Electromagnetic field detector

Abstract

The present invention provides a method for detecting photons in a first frequency range, the method comprising the following steps: exciting a first transmission medium with a first detection signal at a first detection frequency, wherein the first detection signal excites electrons of the first transmission medium from the ground state of the first transmission medium to a first excited state of the first transmission medium; exciting the first transmission medium with a first coupling signal at a first coupling frequency, wherein the first coupling signal overlaps with the first detection signal in the first transmission medium and excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon in the first frequency range and having a first polarization incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein, as part of the subsequent de-excitation of electrons from the another excited state of the first transmission medium, a first photon in a second frequency range is emitted; detecting the first photon in the second frequency range to detect the first photon in the first frequency range and having the first polarization; exciting a second transmission medium with a second detection signal at a second detection frequency, wherein the second detection signal excites electrons of the second transmission medium from the ground state of the second transmission medium to a first excited state of the second transmission medium; exciting the second transmission medium with a second coupling signal at a second coupling frequency, wherein the second coupling signal overlaps with the second detection signal in the second transmission medium and excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon in the first frequency range and having a second polarization incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein, as part of the subsequent de-excitation of electrons in the another excited state of the second transmission medium, a second photon in the second frequency range is emitted; and detecting the second photon in the second frequency range to detect the second photon in the first frequency range and having the second polarization. The present invention also provides a detector and a system for implementing the method.

Description

Technical Field

[0001] The present invention relates to a method for detecting an electromagnetic field, as well as a detector and a system for detecting an electromagnetic field. Background Art

[0002] A Rydberg atom is an atom having one or more electrons excited to a very high principal quantum number (e.g., >10). These Rydberg atoms have several useful properties, such as a very large dipole moment and a long decay period.

[0003] Rydberg atoms can be used to detect an electromagnetic field. An electromagnetic field detector based on Rydberg atoms is based on the electromagnetically induced transparency (EIT) effect. When using a probe laser and a coupling laser to lift the electrons of an atomic medium to a Rydberg state, the EIT effect can be experienced. In this state, the atomic medium becomes transparent to the probe laser. Then, the electromagnetic field incident on the atomic medium can cause another transition of the electrons from the Rydberg state to another Rydberg state. The electrons can then drop from the other Rydberg state to the ground state, making the atomic medium less transparent to the probe laser. Therefore, the electromagnetic field can be detected according to the change in transparency, which is a change in the intensity of the probe laser. In addition, the change in intensity is proportional to the amplitude of the incident electromagnetic field, thus producing an amplitude modulation receiver based on Rydberg atoms. The electromagnetic receiver based on Rydberg atoms also shows the ability to detect frequency-modulated and phase-modulated electromagnetic fields.

[0004] Another form of electromagnetic field detector based on Rydberg atoms utilizes an interferometric technique, in which a probe laser is split into a first path and a second path, where the first path passes through the atomic medium while the second path does not pass through the atomic medium. The coupling laser also passes through the atomic medium to excite the electrons to the Rydberg state. The incident electromagnetic field at the atomic medium causes another transition of the electrons from the Rydberg state to another Rydberg state, which changes the refractive index of the atomic medium. The resulting change in the optical path length difference between the first path and the second path can be detected as a change in the intensity of the combined first path and second path of the probe signal.

[0005] Detecting the incident electromagnetic field at a photodetector by absorption or interferometric techniques requires the ability to detect intensity changes of the probe laser above the background signal.

[0006] There is a desire to improve the sensitivity of electromagnetic field detectors based on Rydberg atoms. Summary of the Invention

[0007] According to a first aspect of the present invention, there is provided a method of exciting a first transmission medium by a first detection signal at a first detection frequency, wherein the first detection signal excites electrons of the first transmission medium from the ground state of the first transmission medium to a first excited state of the first transmission medium; exciting the first transmission medium by a first coupling signal at a first coupling frequency, wherein the first coupling signal overlaps with the first detection signal in the first transmission medium and excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon in the first frequency range and having a first polarization incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein, as part of subsequent de-excitation of electrons from the another excited state of the first transmission medium, a first photon in a second frequency range is emitted; detecting a first photon in the first frequency range by detecting the first photon in the second frequency range; exciting a second transmission medium by a second detection signal at a second detection frequency, wherein the second detection signal excites electrons of the second transmission medium from the ground state of the second transmission medium to a first excited state of the second transmission medium; exciting the second transmission medium by a second coupling signal at a second coupling frequency, wherein the second coupling signal overlaps with the second detection signal in the second transmission medium and excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon in the first frequency range and having a second polarization incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein, as part of subsequent de-excitation of electrons in the another excited state of the second transmission medium, a second photon in the second frequency range is emitted; and detecting a second photon in the first frequency range by detecting the second photon in the second frequency range.

