Electromagnetic field detector

Abstract

The present invention provides a method of detecting an electromagnetic field and an apparatus for performing the method, the method comprising the steps of: transmitting a probe signal at a probe frequency along a first probe signal path and a second probe signal path to an optical receiver, wherein the first probe signal path passes through a transmission medium and the probe frequency is set to excite an electron of the transmission medium from a ground state to a first excited state; transmitting a coupling signal at a coupling frequency along a first coupling signal path, wherein the first coupling signal path overlaps with the first probe signal path in the transmission medium in a first overlapping section, wherein the coupling frequency is set to excite the electron of the transmission medium to a predetermined excited state to induce an electromagnetically induced transparency (EIT) effect in the transmission medium, such that an incident electromagnetic field at the transmission medium causes a change in refractive index in the transmission medium at the first overlapping section, such that an optical path length difference between the first probe signal path and the second probe signal path changes; combining the first path of the probe signal with the second path of the probe signal after the first path of the probe signal passes through the first overlapping section of the transmission medium; and monitoring an intensity of the combined first and second paths of the probe signal at the optical receiver to detect the incident electromagnetic field at the first overlapping section of the transmission medium as an intensity change caused by the change in the optical path length difference between the first probe signal path and the second probe signal path.

Description

Technical Field

[0001] This invention relates to electromagnetic field detectors. In particular, this invention relates to radio frequency (RF) detectors. Background Technology

[0002] Conventional RF detectors, such as dipole antennas, utilize a metallic conductor in which electrons move along the conductor in response to an incident RF electric field, generating a small current. In addition to detecting only RF signals, RF detectors can also be created by converting this current (e.g., using electronic circuitry, mixers, amplifiers, and digitizers) into a demodulated, amplified signal.

[0003] In many applications, minimizing the size of RF receivers is desirable. However, conventional RF receivers based on metallic conductors are limited in size because the relationship between sensitivity, frequency, and conductor length means that for a given application, an RF receiver below a certain size will not function adequately. Electrically small antennas are antennas that operate below their inherent resonant frequency, but they are limited by the Chu limit, which sets a minimum size for any antenna used at a given frequency. Another limitation of conventional RF receivers is that their ability to sense weak RF signals depends on the receiver's gain, which is determined by the antenna design. Therefore, the performance of conventional antennas is limited in terms of size, bandwidth, and sensitivity.

[0004] A novel type of RF receiver is based on the Rydberg atom, an atom with one or more electrons excited to very high principal quantum numbers. These Rydberg atoms possess several useful properties, such as very large dipole moments and long decay periods. These properties can be used to create RF receivers capable of receiving and demodulating amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM) RF electric fields over a very wide frequency range, such as from hundreds of MHz to 1 THz.

[0005] Figure 1 An example of a Rydberg atom-based RF receiver is shown, which operates as follows. An atomic medium is provided, in this example, a glass cell filled with a low-density vapor of alkali metal atoms (e.g., rubidium-85). Each rubidium-85 atom has multiple electronic states, including a ground state (|1>) and multiple excited states. External electrons of the rubidium-85 atom can be excited from the ground state (|1>) to an excited state (e.g., by absorbing a photon of a specific wavelength). The electron can then decay from the excited state to a lower excited state (i.e., an excited state at a lower energy level) or decay back to the ground state (|1>). However, some of these transitions are not allowed because they are dipole-forbidden.

[0006] In the RF receiver, a first laser (called the "probe" laser) passes through the atomic medium at a first wavelength corresponding to the energy required to raise the outer electron of a rubidium-85 atom from its ground state (|1>) to a first excited state (|2>). A second laser (called the "coupling" laser) also passes through the atomic medium in the opposite direction at a relatively high power level (compared to the probe laser) and at a second wavelength corresponding to the energy required to raise the outer electron of a rubidium-85 atom from the first excited state (|2>) to a Rydberg state (|3>). The transition from the Rydberg state (|3>) to the ground state (|1>) is prohibited, causing the ground state (|1>) to become depleted, and thus fewer atoms can absorb the probe laser operating at the first wavelength. Therefore, the atomic medium becomes more transparent to the probe laser, increasing its transmittance, which is observable at the optical detector. This phenomenon is called electromagnetically induced transparency (EIT), and the received signal is called the EIT signal. Specifically, the above description is of the step-by-step EIT effect, but those skilled in the art will understand that the EIT effect can be achieved through alternative electronic transitions, such as the Vee and Lambda schemes.

[0007] Once the atomic medium becomes transparent to the probe laser, further physical effects can be used to detect the RF electric field. Because the outer electrons of rubidium-85 atoms are much farther from the nucleus than in the ground state, they generate a large dipole moment and become responsive to the incident RF electric field. The incident RF field can induce further transitions of electrons from the Rydberg state to adjacent Rydberg states. If transitions from adjacent Rydberg states to the ground state are not prohibited, electrons may subsequently fall back to the ground state, making the atomic medium less transparent to the probe laser and causing a decrease in the amplitude of the EIT signal. This decrease in the amplitude of the EIT signal is proportional to the amplitude of the incident RF electric field, thus creating an AM RF receiver based on Rydberg atoms. Rydberg atom-based RF receivers have also been shown to detect frequency-modulated and phase-modulated RF fields. Regardless of the modulation scheme used, Rydberg atom-based RF detectors can be configured to detect RF fields of a specific frequency by selecting a specific second wavelength of the coupled laser to raise the electrons of the atomic medium to a specific Rydberg state. The Rydberg state is chosen such that a photon at the specific frequency to be detected will lift an electron from that Rydberg state to its neighboring Rydberg state, thus producing a detectable change in the EIT signal that can be observed at the optical detector. The amplitude of the EIT signal, known as the “modulation depth,” is the difference between the EIT signal with and without an incident RF electric field. In conventional systems where the EIT signal is based on the absorbance of the probe laser, the EIT signal is at its optimal modulation depth (i.e., giving the optimal signal-to-noise ratio (SNR)) when the frequency of the RF field matches the energy required for the transition from that particular Rydberg state to its neighboring Rydberg state (i.e., “resonance”). When the frequency of the RF field deviates from the energy required for the transition from that particular Rydberg state to its neighboring Rydberg state (called “RF phase shift”), the amplitude of the EIT signal decreases. Summary of the Invention

