A device for a wireless telecommunications network, a system comprising the device, and a method of operating the device

Abstract

This invention provides a device for a wireless telecommunications network, a system comprising the device and a method of operating the device, the wireless telecommunications network comprising: a Rydberg-atom based electromagnetic field detector; and an electromagnetic signal generator, the electromagnetic signal generator being configured to output a plurality of optical signals, each having a distinct frequency, wherein the electromagnetic signal generator comprises a stabilising module configured to stabilise each optical signal of the plurality of optical signals to their respective frequency; the device comprising: a receiver configured to receive, from the signal generator: a first optical signal having a first frequency for use in the Rydberg-atom based electromagnetic field detector, and a reference signal, the reference signal having been derived from two optical signals of a plurality of optical signals output by the electromagnetic signal generator, the reference signal having a reference frequency; and a frequency controller module configured to receive the reference signal and adjust the frequency of the first optical signal, wherein the frequency adjustment of the first optical signal is derived from the reference frequency of the reference signal.

Description

[0001] A DEVICE FOR A WIRELESS TELECOMMUNICATIONS NETWORK, A SYSTEM COMPRISING THE DEVICE, AND A METHOD OF OPERATING THE DEVICE

[0002] Field of the Invention

[0003] The present invention relates to a device for a wireless telecommunications network, a system comprising the device, and a method of operating the device.

[0004] Background

[0005] A Rydberg atom is an atom with one or more electrons excited to a very high principal quantum number (e.g. >10). These Rydberg atoms have several useful properties, such as very large dipole moments and long lifetimes.

[0006] The Rydberg atom may be used in a Rydberg-atom based ElectroMagnetic (EM) field detector. The Rydberg-atom based EM field detector is based on the Electromagnetically Induced Transparency (EIT) effect. The EIT effect may be experienced when a probe signal and a coupling signal are used to elevate electrons of an atomic medium to a Rydberg state. In this state, the atomic medium becomes transparent to the probe signal. An EM field incident at the atomic medium may then cause a further transition of an electron from the Rydberg state to a further Rydberg state. Electrons may subsequently drop from the further Rydberg state to the ground state so that the atomic medium becomes less transparent to the probe signal. The EM field may therefore be detected from this change in transparency as a change in intensity of the probe signal, thus creating a Rydberg-atom based Amplitude Modulation (AM) EM detector. A more detailed explanation of this effect, and a further explanation of a Rydberg-atom based Frequency Modulation (FM) EM detector, can be found in the article, “A Multiple-Band Rydberg-Atom Based Receiver / Antenna: AM / FM Stereo Reception”, Holloway et al., National Institute of Standards and Technology). A Rydberg-atom based phase-modulated EM detector has been described in “A Rydberg Atom-Based Mixer: Measuring the Phase of a Radio Frequency Wave”, Appl. Phys. Lett. 114, 114101 (2019), Holloway et al, which utilises an additional Local Oscillator (LO) signal. These Rydberg-atom based EM detectors can also be used to receive a data stream by demodulating data encoded in the detected signals, thus creating a Rydbergatom based EM receiver. Performance of the Rydberg-atom based EM detector is a function of several factors, including:

[0007] • A difference between an energy of one or more of the probe signal, coupling signal and (if applicable) LO signal and the energy required for the respective transition between energy state;

[0008] • A linewidth of the probe signal and / or coupling signal;

[0009] • A beam size of the probe signal and / or coupling signal; and

[0010] • A power of the probe signal and / or coupling signal.

[0011] Accordingly, one way in which the Rydberg-atom based EM detector can improve performance is for each of the probe signal, coupling signal and LO signal to be at a target frequency being that required for the respective transition between energy states. It is desirable, therefore, for each source of the probe signal, coupling signal and LO signal to generate the respective signal at the target frequency. Furthermore, as the respective frequency of each signal may drift over time from the target frequency (due to, for example, temperature, mechanical and / or current fluctuations), a stabiliser may also be used to maintain (that is, stabilise or “lock”) the frequency of each signal at the target frequency. A conventional Rydberg-atom based EM detector may therefore comprise a probe signal stabiliser for locking the frequency of the probe signal to the desired probe frequency and a coupling signal stabiliser for locking the frequency of the coupling signal to the desired coupling frequency. The LO signal is typically in the Radio Frequency (RF) range, and the Rydberg-atom based EM detector may also comprise a LO signal stabiliser for locking the frequency of the LO signal to the desired LO frequency.

