 These are "cold" atoms, meaning their temperature is below 1 milliklvin, for example, close to 300 microkelvin. Rydberg atomic vapor lasers cooled to 300 μK achieve temperature-independent microwave detection performance, along with improved correlation signal-to-noise ratio and resolution, and elimination of the Doppler effect within the technical concept. Alternative embodiments use higher-temperature atomic vapor units, such as thermal or room-temperature atomic vapor units.

[0024] Figure 5A flowchart of the microwave direction determination process 500 is provided. At 501, the MDF is calibrated. For example, a microwave wavefront with a known propagation direction and intensity can be guided to a microwave lens. The displacement of the resulting probe intensity pattern relative to a reference probe intensity pattern can be determined, such that the displacement can be mapped to the microwave propagation direction. Similarly, the known intensity of each microwave wavefront can be correlated with the difference between the maximum and minimum values ​​of the probe transmission intensity pattern, such that this difference can be mapped to the microwave wavefront intensity. This calibration process can be repeated for each of the multiple microwave frequencies of interest.

[0025] At position 502, the probe laser beam and the control laser beam can be guided through atoms in the vacuum cell, such as alkali or alkaline-earth atoms. The probe laser causes the ground-state atom to transition to an excited state, while the control laser causes the excited-state atom to transition to a laser-induced Rydberg state.

[0026] At 503, the microwave lens converts the microwave wavefront into a corresponding microwave beam. More precisely, the lens converts microwaves of various frequencies into beams, but typically only one of these frequencies results in direction determination. In the illustrated embodiment, a pair of microwave lenses is used to distinguish different directions along the spacing of the lens optical axes. In other embodiments, one or more additional lenses provide a second dimension for direction determination.

[0027] At position 504, the microwave beam is backpropagated through a group of atoms, for example, contained within a vacuum cell, to obtain a microwave interference pattern. At position 505, a probe laser beam is transmitted through the atoms, such that the microwave interference pattern is applied to the probe beam to produce patterns of high and low transmittance as the probe beam exits the atoms. Generally, multiple interference patterns exist corresponding to various microwave frequencies, but most of these do not induce transitions from laser-induced Rydberg states to microwave-induced Rydberg states, and therefore, the corresponding microwave patterns are not applied to the probe beam.

[0028] The maximum and minimum values ​​of the microwave interferometric pattern correspond to the regions of minimum and maximum values ​​of the probe transmission pattern, respectively. The atomic transitions are narrow enough that the interferometric pattern remains resolvable even when the signal source is broadband. When the interference maximum appears in the image generated by the camera, the phase or position of the interference maximum indicates the direction of the incident signal. When needed, ambiguity due to the presence of antenna sidelobes is resolved by selecting an in-band frequency near the carrier of interest, which produces a unique angle-dependent spatial offset of the interference. This Rydberg detector method greatly simplifies both signal acquisition and electronic processing in correlation interferometry.

[0029] At position 506, the probe beam transmission pattern is captured, for example, using a camera. At position 507, the captured probe transmission pattern is analyzed to determine the propagation direction and intensity of the corresponding microwave wavefront. The propagation direction is determined based on the translational position of the probe transmission pattern, for example, relative to a reference position corresponding to the coaxial microwave propagation direction. The intensity of the microwave wavefront is determined based on the intensity difference between the maximum and minimum values ​​of the captured probe transmission pattern. This completes the determination of direction (and intensity) for a single microwave frequency.

[0030] At 508, the control beam is tuned to change its wavelength, which in turn alters the laser-induced Rydberg states to which atoms in the excited state transition. This, in turn, changes which microwave-induced Rydberg states can be used as transition targets, thus determining which microwave frequencies can be selected as targets for orientation determination. In many cases, the desired target microwave frequency is first selected, and the control laser wavelength is selected based on the desired target microwave frequency. In cases where a suitable control laser wavelength does not exist for the desired target microwave frequency, some embodiments allow changing the probe laser wavelength to provide additional Rydberg transitions from which a match for the desired target microwave frequency can be found. Once the control laser beam frequency has been retuned, process 500 iterates by returning to action 502.