[0008] According to a first aspect of the present invention, a method for detecting an electromagnetic field is provided, the method comprising the steps of: transmitting a detection signal along a first detection signal path and a second detection signal path at a detection frequency to an optical receiver, wherein the first detection signal path passes through a transmission medium, and the detection frequency is set to excite electrons in the transmission medium from a ground state to a first excited state; transmitting a coupling signal along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in the transmission medium in a first overlap section, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state, thereby inducing an electromagnetically induced transparent (EIT) effect in the transmission medium, such that the incident electromagnetic field at the transmission medium causes a change in the refractive index of the transmission medium in the first overlap section, such that the optical path length difference between the first detection signal path and the second detection signal path... The change occurs; after the first path of the probe signal passes through the first overlapping segment of the transmission medium, the first path of the probe signal is combined with the second path of the probe signal; and the intensity of the combined first and second paths of the probe signal is monitored at the optical receiver to detect the incident electromagnetic field at the first overlapping segment of the transmission medium as an intensity change caused by the change in the optical path length difference between the first and second probe signal paths, wherein the first probe signal path traverses the transmission medium in multiple channels, and the first coupling signal overlaps with the first probe signal path in multiple overlapping segments in the multiple channels, and the step of combining the first path of the probe signal with the second path of the probe signal is performed after the first path of the probe signal passes through all overlapping segments of the multiple overlapping segments. The step of monitoring the intensity of the combined first and second paths of the probe signal may include demodulating information contained in the incident electromagnetic field.

[0009] According to a second aspect of the present invention, a method for detecting an electromagnetic field is provided, the method comprising the steps of: transmitting a detection signal along a first detection signal path and a second detection signal path at a detection frequency to a photodetector, wherein the first detection signal path passes through a transmission medium, and the detection frequency is configured to excite electrons in the transmission medium from a ground state to a first excited state; transmitting a coupling signal along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in the transmission medium in a first overlapping segment, wherein the coupling frequency is configured to excite electrons in the transmission medium to a predetermined excited state, thereby inducing an electromagnetically induced transparent (EIT) effect in the transmission medium, such that the transmission... An incident electromagnetic field in the medium causes a change in the refractive index of the transmission medium at the first overlapping section, resulting in a change in the optical path length difference between the first and second detection signal paths; after the first path of the detection signal passes through the first overlapping section of the transmission medium, the first path of the detection signal is combined with the second path of the detection signal; and the intensity of the combined first and second paths of the detection signal is monitored at the optical receiver to demodulate information contained in the incident electromagnetic field at the first overlapping section of the transmission medium as an intensity change caused by the change in the optical path length difference between the first and second detection signal paths.A second aspect of the invention can be defined as a method for detecting electromagnetic fields, the method comprising the steps of: transmitting a detection signal along a first detection signal path and a second detection signal path at a detection frequency to an optical receiver, wherein the first detection signal path passes through a transmission medium, and the detection frequency is set to excite electrons in the transmission medium from a ground state to a first excited state; transmitting a coupling signal along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in the transmission medium in a first overlapping segment, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state, thereby inducing an electromagnetically induced transparent (EIT) effect in the transmission medium, such that the incident electromagnetic field at the transmission medium causes an electromagnetic field in the transmission medium at the first overlapping segment. The change in refractive index causes a change in the optical path length difference between the first and second detection signal paths; after the first path of the detection signal passes through the first overlapping section of the transmission medium, the first path of the detection signal is combined with the second path of the detection signal; and the intensity of the combined first and second paths of the detection signal is monitored at the optical receiver to detect the incident electromagnetic field at the first overlapping section of the transmission medium as an intensity change caused by the change in the optical path length difference between the first and second detection signal paths, wherein the step of monitoring the intensity of the combined first and second paths of the detection signals includes demodulating information from the combined first and second paths of the detection signals.

[0010] According to a third aspect of the present invention, an apparatus for detecting electromagnetic fields is provided, the apparatus comprising: a first optical transmitter, a second optical transmitter, a transmission medium, an optical receiver, and a processor, wherein: the first optical transmitter is configured to transmit a detection signal along a first detection signal path and a second detection signal path at a detection frequency to the optical receiver, wherein the first detection signal path passes through the transmission medium, and the detection frequency is set to excite electrons in the transmission medium from a ground state to a first excited state; the second optical transmitter is configured to transmit a coupling signal along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in the transmission medium in a first overlapping section, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state, thereby inducing an electromagnetically induced transparent (EIT) effect in the transmission medium, such that the incident electromagnetic field at the transmission medium causes a change in the refractive index of the transmission medium in the first overlapping section, thereby inducing the first... The optical path length difference between the probe signal path and the second probe signal path changes; the optical receiver is configured to receive a combination of the first path and the second path of the probe signal after the first path of the probe signal passes through the first overlapping section of the transmission medium; and the processor is configured to monitor the intensity of the combined first and second paths of the probe signal to detect the incident electromagnetic field at the first overlapping section of the transmission medium as an intensity change caused by the change in the optical path length difference between the first and second probe signal paths, wherein the first probe signal path passes through the transmission medium in multiple channels, and the first coupling signal overlaps with the first probe signal path in multiple overlapping sections in the multiple channels, wherein the combination of the first path and the second path of the probe signal is after the first path of the probe signal passes through all overlapping sections of the multiple overlapping sections. The processor can demodulate information contained in the incident electromagnetic field.