[0008] The method includes the steps of: demodulating data based on detecting the first photon in the first frequency range and having the first polarization corresponding to a first bit value, and based on detecting the second photon in the first frequency range and having the second polarization corresponding to a second bit value.

[0009] The method may further include the step of: determining an encryption key based on the demodulated data.

[0010] According to a second aspect of the present invention, there is provided an apparatus for detecting photons in a first frequency range, the detector comprising: a first transmission medium capable of being excited by a first detection signal at a first detection frequency and an overlapping first coupling signal at a first coupling frequency, wherein the first detection frequency excites electrons of the first transmission medium from the ground state of the first transmission medium to a first excited state of the first transmission medium, and the first coupling frequency excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon in the first frequency range and having a first polarization incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein, as part of subsequent de-excitation of electrons in the another excited state of the first transmission medium, a first photon in a second frequency range is emitted; a first detector configured to detect the first photon in the first frequency range and having the first polarization by detecting the first photon in the second frequency range; a second transmission medium capable of being excited by a second detection signal at a second detection frequency and an overlapping second coupling signal at a second coupling frequency, wherein the second detection frequency excites electrons of the second transmission medium from the ground state of the second transmission medium to a first excited state of the second transmission medium, wherein the second coupling frequency excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon in the first frequency range and having a second polarization incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein, as part of subsequent de-excitation of electrons in the another excited state of the second transmission medium, a second photon in the second frequency range is emitted; and a second detector configured to detect the second photon in the first frequency range and having the second polarization by detecting the second photon in the second frequency range.

[0011] The apparatus may further comprise: a housing configured to allow the first photon in the first frequency range to pass through to reach the first transmission medium when the first photon arrives in a predetermined direction.

[0012] The apparatus may further comprise: a processor configured to demodulate data based on detecting a first photon in the first frequency range and having a first polarization corresponding to a first bit value and detecting a second photon in the first frequency range and having a second polarization corresponding to a second bit value.

[0013] The processor may further be configured to determine an encryption key based on the demodulated data.

[0014] The first frequency range can be lower than the second frequency range. The first frequency range can be in the radio frequency (RF), microwave, or infrared range of the electromagnetic spectrum.

[0015] According to a third aspect of the present invention, there is provided a computer program comprising instructions for causing the apparatus of the second aspect of the present invention to perform the steps of the method of the first aspect of the present invention. The computer program can be stored on a computer-readable carrier medium.

[0016] According to a fourth aspect of the present invention, there is provided a system comprising: a first transmission medium capable of being excited by a first detection signal at a first detection frequency and an overlapping first coupling signal at a first coupling frequency, wherein the first detection frequency excites electrons of the first transmission medium from the ground state of the first transmission medium to a first excited state of the first transmission medium, and the first coupling frequency excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon within the first frequency range and having a first polarization incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein, as part of a subsequent de-excitation of the electrons in the another excited state of the first transmission medium, a first photon within the second frequency range is emitted; a first detector configured to detect the first photon within the first frequency range and having the first polarization by detecting the first photon within the second frequency range; a second transmission medium capable of being excited by a second detection signal at a second detection frequency and an overlapping second coupling signal at a second coupling frequency, wherein the second detection frequency excites electrons of the second transmission medium from the ground state of the second transmission medium to a first excited state of the second transmission medium, wherein the second coupling frequency excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon within the first frequency range and having a second polarization incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein, as part of a subsequent de-excitation of the electrons in the another excited state of the second transmission medium, a second photon within the second frequency range is emitted; and a second detector configured to detect the second photon within the first frequency range and having the second polarization by detecting the second photon within the second frequency range. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] To better understand the present invention, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0018] Figure 1 is a schematic diagram of a radio telecommunications network according to a first embodiment of the present invention;

[0019] Figure 2 is a flowchart showing a method according to a first embodiment of the present invention;

[0020] Figure 3 is a schematic diagram of a radio telecommunications network according to a second embodiment of the present invention; and

[0021] Figure 4 is a schematic diagram of a radio telecommunications network according to a third embodiment of the present invention. Detailed Embodiments

[0022] A first embodiment of the radio telecommunications network 100 will now be described with reference to Figure 1 The radio telecommunications network 100 includes a radio transmitter 110 and a Rydberg atom-based RF detector 120. In this embodiment, the radio transmitter 110 is configured to transmit a radio signal at a frequency of 3.6 GHz. The Rydberg atom-based RF detector 120 includes an optical fiber 121, a probe laser 123, a coupling laser 125, and a detector 127. The optical fiber 121 includes a silica bulb 122 filled with a vapor of rubidium-85 atoms.

