Rydberg-atom based electromagnetic field receiver

Abstract

This invention provides a Rydberg-atom based electromagnetic field receiver for a wireless telecommunications network, the electromagnetic field receiver comprising a plurality of coupling laser with one being in active state and the other ones being in preparation state, allowing a fast switching between an active laser and a prepared laser in order to provide a fast frequency hopping scheme.

Description

[0001] RYDBERG-ATOM BASED ELECTROMAGNETIC FIELD RECEIVER

[0002] Field of the Invention

[0003] The present invention relates to a Rydberg-atom based electromagnetic field receiver, a system comprising the Rydberg-atom based electromagnetic field receiver, and a method of controlling the Rydberg-atom based electromagnetic field receiver.

[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 decay periods.

[0006] The Rydberg atom may be used to detect an ElectroMagnetic (EM) field. A Rydbergatom based electromagnetic field detector is based on the Electromagnetically Induced Transparency (EIT) effect. The EIT effect may be experienced when a probe laser and a coupling laser are used to elevate electrons of an atomic medium to a Rydberg state. In this state, the atomic medium becomes transparent to the probe laser. 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 laser. The EM field may therefore be detected from this change in transparency as a change in intensity of the probe laser, 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. These Rydbergatom based EM detectors can also be used to receive a data stream by demodulating data encoded in the detected signals, thus creating a Rydberg-atom based EM receiver.

[0007] It is desirable to implement a secure method of communication between a transmitter and a Rydberg-atom based EM receiver. Summary of the Invention

[0008] According to a first aspect of the invention, there is provided a Rydberg-atom based electromagnetic field receiver for a wireless telecommunications network, the wireless telecommunications network comprising a transmitter configured to transmit an electromagnetic field in a sequence of sub-bands of a frequency band, the Rydbergatom based electromagnetic field receiver comprising: one or more transmission media; one or more probe signals, each probe signal having a probe frequency and being configured to pass through a transmission medium of the one or more transmission media, wherein the probe frequency is configured to excite electrons of the respective transmission medium from a first state to a second state; a plurality of coupling signals, each coupling signal of the plurality of coupling signals having a preparation state and an active state, wherein: in the active state, each coupling signal is configured to pass through a transmission medium of the one or more transmission media and overlap a probe signal of the one or more probe signals so as to induce an Electromagnetically Induced Transparency, EIT, effect in the transmission medium, and in the preparation state, each coupling signal transitions from an inactive state to the active state; and a controller configured to control the receiver such that, in a time period of a first sub-band of the sequence of sub-bands: a first coupling signal of the plurality of coupling signals is in the active state and has a coupling frequency so as to excite electrons of the transmission medium to a first predetermined Rydberg state such that an electromagnetic field at a first frequency in the first sub-band incident at the transmission medium causes a detectable change in the probe signal, and a second coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the second coupling signal comprises configuring a coupling frequency of the second coupling signal to that required to excite electrons of the transmission medium to a second predetermined Rydberg state such that an electromagnetic field at a second frequency incident at the transmission medium causes a detectable change in the probe signal, and, in a time period of a second sub-band of the sequence of sub-bands, the second sub-band succeeding the first sub-band and comprising the second frequency: the second coupling signal of the plurality of coupling signals is in the active state; and the Rydberg-atom based electromagnetic field receiver further comprising a processor configured to predict the second frequency of the second sub-band. A transition of the second coupling signal from the preparation state to the active state may be contemporaneous with a transition between the first sub-band and the second sub-band.

[0009] In the time period of the first sub-band: a third coupling signal of the plurality of coupling signals may be in the preparation state, wherein the transition from the inactive state to the active state of the third coupling signal may comprise configuring a coupling frequency of the third coupling signal to that required to excite electrons of the transmission medium to a third predetermined Rydberg state such that an electromagnetic field at a third frequency incident at the transmission medium causes a detectable change in the probe signal.