[0011] According to a fourth aspect of the present invention, an apparatus for detecting electromagnetic fields is provided, the apparatus comprising: a first optical transmitter, a second optical transmitter, a transmission medium, an optical receiver, and a processor, wherein: the first optical transmitter is configured to transmit a detection signal along a first detection signal path and a second detection signal path at a detection frequency to the optical receiver, wherein the first detection signal path passes through the transmission medium, and the detection frequency is set to excite electrons in the transmission medium from a ground state to a first excited state; the second optical transmitter is configured to transmit a coupling signal along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in the transmission medium in a first overlapping segment, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state, so as to... An electromagnetically induced transparent (EIT) effect is induced in the medium, causing a change in the refractive index of the transmission medium at the first overlapping section, which in turn causes a change in the optical path length difference between the first and second probe signal paths. The optical receiver is configured to receive a combination of the first and second paths of the probe signal after the first path of the probe signal passes through the first overlapping section of the transmission medium. The processor is configured to monitor the intensity of the combined first and second paths of the probe signal in order to demodulate information contained in the incident electromagnetic field at the first overlapping section of the transmission medium as an intensity change caused by the change in the optical path length difference between the first and second probe signal paths.A fourth aspect of the present invention can be defined as a device for detecting electromagnetic fields, the device comprising: a first optical transmitter, a second optical transmitter, a transmission medium, an optical receiver, and a processor, wherein: the first optical transmitter is configured to transmit a detection signal along a first detection signal path and a second detection signal path at a detection frequency to the optical receiver, wherein the first detection signal path passes through the transmission medium, and the detection frequency is set to excite electrons in the transmission medium from a ground state to a first excited state; the second optical transmitter is configured to transmit a coupling signal along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in the transmission medium in a first overlapping segment, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state, thereby inducing an electromagnetically induced transparent (EIT) effect in the transmission medium, such that... The incident electromagnetic field at the transmission medium causes a change in the refractive index of the transmission medium at the first overlapping section, resulting in a change in the optical path length difference between the first detection signal path and the second detection signal path. The optical receiver is configured to receive a combination of the first path and the second path of the detection signal after the first path of the detection signal passes through the first overlapping section of the transmission medium. The processor is configured to monitor the intensity of the combined first and second paths of the detection signal to detect the incident electromagnetic field at the first overlapping section of the transmission medium as an intensity change caused by the change in the optical path length difference between the first and second detection signal paths, wherein the processor is configured to demodulate information from the combined first and second paths of the detection signal.

[0012] The information can be modulated based on frequency modulation, and the demodulation of information from the combined first and second paths can utilize the transmission power value of the incident electromagnetic field.

[0013] The transmission medium may include metal vapor, which may be an alkali metal, and the alkali metal may be one of rubidium, cesium or strontium.

[0014] The electromagnetic field can be a radio frequency (RF) field.

[0015] The interaction volume (including length) of any overlap between the coupled signal and the second detection signal path may differ from the interaction volume (including length) of the first overlapping segment. The coupled signal may not overlap with the second detection signal path, or the coupled signal may overlap with the second detection signal path in a second overlapping portion, such that the incident electromagnetic field causes a change in the refractive index of the transmission medium at the second overlapping portion, and the interaction volume (including length) of the second overlapping segment may differ from the interaction volume (including length) of the first overlapping segment. Attached Figure Description

[0016] To enable a better understanding of the present invention, embodiments thereof will now be described by way of example only with reference to the accompanying drawings, in which:

[0017] Figure 1 This is a schematic diagram of a conventional Rydberg atom-based radio frequency (RF) detector;

[0018] Figure 2 This is a schematic diagram of the Rydberg atom-based RF detector according to the first embodiment of the present invention;

[0019] Figure 3 This is a flowchart of the method according to the first embodiment of the present invention;

[0020] Figure 4 This is an example Figure 2 A graph showing the modulation depth of the RF detector relative to the RF phase shift;

[0021] Figure 5 This is a schematic diagram of a Rydberg atom-based RF detector according to the second embodiment of the present invention;

[0022] Figure 6 This is a schematic diagram of a Rydberg atom-based RF detector according to the third embodiment of the present invention;

[0023] Figure 7 This is a schematic diagram of the Rydberg atom-based RF detector according to the fourth embodiment of the present invention;

[0024] Figure 8 This is a schematic diagram of a Rydberg atom-based RF detector according to the fifth embodiment of the present invention;

[0025] Figure 9 This is a schematic diagram of a Rydberg atom-based RF detector according to the sixth embodiment of the present invention; and

[0026] Figure 10 This is a schematic diagram of a Rydberg atom-based RF detector according to the seventh embodiment of the present invention. Detailed Implementation

[0027] Now refer to Figure 2 A first embodiment of the system 1 of the present invention is described. The system 1 includes a wireless transmitter 10 and a radio frequency (RF) detector 20. In this embodiment, the wireless transmitter is configured to transmit wireless signals at a frequency of 3.46 GHz and a transmission power of 15 microwatts, and the wireless transmitter 10 is located 1 kilometer away from the RF detector 20.

[0028] RF detector 20 is a Rydberg atom-based RF detector and includes a first beam splitter 21a and a second beam splitter 21b, a probe laser 23, a first mirror 24a, a second mirror 24b and a third mirror 24c, a coupling laser 25, a first frequency-correlated mirror 26a and a second frequency-correlated mirror 26b, a vapor chamber 27 containing low-density alkali metal (rubidium-85 in this embodiment) vapor, a first filter 28a and a second filter 28b, and a photodetector 29. In the figures, solid lines indicate only probe laser 23 (i.e., it does not overlap with any other laser such as coupling laser 25), while dashed lines indicate only coupling laser 25 (i.e., it does not overlap with any other laser such as probe laser 23).

