CROSS REFERENCE TO OTHER APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/048,302 entitled RYDBERG-ATOM-BASED MICROWAVE DIRECTION FINDING filed July 06, 2020, and U.S. Patent Application No. 17/021,033 RYDBERG-MOLECULE-BASED MICROWAVE DIRECTION FINDING filed September 15, 2020, both of which are incorporated herein by reference for all purposes.

BACKGROUND

[0002] Microwaves have many applications including those in point-to-point communication links, satellite and spacecraft communications, remote sensing, radio astronomy, radar, and medical imaging. "Microwave”, as broadly defined herein, encompasses electromagnetic radiation of wavelengths of one meter (corresponding to a frequency of 300 megahertz (MHz)) down to 100 micrometers (corresponding to a frequency of three terahertz (THz)); in other words, "microwave”, as defined herein, encompasses ultra-high frequency (UHF), super high frequency (SHF), extremely high frequency (EHF), also known as "millimeter wave”, and tremendously high frequency (THF) frequency ranges defined by the International Telecommunications Union (ITU).

[0003] In many microwave applications, it can be important to determine the propagation direction and electric-field strength of a received microwave wavefront. Herein, "microwave wavefront” refers to a propagating microwave field or field component that can be characterized by a combination of 1) a propagation direction that corresponds to the orientation of the wavefront; and 2) an electric-field intensity that corresponds to the intensity of the wavefront. If the microwave wavefront is information-bearing, then it qualifies as a microwave signal. For example, characterizing the direction and strength of an information-bearing microwave signal can be used to locate its transmitter, e.g., to orient a receiver’s antenna or for geolocation purposes. While microwave sensors have been realized using a variety of technologies, performance has been limited by a lack of sensitivity. What is needed is an approach to microwave sensing that provides for greater sensitivity in direction and intensity measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIGURE 1 is a schematic diagram of a Rydberg-atom-based microwave direction finder.

[0005] FIGURE 2 is a diagram of atom excitation levels applicable to the microwave direction finder of FIG. 1.

[0006] FIGURE 3 is a graph of probe transmission intensity as a function of probe detuning for when the microwave radiation is ON versus OFF.

[0007] FIGURE 4 is a more detailed schematic diagram of the Rydberg-atom-based microwave direction finder of FIG. 1.

[0008] FIGURE 5 is a flow chart of a microwave direction-finding process.

DETAILED DESCRIPTION

[0009] The present invention provides a Rydberg-molecule-based microwave direction finder (MDF) that employs passive correlative interferometry to achieve high sensitivity, high angular resolution, wide tuning bandwidth, and in band and out-of- band selective filtering. Herein, "molecule” refers to the smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms; this definition, which is set forth in the Merriam Webster Dictionary, encompasses monatomic (single-atom) molecules as well as polyatomic molecules. Thus, gas-phase alkali (e.g., potassium, rubidium, and cesium) atoms qualify as molecules under this definition. Not used herein is an alternative and more restrictive definition set forth in the IUPAC Gold Book. "An electrically neutral entity consisting of more than one atom”.

[0010] A probe laser causes molecules in a ground state to transition to an excited state, and a control laser causes molecules in the excited state to transition to a laser-induced Rydberg state. Microwave lenses convert a microwave wavefront into respective microwave beams. The microwave beams are counter-propagated through the molecules so that they interfere to establish a microwave interference pattern of alternating maximum and minimum microwave intensity. In the case that the microwave wavefront has the right frequency to cause molecules to transition from the laser-induced Rydberg state to a microwave-induced Rydberg state, 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] Relative to direction finding systems that use antennas to convert incoming microwaves to electric signals on which the direction determinations are based, the Rydberg-molecule-based MDF provides: (1) high sensitivity; (2) selective in- and out- of-band filtering (due to the narrow bandwidth associated with Rydberg-Rydberg transitions); (3) high angular resolution; and (4) very wide microwave tuning bandwidth. Embodiments provide a frequency range of ~ 1 -1000 GHz, including between 10 GHz and 100 GHz, e.g., for engagement and fire control radar.