[0023] The Rydberg atom-based RF detector 120 can be configured to excite electrons of rubidium-85 atoms contained within the silica bulb 122 to a predetermined Rydberg state. This is achieved by transmitting a probe signal along the optical fiber 121 and through the silica bulb 122 by the probe laser 123, and transmitting a counter-propagating coupling signal along the optical fiber 121 and through the silica bulb 122 by the coupling laser 125. The probe signal and the coupling signal overlap in at least a portion of their respective passages through the silica bulb 122. The probe signal has a specific wavelength (780 nm) selected to excite electrons from the ground state to the first excited state, and the coupling signal has a specific wavelength (480 nm) for exciting electrons from the first excited state to the predetermined Rydberg state. In this embodiment where the Rydberg atom-based RF detector 120 is configured to detect the 3.6 GHz radio signal transmitted by the radio transmitter 110, the predetermined Rydberg state is an electron state having the 83rd principal quantum number 83D_{5 / 2}. In this configuration, the radio signal transmitted by the radio transmitter at 3.6 GHz passing through the silica bulb 122 of the optical fiber 121 excites electrons from this predetermined Rydberg state (83D_{5 / 2}) to another Rydberg state (84P_{3 / 2}).

[0024] The Rydberg-atom-based RF detector 120 includes a controller (not shown) that is configured to control the frequency of the probe signal (i.e., adjust or stabilize the frequency of the probe signal), to control the frequency of the coupling signal (i.e., adjust or stabilize the frequency of the coupling signal), to control the intensity of the probe laser 123 (i.e., the transmission power and / or cross-sectional area of the probe laser 123), and to control the intensity of the coupling laser 125 (i.e., the transmission power and / or cross-sectional area of the coupling laser 125).

[0025] In this embodiment, the detector 127 is a single-photon detector, such as a photomultiplier tube (PMT) (e.g., provided by Hamamatsu Photonics) or a single-photon avalanche diode (SPAD). The controller 126 is also configured to control the spectral window of the detector 127 (i.e., the frequency range in which the single-photon detector 127 can detect individual photons). This spectral window is configured to detect photons (or multiple photons) emitted in the decay path of an electron from another Rydberg state to the ground state (described in more detail below). When the detector 127 is located near the atomic medium (to capture photons emitted during decay), the Rydberg-atom-based RF detector 120 includes a local power supply that powers the detector 127.

[0026] The inner surface of the silica bulb 122 is coated with a dielectric mirror coating (such as hafnium dioxide, as described in "High-reflectivity HfO2 / SiO2 ultraviolet mirrors", Applied Optics, Vol. 41, Issue 16, pp. 3256-3261 (2002)). The coating is selected to have a high reflectivity for photons (or multiple photons) emitted in the decay path from another Rydberg state to the ground state (e.g., a reflectivity of >= 99% in the range from 270 nm to 330 nm). The silica bulb 125 also includes a channel to the detector 127 to enable photons within the silica bulb 122 to enter the detector 127.

[0027] Reference will now be made to Figure 2A first embodiment of the method of the present invention is described. In a first step (S101), an RF detector 120 based on Rydberg atoms operates to excite the electrons of rubidium atoms in a silica bulb 122 to a predetermined Rydberg state. As described above, this is achieved by passing a probe signal and a coupling signal through the silica bulb 122 at their respective frequencies. The transition from this predetermined Rydberg state to the ground state (or at least has a very long decay period). However, the electrons can be further excited to another Rydberg state by an RF signal transmitted by a wireless transmitter 110 at 3.6 GHz. When excited by the RF signal, the electrons will spontaneously decay to their ground state through a fluorescence process. The most likely decay option is to directly decay to the ground state. The direct decay from another Rydberg state to the ground state involves the fluorescence emission of photons with a wavelength of 297.96 nm. This photon is within the spectral window of the detector 127. Other decay options from another Rydberg state to the ground state are possible, such as via one or more intermediate states, where each decay between two states involves the fluorescence emission of photons at a frequency corresponding to the energy difference between the two states. One or more of these characteristic photon frequencies are within the spectral window of the detector 127.