[0012] As the probe and coupling signals are typically generated using a probe laser and coupling laser respectively, then the probe signal stabiliser and coupling signal stabiliser may use laser stabilisation techniques, such as saturated absorption spectroscopy, phase-offset locking, and dichroic atomic vapour spectroscopy locking. These laser stabilisation techniques are typically expensive and are generally suited to a laboratory environment where high quality field control and isolation can be achieved.

[0013] It is desirable to alleviate some or all of the above problems. Summary of the Invention

[0014] According to a first aspect of the invention, there is provided a device for a wireless telecommunications network, the wireless telecommunications network comprising: a Rydberg-atom based electromagnetic field detector; and an electromagnetic signal generator, the electromagnetic signal generator being configured to output a plurality of optical signals, each having a distinct frequency, wherein the electromagnetic signal generator comprises a stabilising module configured to stabilise each optical signal of the plurality of optical signals to their respective frequency; the device comprising: a receiver configured to receive, from the signal generator: a first optical signal having a first frequency for use in the Rydberg-atom based electromagnetic field detector, and a reference signal, the reference signal having been derived from two optical signals of a plurality of optical signals output by the electromagnetic signal generator, the reference signal having a reference frequency; and a frequency controller module configured to receive the reference signal and adjust the frequency of the first optical signal, wherein the frequency adjustment of the first optical signal is derived from the reference frequency of the reference signal.

[0015] The frequency controller module may be configured to: receive a feedback signal indicative of performance of the Rydberg-atom based electromagnetic field detector, wherein the frequency adjustment of the first optical signal is responsive to the feedback signal.

[0016] The receiver may be further configured to receive, from the signal generator: a second optical signal having a second frequency for use in the Rydberg-atom based electromagnetic field detector; and the frequency controller module may be further configured to adjust the frequency of the second optical signal, wherein the frequency adjustment of the second optical signal may be derived from the reference frequency of the reference signal.

[0017] The frequency adjustment of the second optical signal may be responsive to the feedback signal.

[0018] According to a second aspect of the invention, there is provided a system comprising: the device of the first aspect of the invention; and the electromagnetic signal generator. The system may further comprise the Rydberg-atom based electromagnetic field detector, wherein the Rydberg-atom based electromagnetic field detector may comprise a transmission medium and be configured to pass the first optical signal and the second optical signal through the transmission medium to excite electrons of the transmission medium to a predetermined Rydberg state.

[0019] The system may further comprise a local oscillator configured to receive the reference signal from the electromagnetic signal generator and transmit a local oscillator signal through the transmission medium so as to enable phase-modulated electromagnetic signal reception at the Rydberg-atom based electromagnetic field detector, wherein the local oscillator signal may have a frequency derived from the reference frequency of the reference signal.

[0020] According to a third aspect of the invention, there is provided a method of operating a device of the first aspect of the invention, the method comprising the steps of: receiving a first optical signal; receiving a feedback signal indicative of performance of the Rydberg-atom based electromagnetic field detector; determining, from the feedback signal, that performance of the Rydberg-atom based electromagnetic field detector is below a threshold; and, in response, adjusting a frequency of the first optical signal.

[0021] The method may further comprise the steps of: receiving a second optical signal; and, responsive to the determination that performance of the Rydberg-atom based electromagnetic field detector is below a threshold, adjusting a frequency of the second optical signal.

[0022] The method may further comprise the step of: responsive to the determination that performance of the Rydberg-atom based electromagnetic field detector is below a threshold, adjusting a frequency and / or transmission power of the local oscillator signal.