[0029] The RF detector 20 is arranged as an interferometer (more specifically, a Michelson interferometer), in which the probing laser 23 is split into two paths, one of which passes through the vapor chamber 27 and overlaps with the first and second paths of the coupled laser 25 at the vapor chamber 27. This will now be described in more detail.

[0030] The probe laser 23 passes through the first beam splitter 21a, splitting it into a first portion guided along a first path and a second portion guided along a second path (in this embodiment, the splitting ratio is 50:50). The first path of the probe laser 23 passes through the first filter 28a, through the first portion of the vapor chamber 27, through the first frequency-correlated mirror 26a (which does not reflect the probe laser 23), is reflected by the first mirror 24a, passes through the second filter 28b, through the second portion of the vapor chamber 27, through the second frequency-correlated mirror 26b, and terminates at the photodetector 29. The first path of the probe laser 23 can interact with the rubidium-85 atoms in the vapor chamber 27 within the first and second portions, for example, by exciting electrons from the ground state to a first excited state. The second path of the probe laser 23 is reflected by the second mirror 24b and the third mirror 24c and terminates at the photodetector 29. For completeness, it should be noted that the second path of the probe laser 23 does not pass through the vapor chamber 27 and therefore does not interact with the rubidium-85 atoms in the vapor chamber 27.

[0031] At photodetector 29, the first path and the second path of the probe laser 23 combine and interfere. Assuming the optical path length difference between the first and second paths is stable (within a small fraction of the wavelength of the probe laser 23 during the measurement at the photodetector), coherent interference will occur and the intensity of the probe laser 23 at photodetector 29 will remain stable (i.e., have a constant value). However, as explained below, RF detector 20 can be operated such that the intensity of the probe laser 23 changes according to the incident RF signal at vapor chamber 27.

[0032] The coupled laser 25 passes through the second beam splitter 21b, splitting it into a first portion guided along a first path and a second portion guided along a second path (in this embodiment, the splitting ratio is 50:50). The first path of the coupled laser 25 is reflected by the first frequency-dependent mirror 26a, allowing it to pass through the first portion of the vapor chamber 27 and terminate at the first filter 28a. The second path of the coupled laser 25 is reflected by the second frequency-dependent mirror 26b, allowing it to pass through the second portion of the vapor chamber 27 and terminate at the second filter 28b. The coupled laser 25 can also interact with the rubidium-85 atoms of the vapor chamber 27 within both the first and second portions, for example, by exciting electrons from a first excited state to a predetermined Rydberg state.

[0033] Therefore, the probe laser 23 and the coupling laser 25 overlap in the first and second portions of the vapor chamber 27, causing the rubidium-85 atoms within these portions to interact with both the probe laser 23 and the coupling laser 25. In the accompanying drawings, the overlapping portions of the probe laser 23 and the coupling laser 25 are indicated by dashed lines. Thus, these atoms in the first and second portions of the vapor chamber 27 may experience electromagnetically induced transparency (EIT) effects. In this embodiment, the length of the first portion of the vapor chamber 27 is 0.05 m, and the length of the second portion of the vapor chamber 27 is 0.05 m; therefore, EIT effects may occur within a combined distance of 0.1 m between these first and second portions of the vapor chamber 27.

[0034] RF detector 20 is operable to detect RF signals incident on vapor chamber 27 transmitted by wireless transmitter 10. This is achieved by configuring probe laser 23 to transmit a probe signal (which is split by first beam splitter 21a such that a first path of the probe signal passes through a first portion and a second portion of rubidium-85 vapor chamber 27), and further configuring coupling laser 25 to transmit a coupling signal (which is split by second beam splitter 21b such that a first path of the coupling signal passes through the first portion of rubidium-85 vapor chamber 27 and a second path of the coupling signal passes through the second portion of rubidium-85 vapor chamber 27). The frequency of the probe signal is set to correlate with the transition of electrons in rubidium-85 atoms from the ground state to a first excited state, and the frequency of the coupling signal is set to correlate with the transition of electrons in rubidium-85 atoms from the first excited state to a predetermined Rydberg state. In this embodiment, where the RF detector 20 is configured to detect a 3.46 GHz wireless signal from the wireless transmitter 10, the detection signal is set to 780 nm and the coupling signal is set to 480 nm, causing electrons to be excited to a predetermined Rydberg state having the 84th principal quantum number. In this configuration, the wireless signal transmitted by the wireless transmitter 10 at 3.46 GHz through the vapor chamber 27 will excite electrons from the predetermined Rydberg state to an adjacent Rydberg state. As explained in detail below, the RF signal that excites electrons from the predetermined Rydberg state to an adjacent Rydberg state will cause a change in the refractive index of the first and second portions of the vapor chamber 27. Since the first path of the detection laser 23 passes through these first and second portions of the vapor chamber 27, while the second path of the detection laser 23 does not pass through the vapor chamber 27, the incident RF signal at 3.46 GHz from the wireless transmitter 10 causes a change in the optical path length of the first path of the detection laser 23, but does not cause a change in the optical path length of the second path of the detection laser 23. Therefore, the difference in optical path length between the first and second paths when the RF signal is incident on the steam chamber 27 (and thus the phase measured at the photodetector 29) changes compared to the difference in optical path length between the first and second paths when no RF signal is incident on the steam chamber 27. Thus, when the RF signal from the wireless transmitter 10 is incident on the steam chamber 27, the combination of the first and second paths of the probe laser 23 at the photodetector 29 will be different from the combination of the first and second paths of the probe laser 23 at the photodetector 29 when no RF signal is incident on the steam chamber 27. When the RF signal passes through the steam chamber 27 and causes a change in the refractive index of the first and second portions of the steam chamber 27, and thus a change in the difference in optical path length between the first and second paths of the probe laser 23, the RF signal can be detected as a change in intensity value relative to a stable reference value by monitoring the intensity of the combination of the first and second paths of the probe laser 23 at the photodetector 29.

