Detection of signals containing radio frequency pulses

Abstract

In a general aspect, a radar system includes a vapor cell sensor system and a radio frequency (RF) optics. The vapor cell sensor system includes a vapor cell sensor, and the RF optics is configured to direct an RF field to the vapor cell sensor. The RF field includes one or more RF pulses that define a radar signal. The radar system also includes a signal processing system configured to perform operations including generating a digital signal based on a signal from the vapor cell sensor system. The digital signal represents a measured response of the vapor to the RF field over a period of time. The operations also include applying a matched filter to the digital signal to generate a filtered signal, and processing the filtered signal to determine a characteristic of the RF field sensed by the vapor cell sensor over the period of time.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 293,450, entitled "Sensing Radar and Communication Pulses Using Vapor Cells," filed December 23, 2021. The disclosure of the priority application is incorporated herein by reference in its entirety.

[0002] The following description relates to sensing signals including radio frequency pulses, such as by using a vapor cell sensor system. [Background technology]

[0003] A vapor cell contains vapor within an enclosed volume. The vapor is used as a medium to interact with electromagnetic radiation received by the vapor cell. A light beam, such as that produced by a laser, can be directed through the vapor to probe and measure the vapor's response to the received electromagnetic radiation. In this way, the vapor cell can act as a sensor of electromagnetic radiation. [Brief description of the drawings]

[0004] [Figure 1] FIG. 1 is a schematic diagram illustrating an exemplary system for detecting pulse-modulated RF electromagnetic radiation. [Figure 2A] FIG. 2 is a schematic diagram of an exemplary energy level diagram for a cesium-based vapor cell. [Figure 2B] 2B is a graph of the EIT peak obtained from the cesium-based vapor cell of FIG. 2A as a 509 nm laser is scanned across the resonance. [Figure 2C] 2B is a graph of the transient atomic response of the cesium-based vapor cell of FIG. 2A to a 10 μs RF pulse. [Figure 3A] 1 is a graph of the experimental atomic response to a 10 μs RF pulse compared to that simulated using a density matrix model. [Figure 3B]FIG. 3B is a graph showing the variation with IR power for the rising edge of the pulse response of FIG. 3A, as shown by both modeling (left panel) and experiment (right panel). [Figure 4A] 1 is a graph of an exemplary atomic response to a 10 μs RF pulse of varying electric field amplitude. [Figure 4B] 4B is a graph of the output of an exemplary matched filter when applied to the pulse shown in FIG. 4A. [Figure 4C] 4C is a graph of the peak height of the matched filter signal of FIG. 4B as a function of RF electric field for a selection of IR probe laser powers. [Figure 4D] 4C is a graph of the signal-to-noise (SNR) ratio of the matched filter signal of FIG. 4B as a function of RF power as the green laser power is varied. [Figure 5A] 13 is a graph of the signal-to-noise (SNR) ratio of an exemplary matched filter shown for various RF pulse lengths, a burst of three 2 μs pulses spaced 2 μs apart, and a 10 μs pulse acquired at lower IR power. [Figure 5B] 5B is a graph of the standard deviation of a Gaussian fit to the pulse timing measured using the matched filter peaks for the various pulse conditions in FIG. 5A. [Figure 6A] FIG. 1 is a schematic diagram of an exemplary system including a vapor cell sensor system and an RF optical element. [Figure 6B] 1 is a polar graph of an exemplary scenario in which a rotating emitter on an aircraft is represented by a stationary vapor cell receiver as the aircraft flies. [Figure 6C] 6C is a graph of an exemplary signal detected by the fixed vapor cell receiver of FIG. 6B after matched filtering. [Figure 6D] 6D is a graph of a single pulse cluster from the example signal of FIG. 6C showing the pattern formed from a single rotation of the antenna. [Figure 6E] 6E is a graph from the central lobe of a single pulse cluster shown in FIG. 6D. [Figure 7A]1 is a graph showing the effect of laser beam size on pulse shape. [Figure 7B] 1 is a graph showing the effect of laser beam size on pulse shape. [Figure 8A] 1 is a graph showing the dependence of the falling edge of an RF pulse on the green Rabi frequency at various field powers. [Figure 8B] 1 is a graph showing that for an RF pulse, the shape of the falling edge may change with laser beam position, while the shape of the rising edge may remain unchanged. [Figure 8C] 13 is a graph showing the effect of green detuning on the tail of the experimental response to an RF pulse. [Figure 9] 13 is a graph showing the effect of repetition rate on pulse shape for a 2 μs long pulse. [Figure 10A] 13 is a graph showing the effect of atomic transition selection on pulse shape at low and high Rydberg populations. [Figure 10B] 13 is a graph showing the effect of atomic transition selection on pulse shape at low and high Rydberg populations. [Figure 11A] 1 is a graph showing the variation of the RF field pulse shape at 19.4 GHz and 4.2 GHz for small RF electric fields. [Figure 11B] 1 is a graph showing the variation of the RF field pulse shape at 19.4 GHz and 4.2 GHz for small RF electric fields. [Figure 12A-F] 11 is a graph comparing the effect of matched filter templates on atomic pulse shape. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] Rydberg atoms hold considerable promise as the basis for sensitive detectors of radio frequency (RF) electric fields. For example, the time-dependent response of cesium vapor to pulse-modulated RF fields at 19.4 GHz can be studied using a room-temperature cesium vapor cell. Furthermore, density matrix simulations can be used to determine the timescales that shape the transient atomic responses of a vapor of Rydberg atoms under various laser conditions. Such simulations have found that these are governed by dephasing mechanisms including, for example, transition time broadening, Rydberg-Rydberg collisions, and ionization. In some examples, matched filters can be used to detect individual pulses (or sequences thereof) with durations from 10 μs to 50 ns, and amplitudes from 1500 μV / cm to about 170 μV / cm. This detection has been demonstrated in one particular case with a pulse width of 0.24 μV cm -1 Hz -1 / 2 This can result in a sensitivity of less than 100 nm. Such sensitivity can enable the Rydberg atomic vapor cell to function as a precision radar receiver. The Rydberg atomic vapor cell can be a Rydberg photonic crystal vapor cell in some implementations.

[0006] Technologies based on the unique features of quantum systems are attracting much interest across a wide range of applications in computing, communications, and sensing. Rydberg atoms can form the basis of many of these applications due to their uniformity, stability, and well-known properties. In the case of Rydberg atoms based on alkali atoms such as cesium (Cs) and rubidium (Rb), light can optically excite their outer electrons to states that are extremely sensitive to the presence of other atoms and external electric fields. There are many possible options for these excited states, all accessed through laser and radio frequency excitation, which can give great tunability of Rydberg atomic properties. Promising applications include radio frequency (RF) potential measurements, which can be based on optical spectroscopy of Rydberg atoms contained within vapor cell sensors.

[0007] In a vapor cell sensor, quantum interference between a laser field resonating with an atomic or molecular transition to an excited Rydberg state can produce optical transmission, called electromagnetically induced transparency (EIT), in what would otherwise be an absorbing vapor. The presence of an external RF field resonating with another atomic transition can disrupt this interference, splitting the spectrally narrow EIT transmission peak into two peaks by the Ortler-Townes mechanism. (An example of such a splitting is shown in Figure 2B.) These peaks can be further apart in optical frequencies in proportion to the strength of the RF field. As a result, changes in the optical transmission of the vapor cell sensor can identify the presence and amplitude of an RF field with high accuracy and precision.

[0008] Atomic-based potential measurements can be advantageous in that they can be accurately and reproducibly self-calibrated to known atomic properties or constants. Such quantum sensors have high sensitivity (e.g., several nV cm -1 Hz -1 / 2 It is possible to detect electromagnetic fields over a wide MHz-THz frequency range with field amplitudes from about 55 nV / cm to about 1 nV / cm. By applying a strong local auxiliary microwave field, -1 Hz -1 / 2 The sensitivity of the vapor cell sensor can be further improved to attenuate the RF field of the 1000 MHz. These sensors can provide a large frequency bandwidth of up to about 400 MHz around a single electronic transition, but the most sensitive sensing relies on an RF field that is at or near resonant with closely spaced but distinct electronic level transitions. A wide bandwidth around the Rydberg resonance can provide quasi-continuous frequency coverage. There are other methods that can be used to obtain continuous RF coverage. Additionally, an adjustable electric field can be added to dynamically Stark shift the levels. Heterodyne setups with additional RF fields can also be used. Small vapor cells (e.g., 30 mm 3Vapor cells of less than 100 nm (less than 100 nm) can be constructed purely from dielectric materials to enable near-field measurements. Dielectric vapor cells often have sub-wavelength spatial resolution and minimally perturb the incident field, thereby providing opportunities for near-field imaging or direct calibration of small antennas. Representative examples of such vapor cells, including photonic crystal vapor cells, are further described in U.S. Pat. No. 10,859,981, entitled "Vapor Cells Having One or More Optical Windows Bonded to a Dielectric Body," and U.S. Pat. No. 11,209,473, entitled "Sensing Radio Frequency Electromagnetic Radiation."

[0009] While Rydberg potential measurements can be used to detect the amplitude of continuous wave RF fields, it has been found that EIT dynamics occur on timescales well below sub-microseconds, including responses to RF fields. This behavior can, in some instances, allow for detection of modulated RF field envelopes, including pulse modulated, amplitude modulated (AM) and frequency modulated (RF) RF field envelopes. Additionally, the behavior can allow for detection of the polarization or phase of the RF field, which in some instances can also use an additional RF reference field.

[0010] In some implementations, a vapor cell sensor incorporating cesium vapor can be used. This cesium-based vapor cell sensor can be used at room temperature using counter-propagating green and infrared (IR) lasers to detect the change in transmission in response to a pulsed RF field. However, other implementations are possible, such as a rubidium photonic crystal vapor cell, the use of more than two lasers, and the use of other laser frequencies corresponding to different sets of atomic transitions. Density matrix theory can be used to study the time scales that dictate atomic reaction times, including transition time broadening, the effects of collisions and ionization. As described herein, the signal-to-noise ratio (SNR) and timing precision of a single pulse can be improved by applying a matched filter (e.g., through an FPGA, through a microprocessor, or through another type of device). Additionally, the RF field can be as low as 240 nVcm. -1 Hz -1 / 2 It can detect down to less than 170 μV / cm, corresponding to a sensitivity of less than 1000 Hz. This detection capability does not require an auxiliary RF field. The vapor cell sensor may also be used as a radar receiver. It can be used, for example, to detect 1 μs pulses emitted by rotating emitters such as passing aircraft.