[0023] Prior to the step of adjusting a frequency of the first optical signal, the method may further comprise the step of: determining that a frequency and / or transmission power of the local oscillator signal has been adjusted within a time period.

[0024] According to a fourth aspect of the invention, there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of the third aspect of the invention. The computer program may be comprised on a computer readable carrier medium.

[0025] Brief Description of the Figures

[0026] In order that the present invention may be better understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

[0027] Figure 1 is a schematic diagram of a first wireless telecommunications network;

[0028] Figures 2 and 3 illustrate a flow diagram of a method of operating a logic controller of the network of Figure 1;

[0029] Figure 4 is a schematic diagram of a second wireless telecommunications network; and Figure 5 is a schematic diagram of a third wireless telecommunications network.

[0030] Detailed Description

[0031] Figure 1 illustrates a first wireless telecommunications network 100. The first wireless telecommunications network 100 comprises a wireless transmitter 110, a signal generator 120, a Rydberg-atom based Radio Frequency (RF) receiver 130, and a controller 140. The wireless transmitter 110 is configured to transmit phase-modulated wireless signals at a frequency of 3.5GHz. The RF receiver 130 is configured to detect and demodulate the phase-modulated 3.5GHz wireless signal transmitted by the wireless transmitter 110.

[0032] The signal generator 120 is a multi-signal ElectroMagnetic (EM) signal generator configured to generate a plurality of EM signals. In one example, the signal generator 120 comprises a laser frequency comb 121 configured to generate a plurality of optical signals (that is, signals in the optical frequency range of the EM spectrum), each optical signal of the plurality of optical signals being generated at a particular frequency. In this example, the frequency comb 121 generates a plurality of optical signals between, for example, 1540nm (i.e. 194.67THz) and 1580nm (i.e. 189.74THz).r

[0033] The signal generator also comprises a stabilising module 123. The stabilising module 123 is configured to maintain (i.e. stabilise or “lock”) the respective frequency of each optical signal of the plurality of optical signals generated by the laser frequency comb 121. In this example, the stabilising module 123 is configured to lock the laser frequency comb 121 to an external reference, such as an atomic transition or a Global Navigation Satellite System (GNSS) signal.

[0034] In one example, the laser frequency comb 121 and stabilising module 123 are realised in a FC1500-ULNnova frequency comb, which has a fractional stability of 1 x 10'16in 1 second or 1 x 10-18in 1000 seconds when phase locked to an optical reference, or 5 x 10-13in 1 second when phase locked to an RF source.

[0035] Additionally, the signal generator 120 is configured to generate a Radio Frequency (RF) signal which is derived from two of the plurality of optical signals generated by the laser frequency comb 121. A repetition rate of the laser frequency comb 121 is the interval between two adjacent optical signals of the plurality of optical signals generated by the laser frequency comb 121 (that is, between two adjacent “teeth” of the laser frequency comb 121). In this example, the repetition rate is 100MHz. Other common repetition rates of laser frequency combs include 80MHz, 250MHz and 1GHz. A signal at the repetition rate can be extracted from the laser frequency comb 121 by, for example, mode locking two adjacent optical signals of the plurality of optical signals of the laser frequency comb 121. In this example, the extracted 100MHz signal is converted to a 10MHz RF signal by frequency multiplexing the extracted 100MHz signal to the 10thorder to 1GHz, and then mixing the 1GHz signal with a 990MHz signal.

[0036] The 10MHz RF signal is also inherently stabilised by the stabilising module 125 as it is extracted from two adjacent optical signals that are themselves stabilised by the stabilising module 125.

[0037] The signal generator 120 also comprises a conversion module 125. The conversion module 125 is configured to convert a frequency of a first optical signal (of the plurality of optical signals generated by the laser frequency comb 121) from its frequency at generation to an output frequency and is further configured to convert a frequency of a second optical signal (of the plurality of optical signals generated by the laser frequency comb 121) from its frequency at generation to an output frequency. In this example, the conversion module 125 converts a frequency of the first optical signal at 1560nm (e.g. by filtering the 1560nm signal of the laser frequency comb 121) to 780nm by second harmonic generation. The first optical signal at 780nm may therefore be used as a probe signal in the RF receiver 130, as described in more detail below. To generate the coupling signal, highly non-linear optics (not shown) are used to convert the laser frequency comb 121 output to (for example) 960nm. The conversion module 125 converts this 960nm signal to 480nm by second harmonic generation. The second optical signal at 480nm may therefore be used as a coupling signal in the RF receiver

[0038] 130, again described in more detail.