[0012] As shown in FIG. 1, an MDF 100 includes a microwave lens system 102, an ultra- high vacuum (UHV) cell 104 containing atoms 106, a laser system 108, a controller 110, and an analysis system 112. Alternative embodiments use polyatomic molecules instead of atoms. A control laser 113 of laser system 108 outputs a control beam 114 and a probe laser 115 oflaser system 108 outputs a probe beam 116. Control beam 114, which may also be referred to as a "pump” beam or a "coupling” beam, can be tuned to select different microwave frequencies to which direction finding is applied. Probe beam 116 is used to capture an image of an interference pattern associated with the selected microwave frequency.

[0013] Laser system 108 directs beams 114 and 116 through vacuum cell 104, which maintains atoms 106 at a pressure below 10 ^-9 Torr. Probe beam 116 causes atoms in a ground state to transition to an excited state. As shown in diagram 200 of FIG. 2, a probe beam with a wavelength l _R = 780 nanometers (nm) causes a rubidium 87 atom to transition from a ground state 11) 5SI/2 202 to an excited state |2) of 5P3/2 204. The transitions to the excited state are associated with absorption of the probe beam, resulting in an absorption peak (or transmission valley) in the spectrum of the probe beam as it exits atoms 106 (FIG. 1).

[0014] Control beam 114 (FIG. 1) causes atoms in the excited state 204 (FIG. 2) to transition to a laser-induced Rydberg state 206. A control beam with a wavelength Ac = 480 nm causes atoms in excited state 12) 204 to transition to laser-induced Rydberg state |3) of 28D5/2 206. Microwave radiation with a frequency HMW = 104.7 gigahertz (GHz) causes atoms in laser-induced Rydberg state 206 to transition to a microwave-induced Rydberg state |4) 29P3/2 208. This transition to the laser-induced Rydberg state is associated with a phenomenon known as "electromagnetically induced transparency”, abbreviated "EIT”, which is expressed as a transmission peak in the spectrum probe beam as it exits atoms 106 (FIG. 1). To perform direction finding for a different microwave frequency, a different control wavelength can be used to select a different laser-induced Rydberg state, which can have a different microwave-induced Rydberg state associated with the desired different microwave frequency.

[0015] As shown in FIG. 1, microwave lens system 102 includes microwave lenses 124 and 126 with respective optical axes 128 and 130, which are arranged parallel to each other. In an alternative embodiment, the microwave lens system includes more than two microwave lenses. A wavefront with a propagation direction parallel to optical axes 128 and 130 arrives at microwave lenses 114 and 116 at the same time. In an alternative embodiment, a wavefront with a propagation direction parallel to the respective optical axes arrives at the microwave lenses at different times. A wavefront 132 arriving at an angle a with respect to optical axes 128 and 130 arrives at microwave lenses 124 and 126 at different times, resulting in a phase difference 0 corresponding to the propagation direction as represented by angle a.

[0016] Microwave lenses 124 and 126 convert an incoming microwave wavefront 132 to respective microwave beams 134 and 136. Microwave lens system 102 directs microwave beams 134 and 136 so that they counter-propagate ( i.e ., propagate in opposite directions along the same path) through atoms 106 in vacuum cell 104. The counter-propagating microwave beams 134 and 136 produce a microwave interference pattern 140 within vacuum cell 104. [0017] Microwave interference pattern 140 comprises a spatially distributed pattern with alternating maxima (peaks) and minima (valleys) of microwave intensity. The microwave radiation causes a transition from the laser-induced Rydberg state to a microwave-induced Rydberg state. This transition results is an offset to the EIT induced by the transition from the excited state to the laser-induced Rydberg state. In other words, probe transmission intensity is negatively correlated with microwave intensity.

[0018] As indicated in the graph 300 of FIG. 3 and in view of this negative correlation, the microwave minima correspond to maximum transmission intensity, while the microwave maxima correspond to minimum transmission intensity at zero detuning of the probe beam. Thus, the microwave interference pattern is imposed on probe beam 116 (FIG. 1) in the form of a spatially distributed pattern with minima and maxima of transmission intensity. The probe beam transmission intensity pattern is captured by camera 142 of analysis system 112.