[0011] To sense a pulsed RF field (e.g., an amplitude modulated RF field), the vapor cell sensor may be filled with a vapor containing atoms such as cesium or rubidium atoms and conveniently operated at or near room temperature. The vapor cell sensor may be formed in whole or in part of a dielectric material. The Rydberg state is used to generate EIT using two or more laser beams resonant with respective electronic transitions of the atoms. The laser beams that may be generated by the laser system may include a two-photon counterpropagating scheme or a three-photon system. The laser of the laser system may be frequency locked to the electronic transition. Such locking may be achieved, for example, using the Pound-Drever-Hall technique with a reference cell and an external Fabry-Perot cavity. However, other techniques are possible.

[0012] 1 presents a schematic diagram illustrating an exemplary system 100 for detecting pulses of a radio frequency (RF) field. The exemplary system 100 comprises a laser system 102 configured to generate a light beam 104 (e.g., a laser beam). To do so, the laser system 102 can include one or more individual lasers for generating the light beam 104. For example, the one or more individual lasers can include gas lasers, solid-state lasers (e.g., diode lasers), fiber lasers, liquid lasers, etc. The laser system 102 can include one or more optical elements for manipulating the light beam 104, such as mirrors, lenses, fiber optic cables, polarizers, optical filters, frequency amplifiers, etc.

[0013] The light beam includes an optical probe beam 104a, and in many variations also includes an optical combined beam 104b. The light beams 104 may be distinct from each other in frequency (or wavelength), polarization, phase, direction, position, etc. FIG. 1 shows a case where the light beam 104 includes an optical probe beam 104a and an optical combined beam 104b that differ at least in frequency. In this case, a two-photon scheme may be used by the exemplary system 100 to detect, for example, pulse-modulated RF electromagnetic radiation. An example of a two-photon scheme is described in connection with FIG. 2A. However, in other variations, the light beam 104 includes a third light beam, and the exemplary system 100 can operate using a three-photon scheme. Other multi-photon schemes are possible.

[0014] The exemplary system 100 also includes a vapor cell sensor 106 configured to have vapor therein (e.g., atomic or molecular vapor) and to allow the light beam 104 to pass through the vapor. The vapor can include components such as a vapor of alkali metal atoms, a noble gas, a gas of diatomic halogen molecules, or a gas of organic molecules. For example, the vapor can include a vapor of Group IA atoms (e.g., K, Rb, C, etc.), a noble gas (e.g., He, Ne, Ar, Kr, etc.), or both. The light beam 104 can pass through the vapor along an optical path. For example, the optical probe beam 104a and the optical coupling beam 104b can pass through the vapor by propagating along the optical path in opposite directions (e.g., counterpropagation), for example, as shown in FIG. 1 . However, other configurations are possible. For example, the optical probe beam 104a and the optical coupling beam 104b can pass through the vapor and be reflected off an internal mirror (e.g., a dielectric mirror) of the vapor cell sensor 106. In this configuration, the optical probe beam 104a and the optical combining beam 104b may co-propagate together along an optical path in the same direction.

[0015] The exemplary system 100 further comprises a photodetector 108 configured to generate a detector signal 110 based on the optical probe beam 104a after the optical probe beam 104a passes through the vapor. For example, the photodetector 108 may be a photodetector that generates an electrical signal in response to receiving an optical beam (e.g., the optical probe beam 104a). In some variations, the detector signal 110 may be based on the amplitude of the optical probe beam 104a, the deflection of the optical probe beam 104a, the phase of the optical probe beam 104a, or any combination thereof. In some variations, the photodetector 108 receives only the optical probe beam 104a, and the optical combined beam 104b is discarded (e.g., absorbed by an optical aperture). However, in other variations, more than one optical beam 104 may be used by the exemplary system 100. In these variations, the exemplary system 100 may include an instance of the optical detector 108 for each optical beam 104 used. In some variations, such as that shown in FIG. 1, the vapor cell sensor 106 and one or both of the laser system 102 and the photodetector 108 are part of (or all of) a vapor cell sensor system 112 .

[0016] The exemplary system 100 also includes a signal processing system 114 configured to perform operations in response to receiving the detector signal 110. The signal processing system 114 may include a processor based on, for example, a field programmable gate array (FPGA). However, other types of processors are possible. For example, the processor may be based on an application specific integrated circuit (ASIC) or a general processor (e.g., an x86 processor or an ARM processor). The signal processing system 114 may also include one or more memories configured to store instructions representing operations. The one or more memories may also be configured to store data generated by the execution of the operations. In many variations, the one or more memories communicate with the processor. In some variations, the signal processing system 114 includes a high pass filter configured to block portions of the detector signal 110 below a threshold frequency (e.g., portions below 100 Hz). The threshold frequency may be 100 Hz or another frequency that will block DC offset signals and possibly other undesirable low frequency components. The high-pass filter can, for example, enable the signal processing system 114 to eliminate undesirable artifacts that may occur in measurements made by the vapor cell sensor 106, the photodetector 108, or both.

[0017] The operation of the signal processing system 114 includes receiving the detector signal 110 from the photodetector 108 over a period of time and generating a digital signal based on the detector signal 110. The digital signal represents a measured response of the vapor to the RF field 116 experienced by the vapor over a period of time. In some variations, the detector signal 110 is converted to a digital signal by operation of an analog-to-digital converter (ADC). In certain cases, an amplifier may be electrically coupled (e.g., electrically upstream) to the ADC to spread the electrical signal received by the ADC over the dynamic range of the ADC. However, other types of analog components may also process the electrical signal before it is received by the ADC (e.g., an analog filter).

[0018] The RF field 116 can be generated by an RF source 118. Examples of the RF source 118 include an RF horn antenna, a dipole antenna, a source configured to generate an RF pulse, an RF signal reflected from a moving object (e.g., a radar signal), and the like. FIG. 1 illustrates the RF source 118 as an RF horn antenna capable of generating an RF test signal. However, other sources of the RF field 116 are possible. The operations also include applying a matched filter 120 to the digital signal to generate a filtered signal. In some cases, parameters of the matched filter 120 can be stored as data in one or more memories of the signal processing system 114. Additionally, the matched filter 120 can be applied to the digital signal by the processor when the processor executes instructions stored in the one or more memories.

[0019] The filtered signal is generated based on a comparison of the digital signal to a reaction template 122. The reaction template 122 represents a known reaction of the vapor to the target RF pulse. In some variations, the reaction template 122 is based on a computer simulation of the vapor's reaction to the target RF pulse. In some variations, the reaction template 122 is based on empirical data acquired or processed by the exemplary system 100. For example, the exemplary system 100 can include a source 118 of an RF field 116 configured to generate an RF pulse. In these examples, the reaction template 122 can be based on a plurality of reference RF pulses generated by the source 118 over a reference time period. The plurality of reference RF pulses define the RF field 116 and share a common pulse shape. In some variations, the plurality of reference RF pulses can include a subset of reference RF pulses each sharing a different respective common pulse shape.

[0020] The operation of the signal processing system 114 further includes processing the filtered signal to determine characteristics of the RF field 116 experienced by the vapor over the time period. Such processing can include determining at least one of the start time, duration, or amplitude of the RF pulse experienced by the vapor in the time period. Multiple RF pulses are possible (e.g., a sequence of RF pulses), including multiple types of RF pulses. For example, the reaction template 122 may be a first reaction template representing a first known reaction of the vapor to a first type of target RF pulse. In this case, the filtered signal can be generated based on a comparison of the digital signal to the first reaction template and a second reaction template representing a second known reaction of the vapor to a second type of target RF pulse. The first and second types of target pulses can have, for example, different non-overlapping field strength ranges (e.g., a first field strength range that is entirely outside a second field strength range). However, other differences are possible (e.g., duration, phase, etc.).

[0021] 1, the signal processing system 114 can be in communication with the computer 124 of the exemplary system 100. In these embodiments, the computer 124 can include a display device (e.g., a flat panel display) and enable an operator of the exemplary system 100 to interact with the components of the exemplary system 100 (e.g., the signal processing system 114). For example, the computer 124 can enable the operator to observe characteristics of the RF field 116 as determined by the signal processing system 114.

[0022] In some embodiments, the exemplary system 100 includes a heterodyne RF source configured to generate a heterodyne RF field having a known, but different, frequency than that of the RF field 116. The heterodyne RF field can include, for example, a continuous wave (CW) RF field. The heterodyne RF source is directed toward the vapor cell sensor 106 and can be excited simultaneously with the RF source 118 to allow the heterodyne RF field and the RF field 116 to simultaneously interact with the vapor during operation of the exemplary system 100. During such interaction, the two fields may interact with each other, such as by constructive interference or destructive interference. However, because the frequency of the heterodyne RF field is known, this interference can be used in certain instances to improve the ability of the signal processing system 114 to determine the characteristics of the RF field 116.

[0023] In operation, the exemplary system 100 uses the laser system 102 to generate a light beam 104 that passes through the vapor in the vapor cell sensor 106. During such passing, the RF field 116 can interact with the vapor, thereby altering the optical transmission of the vapor, such as through an EIT mechanism. This interaction can occur for all or a portion of a time period. The latter case can correspond to the RF field 116 not being present during a portion of the time period. The optical detector 108 receives at least the optical probe beam 104a after passing through the vapor and responsively generates a detector signal 110. The signal processing system 114 receives the detector signal 110 over the time period and generates a digital signal based on the detector signal 110. The digital signal represents the response of the vapor (as measured by the optical detector 108 via the optical beam 104) to the RF field 116 over the time period. The response of the vapor thus corresponds to a measured response of the vapor. In this example, the measured response is obtained using the optical probe beam 104a. However, other optical beams may be used (eg, instead of or in addition to the optical probe beam 104a).

[0024] The signal processing system 114 then applies a matched filter 120 to the digital signal to generate a filtered signal. To do this, the signal processing system 114 can compare the digital signal to a response template 122. The response template 122 represents a known response of the vapor to a target RF pulse. The signal processing system 114 can also apply the matched filter 120 to generate a filtered signal by applying a convolution function to the digital signal and the response template 122. The period of the convolution function response template may be greater than the expected duration of the pulse of the RF field 116. An example of a convolution function is further described below with respect to equation (4). The filtered signal is then processed by the signal processing system 114 to determine a characteristic of the RF field 116 experienced by the vapor over a period of time. In some situations, the exemplary system 100 may operate for a length of time without detecting an RF field or RF pulse. In these situations, the RF field 116 may not be present or may be too weak to be detected by the exemplary system 100. In some variations, the exemplary system 100 can use a heterodyne RF source to generate a heterodyne RF field, including a continuous wave (CW) RF field over a period of time, which interacts with the vapor and can do so simultaneously with the RF field 116.