[0031] The illustrated embodiment achieves the following: The MDF system has a sensitivity of -194 dBm / Hz (minimum detectable signal) without degradation at high ambient temperatures. From a microwave perspective, Rydberg atomic transitions are practically continuous, and extremely wideband tuning from 1 to 1000 GHz can be achieved using tunable lasers. The instantaneous bandwidth at any transition frequency is approximately 1 MHz, where the filter response has almost infinite out-of-band rejection. This bandwidth is approximately the same at 10 GHz, 30 GHz, and 100 GHz, and therefore, the relative selectivity increases with increasing microwave frequency; this type of spectral selectivity is impossible to achieve using electronic filters.

[0032] Signals from a pair of receiving microwave lenses are correlated to provide interferometric resolution with parallel optical readout, i.e., without any active RF electronics or signal processing. The basic detector resolution of the proposed system spans the 10 GHz–100 GHz spectrum and is less than 0.5° at an incident power of -150 dBm. Higher angular resolution can be achieved by increasing the optical depth of the cold atom cloud, for example, by using a larger or denser optical cloud. Furthermore, a larger Cassegrain disk aperture can achieve correspondingly higher angular resolution. Angular resolution is ultimately limited by the signal-to-noise ratio, which in turn is affected by the field sensitivity and diffraction limit of the target microwave signal.

[0033] In this document, all techniques marked "prior art" (if any) are considered prior art; all techniques not marked "prior art" are not considered prior art. This invention provides illustrated embodiments, variations thereof, and modifications thereof, the scope of which is defined by the appended claims.

Claims

1. A microwave orientation measuring instrument, comprising: Laser systems, including A probe laser provides a probe beam that causes molecules in the ground state to transition to an excited state. A control laser that provides a control beam that causes molecules in an excited state to transition to a laser-induced Rydberg state; A microwave lens system comprising a plurality of microwave lenses that convert a microwave wavefront into a corresponding microwave beam, the microwave lens system causing at least one pair of the microwave beams to propagate in reverse through the molecules to generate a microwave interference pattern, the microwave interference pattern being applied to the probe beam in the form of a probe transmission pattern; as well as An analysis system that determines the propagation direction of the microwave wavefront based on the detected transmission pattern.

2. The microwave orientation measuring instrument according to claim 1, wherein the analysis system determines the intensity of the microwave wavefront based on the detection transmission pattern.

3. The microwave orientation measuring instrument according to claim 1, wherein the spatially varying detection transmission intensity associated with the detection transmission pattern is negatively correlated with the microwave intensity associated with the microwave interference pattern.

4. The microwave direction measuring instrument according to claim 1, further comprising a controller, the controller selecting the microwave frequency to be used for direction measurement by changing the wavelength of the control beam.

5. The microwave orientation measuring instrument according to claim 1, further comprising an ultra-high vacuum (UHV) unit for confining the molecules at a pressure below 10⁻⁹ Torr.

6. A microwave direction determination process, comprising: The probe laser beam is guided through the molecule so that the molecule in the ground state transitions to the excited state; The laser beam is guided and controlled to pass through the molecule, so that the molecule in the excited state transitions to the laser-induced Rydberg state; A microwave lens system comprising multiple microwave lenses is used to convert a microwave wavefront into multiple microwave beams, and a pair of these beams are reverse-propagated through the molecules to induce a transition of molecules in laser-induced Rydberg states to microwave-induced Rydberg states; and The propagation direction of the microwave wavefront is determined based on the detection transmission pattern in the detection laser beam.

7. The microwave direction determination process according to claim 6, wherein the determination includes: The intensity of the microwave wavefront is determined based on the detected transmission pattern.

8. The microwave direction determination process according to claim 6, wherein the spatially varying probe transmission intensity associated with the probe transmission pattern is negatively correlated with the microwave intensity associated with the microwave interference pattern, the microwave interference pattern being generated through the back propagation.

9. The microwave direction determination process according to claim 6 further includes: The microwave frequency for determining the direction of application is changed by altering the wavelength of the control laser beam.

10. The microwave direction determination process according to claim 6 further includes: The molecules are confined using an ultra-high vacuum (UHV) unit.

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