Electromagnetic field detector

Abstract

The invention provides a method for detecting photons in a first frequency range, comprising the steps of: exciting a first transmission medium by means of a first detection signal at a first detection frequency, 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; the first transmission medium is excited by a first coupling signal at a first coupling frequency, the first coupling signal overlapping the first probe signal in the first transmission medium, and electrons of the first transmission medium are excited to a predetermined excitation state of the first transmission medium. A first photon incident on the first transmission medium in a first frequency range and having a first polarization excites electrons in a predetermined excited state of the first transmission medium into another excited state of the first transmission medium as part of a subsequent de-excitation of the electrons from the another excited state of the first transmission medium. Emitting a first photon in a second frequency range; detecting a first photon in the first frequency range and having a first polarization by detecting the first photon in the second frequency range; exciting the second transmission medium by means of a second detection signal at a second detection frequency, the second detection signal exciting 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; the second transmission medium is excited by a second coupling signal at a second coupling frequency, the second coupling signal overlapping with the second detection signal in the second transmission medium, and electrons of the second transmission medium are excited to a predetermined excitation state of the second transmission medium. The second transmission medium is arranged such that a second photon incident on the second transmission medium in a first frequency range and having a second polarization excites electrons in a predetermined excited state of the second transmission medium into another excited state of the second transmission medium, the electrons being excited as part of a subsequent de-excitation of the electrons in the another excited state of the second transmission medium. Emitting a second photon in a second frequency range; and detecting second photons within the first frequency range and having a second polarization by detecting the second photons within the second frequency range. The invention also provides a detector and a system for realizing the method.

Description

Technical Field

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

[0002] Rydberg atoms are atoms with one or more electrons excited to very high principal quantum numbers, e.g. > 10. These Rydberg atoms have several useful properties, such as very large dipole moments and long decay periods.

[0003] Rydberg atoms can be used to detect electromagnetic fields. Electromagnetic field detectors based on Rydberg atoms are based on the electromagnetic induced transparency (EIT) effect. When the electrons of an atomic medium are promoted to a Rydberg state using a probe laser and a coupling laser, 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 electron from the Rydberg state to another Rydberg state. The electron can then drop from the other Rydberg state to the ground state, causing the atomic medium to become less transparent to the probe laser. Therefore, the electromagnetic field can be detected based on the change in transparency as 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, resulting in an amplitude modulation receiver based on Rydberg atoms. Electromagnetic receivers based on Rydberg atoms have also been shown to detect frequency modulated and phase modulated electromagnetic fields.

[0004] Another form of electromagnetic field detector based on Rydberg atoms utilizes an interferometric measurement technique in which a detection laser is split into a first path and a second path, wherein the first path passes through an atomic medium and the second path does not pass through the atomic medium. A coupling laser also passes through the atomic medium to excite electrons to a Rydberg state. The incident electromagnetic field at the atomic medium causes another transition of the electron from a 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 and second paths of the detection signal.

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

[0006] It is hoped that the sensitivity of electromagnetic field detectors based on Rydberg atoms will be improved. Summary of the invention

[0007] According to a first aspect of the present invention, a method is provided for 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 a 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, so that a first photon in the first frequency range and having a first polarization incident on the first transmission medium excites electrons of the first transmission medium in the predetermined excited state to another excited state of the first transmission medium, wherein a first photon in a second frequency range is emitted as part of a subsequent deexcitation of the electrons from the another excited state of the first transmission medium; and a first photon in the first frequency range and having a first polarization is detected by detecting the first photon in the second frequency range. a first photon with a second polarization; 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 a 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, so that a second photon in the first frequency range that is incident on the second transmission medium and has a second polarization excites electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein a second photon in the second frequency range is emitted as part of a subsequent de-excitation of electrons in another excited state of the second transmission medium; and detecting a second photon in the first frequency range with a second polarization by detecting the second photon in the second frequency range.

[0008] The method comprises the steps of demodulating data based on detecting the first photon with the first polarization corresponding to a first bit value within the first frequency range, and based on detecting the second photon with the second polarization corresponding to a second bit value within the first frequency range.

[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, a device for detecting photons in a first frequency range is provided, the detector comprising: a first transmission medium, the first transmission medium being 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 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, so 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 a first photon in the second frequency range is emitted as part of a subsequent de-excitation of electrons in the another excited state of the first transmission medium; and a first detector, the first detector being configured to detect the first frequency range by detecting the first photon in the second frequency range. a first photon with the first polarization within a 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 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, so that a second photon with the second polarization within the first frequency range incident on the second transmission medium excites electrons in a predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein a second photon in the second frequency range is emitted as part of a subsequent de-excitation of electrons in another excited state of the second transmission medium; and a second detector, the second detector being configured to detect the second photon with the second polarization within the first frequency range by detecting the second photon in the second frequency range.