[0039] The signal generator 120 therefore provides, at an output 127:

[0040] 1) A stable first optical signal at its output frequency,

[0041] 2) A stable second optical signal at its output frequency, and

[0042] 3) A stable RF signal at 10MHz.

[0043] In Figure 1 , it is shown that the stable probe signal, stable coupling signal and stable RF signal are provided to the controller 140 and thereafter the RF receiver 130. Operation of the controller 140 is described in more detail below. The RF receiver 130 includes an optical fibre 131 , a Local Oscillator (LO) antenna 135, and a photodetector 137. The optical fibre 131 is part of an optical path which includes, at a point along the optical fibre

[0044] 131 , a transmission region 133 (e.g. vapour cell) containing an atomic vapour. In this embodiment, the atomic vapour is a vapour of Rubidium-85 atoms.. The stable first optical signal, acting as a stable probe signal, is communicated along the optical fibre 131 to the photodetector 137, passing through the transmission region 133 in which it excites electrons of the Rubidium-85 atoms from a ground state to a first excited state. Similarly, the stable second optical signal, acting as a stable coupling signal, is communicated along the optical fibre 131 and passes through the transmission region 133 (counter-propagating the stable probe signal) in which it excites electrons of the Rubidium-85 atoms from the first excited state to a predetermined Rydberg state. The combination of the probe and coupling signals on the electrons of the Rubidium-85 atoms in the transmission region 133 therefore cause an Electromagnetically Induced Transparency (EIT) signal in the monitored probe signal at the photodetector 137.

[0045] The LO antenna 135 is configured to transmit a wireless signal (the “LO signal”) through the transmission region 133 at a target LO frequency. As noted above, the wireless transmitter 110 also transmits a wireless phase-modulated signal at 3.5GHz, which passes through the transmission region 133. The combination of the wireless phase- modulated signal and the LO signal at the transmission region 133 excites electrons of the Rubidium-85 atoms from the predetermined Rydberg state to a further Rydberg state. This change is detectable in the monitored probe signal at the photodetector 137, allowing the wireless phase-modulated signal to be detected and any data demodulated.

[0046] The LO signal is generated by the LO antenna 135 using an internal clock. In particular, the target LO frequency of the LO signal is set based on the internal clock, such as by using a time period (corresponding with the target LO frequency) as measured by the internal clock.

[0047] The LO antenna 135 also receives the stable RF signal from the signal generator 120. The internal clock of the LO antenna 135 is synchronised to the stable RF signal. The target LO frequency of the LO signal is therefore set based on the internal clock that is synchronised to the stable RF signal.

[0048] As noted above, the controller 140 is configured to receive the stable first optical signal, stable second optical signal and stable RF signal from the signal generator 120. The controller 140 comprises a logic controller 141 that is configured to:

[0049] • set a target LO frequency of the LO signal transmitted by an LO antenna 135,

[0050] • adjust (i.e. tune) the stable first optical signal from its output frequency to a target frequency to be used as a probe signal in the RF receiver 130, and

[0051] • adjust (i.e. tune) the stable second optical signal from its output frequency to a target frequency to be used as a coupling signal in the RF receiver 130.

[0052] As noted above, the target LO frequency of the LO signal is set based on the internal clock, such as by using a time period (corresponding with the target LO frequency) as measured by the internal clock. Therefore, a new target LO frequency of the LO signal may be set based on a new time period (corresponding with the new target LO frequency) as measured by the internal clock. Adjustment of the target LO frequency of the LO signal is therefore achieved by adjusting the time period, as measured by the internal clock, on which the target LO frequency is based. As the internal clock is stabilised to the stable RF signal, the LO signal maintains its stability at its new target LO frequency.