[0019] The captured probe transmission intensity pattern 402 for wavefront 122 is shown in graph 400 of FIG. 4. Pattern 402 is shown displaced from a reference pattern 404 that corresponds to a probe transmission intensity pattern produced when the microwave wavefront arrives at the microwave lenses parallel to the optical axes 128 and 130 (FIG. 1) of the microwave lenses. The amount of the displacement corresponds to the angle a between the wavefront and the optical axes. Therefore, the propagation direction of the microwave wavefront 122 can be determined from the displacement of pattern 402 from reference pattern 404. The intensity of the microwave wavefront corresponds to the intensity differential between the maxima 406 and minima 408 of the captured probe transmission intensity pattern 402.

[0020] As shown in FIG. 4, microwave lenses 124 and 126 are Cassegrain lenses, each including a concave "dish” reflector 410 and a convex reflector 412. The dishes are 30 centimeters (cm) in diameter, and their optical axes are 40 cm apart. Other embodiments use different separations, e.g., between 20 and 120 cm apart, different sized dishes, e.g., between 10 and 100 cm, and/or other types of microwave lenses, e.g., phased array lenses that include 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 an orthogonal axis, e.g., so that azimuth and altitude wavefront components can be resolved. Alternatively, one of the pair of lenses can do double duty as part of a second pair with a third lens to provide the extra dimension of direction finding.

[0021] Microwave lens system 102 also includes microwave relays 414 and 416. Microwave relay 414 includes microwave reflectors 420 and 422, which cooperate to direct microwave beam 134 into vacuum cell 104. Microwave relay 416 includes microwave reflectors 424, 426, and 428, which cooperate to direct microwave beam 136 into vacuum cell 104 so that beams 134 and 136 counter-propagate within vacuum cell 104. Generally, there is some angle dependent "walk-off from the microwave lenses. In the illustrated configuration, walk-offs for the beam as they exit the lenses would be in opposite directions, weakening the interference pattern. By using an odd number of microwave mirrors in one relay and not the other, the walk-offs in the beams as they exit the relays are in the same direction, resulting in a stronger interference pattern and, thus, a stronger signal-to-noise ratio for the direction-finder readout.

[0022] Laser system 108 (FIG. 1) includes dichroic reflectors 450 and 452 (FIG. 2) which are used to cause probe beam 116 and control beam 114 to counter-propagate through vacuum cell 106 orthogonal to the path along which microwave beams 134 and 136 counter-propagate. Dichroic mirror 450 allows probe beam 116 to transmit straight through to camera 142 and reflects control laser beam 114. As a result, control beam 114 counter-propagates relative to probe beam 116 as it enters vacuum cell 104. Dichroic mirror 452 also allows probe beam 116 to transmit straight through, while the control laser beam 114, after passing through vacuum cell 104, is reflected out of the probe beam path and thus away from probe laser 115 (FIG. 1). In alternative embodiments, the probe and control beams can co-propagate into the cell through the same wall of the Rydberg cell, or can intersect at a right or other angle within the cell.

[0023] Laser cooling is used so that atoms 106 FIG. 1) are "cold” atoms, that is they have a temperature below one milliKelvin, e.g., closer to 300 microKelvin. Rydberg atom vapor laser cooled to 300 mK enables temperature-independent microwave detection performance, along with improved correlation signal-to-noise ratio and resolution with the elimination of Doppler effects within the technical concept. An alternative embodiment, uses higher temperature, e.g., hot or room-temperature atomic vapor cells.

[0024] A microwave direction-finding process 500 is flow-charted in FIG. 5. At 501, an MDF is calibrated. For example, microwave wavefronts of known propagation direction and intensity can be directed to the microwave lenses. The displacements of the resulting probe intensity patterns relative to a reference probe intensity pattern can be determined so that displacements can be mapped to microwave propagation directions. Likewise, the known strength of each microwave wavefront can be associated with the difference in maxima and minima of the probe transmission intensity pattern so that the difference can be mapped to microwave wavefront intensity. This calibration procedure can be repeated for each of plural microwave frequencies of interest.