[0028] In step S103, the detector 127 detects one or more photons within its spectral window. More specifically, the detector 127 can detect a single photon whose frequency corresponds to the decay path from another Rydberg state directly to the ground state, or detect at least one of two or more photons whose frequencies correspond to the energy state differences in the indirect decay path from another Rydberg state to the ground state via one or more intermediate states. The detection of these one or more characteristic photons is an indirect detection of the photons of the wireless signal transmitted by the wireless transmitter 110, the photons of which excite the electrons from a predetermined Rydberg state to another Rydberg state (since the transition from the predetermined Rydberg state to the ground state is forbidden, and the coupling laser is centered on the transition to the predetermined Rydberg state and has a linewidth less than the interval to another Rydberg state). Therefore, the detection of characteristic photons means that the electrons have been excited from a predetermined Rydberg state to another Rydberg state by the photons of the wireless signal transmitted by the wireless transmitter 110.

[0029] In other words, the Rydberg atom-based RF detector 120 is a single RF photon detector that indirectly detects a single RF photon by directly detecting one or more characteristic photons in the decay path from another Rydberg state to the ground state. By implementing single-photon detection of the wireless signal transmitted by the wireless transmitter 110, the Rydberg atom-based RF detector 120 provides improved sensitivity relative to existing Rydberg atom-based RF detectors that rely on absorption or interferometric detection techniques (both of which, as described above, require a sufficient number of photons to cause a detectable change in the intensity of the detection signal above the background noise). The Rydberg atom-based RF detector 120 utilizes two characteristics to achieve this benefit. First, single-photon detection of photons emitted during decay from another Rydberg state (i.e., in the UV region of the electromagnetic spectrum) is available, while single-photon detectors in the RF range of the electromagnetic spectrum are not; second, the background noise of the single-photon detector for photons emitted during decay from another Rydberg state is much lower than the background noise of existing Rydberg atom-based RF detectors (which rely on absorption or interferometric techniques).

[0030] In this first embodiment, the reflective coating within the silica bulb 122 ensures that any photons emitted in the decay path from another Rydberg state to the ground state that do not directly enter the detector 127 are reflected internally into the detector 127 by one or more reflections.

[0031] Reference will now be made to Figure 3 describe a second embodiment of the wireless telecommunications network 200. The wireless telecommunications network 200 includes a wireless transmitter 210 and a Rydberg atom-based RF detector 220. In this second embodiment, the Rydberg atom-based RF detector 220 includes an optical fiber 221, a probe laser 223, a coupling laser 225, a detector 227, a housing 228, and a vapor cell 229.

[0032] The Rydberg atom-based RF detector 220 of this second embodiment is configured to transmit counter-propagating probe and coupling signals through the vapor cell 229 (the counter-propagating probe and coupling signals overlapping during at least a portion of their respective passages through the vapor cell 229) in order to excite the electrons of the rubidium-85 atoms contained within the vapor cell 229 to a predetermined Rydberg state (i.e., 83D5 / 2). In this configuration, the wireless signal passing through the vapor cell 229 transmitted by the wireless transmitter 210 at 3.6 GHz excites the electrons from this predetermined Rydberg state (83D5 / 2) to another Rydberg state (84P3 / 2).

[0033] The vapor chamber 229 is made of a material (such as acrylic, silicone, quartz, or silica) that is transparent to photons emitted in the decay path from another Rydberg state to the ground state.

[0034] The housing 228 is configured to allow wireless signals inside the housing 228 (and thus passing through the vapor chamber 229) to reach only in a specific direction. In this embodiment, this is achieved by constructing the housing 228 from an insulating material that prevents wireless signals at the frequency of the wireless signals transmitted by the wireless transmitter 210 from passing through, and having one or more channels (e.g., openings or portions made of a material transparent to wireless signals at the frequency used by the wireless transmitter 210) in the housing 228, the channels being positioned and aligned on a predetermined axis so as to allow only wireless signals reaching inside the housing 228 in a specific direction.