[0010] The processor may be further configured to predict the third frequency, wherein the second and third frequencies are both candidate frequencies of the second sub-band.

[0011] The or each prediction may be based on one or more of a group comprising: a relationship with the first frequency of the first sub-band, a characteristic of the wireless telecommunications network, a settling time of the candidate frequency of the second sub-band, and a frequency range of the candidate frequency of the second sub-band.

[0012] According to a second aspect of the invention, there is provided a system comprising: a transmitter; and a Rydberg-atom based electromagnetic field receiver of the first aspect of the invention, wherein: the transmitter is configured to transmit the electromagnetic field at the first frequency during the time period of the first sub-band and to transmit the electromagnetic field at the second frequency during the time period of the second subband.

[0013] According to a third aspect of the invention, there is provided a method of controlling a Rydberg-atom based electromagnetic field receiver of the first aspect of the invention, the method comprising the steps of: during the time period of the first sub-band: configuring the receiver such that the first coupling signal is in the active state, wherein the coupling frequency of the first coupling signal is configured so as to excite electrons of the transmission medium to the first predetermined Rydberg state such that an electromagnetic field at the first frequency in the first sub-band incident at the transmission medium causes a detectable change in the probe signal, and configuring the receiver such that the second coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the second coupling signal comprises configuring the coupling frequency of the second coupling signal to that required to excite electrons of the transmission medium to a second predetermined Rydberg state such that an electromagnetic field at the second frequency incident at the transmission medium causes a detectable change in the probe signal, and predicting the second frequency of the second sub-band; and during the timeperiod of the second sub-band: configuring the receiver such that the second coupling signal of the plurality of coupling signals is in the active state.

[0014] The method may further comprise the step of: during the time-period of the first subband: configuring the receiver such that a third coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the third coupling signal comprises configuring the coupling frequency of the third coupling signal to that required to excite electrons of the transmission medium to a third predetermined Rydberg state such that an electromagnetic field at a third frequency incident at the transmission medium causes a detectable change in the probe signal.

[0015] The method may further comprise the step of: predicting the third frequency, wherein the second and third frequencies are both candidate frequencies of the second sub-band.

[0016] According to a fourth aspect of the invention, there is provided a computer program comprising instructions which, when the program is executed by a Rydberg-atom based electromagnetic field receiver of the first aspect of the invention, cause the Rydbergatom based electromagnetic field receiver to carry out the steps of the method of the third aspect of the invention. The computer program may be stored on a computer readable carrier medium.

[0017] Brief Description of the Figures

[0018] 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:

[0019] Figure 1 is a schematic diagram of a first wireless telecommunications network; Figure 2 is a schematic diagram of a second wireless telecommunications network;

[0020] Figure 3 is a schematic diagram of a Rydberg-atom based RF receiver of the network of Figure 2;

[0021] Figure 4 is a first method of controlling a Rydberg-atom based RF receiver;

[0022] Figure 5 is a schematic diagram of a further Rydberg-atom based RF receiver;

[0023] Figure 6 is a schematic diagram of a further Rydberg-atom based RF receiver; and Figure 7 is a second method of controlling a Rydberg-atom based RF receiver.

[0024] Detailed Description

[0025] A first wireless telecommunications network 100 is illustrated in Figure 1. The first wireless telecommunications network 100 includes a transmitter 110 and a Rydbergatom based Radio Frequency (RF) receiver 120. The transmitter 110 is configured to transmit RF signals to the receiver 120, and the Rydberg-atom based RF receiver 120 is configured to receive the RF signals transmitted by the transmitter 110.