[0035] Now refer to Figure 2 The system and Figure 3 The flowchart illustrates an implementation of the method of the present invention. In a first step S101, the RF detector 20 is operated such that the probe laser 23 and the coupling laser 25 produce an EIT effect in the vapor chamber 27. As described above, the first path of the probe laser 23 passes through the vapor chamber and is affected by the EIT effect, while the second path of the probe laser 23 does not pass through the vapor chamber and is not affected by the EIT effect. In step S103, the intensity of the combined first and second paths of the probe laser 23 is monitored to determine if there is an intensity change. In step S105, the intensity change is detected and recorded as a detection of an RF signal at a specific frequency at the vapor chamber 27 (this specific frequency is the frequency required to excite an electron from a Rydberg state to an adjacent Rydberg state, as determined by the frequencies of the probe signal and the coupling signal). This detection may be a comparison of the intensity change with a threshold. This threshold may be calibrated to be greater than the typical noise level in the system. The process then loops back to step S103 to monitor further changes in the intensity of the combined first and second paths of the probe laser 23.

[0036] This first embodiment represents one implementation of a Rydberg atom-based RF detector that utilizes the change in refractive index of a vapor chamber 27 to detect an incident RF signal. This is achieved using an interferometer design in which, when an RF signal is received at the vapor chamber 27, one path of the probe laser 23 is affected by the change in refractive index of the vapor chamber 27, while the other path of the probe laser 23 is not affected by the change in refractive index of the vapor chamber 27. This is significantly different from prior art designs of Rydberg atom-based RF detectors that utilize the change in absorbance of the vapor chamber 27. The theoretical analysis below will show that the Rydberg atom-based RF detector based on the change in refractive index of the vapor chamber 27 has an unusual response to the incident RF signal because the modulation depth (i.e., the intensity difference of the probe laser 23 measured by the photodetector 29 when the RF signal is incident on the vapor chamber 27 and when no RF signal is incident on the vapor chamber 27) is zero when the RF signal resonates with the frequency required to raise an electron from a predetermined Rydberg state to an adjacent Rydberg state. However, when the RF signal is de-resonant (i.e., the frequency of the RF signal is not equal to the frequency required to raise an electron from a predetermined Rydberg state to an adjacent Rydberg state), its performance is superior to that of existing Rydberg atom-based RF detectors based on absorbance changes.

[0037] The following description provides a theoretical analysis of the invention. As described above, the rubidium-85 atoms in the overlapping probe laser 23 and coupling laser 25 in the first and second portions of the vapor chamber 27 may undergo an EIT effect. In particular, this EIT effect can be achieved according to a stepped EIT configuration, wherein the probe laser 23 excites electrons from the ground state to a first excited state (using, for example, a 780 nm probe signal) and the coupling laser 25 excites electrons from the first excited state to a predetermined Rydberg state (using, for example, a 480 nm coupling signal). The polarizability χ of these rubidium-85 atoms undergoing the EIT effect in the stepped EIT configuration is given by the following equation:

[0038]

[0039] in:

[0040] • N is the number of atoms involved in the interaction.

[0041] · It is the transition dipole moment of the first transition (between the ground state and the first excited state).

[0042] ε₀ is the permittivity of free space.

[0043] · It is a simplified Planck constant.

[0044] ·Δ p ,Δ c and Δ r It is the difference between the angular frequencies of the probe signal, coupling signal, and RF signal and their corresponding resonant atomic transition frequencies (i.e., the frequencies required to raise an electron from the ground state to the first excited state, the frequencies required to raise an electron from the first excited state to a predetermined Rydberg state, and the frequencies required to raise an electron from a predetermined Rydberg state to an adjacent Rydberg state).

[0045] Γ2, Γ3, and Γ4 are the transition rates from the first excited state, the predetermined Rydberg state, and the adjacent Rydberg state, respectively, and

[0046] ·Ω r and Ω c It is the Rabi frequency of the RF signal and the coupled signal, which is the product of the electric field amplitude and the associated transition dipole moment divided by the simplified Planck constant.

[0047] The refractive index of rubidium-85 atoms in vapor chamber 27 undergoing the EIT effect is described by the real part of this complex polarizability. It is helpful to define some intermediate expressions as follows:

[0048]

[0049] C=Γ3(Δ p +Δ c+Δ r )+Γ4(Δ p +Δ c )

[0050]

[0051]

[0052]

[0053] Thus, the complex polarizability is given by the following formula:

[0054] χ=k{2(CS-BT)+i(BS+4CT)} / (S 2 +4T 2 )=D+iA

[0055] Therefore, the real part of the polarizability is:

[0056] D = 2k(CS - BT) / (S) 2 +4T 2 )

[0057] The modulation depth M can be defined as the intensity difference of the probe laser 23 measured by the photodetector 29 when the RF signal is incident on the steam chamber 27 and when no RF signal is incident on the steam chamber 27.