[0035] The inner surface of the housing 228 is coated with a dielectric mirror coating (such as hafnium dioxide, as described above). The coating is selected to have a high reflectivity (e.g., >= 99% reflectivity) for photons (or multiple photons) emitted in the decay path from another Rydberg state to the ground state. As Figure 3 shown, the housing 228 is shaped to reflect photons (emitted during decay from another Rydberg state to the ground state) transmitted out of the vapor chamber 229 to the detector 227. The shape may include a spherical portion and a parabolic portion so as to reflect photons to the detector 227.

[0036] In this second embodiment, the detector 227 is also a single-photon detector, such as a single-photon avalanche diode (SPAD) or a photomultiplier tube (PMT) as described above. The spectral window of the detector 227 (i.e., the frequency range in which the single-photon detector 227 can detect individual photons) is configurable so as to detect photons emitted in the electron decay path from another Rydberg state to the ground state. Since the detector 227 is located near the atomic medium (so as to capture photons emitted during decay), the Rydberg-atom-based RF detector 220 still includes a local power source to power the detector 227.

[0037] As described above, the method of the first embodiment can be applied to this second embodiment such that single photons of the wireless signals transmitted by the wireless transmitter 210 in the RF region of the electromagnetic spectrum can be indirectly detected by the detector 227. In this second embodiment, a detection event indicates a single photon of a wireless signal reaching the Rydberg-atom-based RF detector 220 in a specific direction corresponding to the axis of the channel in the housing 228.

[0038] Now the third embodiment of the present invention will be described with reference to Figure 4 to. Figure 4An end-to-end quantum key distribution (QKD) system 300 with a transmitter 310 and a receiver 320 is shown. The transmitter 310 is configured to transmit a wireless signal at 3.6 GHz (including polarization-encoded qubits, as described below). The receiver 320 includes a photon polarization detector module 327. The photon polarization detector module 327 includes a first Rydberg-atom-based RF detector and a second Rydberg-atom-based RF detector, each Rydberg-atom-based RF detector being based on the Rydberg-atom-based RF detector of the first or second embodiment described above and thus configured to detect the wireless signal at 3.6 GHz transmitted by the transmitter 310.

[0039] The first Rydberg-atom-based RF detector and the second Rydberg-atom-based RF detector include polarization filters that only allow wireless signals with a specific polarization to pass through the detector and excite the atomic medium. In the first state, the first Rydberg-atom-based RF detector uses a first polarization filter such that a wireless signal with a first polarization (i.e., 0 degrees) can pass through the first polarization filter and through the atomic medium of the silica bulb / vapor cell (so as to cause a transition from a predetermined Rydberg state to another Rydberg state), but a wireless signal with any other polarization (including 45 degrees, 90 degrees, or 135 degrees) cannot pass through the first polarization filter (and thus does not pass through the atomic medium of the silica bulb / vapor cell, which would otherwise cause a transition from a predetermined Rydberg state to another Rydberg state). Additionally, in the first state, the second Rydberg-atom-based RF detector uses a second polarization filter such that a wireless signal with a second polarization (i.e., 90 degrees) can pass through the second polarization filter and through the atomic medium of the silica bulb / vapor cell (so as to cause a transition from a predetermined Rydberg state to another Rydberg state), but a wireless signal with any other polarization (including 0 degrees, 45 degrees, or 135 degrees) cannot pass through the second polarization filter (and thus does not pass through the atomic medium of the silica bulb / vapor cell, which would otherwise cause a transition from a predetermined Rydberg state to another Rydberg state).

[0040] In the second state, the RF detector based on the first Rydberg atom uses a third polarization filter such that a wireless signal having a third polarization (i.e., 45 degrees) can pass through the third polarization filter and through the atomic medium of the silica bulb / vapor cell (so as to cause a transition from a predetermined Rydberg state to another Rydberg state), but a wireless signal having any other polarization (including 0 degrees, 90 degrees, or 135 degrees) cannot pass through the third polarization filter (and thus cannot pass through the atomic medium of the silica bulb / vapor cell, which would otherwise cause a transition from a predetermined Rydberg state to another Rydberg state). Additionally, in the second state, the RF detector based on the second Rydberg atom uses a fourth polarization filter such that a wireless signal having a fourth polarization (i.e., 135 degrees) can pass through the fourth polarization filter and through the atomic medium of the silica bulb / vapor cell (so as to cause a transition from a predetermined Rydberg state to another Rydberg state), but a wireless signal having any other polarization (including 0 degrees, 45 degrees, or 90 degrees) will not pass through the fourth polarization filter (and thus cannot pass through the atomic medium of the silica bulb / vapor cell, which would otherwise cause a transition from a predetermined Rydberg state to another Rydberg state).