[0025] The signal processing system 110 can process the filtered signal to determine at least one of the start time, duration, or amplitude of the RF pulse experienced by the vapor in time period. However, other characteristics of the RF pulse are possible (e.g., frequency, polarization, etc.). For example, the frequency of the repetition rate of the RF pulse can be used to detect the Doppler shift. One technique that can be used is pulse Doppler processing. Using this technique, range data from the return pulses is binned. The pulses in a particular bin of multiple pulses are then analyzed. The Doppler shift resolution for this method is + / - PRF, where PRF refers to the pulse repetition rate. The RF pulses in a particular bin can be Fourier transformed by the signal processing system 110 into the frequency domain to determine the Doppler shift.

[0026] Similarly, matched filters are useful for polarization because two different cross-polarized receivers can be used to determine the polarization. Time correlation between received pulses can also be used to reduce noise.

[0027] In a typical embodiment, the EIT is sensitive to the presence and intensity of an incident RF field, and the resulting transient change upon transmission of one of the laser beams through the vapor cell sensor is optically read out using a high bandwidth photodetector. The photodetector signal is input to a digital processing system, for example an FPGA, which applies a matched filter to the data. A DC block can also be added after the photodetector but before the FPGA. In these cases, the DC block includes a high-pass filter with a sufficiently low cutoff frequency, below a few kHz, to avoid distorting the measured pulse shape. Such a block can remove any DC offset in the optical response of the photodetector and thus in the output of the matched filter, which can facilitate subsequent processing such as thresholding or amplification.

[0028] A template for the matching filter, e.g., a "known pulse shape", can be obtained through density matrix modeling of the atomic response of the vapor or through experimental measurements taken with the same laser conditions used for sensing. The experimental measurements can be averaged over many cycles to reduce noise. The template atomic response may be used rather than the experimental output RF pulse because the pulse shape, despite approximating the envelope of the RF pulse, may differ due to finite atomic response times. The matched filter may be implemented as a convolution process of a finite impulse response filter and the template data points used as coefficients. The template data may be discretized to have a sampling rate that matches the incoming photodetector data, after any downsampling that may be done to reduce processor resources. By using a longer pulse (or a pulse with a higher sampling rate), a larger portion of the processor may be utilized due to the larger number of template data points and therefore the multiplication and addition steps required. The matched filter outputs a peak when an RF pulse is detected, and the timing of its maximum is identified as the best match point between the pulse template and the experimental pulse, which corresponds to the end of the pulse. The maximum value can be identified in digital post-processing, such as through peak finding algorithms, or can be identified in real time using techniques such as thresholding or constant fraction discrimination.

[0029] The incident RF field can have a frequency that is resonant or nearly resonant with another electronic transition for best sensitivity, the transition being between two levels, one of which matches the top atomic energy level bounded by the laser. The vapor cell sensor can be operated independently with no gain or filtering applied to the incident RF field prior to vapor cell sensing, but can be implemented in a system having a receiving antenna or dish to amplify the incoming RF field that enters the cell. An example of such a system is described below with respect to FIG. 6A. In some cases, the vapor cell sensor can be based on a photonic crystal vapor cell. An example of a photonic crystal vapor cell, including its operation, is further described in U.S. Pat. No. 11,137,432, entitled "Photonic Crystal Receivers."

[0030] In some embodiments, a 3 cm long rectangular glass-blown cell can be filled with cesium vapor at room temperature. EIT in the vapor cell is generated using counter-propagating laser beams, where the probe laser beam has an IR wavelength of 852.35 nm and the combined laser beam has a green wavelength of 509.31 nm. The probe and combined laser beams have wavelengths of approximately 160 μm and 140 μm, respectively. 2 These wavelengths are resonant with the transitions between atomic states in the ladder system shown in Figure 2A. In all measurements, the IR laser beam is offset-locked on resonance with the D2 transitions of Cs F=4 to F'=5 using an external Fabry-Perot cavity and the Pound-Drever-Hall technique. The transmission of the IR laser through the vapor cell is detected using an avalanche photodetector with a bandwidth of 10 MHz. The detector signal from the photodetector is then input into the Red Pitaya FPGA for further processing using a matched filter.

[0031] For testing, RF pulses with rise and fall times of less than 100 ns can be applied using pulse modulation on the RF synthesizer and output to an RF horn antenna with 15 dB gain located approximately 25 cm from the vapor cell. The output of the RF horn has a frequency of 19.4 GHz (K band), which corresponds to the 55D of C shown in Figure 2A. 5 / 2 and 53F 7 / 2 The Rydberg states are coupled at a pulse repetition rate of 5 Hz.

[0032] Referring now to FIG. 2A, a schematic diagram is depicted for an exemplary reduced energy level diagram of a cesium-based vapor cell. FIG. 2B presents a graph of the EIT peak obtained from a cesium-based vapor cell using the atomic field interaction system shown in FIG. 2A as a 509 nm laser is scanned across the resonance. The Ortler-Townes splitting of the peak is shown for two different continuous wave RF fields. A green laser is locked on resonance to measure the RF pulse (dashed line). FIG. 2C presents a graph of the transient atomic response of the cesium-based vapor cell whose atomic field interaction is shown in FIG. 2A to a 10 μs RF pulse measured as a change in transmission in an 852 nm laser, both a single trace (red light) and averaged over 104 cycles for a 1.45 V / m RF field (purple), with a peak of Ω. RF =2π×117MHz, Ω p = 2π × 3.5MHz, Ω c =2π×8.4MHz.

[0033] FIG. 2B shows the EIT measured from the vapor cell with no RF field applied (line labeled 0 V / m) as the green laser is scanned across the resonance. The full width at half maximum (FWHM) of the EIT peak for these laser conditions is about 4.5 MHz and Ω p or Ω cUpon application of a continuous wave RF field, Ortler-Townes splitting of the EIT peaks is observed and is shown in Figure 2B for two field strengths, 0.20 V / m and 1.45 V / m. The slight symmetry in the peak height arises from the background DC electric field present in the vapor cell. The peak splitting is given by ν=Eμ 21 / h, where μ 21 = 6294.3ea0, 55D 5 / 2 ⇔53F 7 / 2 is the dipole moment of the transition, h is the Planck constant, and E is the amplitude of the RF field. The splitting of the EIT peaks is measured in response to continuous wave RF fields of various intensities, and a calibration factor is extracted to convert the nominal output field of the RF generator to the field experienced in the vapor cell. The difference between the two fields can arise mainly from cable losses and antenna gain. At low RF intensities, the peak splitting is not discernible (e.g., less than 0.1 V / m), and this calibration factor can be used to extrapolate the RF field / pulse detected by the atoms in the vapor cell.

[0034] To detect the RF pulse, a green laser can be locked on resonance to the center of the EIT peak at the location marked by the vertical dashed line in FIG. 2B. Upon application of the RF pulse, the EIT peak splits and the transmission at the resonance undergoes a large drop, which is detected as the pulse shown in FIG. 2C. At lower RF fields, the splitting is reduced and the peak overlap increases, resulting in a lower change in transmission and therefore a lower pulse depth in the detected optical transmission. The atomic response shape approximates the inverse of the RF pulse envelope, but with slower rising and falling edges. These edges can each take about 2 μs to reach a steady state after an initial rapid sub-100 nm transient. A well-averaged pulse with minimal noise, such as the smooth line shown in FIG. 2C, can form the basis of a matched filter template.

[0035] Alternatively, a simulated atomic reaction (or a model of the reaction) can be used as a template input to a matched filter. This approach can provide a rapid means to change the laser conditions used without requiring an RF test setup to obtain an experimental pulse. An exemplary model of the vapor and its reaction that may occur as a pulse in EIT can be represented, for example, by a five-level atomic system. To model this pulse and better understand the origin of the atomic timescale, a density matrix simulation of the five-level system can be performed, which follows the time-dependent master equation shown in equation (1).

[0036]

number

number

[0037]

number

[0038] The Lindblad operator L can account for the decay and dephasing of atomic states. 21 = 2π × 5.2MHz is 6P 3 / 2 ~6S 1 / 2 can be used as the well-known damping factor of Γ 32 can be used as the radiative decay rate from the Rydberg state to the excited state. In the exemplary model, the rate Γ 31 = Γ 41 = Γ 51 is fitted to the experiment and represents the transition time through the beam, where atoms in |3>, |4>, and |5> states are replaced by ground state |1> atoms as they drift out of the beam. Finally, Γ 35represents the rate of production of atoms and / or ions in the dark state, occurring primarily through Rydberg-Rydberg collisions. This rate can be implemented as a fixed average value, or as a time-varying rate that relies on the Rydberg state population. Decay from state |4> can also be included explicitly, but its value is small for the exemplary case, so in the exemplary model, decay and dephasing from level |4> is ignored.

[0039] Referring now to FIG. 3A, a graph is presented comparing the experimental atomic response to a 10 μs RF pulse (an example of the measured response of steam) with that simulated using a density matrix model. The former is represented by the solid line in FIG. 3A, the latter by the dashed line. Such a model is able to capture well the overall shape of the pulse, except for the slow field effects that are noticeable at the end of the pulse, and can be used as a matched filter template. For the exemplary case shown in FIG. 3A, Ω p = 2π × 1.8MHz, Ω c = 2π × 8.4MHz, Ω MW = 2π × 119 MHz. Figure 3B presents a graph showing the variation with IR power for the rising edge of the pulse response of Figure 3A, as shown by both modeling (left panel) and experiment (right panel). Here, Ω c = 2π × 8.4MHz and Ω MW = 2π × 119 MHz.

[0040] An exemplary simulated atomic reaction template for a 10 μs RF pulse is shown in FIG. 3A along with experimental results for comparison. The imaginary part of the density matrix element ρ 21The change in is proportional to the absorption coefficient α. This change closely approximates the total change in transmitted intensity of the probe laser after passing through the vapor cell under weak absorption conditions. The absolute value of the total absorption can be difficult to model due to uncertainties in the atomic number density, optical losses (including reflections and absorptions due to the vapor cell walls and downstream optical components), detector sensitivity and detector gain. For example, as shown by equation (3), integral simulations performed at different velocities ν over the Boltzmann distribution give the following: 21 The Doppler average value can be obtained.