[0053] Adjustment of the target LO frequency may be in response to a configuration signal sent by the logic controller 141 of the controller 140, indicating a new target LO frequency for the LO signal, which may be triggered by a method described in more detail below. The stable first optical signal and the stable second optical signal are received at the controller 140 at their respective output frequencies (as output by the signal generator 120). A frequency of the stable first optical signal may be adjusted from its output frequency to a target frequency by cooperation of a first frequency synthesizer 143 and a first Acousto-Optic Modulator (AOM) 145 of the controller 140. Similarly, the frequency of the stable second optical signal may be adjusted from its output frequency to a target frequency by cooperation of a second frequency synthesizer 147 and a second AOM 149 of the controller 140. This will now be described in more detail.

[0054] The first frequency synthesizer 143 receives the stable RF signal from the signal generator 120, to which an internal clock of the first frequency synthesizer 143 is synchronised. The frequency synthesizer 143 generates an electrical signal at a particular frequency. The frequency of the electrical signal is set based on the internal clock, such as by using a time period (corresponding with the frequency) as measured by the internal clock. The frequency of the electrical signal is therefore set based on the internal clock that is synchronised to the stable RF signal.

[0055] The frequency synthesizer 143 provides the electrical signal to the first AOM 145. The first AOM 145 uses the electrical signal to generate an acoustic wave. A frequency of the acoustic wave corresponds with a frequency shift between the output frequency of the first optical signal and a target frequency of the first optical signal. As the first optical signal passes through the first AOM 145, the interaction between the first optical signal and the acoustic wave adjusts the frequency of the first optical signal from the output frequency to the target frequency.

[0056] The frequency of the stable second optical signal is adjusted from its output frequency to a target frequency in a similar manner by cooperation of a second frequency synthesizer 147 and a second AOM 149.

[0057] As noted above, the first frequency synthesizer 143 (and similarly the second frequency synthesizer 147) are configured to generate their respective electrical signals at a particular frequency. The frequencies of these respective electrical signals are set so as to adjust the frequency of the respective first and second optical signals by a certain amount. The first wireless telecommunications network 100 therefore provides a common stabilising source for 1) locking a plurality of optical signals generated by the signal generator 121 to their respective output frequencies, 2) synchronising an internal clock of a LO antenna 135 for precise frequency setting of the LO signal, 3) synchronising an internal clock of the first frequency synthesizer 143 for precise frequency adjustment of the first optical signal, and 4) synchronising an internal clock of the second frequency synthesizer 147 for precise frequency adjustment of the second optical signal. The RF signal used to stabilise the internal clocks is inherently stabilised by the stabilising module 125 as it is extracted from two optical signals generated by the signal generator 120 that are themselves stabilised by the stabilising module 125. The wireless telecommunications network 100 therefore provides both frequency locking (of the optical signals output by the signal generator 120) and frequency tuning (between the respective output frequencies of the optical signals and their respective target frequencies) based on the same stabilising source. An alternative wireless telecommunications network may comprise a plurality of stabilising sources, such as a stabilising source for the frequency locking process for each optical signal, a stabilising source for the frequency setting process for the LO signal, and a stabilising source for the frequency tuning process. By comparison, the first wireless telecommunications network 100 comprises fewer stabilising sources. Furthermore, by using a common stabilising source, the first wireless telecommunications network 100 ensures that multiple components are universally stabilised, which may be beneficial compared to the alternative wireless telecommunications network in which each stabilising source may exhibit very different properties. In other words, the respective frequencies of the probe, coupling, and LO signals do not drift with respect to one another in the first wireless telecommunications network 100.