[0026] The Rydberg-atom based RF receiver 120 is configured to detect an RF signal at a particular frequency. This is achieved by exciting electrons of a transmission medium comprising Rydberg-atoms (such as Rubidium) to a predetermined Rydberg state using a probe optical signal and a coupling optical signal. The frequencies of the probe and coupling optical signals are selected so as to elevate electrons to the predetermined Rydberg state. In this state, in which the transmission medium experiences the Electromagnetically Induced Transparency (EIT) effect, an RF signal at the particular frequency elevates an electron in the predetermined Rydberg state to a further Rydberg state. This interaction of the RF signal at the particular frequency with the transmission medium in the predetermined Rydberg state may be detected in the probe optical signal.

[0027] A secure communication method is implemented in the first wireless telecommunications network 100 by an implementation of Frequency Hopping Spread Spectrum (FHSS). FHSS involves a frequency band (being a contiguous or non-contiguous frequency range of the electromagnetic spectrum) of communications between the transmitter 110 and Rydberg-atom based RF receiver 120 being divided into a plurality of sub-bands. The transmitter 1 10 and Rydberg-atom based RF receiver 120 are configured to communicate by switching (“hopping”) a carrier frequency of communications between these sub-bands. An order of switching implemented by the transmitter 110, often referred to as the frequency hopping code, may be communicated to the Rydberg-atom based RF receiver 120. The Rydberg-atom based RF receiver 120 may switch from a first sub-band to a second sub-band at the appropriate time, as indicated in the frequency hopping code, for continuous signal reception. The switch from the first sub-band to the second sub-band may be implemented by the Rydberg-atom based RF receiver 120 by switching from a first predetermined Rydberg state (such that an RF signal in the first sub-band excites electrons from the first predetermined Rydberg state such that the RF signal in the first sub-band may be detected) to a second predetermined Rydberg state (such that an RF signal in the second sub-band excites electrons from the second predetermined Rydberg state such that the RF signal in the second sub-band may be detected). The switch from the first predetermined Rydberg state to the second predetermined Rydberg state may be implemented by, for example, changing the frequency of the probe and / or coupling optical signals.

[0028] It is beneficial to implement FHSS in a wireless telecommunications network comprising a Rydberg-atom based RF receiver. FHSS is a secure communication method that enjoys interference-tolerance, anti-jamming and detection avoidance by virtue of switching between different sub-bands of the frequency band. Rydberg-atom based RF receivers enjoy a much wider frequency band for signal reception (relative to conventional RF receivers based on dipole antennas) without any change in device dimensions or topology. The frequency band for implementing FHSS may therefore be much wider when using a Rydberg-atom based RF receiver, such that many more subbands may be used (of equivalent sub-band width), which enhances the interferencetolerance, anti-jamming and detection avoidance capabilities of FHSS.

[0029] However, there is a problem with the first wireless telecommunications network 100 implementing FHSS in that there is a time period between a frequency hop to a new subband occurring and the Rydberg-atom based RF receiver 120 satisfactorily receiving communications transmitted in the new sub-band. During this time-period, the Rydbergatom based RF receiver 120 reconfigures the probe and / or coupling signals such that electrons of the transmission medium are excited to a new predetermined Rydberg state (corresponding with the new sub-band). This reconfiguration of the probe and / or coupling optical signals therefore comprises a settling time during which various control parameters, such as diode temperature, diode current, cavity piezo alignment, must be set, achieved and maintained. During this settling time, the excitation of electrons to the new predetermined Rydberg state may be sub-optimal, corresponding with a sub-optimal reception of signals in the new sub-band (e.g. failure or significant erroring at the data transport layer). The following second wireless telecommunications network 200 may alleviate some or all of these problems.

[0030] A second wireless telecommunications network 200 will now be described with reference to Figure 2. The wireless telecommunications network 200 comprises a transmitter 210 and a Rydberg-atom based RF receiver 220. The transmitter 210 is configured to transmit RF signals to the Rydberg-atom based RF receiver 220, and the Rydberg-atom based RF receiver 220 is configured to receive the RF signals transmitted by the transmitter 210.