[0041]

number

[0042] The rising edge of the pulse may include two time scales. For example, the first time scale may include a sharp decrease in transmission occurring over about 50 ns, and the subsequent time scale may include a slower exponential decrease in transmission over several microseconds. The first sharp transient is driven by the sudden absorption of the probe laser in response to the EIT effectively modified by the RF field, and is visible at low optical Rabi frequencies (e.g., low laser powers). Here, 6P 3 / 2 and 6S 1 / 2 Based on the population in the states, as well as the coherence at which the EIT is shifted, the two-level system equilibrates itself at the D2 transition. The calculated transient spike may not be fully observed experimentally due to limited detection bandwidth. The relative presence and depth of this fast transient current is shown in both the model and the experiment as Ω p Therefore, when rapid detection of the arrival of an RF pulse is desired, or when detecting short, sub-microsecond RF pulses, a lower Ωp It may be advantageous to use conditions.

[0043] The slower subsequent dynamics can be accounted for by considering the repopulation of the interaction region in the vapor cell due to the motion of the atoms through the laser beam and population of the dark state |5>. Before the RF pulse, a certain percentage of the atoms may end up in the dark state due to collisions, radiation damping, and blackbody processes and may not be able to participate in the optical dynamics at the D2 transition. However, when the RF field is turned on, the Rydberg state is shifted off-resonance due to the Ortler-Townes effect and the atomic system determines that the green laser is effectively turned off, preventing the creation of further Rydberg states |3> and their associated collisional by-products. The dark state atoms eventually drift out of the laser beam and are replaced with new ground state atoms at a rate that depends on the transition time. An exponential fit to the portion of the pulse after the initial transient allows us to obtain an effective time constant that scales linearly as the laser beam diameter increases and thus the transition time increases (see Figures 7A and 7B).

[0044] The recovery time at the falling edge of the pulse can be significantly longer than the dynamics at the rising edge of the pulse, and at higher laser powers, the pulse often exhibits periods of enhanced transmission. This behavior may be due in part to collision-dependent ionization and field effects in the vapor cell, which can take a significant amount of time (e.g., about 100 μs) to reach equilibrium again after a change in ionization rate occurs during a pulse. As a result, the pulse shape may only be weakly dependent on the pulse rate. These effects are illustrated in Figures 8-12F and Ω c It becomes stronger as the green laser detuning Δ CCombined with a strong dependence on the amount and direction of Γ, the effect suggests a collision origin. Furthermore, the exact form of the long transient signal tail can depend on the position of the laser beam as it passes through the vapor cell, suggesting that any background electric field in the vapor cell may have an effect. One approach to approximately model this slow reaction is to use a high Γ 35 However, given that these effects depend on the particular vapor cell and may be non-trivial to model accurately, better performance of the matched filter is obtained by using experimental templates.

[0045] In some embodiments, the exemplary system 100 can be configured to detect a single RF pulse or a sequence of RF pulses, such as those intended for communication and radar receiver applications. FIG. 4A shows the atomic response of the exemplary system 100 to 19.4 GHz pulses of different amplitudes, with the corresponding EIT peaks splitting in response to the continuous RF field, as shown in the inset. Since lower RF amplitudes lead to lower EIT peak splitting, the main change may only be in the depth of the pulse, with only minor changes to the second, slower time constant of the rising edge. Such behavior is further explained in connection with FIGS. 7A-12F. Any changes to the non-uniformity at the bottom of the pulse of about 3 μs can be picked up from small amplitude variations in the RF power itself.

[0046] To improve the detection of weak RF pulses and improve the signal-to-noise ratio (SNR), a matched filter can be applied to each individual pulse. In certain cases, this technique can be used to extract a known pulse shape from white noise. Mathematically, a matched filter filters the noisy waveform p data (t) and the time-reversal prediction pulse template p temp (t) with the convolution function MF(t) as shown below in equation (4).

[0047]

number

[0048] As demonstrated in Figures 2C and 4C, the optical transmission does not rapidly restore to its pre-pulse value when the RF field is turned off, but rather the atoms react on a finite timescale of a few microseconds. For this reason, in some embodiments of the reaction template, a period of 0.5-2 μs is added beyond the end of the RF pulse to match the complete atomic reaction shape. As a result, t pulse is equal to the expected duration of the RF pulse plus this extra added time.

[0049] In some variations, the matched filter is implemented as a 10 3The proposed method is to perform real-time analysis using previously measured waveforms averaged over 1000 cycles. In these variations, the outgoing RF pulse is not used as a predictive template because the atomic response shape does not closely match the rectangular RF pulse envelope. Furthermore, unlike conventional radar receivers, the FPGA can be directly coupled to the vapor cell sensor without intermediate filters or amplifiers. The same predictive pulse template can be used for matched filters at different RF amplitudes because the overall pulse shape varies only minimally with RF amplitude. However, mismatch between the pulse edge time constants and the template may result in a slight shift in the extracted pulse timing combined with a slight noise reduction. The slight shift and noise reduction are discussed further with respect to Figs. 12A-F. These effects become more apparent as the RF field amplitude is reduced and can be remedied by running the matched filter with more than one reaction template. For example, one reaction template can be tuned to detect large RF fields and another reaction template can be run in parallel to sense weaker fields and obtain maximum overlap of the two.

[0050] Now referring to FIG. 4A, a single trace (jagged line) and 10 4 A graph of an exemplary atomic response to a 10 μs RF pulse of varying electric field amplitude is presented, shown both as a trace averaged over Ω cycles (approximately a smooth line). The inset shows the corresponding Ortler-Townes splitting of the EIT peak in response to continuous wave RF. Here, Ω p = 2π × 3.5MHz and Ω c = 2π × 8.4 MHz. Figure 4B shows a graph of the output of a matched filter (MF) when applied to the pulse shown in Figure 4A. The peaks of the matched filter output give the pulse timing. Figure 4C shows a graph of the output of a matched filter when applied to the pulse shown in Figure 4A for a fixed Ω cFigure 4D presents a graph of the peak height of the matched filter signal of Figure 4B as a function of IR laser power ranging from 0.6 to 73 µW, normalized to the value at perfect EIT peak splitting (large RF field) at Ω = 2π × 8.4 MHz. Figure 4E shows the results for a fixed Ω match filter signal with a green laser power (at a fixed Ω p 4B presents a plot of the signal-to-noise (SNR) ratio of the matched filter signal of FIG. 4B as it varies from 1 to 25 mW (at .times..times.2.5 MHz).

[0051] Figure 4B shows the output of a matched filter applied to an individual pulse under the same conditions as shown in Figure 4A. A predictive pulse template is used that is 1.1 μs longer than the 10 μs pulse to include matching of the slow falling edge of the atomic response, so the pulse timing output by the matched filter is 11.1 μs. The matched filter can suppress much of the noise seen in the single pulse, allowing the timing of the weaker amplitude RF pulse to be extracted.

[0052] Since the EIT peak width is strongly affected by the combination of the probe and coupling Rabi frequencies (i.e., IR and green laser powers), these frequencies are varied to determine the optimal conditions for detecting the weak RF field. Figure 4C shows the relationship between Ω and the EIT peak width for a fixed intermediate green power. p Figure 1 shows the matched filter peak height as a function of Ω. In general, the total absorption scales with the beam intensity, so for larger Ω p leads to a larger raw pulse depth and therefore a larger matched filter peak and SNR. However, avalanche photodetectors can saturate at high IR powers, requiring the addition of a neutral density filter in front of the photodetector. Thus, for a given Ω p The matched filter peak for Ω was normalized to the value at the largest RF field. Thus, Figure 4C shows the relationship between RF amplitude and Ω p The change in EIT peak width and overlap can be mapped as a function of Ω. pAt Ω, the EIT peaks are narrower, so the Ortler-Townes regime is extended and lower RF fields can be reached before the peaks start to overlap and the pulse amplitude decreases. If the goal is to maximize sensitivity to weak RF fields, a lower Ω p In contrast, if the goal is to distinguish between a wide range of RF field strengths, a higher Ω is required because the pulse depth varies gradually with the RF field due to the wide overlapping EIT peaks. p is preferred.

[0053] Figure 4D shows the p While is held at an intermediate value, Ω c Ω shows the change in signal-to-noise ratio (SNR) as Ω is changed, where SNR is defined as the ratio of the matched filter peak height to the standard deviation of the matched filter noise. c A reduction in Ω improves the SNR at the lowest RF fields by narrowing the EIT peak. However, such an improvement may reduce the SNR at higher RF fields due to the lower EIT peak amplitude. Furthermore, for a given Ω c A reduction of more than 1.0, here about 2π×3.5 MHz, can be detrimental to the SNR in all RF fields.

[0054] 5A, a graph is presented of the signal-to-noise (SNR) ratio of an exemplary matched filter shown for various RF pulse lengths, a burst of three 2 μs pulses spaced 2 μs apart (white triangles), and a 10 μs pulse acquired at lower IR power (white diamonds), where Ω p = 2π × 1.7 MHz, not 2π × 3.5 MHz. Figure 5B presents a graph of the standard deviation of the Gaussian fit to the pulse timing measured from the matched filter peak for the various pulse conditions shown in Figure 5A. The dashed horizontal lines correspond to limitations due to the finite sampling rate of the FPGA implementing the matched filter.

[0055] In FIG. 5A and FIG. 5B, the SNR and timing accuracy of different pulse lengths and sequences are compared to determine the laser condition that gives the highest overall SNR, e.g., Ω p = 2π × 3.5MHz and Ω c = 2π × 8.4 MHz. The timing accuracy of the detection is evaluated as the standard deviation of a Gaussian fit from the matched filter maximum to the distribution of 300 pulse timings extracted during post-processing. At larger RF fields, the timing accuracy can be limited by the downsampling required to implement the matched filter on the FPGA. The FPGA may be constrained by the maximum number of data points in the predicted pulse template due to finite resources to simultaneously perform the necessary multiplications and additions in the convolution. Thus, for shorter pulses, the sampling rate of the FPGA can be increased. The limitation from the sampling rate is shown as a horizontal dashed line in Fig. 5B, which shows half the sampling rate as a limit on the timing resolution for comparison with the extracted standard deviation.