[0058] A goal of the RF receiver 130 may be defined as to maximise a data rate and / or minimise an error rate. The RF receiver 130 may be initially optimised for receiving the wireless phase-modulated signal from the wireless transmitter 110. However, performance (in terms of data rate and / or error rate) may become sub-optimal, which is likely due to a change in intensity and / or frequency of the wireless phase-modulated signal from the wireless transmitter 110. The RF receiver 130 may be reconfigured to improve performance in response to this change by operation of the logic controller 141. A method of operating the logic controller 141 will now be described with reference to Figures 2 and 3. In step S101 as shown in Figure 2, the logic controller 141 receives a performance measurement from the RF receiver 130. The performance measurement may be, for example, a modulation depth of the probe signal at the photodetector 137, a signal to noise ratio of the probe signal at the photodetector 137, and / or a bit error rate for data demodulated from the probe signal at the photodetector 137. In step S103, the performance measurement is compared to a performance threshold. If this comparison indicates that performance is satisfactory (i.e. the measurement is above the threshold), then the process ends. If this comparison indicates that performance is unsatisfactory (i.e. the measurement is below the threshold), then the process proceeds to step S105.

[0059] In step S105, the logic controller 141 determines whether the LO signal has been optimised within a predetermined time interval. This will be explained in more detail below. If the determination of step S105 is negative, then the process proceeds to step S107. If the determination of step S105 is affirmative, then the process proceeds to step S115 (shown in Figure 3).

[0060] In step S107, the logic controller 141 enters an outer iterative loop to optimise the LO signal. This outer iterative loop comprises a first inner iterative loop in which a frequency of the LO signal is set (step S109a) to each of a plurality of candidate frequencies (typically being a contiguous range of frequencies) and a performance of the RF receiver 130 is measured (step S109b) at each candidate frequency. The performance measurement at each candidate frequency is recorded such that the candidate frequency associated with the greatest performance measurement can be identified. In step S109c, the frequency of the LO signal is set to the identified candidate frequency.

[0061] These adjustments to the frequency of the LO signal are caused by the logic controller 141 sending a configuration signal to the LO antenna 135 identifying a new frequency for the LO signal, and the LO antenna 135 responding by performing a corresponding adjustment to the frequency of the LO signal (in the manner described above).

[0062] The outer iterative loop also comprises a second inner iterative loop in which a transmission power of the LO signal is set (step S111 a) to each of a plurality of candidate transmission powers (typically being a contiguous range of transmission powers) and a performance of the RF receiver 130 is measured (step S111 b) at each candidate transmission power. The performance measurement at each candidate transmission power is recorded such that the candidate transmission power associated with the greatest performance measurement can be identified. In step S111c, the transmission power of the LO signal is set to the identified candidate transmission power.

[0063] These adjustments to the transmission power of the LO signal are caused by the logic controller 141 sending a configuration signal to the LO antenna 135 identifying a new transmission power for the LO signal, and the LO antenna 135 responding by performing a corresponding adjustment to the transmission power of the LO signal.

[0064] In step S113, it is determined whether one or more termination conditions has been met. The termination condition may be:

[0065] 1) a number of iterations of the outer iterative loop has reached a threshold,

[0066] 2) a performance measurement of the RF receiver 130 in the current iteration meets a performance threshold, and / or

[0067] 3) a performance measurement of the RF receiver 130 in the current iteration does not exceed a performance measurement of the RF receiver 130 in the previous by a particular amount (e.g. zero).

[0068] If the determination of step S113 is answered in the negative, then the process loops back to step S107 for a further iteration. If the determination of step S113 is answered in the affirmative, then the process ends.

[0069] As noted above, if the determination of step S105 (in which the logic controller 141 determines whether the LO signal has been optimised within the predetermined time interval) is affirmative, then the process proceeds to step S115. This will now be described.

[0070] In step S115, the logic controller 141 enters an outer iterative loop to optimise the first optical signal. This outer iterative loop comprises a first inner iterative loop in which a frequency of the first optical signal is set (step S117a) to each of a plurality of candidate frequencies (typically being a contiguous range of frequencies) and a performance of the RF receiver 130 is measured (step S117b) at each candidate frequency. The performance measurement at each candidate frequency is recorded such that the candidate frequency associated with the greatest performance measurement can be identified. In step S117c, the frequency of the first optical signal is set to the identified candidate frequency.