[0058] M = D – D0

[0059] The goal is to maximize the modulation depth M, thereby maximizing the intensity change in response to the incident RF signal at vapor chamber 27 and making it easier to detect. This is achieved if the probe laser 23 is stabilized to raise electrons from the ground state to the first excited state (i.e., Δ). p =0) the required frequency and the coupled laser 25 is stabilized to raise electrons from the first excited state to a predetermined state (i.e., Δ c The frequency required for (=0) can be proven to be:

[0060] (S 2 +4T 2 M = -2kΔ r Ω r 2 Ω c 2

[0061] Furthermore, it can be seen that when the following equation is satisfied, the modulation depth M can be related to the power of the coupled laser 25 (by Ω). c 2 (Indicates) is maximized:

[0062] Γ2 2 {(Γ3Γ4+Ω r2 ) 2 +4Δ r 2 Γ3 2}=(4Δ r 2 +Γ4 2 )Ω c 4

[0063] Therefore, for any given RF power, there exists an optimal value for the power of the coupling laser 25. However, since the RF power is unknown, may be variable, and is likely to be very weak (in many use cases of Rydberg atom-based RF detectors), the power of the coupling laser 25 can be set based on a negligible RF power, such that Ω r 2 =0, and Ω c 2 =Γ2Γ3.

[0064] In Δ p =Δ c =0 and Ω c 2 In the case of Γ2Γ3, the modulation depth M in the real part of the polarizability is: M = -2kΓ3Δ r Ω r 2 / Γ2{(2Γ3Γ4+Ω r 2 ) 2 +(4Γ3Δ r ) 2}

[0065] This indicates that the modulation depth of the refractive index-based RF detector is related to Δ r -1 (The frequency of the RF signal is proportional to the difference in frequency between the corresponding resonant atomic transitions of an electron from a Rydberg state to an adjacent Rydberg state.) In contrast, the modulation depth of an absorbance-based RF detector is proportional to Δ... r -2 The refractive index is proportional to the signal. Therefore, refractive index-based RF detectors are more suitable for detecting RF signals with significant RF phase shifts.

[0066] Figure 4 against Figure 2The system shown illustrates how the modulation depth varies as a function of the RF phase shift. The figure shows that when the RF phase shift is zero (in other words, the RF signal frequency is exactly 3.46 GHz, perfectly matching the energy required to excite an electron from a Rydberg state to an adjacent Rydberg state), the modulation depth is zero, indicating that the RF detector 20 is unsuitable for detecting RF signals at the resonant frequency. However, when the RF phase shift is non-zero, the modulation depth is non-zero, indicating that the RF detector 20 is suitable for detecting RF signals not at the resonant frequency. This contrasts sharply with conventional Rydberg atom-based RF receivers, which utilize changes in the detected signal absorption and have maximum modulation depth at the incident RF signal's resonant frequency.

[0067] Figure 4 Further examples illustrate the existence of two modulation depth maxima at ±100 kHz RF phase shifts, with the modulation depth decaying as the RF phase shift increases above +100 kHz or decreases below -100 kHz. As described above, the modulation depth decays at a lower rate with larger RF phase shifts compared to the decay rate of conventional Rydberg atom-based RF detectors (utilizing the absorption of the probe signal) with increasing modulation depth.

[0068] Figure 4 It is also illustrated that, in this case, the modulation depth is 10. -5 The order of magnitude. It is known that interferometer devices (e.g.) Figure 2 The Michelson-type device shown can measure 10 -9 Orders of magnitude of refractive index (e.g., see the article "Absolute refractometry of dry gas to ±3 parts in 10) 9 ", Patrick Egan et al., Appl. Opt. 50, 3076-3086 (2011)). Therefore, such an interferometer device can detect approximately 10 -8The modulation depth changes by orders of magnitude (due to the change in refractive index of rubidium-85 atoms undergoing the EIT effect in vapor chamber 27) (because the change in the refractive index of the material causes the change in the real part of the material's polarizability to double). Therefore, the RF detector 20 of the above embodiment can detect RF signals with an RF phase shift of many megahertz on either side of the resonant frequency. If the system noise is low enough, an RF signal can be detected where the RF phase shift reaches the midpoint between 1) the frequency required to raise an electron from a Rydberg state of the nth principal quantum number to an adjacent Rydberg state and 2) the frequency required to raise an electron from a Rydberg state of the (n+1)th principal quantum number to an adjacent Rydberg state, or the midpoint between 1) the frequency required to raise an electron from a Rydberg state of the nth principal quantum number to an adjacent Rydberg state and 2) the frequency required to raise an electron from a Rydberg state of the (n-1)th principal quantum number to an adjacent Rydberg state. For example, consider a system where the detection and coupling signals are configured to raise an electron to a Rydberg state with the 84th principal quantum number, such that a 3.46 GHz RF signal will raise the electron to a neighboring Rydberg state with the 85th principal quantum number. If the system noise is sufficiently low, this system can detect such an RF signal in the range of 3.3985 GHz to 3.5245 GHz, where 3.3985 GHz is the midpoint between the resonant frequency (3.337 GHz) for the electron transition from the 85th to the 86th principal quantum number and the resonant frequency (3.460 GHz) for the electron transition from the 84th to the 85th principal quantum number, and where 3.5245 GHz is the midpoint between the resonant frequency (3.589 GHz) for the electron transition from the 83rd to the 84th principal quantum number and the resonant frequency (3.460 GHz) for the electron transition from the 84th to the 85th principal quantum number. Once the frequency of the RF signal is below 3.3985 GHz or above 3.5245 GHz, the coupling signal can be set to the next corresponding Rydberg state (i.e., the Rydberg state of the 83rd principal quantum number if the frequency of the RF signal is above 3.5245 GHz; and the Rydberg state of the 85th principal quantum number if the frequency of the RF signal is below 3.3985 GHz) in order to increase the modulation depth when the RF signal is received (relative to the modulation depth when the system is not recalibrated).