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

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

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

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

[0015] According to a third aspect of the present invention, a computer program is provided, the 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 may be stored on a computer readable carrier medium.

[0016] According to a fourth aspect of the present invention, a system is provided, which includes: a first transmission medium, which can be 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 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, so 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, wherein a first photon in a second frequency range is emitted as part of a subsequent de-excitation of electrons in another excited state of the first transmission medium; and a first detector, which is 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 first photon of a first polarization; 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 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, so 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 a second photon in the second frequency range is emitted as part of a subsequent de-excitation of electrons in another excited state of the second transmission medium; and a second detector, the second detector being 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order that the present invention may be better understood, 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 wireless telecommunications network according to a first embodiment of the present invention;

[0019] Figure 2 is a flow chart illustrating a method of a first embodiment of the present invention;

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

[0021] Figure 4 is a schematic diagram of a wireless telecommunication network according to a third embodiment of the present invention. DETAILED DESCRIPTION

[0022] Now refer to Figure 1 A first embodiment of a wireless telecommunication network 100 is described. The wireless telecommunication network 100 includes a wireless transmitter 110 and a Rydberg atom-based RF detector 120. In this embodiment, the wireless transmitter 110 is configured to transmit a wireless signal at a frequency of 3.6 GHz. The Rydberg atom-based RF detector 120 includes an optical fiber 121, a detection 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 in the silica bulb 122 to a predetermined Rydberg state. This is achieved by transmitting a detection signal along the optical fiber 121 and through the silica bulb 122 by the detection laser 123, and transmitting a reverse-propagating coupling signal along the optical fiber 121 and through the silica bulb 122 by the coupling laser 125. The detection signal and the coupling signal overlap in at least a portion of their respective passages through the silica bulb 122. The detection 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 a 3.6 GHz wireless signal transmitted by the wireless transmitter 110, the predetermined Rydberg state is an electronic state having the 83rd principal quantum number 83D_{5 / 2}. In this configuration, a wireless signal transmitted by a wireless transmitter at 3.6 GHz through the silica bulb 122 of the optical fiber 121 excites electrons from the 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) for controlling the frequency of the detection signal (i.e., regulating or stabilizing the frequency of the detection signal), for controlling the frequency of the coupling signal (i.e., regulating or stabilizing the frequency of the coupling signal), for controlling the intensity of the detection laser 123 (i.e., the transmission power and / or cross-sectional area of ​​the detection laser 123), and for controlling 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 a single photon). The spectral window is configured (e.g., by selecting the photocathode material of the PMT) to detect a photon (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 (so as to capture the photons emitted during the decay), the Rydberg atom-based RF detector 120 includes a local power supply for powering the detector 127.

[0026] The inner surface of the silica bulb 122 is coated with a dielectric mirror coating (e.g., 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 (e.g., a reflectivity of >= 99% in the range from 270nm to 330nm) of a photon (or photons) emitted in the decay path from another Rydberg state to the ground state. The silica bulb 125 also includes a channel to the detector 127 so that the photons within the silica bulb 122 can enter the detector 127.

[0027] Now refer to Figure 2To describe a first embodiment of the method of the present invention. In the first step (S101), the Rydberg atom-based RF detector 120 is operated to excite the electrons of the rubidium atoms in the silica bulb 122 to a predetermined Rydberg state. As described above, this is achieved by passing the detection signal and the coupling signal through the silica bulb 122 at their respective frequencies. The transition from the 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 at 3.6 GHz by the wireless transmitter 110. 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 decay directly to the ground state. The direct decay from another Rydberg state to the ground state involves the fluorescence emission of a photon with a wavelength of 297.96 nm. The photon is within the spectral window of the detector 127. Other decay options from another Rydberg state to the ground state are possible, for example via one or more intermediate states, where each decay between two states involves the fluorescence emission of a photon 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. In more detail, the detector 127 may detect a single photon whose frequency corresponds to an attenuation path from another Rydberg state directly to the ground state, or detect at least one of two or more photons whose frequency corresponds to an energy state difference in an indirect attenuation 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 photons of a wireless signal transmitted by the wireless transmitter 110, which photons of the wireless signal excite electrons from a predetermined Rydberg state to another Rydberg state (because the transition from the predetermined Rydberg state to the ground state is prohibited, and the coupled laser is centered on the transition to the predetermined Rydberg state and has a line width smaller than the interval to another Rydberg state). Therefore, the detection of the characteristic photons means that the electrons have been excited from the 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 enabling single-photon detection of wireless signals 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 interferometry detection techniques (as described above, both require a sufficient number of photons to cause detectable changes 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 single-photon detectors 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 interferometry 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 enter the detector 127 directly are reflected internally into the detector 127 by one or more reflections.