[0071] These adjustments to the frequency of the first optical signal are caused by the logic controller 141 sending a configuration signal to the first frequency synthesizer 143 identifying a new frequency for the first optical signal, and the first frequency synthesizer 143 responding by generating a corresponding electrical signal so as to cause (in cooperation with the first AOM 145) an adjustment to the frequency of the first optical signal (as described in more detail above).

[0072] The outer iterative loop also comprises a second inner iterative loop in which a frequency of the second optical signal is set (step S119a) to each of a plurality of candidate frequencies (typically being a contiguous range of frequencies) and a performance of the RF receiver 130 is measured (step S119b) at each candidate frequency. The performance measurement at each candidate frequency is recorded such that the candidate frequency associated with the greatest performance measurement can be identified. In step S119c, the frequency of the second optical signal is set to the identified candidate frequency.

[0073] These adjustments to the frequency of the second optical signal are caused by the logic controller 141 sending a configuration signal to the second frequency synthesizer 147 identifying a new frequency for the second optical signal, and the second frequency synthesizer 147 responding by generating a corresponding electrical signal so as to cause (in cooperation with the second AOM 149) an adjustment to the frequency of the second optical signal (as described in more detail above).

[0074] In step S121 , it is determined whether a termination condition has been met. This termination condition may be:

[0075] 1) a number of iterations of the outer iterative loop has reached a threshold,

[0076] 2) a performance measurement of the RF receiver 130 in the current iteration meets a performance threshold, or

[0077] 3) a performance measurement of the RF receiver 130 in the current iteration does not exceed a performance measurement of the RF receiver 130 in the previous by a particular amount (e.g. zero). If the determination of step S121 is answered in the negative, then the process loops back to step S115 for a further iteration. If the determination of step S121 is answered in the affirmative, then the process ends.

[0078] The above process may be triggered periodically. The method of Figures 2 and 3 therefore enables the wireless telecommunications network 100 to continually optimise the RF receiver 130 to compensate for any drift in frequency of the wireless signal to be detected, a drift in frequency of the LO signal, and / or drift in frequency of the probe and / or coupling signals.

[0079] The skilled person will understand that the signals generated by the signal generator 120 may be provided to multiple RF receivers 130, such as by splitting the signals and providing each split signal to a particular RF receiver 130. A second wireless telecommunications network 200 having this architecture is illustrated in Figure 4. Furthermore, the stabilising module may be remotely located to the signal generator 120 and provide a stabilised signal (to which the signal generator 120 locks its optical signals) to the signal generator 120. This stabilised signal may be provided to multiple signal generators 120, as shown in Figure 5.

[0080] The skilled person will understand that the method of Figures 2 and 3 is non-essential and the controller 140 may be used to set / tune the frequencies of the LO signal, first optical signal and second optical signal in response to any trigger (e.g. manual intervention).

[0081] Furthermore, the skilled person will understand that it is non-essential the RF receiver 130 comprises a LO antenna 135, for example if the RF receiver 130 is receiving amplitude or frequency modulated signals. Furthermore, the RF receiver 130 may be configured as a detector only.

[0082] Furthermore, it is non-essential that the atomic medium is comprised of Rubidium-85 atoms. Instead, the atomic medium may be comprised of any other alkali metal, such as Caesium or Strontium or Rubidium-87. The skilled person will understand that it is non- essential that the atomic medium of the RF detector that may be excited to the predetermined Rydberg states is contained in a vapour cell. That is, the transmission medium may be a capillary system or a hollow-core fibre segment of the optical fibre 131.

[0083] The skilled person will also understand that the above methods may be applied to many other electromagnetic signals of different frequencies. That is, for a particular target frequency, a system may be configured such that an EIT signal is produced on a probe signal by an incident electromagnetic signal at that target frequency (such as by selecting an appropriate atomic medium (e.g. Rubidium, Caesium or Strontium) having Rydberg states that correspond with that target frequency). Furthermore, it is also non-essential that the EIT signal is produced following a ladder configuration of electron transitions. That is, any configuration (e.g. Lambda, Vee) may be used. The concepts discussed above may be extended to a three (or more) level system, such as by using one or more further optical signals generated by the signal generator 120.