[0056] If the EIT peaks are completely split, the SNR remains flat as the RF field varies. However, as overlap occurs in weaker RF fields, the SNR begins to drop. At an SNR around 15 dB, evidence of false alarms begins to appear. This evidence becomes more frequent at lower SNRs. Pulses shorter than 2 μs may not have time to reach the full pulse depth due to the long time constant of the atomic response. Thus, the SNR of short pulses may be disadvantageous in all RF fields, as may the timing accuracy of pulses shorter than 0.5 μs, which may be limited by noise rather than sampling rate in high RF fields. Despite these possibilities, short pulses can still be detected up to 50 ns wide.

[0057] Matched filtering can also be applied to bursts of pulses. As an example, a matched field can be applied to a series of three 2 μs pulses spaced by 2 μs, with the total pattern being 10 μs long but containing lower total energy than a single 10 μs pulse. The burst pattern has a lower SNR than a single 10 μs pulse at high RF fields due to the lower energy, but maintains a similar SNR at low RF fields. The burst pattern produces narrower peaks in the matched filter output, thereby improving timing accuracy in low to moderate RF fields over either a single 2 μs or 10 μs pulse. However, due to the addition of side lobes in the matched filter pattern, the use of a burst sequence may in some cases entail an additional false alarm rate of about 5%, such as when the side lobes exceed the central peak. Improvements can be achieved by varying the amplitude of each pulse in the sequence based on a comparison technique.

[0058] As shown in FIG. 5A, the optimal laser conditions for detecting weak RF pulses (white diamonds in FIG. 5A-FIG. 5B) are, e.g., Ω p = 2π × 1.7MHz and Ω c = 2π × 8.4 MHz, the RF field sensitivity can reach a limit of about 170 μV / cm when the SNR approaches 1. For a 2 μs detection time, this sensitivity is about 240 nVcm -1 Hz -1 / 2 When the SNR reaches 15 dB and accidental false alarms become more likely (e.g., corresponding to an RF field limit of about 330 μV / cm), a detection time of 2 μs results in a sensitivity of 470 nV cm -1 Hz -1 / 2An effective sensitivity of 1000 Hz can be obtained. Such sensitivity can be obtained in real time on a single pulse without the need for an auxiliary reference RF field. The detectable field range can be further extended by amplifying the RF field in the vapor cell using an amplifier as part of the vapor cell receiver, e.g., a dish, or by averaging over several pulses. Photonic crystal receivers can also be used.

[0059] A matched filter may be useful in detecting RF pulses associated with radar signals. For example, FIG. 6A presents a schematic diagram of an exemplary system 600 including a vapor cell sensor system 602 and an RF optics 604. The exemplary system 600 may be configured to detect a signal, such as a radar signal or a communication signal. The vapor cell sensor system 602 includes a vapor cell sensor 606 that may be similar to the vapor cell sensor 106 described in connection with FIG. 1. In some variations, the vapor cell sensor system 602 includes a laser system configured to generate a light beam (e.g., an optical probe beam) that passes through the vapor of the vapor cell sensor 606. The RF optics 604 is configured to direct an RF field 608 toward the vapor cell sensor 606. The RF field 608 may include one or more RF pulses that define a signal, such as a radar signal or a communication signal. In some cases, the RF field 608 is generated by a remotely located RF source 610 (e.g., a radar tower). In some cases, the RF field 608 is reflected from a moving object 612 (e.g., an aircraft). When the RF field 608 is directed at the vapor cell sensor 606, it interacts with the vapor in the vapor cell sensor 606 and alters (e.g., by an EIT mechanism) the optical transmission through the vapor, which in turn can change the optical properties of one or more of the optical beams (e.g., the optical probe beam) as they pass through the vapor.

[0060] In some variations, such as shown in FIG. 6A, the RF optical element 604 is a dish. The dish can include, for example, a hemispherical wall that curves around a focal point of the dish. The hemispherical wall can have dimensions suitable for receiving an RF field 608 from an environment surrounding the exemplary system 600. The dimensions can also enable the hemispherical wall to direct the RF field 608 onto the focal point of the dish. In certain cases, the hemispherical wall can be formed from a metal (e.g., aluminum, steel, etc.). In some variations, such as shown in FIG. 6A, the vapor cell sensor 606 is disposed at the focal point of the dish. For example, the dish can include a support arm that extends from an edge of the dish and terminates at an end at or near the focal point. The vapor cell sensor 606 can be coupled to this end. In some variations, the hemispherical wall focuses the RF field 608 onto the vapor cell sensor 606 (and the vapor therein) when directing the RF field 608 onto the focal point. Such concentration may be beneficial in situations where the RF field 608 is weak. In some variations, the RF optical element 604 is an RF lens. Examples of RF lenses include a Teflon lens, a metamaterial lens, or a gradient index lens. However, other types of RF optical elements 604 are possible. Combinations of various types of RF optical elements are also possible.

[0061] The exemplary system 600 also includes a signal processing system 614 and, in some variations, a computer system 616 in communication with the signal processing system 614. The computer system 616 allows a user to interact with and control the exemplary system 600. The signal processing system 614 is configured to perform operations including generating a digital signal based on a signal from the vapor cell sensor system 602. The digital signal represents a measured response of the vapor of the vapor cell sensor 606 to the RF field 608 over a period of time. The operations also include applying a matched filter to the digital signal to generate a filtered signal. The filtered signal is generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to a target RF pulse. The operations further include processing the filtered signal to determine a characteristic of the RF field sensed by the vapor cell sensor over a period of time. In some variations, processing the filtered signal includes detecting a sequence of RF pulses. For example, the one or more RF pulses may define a radar signal, and the signal processing system 614 may be configured to identify an aircraft as the source of the sequence of RF pulses. As another example, the one or more RF pulses may define a communication signal transmitted from a communication station (e.g., a satellite, an aircraft, a vehicle, a boat, a broadcast antenna, etc.). In these cases, the signal processing system 614 may be configured to identify data represented by the communication signal.

[0062] FIG. 6B presents a polar graph of an exemplary scenario in which a rotating emitter on an aircraft is represented by a fixed vapor cell receiver as the aircraft flies. The fixed vapor cell receiver emits 1 μs pulses at a repetition rate of 10 kHz. FIG. 6C presents a graph of an exemplary signal detected by the fixed vapor cell receiver of FIG. 6B after matched filtering. The fixed vapor cell receiver produces a cluster of pulses whenever the emitter rotates so that its antenna faces the receiver. FIG. 6D presents a graph of a single pulse cluster from the exemplary signal of FIG. 6C showing the pattern formed from a single rotation of the antenna. The pattern includes a large central peak with multiple weaker side lobes. FIG. 6E presents a graph from the central lobe of a single pulse cluster shown in FIG. 6D. The central lobe is approximately 8.5 s and FIG. 6D shows the matched filter output from an individual 1 μs pulse. Here, Ω p = 2π × 7.8MHz, Ω c = 2π × 8.4 MHz.

[0063] FIG. 6B demonstrates detection of a radar signal generated by an airplane flying past a vapor cell receiver (or sensor) along a flight path. The radar signal is generated by a vector signal generator that simulates the power and timing of a pulse that would reach the vapor cell receiver. The vector signal generator accounts for varying distances and generates a peak power of about 6 dBm, which is transmitted to an RF horn antenna near the vapor cell receiver. In an exemplary scenario, the aircraft travels at a typical commercial aircraft speed of 200 m / s, about 1 km from the vapor cell receiver. The aircraft is equipped with a rotating antenna rotating at 30 rpm that emits 1 μs long pulses at a repetition rate of 10 kHz. The laser conditions for detecting pulses from the aircraft, corresponding to the highest RF field sensitivity, are, for example, Ω p = 2π × 1.7MHz and Ω c = 2π × 8.4 MHz.

[0064] FIG. 6C shows a 15 s long radar pattern seen through the atomic response of the vapor cell receiver, processed with a matched filter implemented by an FPGA. The pulses are visible when the emitter is facing the receiver, producing a cluster of spikes with varying amplitudes shown in FIG. 6D, corresponding to a typical antenna pattern. The peaks of the individual pulses output by the atomic system and matched filter generally scale with the received RF pulse amplitude, but in a nonlinear fashion as shown in FIG. 4C. A possible consequence of the scaling is that fields above about 0.2 V / m (or about -10.7 dBm of power) cannot be distinguished due to saturation in the laser conditions used in the demonstration. This behavior is less likely at lower RF fields where the peak splitting and pulse depth are roughly linear, e.g., 0.04 to 0.2 V / m (or -24.8 to -13.3 dBm of power), or quadratic, e.g., below 0.04 V / m (or -24.8 dBm of power). Due to saturation at the pulse depth after the Ortler-Townes peak is completely split, the central lobe in Fig. 6D is flat rather than rounded like the side lobes. Furthermore, the first side lobe has a slightly higher amplitude than the central lobe due to enhanced absorption at the sides of the EIT peak.

[0065] Side lobes of decreasing field strength can be observed as the antenna spins. As the aircraft approaches the receiver, the RF field at the vapor cell receiver increases and more side lobes can be distinguished above the noise floor. At its closest approach, four side lobes can be detected on each side of the central peak compared to only one at the extremes of the flight path. To detect aircraft from further away or to detect weaker side lobes, increasing the emitted pulse length, increasing the vapor cell depth, adding amplifiers in the receiver, or reducing noise sources, or photonic crystal vapor cells may be required. In certain cases, the detection system may become dominated by the optical frequency and amplitude noise from the lasers and their locking process, with some smaller delays and noise added by the FPGA processing.

[0066] Applications of Rydberg atom-based sensors include radar receivers (e.g., detecting pulses emitted by aircraft antennas) and communication systems. Rydberg atom-based sensors can be based on Cs vapor cells as described above. Cs vapor cells at room temperature can have a fast transient response to the application of a pulse-modulated RF field, and can detect RF pulses down to less than 50 ns. The atomic response time to the RF field being turned on can in some cases include two time scales: a short, roughly 50 ns transient, followed by a longer microsecond delay depending on the transition time of by-products from Rydberg-Rydberg collisions from the laser beam. The response to the RF field being removed can be slower due to Rydberg-Rydberg interactions and the sudden increase in the electric field. Using a matched filter based on the atomic response geometry, the vapor cell sensor can detect single-shot RF pulses down to amplitudes of about 170 μV / cm, with a sensitivity of about 240 nVcm. -1 Hz -1 / 2with a timing accuracy of about 30 ns, all without an auxiliary RF field. In many cases, a narrow EIT peak and weak laser conditions are optimal for highest sensitivity to low target fields, while higher laser powers are preferred for calibration over a wider amplitude range.