[0031] Now refer to Figure 3 1 and 2 to describe a second embodiment of a wireless telecommunication network 200. The wireless telecommunication 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 detection laser 223, a coupling laser 225, a detector 227, a housing 228, and a vapor chamber 229.

[0032] The Rydberg atom-based RF detector 220 of the second embodiment is configured to transmit counter-propagating detection signals and coupling signals through the vapor chamber 229 (the counter-propagating detection signals and coupling signals overlap during at least a portion of their respective passages through the vapor chamber 229) so as to excite electrons of rubidium-85 atoms contained in the vapor chamber 229 to a predetermined Rydberg state (i.e., 83D_{5 / 2}). In this configuration, the wireless signal transmitted by the wireless transmitter 210 at 3.6 GHz through the vapor chamber 229 excites the electrons from the predetermined Rydberg state (83D_{5 / 2}) to another Rydberg state (84P_{3 / 2}).

[0033] Vapor cell 229 is composed of a material that is transparent to photons emitted in the decay path from another Rydberg state to the ground state, such as acrylic, silicone, quartz, or silicon dioxide.

[0034] The housing 228 is configured to allow only wireless signals inside the housing 228 (and therefore through the vapor chamber 229) to arrive in a particular direction. In this embodiment, this is achieved by constructing the housing 228 from an insulating material that prevents the passage of wireless signals at the frequency of the wireless signals transmitted by the wireless transmitter 210, and having one or more passages (e.g., openings or portions constructed from a material that is transparent to wireless signals at the frequency used by the wireless transmitter 210) in the housing 228, the passages being positioned and aligned on a predetermined axis so as to allow only wireless signals arriving within the housing 228 along a particular direction.

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

[0037] As described above, the method of the first embodiment can be applied to this second embodiment, so that a single photon of a wireless signal 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, the detection event indicates a single photon of a wireless signal arriving at the Rydberg atom-based RF detector 220 in a specific direction corresponding to the axis of the channel in the housing 228.

[0038] Now refer to Figure 4 A third embodiment of the present invention will be described. Figure 4An end-to-end quantum key distribution (QKD) system 300 is shown having a transmitter 310 and a receiver 320. The transmitter 310 is configured to transmit a 3.6 GHz wireless signal (containing polarization-encoded quantum bits, 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 of which is based on the Rydberg atom-based RF detector of the first embodiment or the second embodiment described above, and is therefore configured to detect the 3.6 GHz wireless signal 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 specific polarizations to pass through the detector and excite the atomic medium. In a first state, the first Rydberg atom-based RF detector uses a first polarization filter so that wireless signals with a first polarization (i.e., 0 degrees) can pass through the first polarization filter and pass through the atomic medium of the silica bulb / vapor chamber (so as to cause a transition from a predetermined Rydberg state to another Rydberg state), but wireless signals with any other polarization (including 45 degrees, 90 degrees, or 135 degrees) may not pass through the first polarization filter (and therefore not pass through the atomic medium of the silica bulb / vapor chamber, otherwise causing a transition from a predetermined Rydberg state to another Rydberg state). In addition, in the first state, the RF detector based on the second Rydberg atom uses a second polarization filter, so that a wireless signal having a second polarization (i.e., 90 degrees) can pass through the second polarization filter and pass through the atomic medium of the silica bulb / vapor chamber (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 135 degrees) may not pass through the second polarization filter (and therefore not pass through the atomic medium of the silica bulb / vapor chamber, otherwise it would 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 so 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 chamber (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) may not pass through the third polarization filter (and therefore not pass through the atomic medium of the silica bulb / vapor chamber, otherwise causing a transition from a predetermined Rydberg state to another Rydberg state). In addition, in the second state, the RF detector based on the second Rydberg atom uses a fourth polarization filter, so that a wireless signal having a fourth polarization (i.e., 135 degrees) can pass through the fourth polarization filter and pass through the atomic medium of the silica bulb / vapor chamber (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 therefore not pass through the atomic medium of the silica bulb / vapor chamber, otherwise causing 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 which the photon polarization detector module 327 is configured to detect 0 degree polarization at the RF detector based on the first Rydberg atom and to detect 90 degree polarization at the RF detector based on the second Rydberg atom, and in the second state, the photon polarization detector module 327 is configured to detect 45 degree polarization at the RF detector based on the first Rydberg atom and to detect 135 degree polarization at the RF detector based on the second Rydberg atom.