[0041] In summary, the photon polarization detector module 327 can be switched between a first state and a second state. In the first state, the photon polarization detector module 327 is configured to detect 0-degree polarization with the RF detector based on the first Rydberg atom and 90-degree polarization with the RF detector based on the second Rydberg atom. In the second state, the photon polarization detector module 327 is configured to detect 45-degree polarization with the RF detector based on the first Rydberg atom and 135-degree polarization with the RF detector based on the second Rydberg atom.

[0042] The QKD system 300 of this third embodiment transmits an encryption (i.e., cryptographic) key between a transmitter 310 and a receiver 320 by using polarization - encoded qubits. This can be based on, for example, the BB84 protocol, where the transmitter 310 sends qubits encoded according to a first basis (where a polarization of 0 degrees corresponds to a bit value of 0 and a polarization of 90 degrees corresponds to a bit value of 1) or a second basis (where a polarization of 45 degrees corresponds to a bit value of 0 and a polarization of 135 degrees corresponds to a bit value of 1) to the receiver 320. The receiver 320 measures the qubits using the first basis (where the photon polarization detector module 327 is in a first state such that a 0 - degree - polarized qubit is detected at the RF detector based on the first Rydberg atom and a 90 - degree - polarized qubit is detected at the RF detector based on the second Rydberg atom) or the second basis (where the photon polarization detector module 327 is in a second state such that a 45 - degree - polarized qubit is detected at the RF detector based on the first Rydberg atom and a 135 - degree - polarized qubit is detected at the RF detector based on the second Rydberg atom). After the measurement, the transmitter 310 and the receiver 320 can communicate to determine which basis was used to transmit each qubit, so that the receiver 320 can identify the successfully received qubits. This protocol can be used to transmit an encryption key between the transmitter 310 and the receiver 320.

[0043] The above - mentioned third embodiment is set to distinguish four different polarizations in order to implement the QKD protocol. In another embodiment, the receiver may include a first Rydberg - atom - based RF receiver based on the above - mentioned first or second embodiment and configured to detect a first polarization, and the receiver may also include a second Rydberg - atom - based RF receiver based on the above - mentioned first or second embodiment and configured to detect a second polarization. The first polarization and the second polarization may be perpendicular. The receiver can then be used to receive (i.e., demodulate) data in a photon sequence encoded by polarization (e.g., any photon with the first polarization corresponds to a bit value of 0, and any photon with the second polarization corresponds to a bit value of 1). In other words, this other embodiment can be used to transmit any form of data between a transmitter and a receiver, and the QKD implementation of the third embodiment is just an example. Those skilled in the art will also understand that other QKD implementations using multiple Rydberg - atom - based (RF detectors for Rydberg atoms, each RF detector being configured to detect a specific polarization) polarization - encoded qubits are possible.

[0044] In the above - mentioned third embodiment, the Rydberg - atom - based RF detector is adapted to detect photons of a specific polarization by using a polarization filter. However, this is not necessary, and the detector can be configured to detect photons of a specific polarization in other ways, such as by being excited by a probe signal and a coupling laser having a specific polarization.

[0045] In the above embodiments, the wireless signal transmitted to the Rydberg atom-based RF detector has a frequency of 3.6 GHz, and each photon excites an electron from a predetermined Rydberg state (83D5 / 2) to another Rydberg state (84P3 / 2). As described above, the subsequent decay from the other Rydberg state to the ground state involves fluorescence emission of photons at 297.96 nm, which can be detected using a single-photon detector. However, the Rydberg atom-based RF detector can be configured to detect single photons at other frequencies using the same technique (i.e., by detecting one or more characteristic photons emitted during the decay to the ground state). For example, a predetermined Rydberg state with a relatively high quantum number, such as the Rb149D5 / 2 state, can be selected (by configuring the probe signal and the coupling signal). Then, an electron can be excited from this predetermined Rydberg state to another Rydberg state (Rb150P3 / 2) by an RF photon with a frequency of 605 MHz, and the subsequent direct decay to the ground state involves fluorescence emission of photons at 296.86 nm, which can be detected again using a single-photon detector. In another example, alternatively, a predetermined Rydberg state (by configuring the probe signal and the coupling signal) can be selected with a relatively low quantum number (such as Rb6D5 / 2). Then, an electron can be excited from this predetermined Rydberg state to another Rydberg state (Rb7P3 / 2) by an RF photon with a frequency of 24.6 THz, and the subsequent direct decay to the ground state involves fluorescence emission of photons at 359 nm, which can be detected again using a single-photon detector. Therefore, a single-photon detector operating in the range of 296 nm to 359 nm can be used to indirectly detect incident photons in the range of 605 MHz to 24.6 THz. In other words, by detecting characteristic photons with a relatively short wavelength (or relatively high frequency) emitted during the decay to the ground state, the Rydberg atom-based detector can be used to indirectly detect photons with a relatively long wavelength (or relatively low frequency). The photons indirectly detected can be in the RF, microwave, and infrared ranges of the electromagnetic spectrum, and the photons detected during the decay to the ground state can be in the visible and ultraviolet ranges of the electromagnetic spectrum.