[0067] Now referring to Figures 7A and 7B, graphs are presented showing the effect of laser beam size on pulse shape. In Figure 7A, the graph shows that increasing the radius of the interaction volume (e.g., the overlap of the probe and combined beams in the exemplary case) radius slows down the atomic reaction at the rising edge of the pulse. Figure 7B shows that the time constant extracted from the exponential fit to the rising edge of the pulse scales linearly with the IR beam radius. In this example, the combined beam is larger than any of the IR beams. As shown in Figure 7A, the second slower time constant that constitutes the rising edge of the atomic pulse reaction can be sensitive to the choice of laser beam radius. Here, the infrared beam radius is changed, which is smaller than the green laser beam. A constant Rabi frequency is maintained even by increasing the infrared laser power. The initial, less than 50 ns, transient current present at the rising edge of the pulse appears to be unaffected by the laser beam size, up to within the bandwidth resolution of the photodetector. However, a time constant can be extracted from an exponential fit to the second, slower decay of the pulse, and this time constant is found to scale monotonically with the radius of the IR laser beam, as shown in Figure 7 B. Since the transit time of an atom through the beam is directly proportional to its radius, the monotonic scaling demonstrates that this slower transient timescale of the pulse is dominated by the atomic transition time.

[0068] 8A-8C, graphs are presented showing the effect of an electric field at different Rabi frequencies. In FIG. 8A, the trailing edge of the pulse is at the green Rabi frequency and has a stronger residual overshoot at higher Rabi frequencies. Here, Ω p = 2π × 1.8 MHz. In FIG. 8B, the shape of the rising edge of the pulse does not change with laser beam position, but the shape of the falling edge does. The inset shows the approximate location for the laser beam within the short 1 cm × 1 cm face of the cell as the laser steps in and out of the page. In FIG. 8B, Ω p = 2π × 1.8MHz and Ω c = 2π × 10.5 MHz. Figure 8C shows that green detuning has a strong effect on the tail of the experimental response to the RF pulse. In Figure 8C, Ω p = 2π × 34MHz, Ω c = 2π × 8.7MHz, and Ω MW = 2π × 119 MHz.

[0069] The shape of the falling edge depends on the green laser power, as shown in FIG. 8A, which will change the Rydberg state population. Similarly, the shape can be highly dependent on the direction (blue or red detuning) and the amount of green laser frequency detuning from resonance, as shown in FIG. 8C. Furthermore, the shape and timescale of the falling edge can depend on the laser position relative to the vapor cell wall, as shown in FIG. 8B. The pulse depth changes mainly due to changes in optical transmission due to the electric field shift, which depends on the distribution of charge in the vapor cell, mostly attached to the vapor cell wall. All these observations highlight that the slow falling edge of the pulse can be highly sensitive to factors such as collisions, background electric fields, ion generation and / or blockage in the vapor cell.

[0070] Referring now to FIG. 9, a graph is presented showing the effect of repetition rate on pulse shape for a 2 μs long pulse. The pulse shape will change slightly if the interval between pulses is shorter, since the falling edge of the pulse can linger for up to about 100 μs before reaching a steady state. FIG. 9 shows that the overall pulse depth, as well as the time scales of the rising and falling edges, vary slightly with pulse rate. Pulses with higher repetition rates appear to be slightly easier to detect due to the increased pulse depth.

[0071] Referring now to FIG. 10A, a graph is presented showing the effect of the choice of atomic transition on pulse shape at low optical Rabi frequencies. In FIG. p = 2π × 1.8MHz and Ω c = 2π × 2.5 MHz. FIG. 10B presents a graph of the corresponding effect at high optical Rabi frequencies, where Ω p = 2π × 12.2MHz and Ω c = 2π × 5.7 MHz. Changes to the Rydberg and / or RF transitions have minimal effect on the rising edge of the experimental atomic reaction, but may result in a higher optical Rabi frequency (e.g., Ω MW = 2π × 60 MHz).

[0072] 10A and 10B provide a characterization of whether the same pulse template can be used across different atomic transitions and therefore at different RF frequencies. 5 / 2 ⇔53F 7 / 2 In addition to the transition (K-band 19.40 GHz microwave), 55D 5 / 2 ⇔56P 3 / 2 Transition (C-band 4.24GHz microwave) and 58S 1 / 2 ⇔58P 3 / 2The green laser is shifted to a different Rydberg state in a large RF field when the transition (18.94 GHz microwave) is probed and the EIT peak is completely split. At low laser powers (hence the Rabi frequency), which are optimal for weak RF pulse detection, the pulse shape appears to be identical across the varying Rydberg and RF atomic state transitions, as shown in Figure 10A. However, at higher optical powers, some changes to the trailing edge of the pulse may appear, possibly due to varying collision rates and polarizabilities between the states.

[0073] 11A and 11B, graphs are presented showing the variation of pulse shapes for different RF fields at 19.4 GHz and 4.2 GHz, respectively. Both graphs show that the atomic response is slightly more rapid in the weaker RF fields. As the RF field is further reduced, an overshoot begins to develop at each edge. Here, Ω p = 2π × 3.5MHz and Ω c = 2π × 8.4 MHz. The graphs in Figures 11A and 11B provide insight into whether the pulse shape remains the same at all RF field amplitudes. For the larger range of RF fields, the pulse shape remains mostly the same, with only a slight change to a slower time constant after the initial transient. This behavior indicates that the same matched filter template can be used for a large range of RF fields. At the smallest RF fields, the pulse shapes start to differ more substantially, with rapid overshoots on both the rising and falling edges. The changes to the non-uniformity of the pulse from about 2.5 μs to about 10 μs may be due to amplitude variations in the RF generator used, where the atoms are more sensitive to weak RF fields (e.g., in the amplitude region of Figure 4C).

[0074] 12A-F, graphs are presented comparing the effect of a matched filter template on the atomic pulse shape. These graphs show that the effect of using a matched filter template is not perfectly matched to the atomic pulse shape. FIGS. 12D-F show the matched filter output as a solid line when the pulse shown in the top panel (light solid line) is matched to itself for an ideal fit. For the scenario where the pulse in the top panel (light solid line) is matched to the pulse template in the top panel (dark solid line), the matched filter output is shown as a dotted line. The overall filter output remains largely the same, but may slightly shift the extracted pulse timing.

[0075] In general, as discussed above in connection with Figures 7A-11B, the exact timescales of the rising and falling edges of the pulses can depend on many factors related to the vapor cell sensor itself (e.g., electric field, temperature, vapor pressure), the laser (e.g., power, radius, frequency detuning), the choice of atomic state, and the RF field amplitude. Figures 12A-F show what can happen when the matched filter template does not perfectly match the underlying pulse shape being detected. The difference in timescales, including the presence of overshoots in the pulse, or edges, results in a slight shift to the extracted pulse time at the maximum of the filter output, typically less than 0.5 μs. The direction of the shift can be slower or faster depending on the nature of the pulse template mismatch.

[0076] Some of the subject matter and operations described herein, including the structures disclosed herein and their structural equivalents, or any combination or combinations of one or more of these, can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware. Some of the subject matter described herein can be implemented as one or more computer programs, i.e., as one or more modules of computer program instructions encoded on a computer storage medium that are executed by or control the operation of a data processing apparatus. The computer storage medium can be or be included in a computer readable storage device, a computer readable storage substrate, a random or serial access memory array or device, or a combination or combinations of one or more of these. Furthermore, the computer storage medium can be a source or destination of computer program instructions encoded in an artificially generated propagated signal rather than a propagating signal. The computer storage medium can be or be included in one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices).

[0077] Some of the operations described herein may be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources.

[0078] The term "data processing apparatus" encompasses all kinds of apparatus, devices and machines for data processing, including, by way of example, a programmable processor, a computer, a system on a chip, or a combination of these. An apparatus may include, for example, a special-purpose logic circuit, such as an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). In addition to hardware, an apparatus may also include code that forms an execution environment for a computer program of interest, such as code that constitutes a processor firmware, a protocol stack, a database management system, an operating system, a cross-platform execution environment, a virtual machine, or one or more combinations of these.

[0079] A computer program (also known as a program, software, software application, script or code) can be written in any form of programming language, including compiled or interpreted, declarative or procedural, and can be arranged in any form, whether included as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program can correspond to a file in a file system, but this is not required. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program, or in a portion of a file that stores multiple organized files (e.g., one or more modules, subprograms, or portions of code). A computer program can be arranged to be executed on one computer, or on multiple computers that are located in one location or distributed across multiple locations and connected together by a communications network.

[0080] Some of the processes and logic flows described herein may be performed by one or more programmable processors executing one or more computer programs to perform operations by operating on input data and generating output. The processes and logic flows may also be performed by, and an apparatus may be implemented as, special purpose logic circuitry, such as an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[0081] In some aspects of what has been described, the system can be illustrated by the following examples. (Example 1) 1. A system comprising: a laser system configured to generate an optical beam including an optical probe beam; a vapor cell sensor having vapor therein and configured to allow a light beam to pass through the vapor; an optical detector configured to generate a detector signal based on the optical probe beam after the probe beam passes through the vapor; 1. A signal processing system configured to perform operations, the operations including: receiving a detector signal from the photodetector over a period of time; generating a digital signal based on the detector signal, the digital signal representing a measured response of the vapor to the RF field to which the vapor was subjected over a period of time; applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to the target RF pulse; processing the filtered signal to determine a characteristic of the RF field experienced by the vapor over a period of time; a signal processing system including: A system comprising: (Example 2) Processing the filtered signal includes: The system of Example 1, comprising determining at least one of a start time, a duration, or an amplitude of an RF pulse to which the vapor is subjected in a period of time. (Example 3) a source of an RF field configured to generate RF pulses; The system of Examples 1 or 2, wherein the response template is based on a plurality of reference RF pulses generated by the source over a reference period, the plurality of reference RF pulses defining an RF field and sharing a common pulse shape. (Example 4) The system of Example 1 or Example 2, wherein the response template is based on a computer simulation of the vapor's response to the target RF pulse. (Example 5) The system of Example 1 or any one of Examples 2-3, wherein the signal processing system comprises a high pass filter configured to block a portion of the detector signal having a frequency below a threshold frequency. (Example 6) The system of Example 5, wherein the threshold frequency is less than or equal to 100 Hz. (Example 7) the response template is a first response template representing a first known response of the vapor to the first type of target RF pulse; The filtered signal is a digital signal; A first reaction template, and a second response template representing a second known response of the vapor to a second type of target RF pulse; The system of any one of Example 1 or Examples 2 to 6, which is generated based on a comparison with (Example 8) The system of Example 7, wherein the first and second types of target RF pulses have different non-overlapping ranges of field strengths. (Example 9) The system of Example 1 or any one of Examples 2-8, wherein the detector signal is based on at least one of an amplitude, a polarization, or a phase of the optical probe beam.