[0042] The QKD system 300 of this third embodiment utilizes polarization-encoded qubits to transmit encryption (i.e., cryptographic) keys between a transmitter 310 and a receiver 320. This can be based on, for example, the BB84 protocol, where the transmitter 310 sends qubits encoded according to a first basis (where 0 degrees of polarization corresponds to a bit value of 0 and 90 degrees of polarization corresponds to a bit value of 1) or a second basis (where 45 degrees of polarization corresponds to a bit value of 0 and 135 degrees of polarization 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 polarization qubit is detected at a first Rydberg atom-based RF detector and a 90 degree polarization qubit is detected at a second Rydberg atom-based RF detector) or a second basis (where the photon polarization detector module 327 is in a second state such that a 45 degree polarization qubit is detected at a first Rydberg atom-based RF detector and a 135 degree polarization qubit is detected at a second Rydberg atom-based RF detector). 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 qubits that were successfully received. The protocol can be used to transfer encryption keys between the transmitter 310 and the receiver 320.

[0043] The third embodiment described above is configured 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 first or second embodiment described above and configured to detect a first polarization, and the receiver may also include a second Rydberg atom-based RF receiver based on the first or second embodiment described above 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 encoded in a sequence of photons by polarization (e.g., any photon with a first polarization corresponds to a bit value of 0, and any photon with a 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 polarization-encoded quantum bits are possible using multiple Rydberg atom-based RF detectors (each RF detector is configured to detect a specific polarization).

[0044] In the third embodiment described above, 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 detection signal with a specific polarization and a coupled laser.

[0045] In the above embodiment, 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 (83D_{5 / 2}) to another Rydberg state (84P_{3 / 2}). As described above, the subsequent decay from another Rydberg state to the ground state involves the fluorescence emission of the photon 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 of other frequencies using the same technology (i.e., by detecting one or more characteristic photons emitted during the decay to the ground state). For example, a predetermined Rydberg state of a relatively high quantum number, such as the Rb149D_{5 / 2} state, can be selected (by configuring the detection signal and the coupling signal). The electrons can then be excited from this predetermined Rydberg state to another Rydberg state (Rb150P_{3 / 2}) by RF photons with a frequency of 605 MHz, and the subsequent direct decay to the ground state involves the fluorescence emission of the photon at 296.86 nm, which can be detected again using a single photon detector. In another example, the predetermined Rydberg state can be selected (by configuring the detection signal and the coupling signal) with a relatively low quantum number (e.g., Rb6D_{5 / 2}). The electrons can then be excited from this predetermined Rydberg state to another Rydberg state (Rb7P_3 / 2) by RF photons with a frequency of 24.6 THz, and the subsequent direct decay to the ground state involves the fluorescence emission of the photon 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 a characteristic photon with a relatively short wavelength (or relatively high frequency) emitted during the decay to the ground state, a Rydberg atom-based detector can be used to indirectly detect photons with a relatively long wavelength (or relatively low frequency). The indirectly detected photons 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 a trapezoidal EIT effect using a detection signal and a coupling signal. However, those skilled in the art will appreciate that any other form of the EIT effect (e.g., λ or V-shaped) may be used to raise electrons to Rydberg states, and that more than two signals may be used. Furthermore, those skilled in the art will appreciate that it is not essential that the detector uses rubidium-85 atoms, as other alkali metals, such as cesium or strontium, may be used instead.