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

[0026] At 503, microwave lenses convert a microwave wavefront into respective microwave beams. More precisely, the lenses convert microwaves of various frequencies into the beams, but typically only one of the frequencies results in a direction determination. In the illustrated embodiment, one pair of microwave lenses is used to distinguish directions that differ in the direction of separation of the optical axes of the lenses. In other embodiments, one or more additional lenses provide for a second dimension of direction finding.

[0027] At 504, the microwave beams are counter-propagated through a population of atoms, e.g., contained within a vacuum cell, resulting in a microwave interference pattern. At 505, a probe laser beam is transmitted through the atoms so that the microwave interference pattern is imposed on the probe beam to produce a pattern of high and low transmissivity for the probe beam as it exits the atoms. In general, there are multiple interference patterns corresponding to a variety of microwave frequencies, but most of these do not cause transitions from the laser-induced Rydberg state to a microwave-induced Rydberg state and, so, the corresponding microwave patterns are not imposed on the probe beam. [0028] The maxima and minima of the microwave interference pattern correspond to regions of minima and maxima, respectively, of the probe transmission pattern. The atomic transition is sufficiently narrow that the interference pattern is always resolvable even if the signal source is broadband. The phase, or position of interference maxima as they appear on an image produced by a camera, is indicative of the incoming signal direction. Ambiguities due to the presence of antenna side lobes are resolved when needed by selecting in-band frequencies near the carrier of interest, which produces a spatial shift of the interference uniquely depending upon angle. This Rydberg detector approach simplifies considerably both signal acquisition and electronic processing of correlative interferometry.

[0029] At 506, the probe beam transmission pattern is captured, e.g., by a camera. At 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, e.g., relative to a reference position corresponding to an on-axes microwave propagation direction. The intensity of the microwave wavefront is determined based on the intensity difference between the maxima and minima of the captured probe transmission pattern. This completes the direction (and intensity) finding for a single microwave frequency.

[0030] At 508, the control beam is tuned to change its wavelength, which in turn changes the laser-induced Rydberg state to which atoms in the excited state are transitioned. This in turn, changes which microwave-induced Rydberg states are available as transition targets, which determines which microwave frequency can be selected as a target for direction finding. In many cases, the desired target microwave frequency is selected first and the control laser wavelength is selected as a function of the desired target microwave frequency. In case a suitable control laser wavelength does not exist for a desired target microwave frequency, some embodiments allow the probe laser wavelength to be changed to provide additional Rydberg transitions from which a match for the desired target microwave frequency may 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) with no high ambient temperature degradation. Rydberg atom transitions are practically continuous from a microwave perspective and, using a tunable laser, can achieve extremely wide band tuning of 1-1000 GHz. The instantaneous bandwidth at any transition frequency is on the order of 1 MHz with a filter response that has practically infinite rejection out of band. This bandwidth is approximately the same at 10, 30 and 100 GHz, and therefore, relative selectivity increases with increasing microwave frequency; this type of spectral selectivity has not been achievable with 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 radio frequency electronics or signal processing. Fundamental detector resolution of the proposed system is less than 0.5° at an incident power of -150 dBm (decibel milliwatts) across the spectrum 10 GHz - 100 GHz. Higher angular resolution can be achieved by increasing the optical depth of the cold-atom cloud, e.g., by using a larger or more dense optical cloud. In addition, larger Cassegrain dish apertures can achieve correspondingly higher angular resolution. The angular resolution is ultimately limited the signal-to- noise ratio, which, in turn, is affected field sensitivity and a diffraction limit of the targeted microwave signal.

[0033] Herein, all art labeled "prior art”, if any, is admitted prior art; all art not labelled "prior art” is not admitted prior art. The illustrated embodiments, variations thereupon, and modifications thereto are provided for by the present invention, the scope of which is defined by the following claims.