[0069] The system described above uses an interferometer to detect the intensity change between the two paths of a laser beam that has split into two paths. One path of the laser acts as the detection signal in a Rydberg atom-based RF detector, while the other path does not. This is because the phase (and therefore the optical path length) between the two paths changes due to the presence of an RF signal at the Rydberg atom-based RF detector. This intensity change can be recorded as the detection of the RF signal. The specific device used in the above embodiment is an example of a Michelson-type interferometer. However, those skilled in the art will understand that any form of interferometer can be used alternatively, wherein the interferometer is designed to detect the change in optical path length between the two paths of the detection laser. These interferometer designs include Fabry-Perot, Mach-Zehnder, Sagnac, and Fizeau configurations. The operating principle of these interferometer designs is to combine two laser signals of the same frequency such that the interference pattern of the combined signal is determined by the phase difference between the two signals. Therefore, this property can be utilized so that the RF signal causes a phase difference, as described above, so that the change in the interference pattern (and therefore the intensity) of the combined signal can be recorded as the detection of the RF signal.

[0070] Figures 5 to 10 Further embodiments of the RF detector of the present invention based on an alternative interferometer design are illustrated. Figures 5 to 10 In each figure, solid lines represent only probe laser 23 (i.e., not overlapping with any other laser such as coupling laser 25), dashed lines represent only coupling laser 25 (i.e., not overlapping with any other laser such as probe laser 23), and long and short dashed lines represent the overlap between probe laser 23 and coupling laser 25. Figure 5 and Figure 6 An example is the Mach-Zehnder interferometer, in which one path of the split probe laser passes through a vapor chamber of rubidium-85 atoms. Figure 7 A stabilizing laser (where a short dash indicates only the stabilizing laser (i.e., not overlapping with the probe laser and / or coupling laser), a single dash indicates overlap between the probe laser 23 and the stabilizing laser, and a double dash indicates overlap between the probe laser 23, the stabilizing laser, and the coupling laser 25) and a piezoelectric transducer are introduced into the Mach-Zehnder interferometer. The purpose of the stabilizing laser and the piezoelectric transducer is to keep the probe laser stable and resistant to the effects of vibration, temperature changes, and / or pressure changes (by altering the position of one of the mirrors along one path of the probe laser based on a feedback loop generated by a photodiode that measures the intensity of the stabilizing laser). Figure 8 The device shown is with Figure 7 The device is similar, but it uses balanced photodiodes instead of piezoelectric transducers as part of the stabilization technology. Figure 9 and Figure 10 The device shown is Figure 7 and Figure 8 The device is similar, but uses hollow-core optical fiber filled with rubidium-85 to transmit probe and coupling signals. Figure 9 This includes fiber optic stretchers as part of stabilization technology, where the fiber optic stretcher alters the length of the fiber to compensate for length variations caused by pressure, temperature, and / or vibration. Figure 10 Two photodiodes are used to eliminate noise (the noise of the probe laser is slightly different from the noise of the stabilized laser).

[0071] exist Figure 2 In this implementation, the probe laser 23 and the coupling laser 25 overlap twice in the vapor chamber 27. By increasing the effective length of the overlapping segments of the probe laser 23 and the coupling laser 25, more rubidium-85 atoms can undergo the EIT effect, which increases the intensity variation of the RF signal as it passes through these overlapping segments (due to the larger corresponding changes in refractive index, phase, and optical path length). However, this is not necessary, and the probe laser and the coupling laser can overlap in any configuration, as long as the interaction volume (i.e., the length of the overlapping segment multiplied by the overlapping area) is large enough for the photodetector 29 to detect the RF signal (calculated based on the sensitivity of the photodetector and the expected power of the RF signal).

[0072] Figure 2 The implementation describes a detector in which changes in the strength of a first and second path of the combined probe signal relative to a stable reference value indicate the presence of an RF signal at steam chamber 27. The RF detector 20 can also be used as a receiver to receive (i.e., demodulate) information contained in the wireless signal, for example, by using an on / off keying communication protocol (where the absence of an RF signal at steam chamber 27 in a time slot can indicate a value of 0, while the presence of an RF signal at steam chamber 27 in a time slot can indicate a value of 1). Those skilled in the art will understand that other communication protocols and modulation schemes can be used. For example, the RF detector 20 can be used to receive frequency-modulated data by determining the value of the RF phase shift based on the intensity change at photodetector 29 (which can be determined if the RF signal has a known received power), thereby tracking frequency changes in the RF signal sequence from the wireless transmitter 10.

[0073] Figure 2The implementation described above illustrates an example system for detecting 3.46 GHz wireless signals. However, those skilled in the art will understand that the method described above can be applied to many other electromagnetic signals of different frequencies, and the 3.46 GHz signal is merely an example. That is, for a specific target frequency, the system can be configured such that the EIT signal is generated by an incident electromagnetic signal at that target frequency (e.g., by selecting a suitable atomic medium (e.g., rubidium, cesium, or strontium) having Rydberg states corresponding to that target frequency). Once the relevant principal quantum number is known, the values ​​of other physical quantities (resonant frequency, transition rate, dipole moment) can be determined. Furthermore, it is not necessary to generate the EIT signal following a ladder configuration of electronic transitions. That is, any configuration (e.g., Lambda, Vee) can be used.

[0074] In the above embodiment, the probe laser and the coupling laser pass through a vapor chamber containing low-density rubidium-85 atoms. As mentioned above, other alkali metals can be used. Furthermore, those skilled in the art will understand that one branch of the probe laser can be subjected to the EIT effect using other devices, such as an optical fiber with a hollow core segment filled with alkali metal vapor.

[0075] Those skilled in the art will also understand that it is not necessary for the second path of the probe signal not to overlap with the coupled signal. That is, as long as there is a change in the optical path length difference between the first and second paths of the probe signal, the intensity change of the probe signal in response to the incident RF signal can still be detected at the optical receiver. Therefore, the interferometer design of the Rydberg atom-based RF receiver can utilize the different changes in the optical path length of the first path of the probe signal relative to the optical path length of the second path of the probe signal when the incident signal is received. As in the above embodiment, this can be achieved by the first path of the probe signal passing through the vapor chamber while the second path of the probe signal does not, or in an alternative implementation by such that both the first and second paths pass through the vapor chamber, but the interaction volume of the overlapping portion of the first paths of the coupled signal and the probe signal is different from the interaction volume of the overlapping portion of the second paths of the coupled signal and the probe signal.