[0047] In the above description, the photons of the wireless signal to be detected excite the electrons of the Rydberg atoms from a predetermined Rydberg state to another Rydberg state. This covers a first scenario, in which the wireless signal has a signal strength less than a threshold value, so that the predetermined Rydberg state and the another Rydberg state are not separated after interacting with the wireless signal, and a second scenario, in which the wireless signal has a signal strength greater than a threshold value, so that the predetermined Rydberg state and the another Rydberg state are separated after interacting with the wireless signal. In the first scenario, the photons of the wireless signal excite the electrons from a predetermined Rydberg state to another Rydberg state (as broadly described above). In the second scenario, the predetermined Rydberg state is split into two (which can be represented as R1-d and R1+d), and the other Rydberg state is split into two (which can be represented as R2-d and R2+d). This allows four transitions (R1-d→R2-d, R1-d→R2+d, R1+d→R2-d, and R1+d→R2+d). In addition, if the two states are split equally, there are three different energy differences for these four transitions (R2-R1-2d, R2-R1, and R2-R1+2d), which is called a Stark triplet. The photons of the wireless signal can then excite electrons through any of these transitions (i.e., from any of the predetermined Rydberg split states to any of the other Rydberg split states), and then 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. Direct decay from another Rydberg state to the ground state is the most likely transition, but there is generally a probability distribution of transitions from any particular another 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 to be an indirect detection of RF photons at the Rydberg atom-based RF detector.

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

Claims

1. A method for detecting photons within a first frequency range, the method comprising the following steps: 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 a 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 first photons in the first frequency range and having a first polarization incident on the first transmission medium excite electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein first photons in a second frequency range are emitted as part of a subsequent deexcitation of the electrons from the another excited state of the first transmission medium; detecting a first photon within the first frequency range and having the first polarization by detecting a first photon within 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 a 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 second photons in the first frequency range incident on the second transmission medium and having a second polarization excite electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein second photons in the second frequency range are emitted as part of subsequent de-excitation of electrons in the other excited state of the second transmission medium; and Second photons with the second polarization within the first frequency range are detected by detecting second photons within the second frequency range.

2. The method according to claim 1, further comprising the steps of: Data is demodulated based on detecting a first photon with the first polarization corresponding to a first bit value within the first frequency range and based on detecting a second photon with the second polarization corresponding to a second bit value within the first frequency range.

3. The method according to claim 2, further comprising the steps of: An encryption key is determined based on the demodulated data.

4. A method according to any one of the preceding claims, wherein: The first frequency range is lower than the second frequency range.

5. A method according to any one of the preceding claims, wherein: The first frequency range is in the radio frequency (RF), microwave or infrared range of the electromagnetic spectrum.

6. A device for detecting photons within 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 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 first photons in a first frequency range and having a first polarization incident on the first transmission medium excite electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein first photons in a second frequency range are emitted as part of a subsequent de-excitation of electrons in the another excited state of the first transmission medium; a first detector configured to detect first photons having the first polarization within the first frequency range by detecting first photons 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 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 second photons with a second polarization in the first frequency range incident on the second transmission medium excite electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein second photons in the second frequency range are emitted as part of a subsequent de-excitation of electrons in the other excited state of the second transmission medium; and A second detector is configured to detect second photons with the second polarization within the first frequency range by detecting second photons within the second frequency range.

7. The device according to claim 6, further comprising: A housing is 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.

8. The device according to claim 6 or claim 7, further comprising: A processor configured to demodulate data based on detecting a first photon having the first polarization corresponding to a first bit value within the first frequency range and detecting a second photon having the second polarization corresponding to a second bit value within the first frequency range.

9. The device according to claim 8, wherein: The processor is also configured to determine an encryption key based on the demodulated data.

10. The device according to any one of claims 6 to 9, wherein: The first frequency range is lower than the second frequency range.

11. The device according to any one of claims 6 to 10, wherein: The first frequency range is in the radio frequency (RF), microwave or infrared range of the electromagnetic spectrum.

12. A computer program comprising instructions for causing an apparatus according to any one of claims 6 to 11 to perform the steps of a method according to any one of claims 1 to 5.

13. A computer readable carrier medium comprising a computer program according to claim 12.

14. 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 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 first photons in a first frequency range and having a first polarization incident on the first transmission medium excite electrons in the predetermined excited state of the first transmission medium to another excited state of the first transmission medium, wherein first photons in the second frequency range are emitted as part of a subsequent de-excitation of electrons in the another excited state of the first transmission medium; a first detector configured to detect first photons within the first frequency range and having the first polarization by detecting first photons 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 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 second photons in the first frequency range and having a second polarization incident on the second transmission medium excite electrons in the predetermined excited state of the second transmission medium to another excited state of the second transmission medium, wherein second photons in the second frequency range are emitted as part of a subsequent de-excitation of electrons in the other excited state of the second transmission medium; and A second detector is configured to detect second photons in the first frequency range and having the second polarization by detecting second photons in the second frequency range.

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