[0084] The skilled person will also understand that it is non-essential for the logic controller 141 to adjust the respective frequencies of each optical signal for the RF receiver 130. That is, a frequency of a subset (including only one) of these optical signals may be adjusted.

[0085] The skilled person will understand that any combination of features is possible within the scope of the invention, as claimed.

Claims

CLAIMS1. A device for a wireless telecommunications network, the wireless telecommunications network comprising: a Rydberg-atom based electromagnetic field detector; and an electromagnetic signal generator, the electromagnetic signal generator being configured to output a plurality of optical signals, each having a distinct frequency, wherein the electromagnetic signal generator comprises a stabilising module configured to stabilise each optical signal of the plurality of optical signals to their respective frequency; the device comprising: a receiver configured to receive, from the signal generator: a first optical signal having a first frequency for use in the Rydberg-atom based electromagnetic field detector, and a reference signal, the reference signal having been derived from two optical signals of a plurality of optical signals output by the electromagnetic signal generator, the reference signal having a reference frequency; and a frequency controller module configured to receive the reference signal and adjust the frequency of the first optical signal, wherein the frequency adjustment of the first optical signal is derived from the reference frequency of the reference signal.

2. A device as claimed in Claim 1 , wherein the frequency controller module is configured to: receive a feedback signal indicative of performance of the Rydberg-atom based electromagnetic field detector, wherein the frequency adjustment of the first optical signal is responsive to the feedback signal.

3. A device as claimed in either Claim 1 or Claim 2, wherein: the receiver is further configured to receive, from the signal generator: a second optical signal having a second frequency for use in the Rydberg-atom based electromagnetic field detector; and the frequency controller module is further configured to adjust the frequency of the second optical signal, wherein the frequency adjustment of the second optical signal is derived from the reference frequency of the reference signal.

4. A device as claimed in Claim 3, wherein the frequency adjustment of the second optical signal is responsive to the feedback signal.

5. A system comprising: the device as claimed in any one of the preceding claims; and the electromagnetic signal generator.

6. A system as claimed in Claim 5, further comprising: the Rydberg-atom based electromagnetic field detector, wherein the Rydbergatom based electromagnetic field detector comprises a transmission medium and is configured to pass the first optical signal and the second optical signal through the transmission medium to excite electrons of the transmission medium to a predetermined Rydberg state.

7. A system as claimed in Claim 6, further comprising: a local oscillator configured to receive the reference signal from the electromagnetic signal generator and transmit a local oscillator signal through the transmission medium so as to enable phase-modulated electromagnetic signal reception at the Rydberg-atom based electromagnetic field detector, wherein the local oscillator signal has a frequency derived from the reference frequency of the reference signal.

8. A method of operating a device as claimed in any one of the preceding claims, the method comprising the steps of: receiving a first optical signal; receiving a feedback signal indicative of performance of the Rydberg-atom based electromagnetic field detector; determining, from the feedback signal, that performance of the Rydberg-atom based electromagnetic field detector is below a threshold; and, in response, adjusting a frequency of the first optical signal.

9. A method as claimed in Claim 8 when dependent on Claim 3, the method further comprising the steps of:receiving a second optical signal; and, responsive to the determination that performance of the Rydberg-atom based electromagnetic field detector is below a threshold, adjusting a frequency of the second optical signal.

10. A method as claimed in either Claim 8 or Claim 9, when dependent on Claim 7, further comprising the step of: responsive to the determination that performance of the Rydberg-atom based electromagnetic field detector is below a threshold, adjusting a frequency and / or transmission power of the local oscillator signal.

11. A method as claimed in Claim 10, wherein, prior to the step of adjusting a frequency of the first optical signal, the method further comprises the step of: determining that a frequency and / or transmission power of the local oscillator signal has been adjusted within a time period.

12. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of any one of Claims 8 to 11.

13. A computer readable carrier medium comprising the computer program of Claim

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