[0082] In some embodiments of what has been described, the methods can be illustrated by the following examples. (Example 10) 1. A method comprising: passing a light beam through a vapor in the vapor cell sensor, the light beam including an optical probe beam; allowing an RF field to interact with the vapor; generating a detector signal based on the optical probe beam after the probe beam passes through the vapor by operation of the optical detector; receiving a detector signal at a signal processing system over a period of time; The operation of the signal processing system generating a digital signal based on the detector signal, the digital signal representing a measured response of the vapor to the RF field over a period of time; applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to the target RF pulse; processing the filtered signal to determine a characteristic of the RF field experienced by the vapor over a period of time; A method comprising: (Example 11) The method of Example 10, wherein applying the matched filter includes applying a convolution function to the digital signal and the response template to generate a filtered signal. (Example 12) The method of example 11, wherein the period of the response template of the convolution function is greater than the expected duration of the pulse of the RF field. (Example 13) Processing the filtered signal includes: The method of Example 10 or any one of Examples 11-12, comprising determining at least one of a start time, a duration, or an amplitude of an RF pulse to which the vapor is subjected in a period of time. (Example 14) allowing a reference RF pulse to interact with the vapor, the reference RF pulse sharing a common pulse shape; receiving a reference detector signal at a signal processing system over a reference period; The operation of the signal processing system generating a reference digital signal based on the reference detector signal, the reference digital signal representing a measured response of the vapor to the reference RF pulse over a reference time period; generating a response template based on a reference digital signal; The method of Example 10 or any one of Examples 11-13, comprising: (Example 15) Generating a reaction template includes: determining, for each reference pulse, a pulse shape based on a portion of the digital signal associated with a duration of the reference pulse; Averaging the pulse shapes to generate a response template; The method of Example 14, comprising: (Example 16) The method of Example 10 or any one of Examples 11-13, comprising generating a reaction template based on a computer simulation of a reaction of the vapor to the target RF pulse. (Example 17) The method of Example 10 or any one of Examples 11-16, wherein the detector signal is based on at least one of an amplitude, a polarization, or a phase of the optical probe beam. (Example 18) The method of Example 10 or any one of Examples 11-17, comprising filtering the detector signal to remove portions having frequencies below a threshold frequency. (Example 19) The method of Example 18, wherein the threshold frequency is less than or equal to 100 Hz. (Example 20) The method of Example 10 or any one of Examples 11-19, wherein interacting the RF field includes interacting the RF field with the vapor for at least a portion of the time period. (Example 21) allowing a heterodyne RF field to interact with the vapor, the heterodyne RF field comprising a continuous wave (CW) RF field over a period of time; the digital signal represents a measured response of the vapor to a mixed RF field over a period of time, the mixed RF field including a heterodyne RF field and an RF field; The method of Example 10 or any one of Examples 11-20, wherein processing the filtered signal includes processing the filtered signal to determine a characteristic of the mixed RF field experienced by the vapor over a period of time. (Example 22) the response template is a first response template representing a first known response of the vapor to the first type of target RF pulse; The filtered signal is a digital signal; A first reaction template, and a second response template representing a second known response of the vapor to a second type of target RF pulse; The method of any one of Example 10 or Examples 11 to 21, wherein the method is generated based on a comparison with (Example 23) 23. The method of Example 22, wherein the first and second types of targeted RF pulses have different non-overlapping ranges of field strengths.

[0083] In some aspects of what is described, the non-transitory computer readable medium can be illustrated by the following examples. (Example 24) A non-transitory computer-readable medium comprising instructions operable to perform operations when executed by a data processing apparatus, the operations comprising: generating a digital signal representative of a measured response of the vapor to the RF field experienced by the vapor over a period of time, the digital signal being based on a detector signal from the photodetector over the period of time; the vapor is within a vapor cell sensor configured to allow a light beam to pass through the vapor, the light beam including an optical probe beam; the optical detector is configured to receive the optical probe beam after it passes through the vapor and, in response, generate a detector signal; Generating and applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to the target RF pulse; processing the filtered signal to determine a characteristic of the RF field experienced by the vapor over a period of time; A non-transitory computer readable medium comprising: (Example 25) 25. The non-transitory computer-readable medium of Example 24, wherein applying the matched filter includes applying a convolution function to the digital signal and the response template to generate a filtered signal. (Example 26) 26. The non-transitory computer readable medium of example 25, wherein the period of the response template of the convolution function is greater than the expected duration of the pulse of the RF field. (Example 27) Processing the filtered signal includes: The non-transitory computer readable medium of Example 24 or any one of Examples 25-26, comprising determining at least one of a start time, a duration, or an amplitude of an RF pulse to which the vapor is subjected in a period of time. (Example 28) generating a reference digital signal representative of a measured response of the vapor to reference RF pulses over a reference time period, the reference RF pulses sharing a common pulse shape; generating a response template based on a reference digital signal; The non-transitory computer-readable medium of Example 24 or any one of Examples 25 to 27, comprising: (Example 29) The non-transitory computer readable medium of Example 24 or any one of Examples 25-27, wherein the reaction template is based on a computer simulation of a reaction of the vapor to the target RF pulse. (Example 30) The non-transitory computer-readable medium of Example 24 or any one of Examples 25-29, wherein the detector signal is based on at least one of an amplitude, a polarization, or a phase of the optical probe beam. (Example 31) the response template is a first response template representing a first known response of the vapor to the first type of target RF pulse; The filtered signal is a digital signal; A first reaction template, and a second response template representing a second known response of the vapor to a second type of target RF pulse; The non-transitory computer-readable medium of any one of Example 24 or Examples 25-30, generated based on a comparison with (Example 32) The non-transitory computer-readable medium of Example 31, wherein the first and second types of targeted RF pulses have different non-overlapping ranges of field strengths.

[0084] In some aspects of what has been described, the system can be illustrated by the following examples. (Example 33) 1. A system comprising: a vapor cell sensor system including a vapor cell sensor; a radio frequency (RF) optical element configured to direct a radio frequency (RF) field onto the vapor cell sensor, the RF field including one or more RF pulses defining a radar signal, a communication signal, or other type of signal; 1. A signal processing system configured to perform operations, the operations including: generating a digital signal based on a signal from the vapor cell sensor system, the digital signal representing a measured response of the vapor of the vapor cell sensor to the RF field over a period of time; applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to the target RF pulse; processing the filtered signal to determine a characteristic of the RF field sensed by the vapor cell sensor over a period of time; a signal processing system including: A system comprising: (Example 34) The system of example 33, wherein the RF optical element comprises a dish. (Example 35) The system of example 33 or example 34, wherein the RF optical element comprises an RF lens. (Example 36) The system of Example 33 or any one of Examples 34-35, wherein processing the filtered signal includes detecting a sequence of RF pulses. (Example 37) The system of Example 33 or any one of Examples 34-36, wherein the system is a radar system and the other RF pulse defines a radar signal. (Example 38) Processing the filtered signal includes detecting a sequence of RF pulses; 38. The system of example 37, wherein the signal processing system is configured to identify the aircraft as a source of the sequence of RF pulses. (Example 39) The system of Example 33 or any one of Examples 34-36, wherein the system is a communications system and the one or more RF pulses define a communications signal. (Example 40) 40. The system of example 39, wherein the signal processing system is configured to identify data represented by the communication signal. (Example 41) The operation is The system of Example 33 or any one of Examples 34-40, comprising determining at least one of a start time, a duration, or an amplitude of an RF pulse sensed by the vapor cell sensor over a period of time. (Example 42) The system of Example 33 or any one of Examples 34-41, wherein the response template is based on a plurality of reference RF pulses generated by the RF source over a reference time period, the plurality of reference RF pulses sharing a common pulse shape. (Example 43) The system of Example 33 or any one of Examples 34-41, wherein the reaction template is based on a computer simulation of the reaction of the vapor to the target RF pulse. (Example 44) the response template is a first response template representing a first known response of the vapor to the first type of target RF pulse; The filtered signal is a digital signal; A first reaction template, and a second response template representing a second known response of the vapor to a second type of target RF pulse; The system of any one of Example 33 or Examples 34 to 43, which is generated based on a comparison with (Example 45) 45. The system of example 44, wherein the first and second types of target RF pulses have different non-overlapping ranges of field strengths. (Example 46) The system of any one of Example 33 or Examples 34-45, wherein the vapor comprises a vapor of a Group IA atom. (Example 47) The system of any one of Example 33 or Examples 34-46, wherein the vapor cell sensor system includes a laser system configured to generate a light beam that passes through a vapor of the vapor cell sensor, the light beam including an optical probe beam. (Example 48) 48. The system of example 47, wherein the signal from the vapor cell sensor system is based on at least one of an amplitude, a deflection, or a phase of the optical probe beam. (Example 49) the vapor cell sensor system includes a photodetector, and the signal from the vapor cell sensor system is a detector signal generated by the photodetector; The system of Example 33 or any one of Examples 34-48, wherein the operations include receiving a detector signal at a signal processing system over a period of time, the detector signal being based on an optical probe beam passing through vapor in the vapor cell sensor.