[0046] The above embodiments describe the ladder EIT effect using the probe signal and the coupling signal. However, those skilled in the art will understand that any other form of EIT effect (e.g., λ or V-shaped) can be used to raise the electron to the Rydberg state, and more than two signals can be used. In addition, those skilled in the art will also understand that it is not necessary for the detector to use rubidium-85 atoms, as other alkali metals, such as cesium or strontium, can be used alternatively.

[0047] In the above description, photons of the wireless signal to be detected excite electrons of a Rydberg atom from a predetermined Rydberg state to another Rydberg state. This covers a first scenario and a second scenario. In the first scenario, the wireless signal has a signal strength less than a threshold such that the predetermined Rydberg state and the other Rydberg state are not separated after interaction with the wireless signal. In the second scenario, the wireless signal has a signal strength greater than the threshold such that the predetermined Rydberg state and the other Rydberg state are separated after interaction with the wireless signal. In the first scenario, photons of the wireless signal excite electrons from a predetermined Rydberg state to another Rydberg state (as described extensively above). In the second scenario, the predetermined Rydberg state is split into two (which can be denoted as R1 - d and R1 + d), and the other Rydberg state is split into two (which can be denoted as R2 - d and R2 + d). This allows for four transitions (R1 - d → R2 - d, R1 - d → R2 + d, R1 + d → R2 - d, and R1 + d → R2 + d). Additionally, if the two states are split equally, there are three different energy differences (R2 - R1 - 2d, R2 - R1, and R2 - R1 + 2d) for these four transitions, which is referred to as a Stark triplet. Then, photons of the wireless signal can excite electrons through any one of these transitions (i.e., from any one of the predetermined Rydberg split states to any one of the other Rydberg split states) and subsequently de - excite as described above.

[0048] As described above, the single - photon detector in each embodiment can be configured with a spectral window to capture one or more characteristic photons of the decay path from another Rydberg state to the ground state. The direct decay from another Rydberg state to the ground state is the most probable transition, but there is typically a probability distribution of transitions from any particular other Rydberg state to the ground state, which involves the emission of one or more characteristic photons. The detection of at least one of these characteristic photons is considered an indirect detection of RF photons at the Rydberg - atom - based RF detector.

[0049] Those skilled in the art will understand that any combination of features is possible within the scope of the claimed invention.

Claims

1. A method for detecting photons in a first frequency range, the method comprising the steps of: Exciting a first transmission medium with a first detection signal at a first detection frequency, wherein the first detection signal excites electrons of the first transmission medium from a ground state of the first transmission medium to a first excited state of the first transmission medium; Exciting the first transmission medium with a first coupling signal at a first coupling frequency, wherein the first coupling signal overlaps with the first detection signal in the first transmission medium and excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon in the first frequency range and having a first polarization incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein, as part of subsequent de-excitation of the electrons from the another excited state of the first transmission medium, a first photon in a second frequency range is emitted; Detecting a first photon in the first frequency range and having the first polarization by detecting the first photon in the second frequency range; Exciting a second transmission medium with a second detection signal at a second detection frequency, wherein the second detection signal excites electrons of the second transmission medium from a ground state of the second transmission medium to a first excited state of the second transmission medium; Exciting the second transmission medium with a second coupling signal at a second coupling frequency, wherein the second coupling signal overlaps with the second detection signal in the second transmission medium and excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon in the first frequency range and having a second polarization incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein, as part of subsequent de-excitation of the electrons in the another excited state of the second transmission medium, a second photon in the second frequency range is emitted; Detecting a second photon in the first frequency range and having the second polarization by detecting the second photon in the second frequency range; and Demodulating data based on detecting a first photon in the first frequency range and having the first polarization corresponding to a first bit value and based on detecting a second photon in the first frequency range and having the second polarization corresponding to a second bit value.