[0076] Those skilled in the art will understand that any combination of features is possible within the scope of the invention as claimed.

Claims

1. A method for detecting an electromagnetic field, the method comprising the following steps: A detection signal is transmitted to an optical receiver along a first detection signal path and a second detection signal path at a detection frequency, wherein the first detection signal path passes through a transmission medium, and the detection frequency is set to excite electrons in the transmission medium from the ground state to a first excited state. A coupling signal is transmitted along a first coupling signal path at a coupling frequency, wherein the first coupling signal path overlaps with the first detection signal path in a first overlap section in the transmission medium, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state, so as to induce an electromagnetically induced transparent (EIT) effect in the transmission medium, such that the incident electromagnetic field at the transmission medium causes a change in the refractive index of the transmission medium in the first overlap section, thereby causing a change in the optical path length difference between the first detection signal path and the second detection signal path; After the first detection signal path of the detection signal passes through the first overlapping section of the transmission medium, the first detection signal path of the detection signal is combined with the second detection signal path of the detection signal; and The intensity of the combined first and second detection signal paths of the detection signals is monitored at the optical receiver to detect the incident electromagnetic field at the first overlapping section of the transmission medium based on the intensity change caused by the change in the optical path length difference between the first and second detection signal paths. Wherein, the first detection signal path passes through the transmission medium in multiple channels, and the first coupling signal overlaps with the first detection signal path in multiple overlapping segments in the multiple channels, and the step of combining the first detection signal path of the detection signal with the second detection signal path of the detection signal is performed after the first detection signal path of the detection signal passes through all overlapping segments of the multiple overlapping segments.

2. The method according to claim 1, wherein, The step of monitoring the strength of the combined first and second detection signal paths of the detection signals includes demodulating information contained within the incident electromagnetic field.

3. The method according to claim 2, wherein, The information is modulated based on frequency modulation, and the step of demodulating the information contained in the incident electromagnetic field utilizes the transmission power value of the incident electromagnetic field.

4. The method according to any one of claims 1 to 3, wherein, The electromagnetic field is a radio frequency (RF) field.

5. The method according to claim 1, wherein, The interaction volume of any overlap between the coupled signal and the second detection signal path is different from the interaction volume of the first overlapping segment.

6. The method according to claim 5, wherein, The coupling signal does not overlap with the path of the second detection signal.

7. The method according to claim 5, wherein, The coupling signal overlaps with the path of the second detection signal in the second overlapping section, causing the incident electromagnetic field to cause a change in the refractive index of the transmission medium in the second overlapping section, and the interaction volume of the second overlapping section is different from the interaction volume of the first overlapping section.

8. A device for detecting electromagnetic fields, the device comprising: First optical transmitter, Second optical transmitter, Transmission medium Optical receiver, and processor, in: The first optical transmitter is configured to transmit a probe signal along a first probe signal path and a second probe signal path at a probe frequency to the optical receiver, wherein the first probe signal path passes through the transmission medium, and the probe frequency is set to excite electrons in the transmission medium from the ground state to a first excited state. The second optical transmitter is configured to transmit a coupled signal along a first coupled signal path at a coupling frequency, wherein the first coupled signal path overlaps with the first probe signal path in a first overlapping section in the transmission medium, wherein the coupling frequency is set to excite electrons in the transmission medium to a predetermined excited state in order to induce an electromagnetically induced transparent (EIT) effect in the transmission medium, such that the incident electromagnetic field at the transmission medium causes a change in the refractive index of the transmission medium in the first overlapping section, thereby causing a change in the optical path length difference between the first probe signal path and the second probe signal path; The optical receiver is configured to receive a combination of the first detection signal path and the second detection signal path after the first detection signal path of the detection signal passes through the first overlapping section of the transmission medium; and The processor is configured to monitor the intensity of a first and a second combined detection signal path of the detection signals, in order to detect the incident electromagnetic field at the first overlapping segment of the transmission medium based on the intensity change caused by the change in the optical path length difference between the first and second detection signal paths. The first detection signal path passes through the transmission medium in multiple channels, and the first coupling signal overlaps with the first detection signal path in multiple overlapping segments in the multiple channels. The combination of the first detection signal path and the second detection signal path of the detection signal is performed after the first detection signal path of the detection signal passes through all overlapping segments of the multiple overlapping segments.

9. The device according to claim 8, wherein, The processor is configured to demodulate information contained within the incident electromagnetic field.

10. The device according to claim 9, wherein, The information is modulated based on frequency modulation, and the demodulation of the information contained in the incident electromagnetic field utilizes the transmission power value of the incident electromagnetic field.

11. The device according to any one of claims 8 to 10, wherein, The transmission medium includes metal vapor.

12. The device according to claim 11, wherein, The metal vapor is an alkali metal vapor.

13. The device according to claim 12, wherein, The alkali metal vapor is one of rubidium, cesium, or strontium.

14. The device according to claim 8, wherein, The incident electromagnetic field is a radio frequency (RF) field.

15. The device according to claim 8, wherein, The interaction volume of any overlap between the coupled signal and the second detection signal path is different from the interaction volume of the first overlapping segment.

16. The device according to claim 15, wherein, The coupling signal does not overlap with the path of the second detection signal.

17. The device according to claim 15, wherein, The coupling signal overlaps with the path of the second detection signal in the second overlapping section, causing the incident electromagnetic field to cause a change in the refractive index of the transmission medium in the second overlapping section, and the interaction volume of the second overlapping section is different from the interaction volume of the first overlapping section.

Images