[0085] In some embodiments of what has been described, the manner in which the system operates can be illustrated by the following examples. (Example 50) 1. A method of operating a system, comprising: directing a radio frequency (RF) field onto a vapor cell sensor of the vapor cell sensor system, the vapor cell sensor including vapor, the RF field including one or more RF pulses that define a radar signal, a communication signal, or other type of signal; The operation of the signal processing system generating a digital signal based on a signal from the vapor cell sensor system, the digital signal representing a measured response of the vapor to the RF field over a period of time; applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to the target RF pulse; processing the filtered signal to determine a characteristic of the RF field experienced by the vapor over a period of time; A method comprising: (Example 51) Directing the RF field The method of example 50, including directing an RF field onto a focal point of the dish by movement of the dish, the vapor cell sensor being disposed at the focal point of the dish. (Example 52) Directing the RF field The method of example 50 or example 51, comprising directing an RF field onto the vapor cell sensor by operation of an RF lens. (Example 53) The method of Example 50 or any one of Examples 51-52, wherein processing the filtered signal includes detecting a sequence of RF pulses. (Example 54) Example 50 or the method of any one of Examples 51-53, wherein the system is a radar system and the other RF pulse defines a radar signal. (Example 55) Processing the filtered signal includes detecting a sequence of RF pulses; 55. The method of example 54, wherein the method includes identifying the aircraft as a source of the sequence of RF pulses by operation of the signal processing system. (Example 56) Example 50 or the method of any one of Examples 51-53, wherein the system is a communications system and the one or more RF pulses define a communications signal. (Example 57) 57. The method of example 56, wherein the signal processing system is configured to identify data represented by the communication signal. (Example 58) The method of Example 50 or any one of Examples 51-57, wherein applying the matched filter includes applying a convolution function to the digital signal and the response template to generate a filtered signal. (Example 59) 59. The method of example 58, wherein the period of the response template of the convolution function is greater than the expected duration of the RF pulse of the RF field. (Example 60) The method of Example 50 or any one of Examples 51-59, comprising determining at least one of a start time, a duration, or an amplitude of an RF pulse sensed by the vapor cell sensor in a period of time. (Example 61) receiving reference pulses of an RF field at the vapor cell sensor for a reference time period, the reference pulses sharing a common pulse shape; generating a response template based on a digital signal associated with a reference pulse over a reference period by operation of a signal processing system; The method of any one of Example 50 or Examples 51 to 60, comprising: (Example 62) Generating the pulse template includes: determining, for each reference pulse, a pulse shape based on a portion of the digital signal associated with a duration of the reference pulse; Averaging the pulse shapes to generate a response template; The method of Example 61, (Example 63) The method of Example 50 or any one of Examples 51-60, comprising generating a reaction template based on a computer simulation of a reaction of the vapor to the target RF pulse. (Example 64) The vapor cell sensor system includes a laser system. The method is: generating a light beam by operating a laser system; passing a light beam through a vapor in the vapor cell sensor, the light beam including an optical probe beam; Including, The method of Example 50 or any one of Examples 51-63, wherein the signal from the vapor cell sensor system is based on the optical probe beam after the probe beam passes through the vapor. (Example 65) Directing the RF field The method of example 64, comprising modifying the optical transmission of the optical probe beam through the vapor as the optical beam passes through the vapor by operation of an RF field on the vapor. (Example 66) The method of example 64 or example 65, wherein the signal from the vapor cell sensor system is based on at least one of an amplitude, a deflection, or a phase of the optical probe beam. (Example 67) The vapor cell sensor system includes a photodetector. The signal from the vapor cell sensor system is a detector signal from an optical detector, the detector signal being based on the optical probe beam after the probe beam passes through the vapor; The method of Example 64 or any one of Examples 65-66, wherein the method includes receiving a detector signal at a signal processing system over a period of time. (Example 68) The method of Example 50 or any one of Examples 51-67, wherein the vapor comprises vapor of a Group IA atom. (Example 69) the response template is a first response template representing a first known response of the vapor to the first type of target RF pulse; The filtered signal is a digital signal; A first reaction template, and a second response template representing a second known response of the vapor to a second type of target RF pulse; The method of any one of Example 50 or Examples 51-68, wherein the method is generated based on a comparison with (Example 70) 70. The method of example 69, wherein the first and second types of targeted RF pulses have different non-overlapping ranges of field strengths.

[0086] Although the present specification contains many details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular examples. Certain features described herein or shown in the drawings in the context of separate implementations may also be combined. Conversely, various configurations that are described or shown in the drawings in the context of a single implementation may also be implemented in multiple embodiments separately or in any suitable subcombination.

[0087] Similarly, although operations are shown in a particular order, this should not be construed as requiring that such operations be performed in the particular order shown, or sequentially, or that all of the operations shown be performed, to achieve desirable results. In certain situations, multitasking and parallel processing may be advantageous. Furthermore, the separation of various system components in the above-described implementations should not be understood as requiring such separation in all embodiments, and it should be understood that the program components and systems described may generally be integrated into a single product or packaged into multiple products.

[0088] Although several embodiments have been described, it will be understood that various modifications may be made and, therefore, other embodiments are within the scope of the following claims.

Claims

1. a vapor cell sensor system including a vapor cell sensor; a radio frequency (RF) optical element configured to direct a RF field onto the vapor cell sensor, the RF field including one or more RF pulses that define a radar signal; 1. A signal processing system configured to perform operations, the operations including: generating a digital signal based on a signal from the vapor cell sensor system, the digital signal representing a measured response of the vapor of the vapor cell sensor to the RF field over a period of time; applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to a target RF pulse; processing the filtered signal to determine a characteristic of the RF field sensed by the vapor cell sensor over the period of time; 2. A radar system comprising:

2. The radar system of claim 1 , wherein processing the filtered signal includes detecting a sequence of RF pulses.

3. The radar system of claim 2 , wherein the signal processing system is configured to identify an aircraft as a source of the sequence of RF pulses.

4. The operation includes: A radar system according to any one of claims 1 to 3, comprising determining at least one of a start time, a duration or an amplitude of an RF pulse detected by the vapor cell sensor during said period.

5. The radar system of any one of claims 1 to 3, wherein the response template is based on a plurality of reference RF pulses generated by an RF source over a reference time period, the plurality of reference RF pulses sharing a common pulse shape.

6. The radar system of any one of claims 1 to 3, wherein the response template is based on a computer simulation of the response of the vapor to the target RF pulse.

7. the response template is a first response template representing a first known response of the vapor to a first type of target RF pulse; The filtered signal is a digital signal. the first reaction template; and a second response template representing a second known response of the vapor to a second type of target RF pulse; The radar system according to any one of claims 1 to 3, wherein the signal is generated based on a comparison with

8. 8. The radar system of claim 7, wherein the first and second types of target RF pulses have different non-overlapping ranges of field strengths.

9. The radar system of any one of claims 1 to 3, wherein the vapor comprises a vapor of Group IA atoms.

10. The radar system of any one of claims 1 to 3, wherein the vapor cell sensor system comprises a laser system configured to generate a light beam that passes through the vapor of the vapor cell sensor, the light beam including an optical probe beam.

11. The radar system of claim 10 , wherein the signal from the vapor cell sensor system is based on at least one of an amplitude, a deflection, or a phase of the optical probe beam.

12. the vapor cell sensor system includes a photodetector, and the signal from the vapor cell sensor system is a detector signal generated by the photodetector; The radar system of any one of claims 1 to 3, wherein the operation includes receiving the detector signal in the signal processing system over the period of time, the detector signal being based on an optical probe beam passing through vapor in the vapor cell sensor.

13. 1. A method of operating a radar system, comprising: directing a radio frequency (RF) field onto a vapor cell sensor of a vapor cell sensor system, the vapor cell sensor including vapor, the RF field including one or more RF pulses that define a radar signal; By the operation of the signal processing system, generating a digital signal based on a signal from the vapor cell sensor system, the digital signal representing a measured response of the vapor to the RF field over a period of time; applying a matched filter to the digital signal to generate a filtered signal, the filtered signal being generated based on a comparison of the digital signal to a response template, the response template representing a known response of the vapor to a target RF pulse; processing the filtered signal to determine a characteristic of the RF field experienced by the vapor over the period of time; A method comprising:

14. Directing the RF field includes:

14. The method of claim 13, comprising directing the RF field onto a focal point of a dish by movement of the dish, the vapor cell sensor being disposed at the focal point of the dish.

15. Directing the RF field includes: The method of claim 13 , comprising directing the RF field onto the vapor cell sensor by operation of an RF lens.

16. The method of any one of claims 13 to 15, wherein processing the filtered signal comprises detecting a sequence of RF pulses.

17. 17. The method of claim 16, including identifying an aircraft as a source of the sequence of RF pulses by operation of the signal processing system.

18. The method of any one of claims 13 to 15, wherein applying a matched filter comprises applying a convolution function to the digital signal and the response template to generate the filtered signal.

19. 20. The method of claim 18, wherein a period of the convolution function is greater than an expected duration of an RF pulse in the RF field.

20. The method of any one of claims 13 to 15, comprising determining at least one of a start time, a duration, or an amplitude of an RF pulse detected by the vapor cell sensor during said period.

21. receiving reference pulses of the RF field at the vapor cell sensor for a reference time period, the reference pulses sharing a common pulse shape; generating the response template based on the digital signal associated with the reference pulse over the reference period by operation of the signal processing system; The method according to any one of claims 13 to 15, comprising:

22. Generating the pulse template comprises: for each reference pulse, determining a pulse shape based on a portion of the digital signal associated with a duration of the reference pulse; averaging the pulse shapes to generate the response template; 22. The method of claim 21 , comprising:

23. A method according to any one of claims 13 to 15, comprising generating the reaction template based on a computer simulation of the reaction of the vapour to the target RF pulse.

24. the vapor cell sensor system includes a laser system; The method comprises: generating a light beam by operating the laser system; passing the light beam through the vapor in the vapor cell sensor, the light beam including an optical probe beam; Including, The method of any one of claims 13 to 15, wherein the signal from the vapour cell sensor system is based on the optical probe beam after it has passed through the vapour.

25. Directing the RF field includes:

25. The method of claim 24, comprising modifying the optical transmission of the optical probe beam through the vapor as the optical beam passes through the vapor by operation of the RF field on the vapor.

26. The method of any one of claims 13 to 15, wherein the signal from the vapor cell sensor system is based on at least one of the amplitude, deflection, or phase of the optical probe beam.

27. the vapor cell sensor system includes a photodetector; the signal from the vapor cell sensor system is a detector signal from the optical detector, the detector signal being based on the optical probe beam after the probe beam has passed through the vapor; A method according to any one of claims 13 to 15, wherein the method comprises receiving the detector signal in the signal processing system over the period of time.

28. The method of any one of claims 13 to 15, wherein the vapor comprises vapor of Group IA atoms.

29. the response template is a first response template representing a first known response of the vapor to a first type of target RF pulse; The filtered signal is a digital signal. the first reaction template; and a second response template representing a second known response of the vapor to a second type of target RF pulse; The method according to any one of claims 13 to 15, wherein the generated value is based on a comparison with

30. 30. The method of claim 29, wherein the first and second types of targeted RF pulses have different non-overlapping ranges of field strengths.

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