2. The method according to claim 1, the method further comprising the steps of: Determining an encryption key based on the demodulated data.

3. The method according to claim 1, wherein The first frequency range is lower than the second frequency range.

4. The method according to claim 1, wherein The first frequency range is in the radio frequency (RF), microwave, or infrared range of the electromagnetic spectrum.

5. A device for detecting photons in a first frequency range, the device comprising: A first transmission medium that can be excited by a first detection signal at a first detection frequency and an overlapping first coupling signal at a first coupling frequency, where the first detection frequency excites electrons of the first transmission medium from the ground state of the first transmission medium to a first excited state of the first transmission medium, and the first coupling frequency excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon in a first frequency range and having a first polarization incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, where, as part of subsequent de-excitation of electrons in the another excited state of the first transmission medium, a first photon in a second frequency range is emitted; A first detector configured to detect a first photon in the first frequency range and having the first polarization by detecting the first photon in the second frequency range; A second transmission medium that can be excited by a second detection signal at a second detection frequency and an overlapping second coupling signal at a second coupling frequency, where the second detection frequency excites electrons of the second transmission medium from the ground state of the second transmission medium to a first excited state of the second transmission medium, where the second coupling frequency excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon in the first frequency range and having a second polarization incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, where, as part of subsequent de-excitation of electrons in the another excited state of the second transmission medium, a second photon in the second frequency range is emitted; A second detector configured to detect a second photon in the first frequency range and having the second polarization by detecting the second photon in the second frequency range; and A processor configured to demodulate data based on detecting a first photon in the first frequency range and having the first polarization corresponding to a first bit value and detecting a second photon in the first frequency range and having the second polarization corresponding to a second bit value.

6. The apparatus according to claim 5, the apparatus further comprising: A housing configured to allow a first photon in the first frequency range to pass through to reach the first transmission medium when the first photon arrives along a predetermined direction.

7. The apparatus according to claim 5, wherein, The processor is further configured to determine an encryption key based on the demodulated data.

8. The apparatus according to claim 5, wherein The first frequency range is lower than the second frequency range.

9. The device according to claim 5, wherein, The first frequency range is in the radio frequency (RF), microwave, or infrared range of the electromagnetic spectrum.

10. A computer-readable carrier medium, the computer-readable carrier medium comprising a computer program, the computer program comprising instructions for causing the apparatus according to any one of claims 5 to 9 to perform the steps of the method according to any one of claims 1 to 4.

11. A wireless telecommunications system, the wireless telecommunications system comprising: a transmitter configured to transmit a wireless signal comprising polarisation-encoded data; and a receiver configured to receive the wireless signal, and the receiver comprising a photon polarisation detector module, the photon polarisation detector module comprising: a first transmission medium capable of being excited by a first probe signal at a first probe frequency and an overlapping first coupling signal at a first coupling frequency, wherein the first probe frequency excites electrons of the first transmission medium from a ground state of the first transmission medium to a first excited state of the first transmission medium, and the first coupling frequency excites electrons of the first transmission medium to a predetermined excited state of the first transmission medium, such that a first photon in a first frequency range and having a first polarisation incident on the first transmission medium excites electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein, as part of a subsequent de-excitation of the electrons in the another excited state of the first transmission medium, a first photon in a second frequency range is emitted; a first detector configured to detect a first photon in the first frequency range and having the first polarisation by detecting the first photon in the second frequency range; a second transmission medium capable of being excited by a second probe signal at a second probe frequency and an overlapping second coupling signal at a second coupling frequency, wherein the second probe frequency excites electrons of the second transmission medium from a ground state of the second transmission medium to a first excited state of the second transmission medium, wherein the second coupling frequency excites electrons of the second transmission medium to a predetermined excited state of the second transmission medium, such that a second photon in the first frequency range and having a second polarisation incident on the second transmission medium excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein, as part of a subsequent de-excitation of the electrons in the another excited state of the second transmission medium, a second photon in the second frequency range is emitted; a second detector configured to detect a second photon in the first frequency range and having the second polarisation by detecting the second photon in the second frequency range; and a processor configured to demodulate data based on detecting a first photon in the first frequency range having the first polarisation corresponding to a first bit value and detecting a second photon in the first frequency range having the second polarisation corresponding to a second bit value.

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