[0031] The Rydberg-atom based RF receiver 220 is shown in more detail in Figure 3. The Rydberg-atom based RF receiver 220 includes a probe laser 221 , a transmission medium 223, a first coupling laser 225, a second coupling laser 227, an optical switch 228 and a photodetector 229. In this example, the transmission medium 223 is a vapour cell containing Rubidium-85 atoms. The probe laser 221 transmits a probe optical signal through the transmission medium 223 to the photodetector 229. The transmission medium may be further excited by a first or second coupling signal, transmitted by the first and second coupling laser 225, 227 respectively, which is selectively directed to the transmission medium by the optical switch 228. The first or second coupling signal overlaps and counter-propagates the probe signal within the transmission medium. If excited by the first coupling optical signal, then the combination of the probe optical signal and first coupling optical signal excites the transmission medium to a first predetermined Rydberg state. If excited by the second coupling optical signal, then the combination of the probe optical signal and second coupling optical signal excites the transmission medium to a second predetermined Rydberg state. The first and second predetermined Rydberg states are non-adjacent Rydberg states. Put another way, a difference between a principal quantum number of the first predetermined Rydberg state and a principal quantum number of the second predetermined Rydberg is greater than one.

[0032] The probe laser 221 , first coupling laser 225 and second coupling laser 227 of the Rydberg-atom based RF receiver 220 may be in an inactive state or an active state. The first and second coupling lasers 225, 227 have an additional preparation state. These three states may be characterised as: • An inactive state in which the respective laser does not excite electrons of the transmission medium, such as by being powered down and / or having zero or negligible transmission power;

[0033] • An active state in which the respective laser excites electrons of the transmission medium, such as by generating its respective optical signal and (for the first and second coupling lasers 225, 227) the optical switch 228 permitting passage of the respective optical signal to the transmission medium 223; and

[0034] • A preparation state, in which the respective laser transitions from the inactive state to the active state. This transition may include one or more of: the first / second coupling laser 225, 227 powering on, the first / second coupling laser 225, 227 setting their control parameters (such as frequency, diode temperature, diode current, cavity piezo alignment), the first / second coupling laser 225, 227 achieving and maintaining an optical signal consistent with these control parameters, and triggering a switch of the optical switch 228 between the first and second coupling lasers 225 / 227.

[0035] The Rydberg-atom based RF receiver 220 is controllable (by a controller, not shown) such that one of the first or second coupling lasers 225, 227 is in an active state and the other of the first and second coupling lasers 225, 227 is in a preparation state at a given point in time.

[0036] The benefits of the Rydberg-atom based RF receiver 220 of the second wireless telecommunications network 200 when implementing FHSS will become clear through the following description of a method, as illustrated in Figure 4.

[0037] Prior to performance of the method of Figure 4, the probe laser 221 , first coupling laser 225 and second coupling laser 227 are all in an inactive state. In a first step (S101 ) of the method of Figure 4, the transmitter 210 and Rydberg-atom based RF receiver 220 both obtain data indicating a frequency hopping code to be used in communications between the transmitter 210 and Rydberg-atom based RF receiver 220.

[0038] In a second step (S103), the transmitter 210 and Rydberg-atom based RF receiver 220 synchronise to a common timing protocol (such as a Network Timing Protocol (NTP)). On completion of steps S101 and S103, both the transmitter 210 and Rydberg-atom based RF receiver 220 store data indicating the order of sub-bands which will be used to communicate signals from the transmitter 210 to the Rydberg-atom based RF receiver 220 and further indicating the time of switching to each sub-band.

[0039] In a third step (S105), the Rydberg-atom based RF receiver 220 is initialised and configured for receiving a signal from the transmitter 210 in the first sub-band of the frequency hopping code. The Rydberg-atom based RF receiver 220 therefore activates both the probe laser 221 and first coupling laser 225 and configures the optical switch 228 to permit passage of the first coupling laser 225 to the transmission medium 223, so as to excite electrons of the transmission medium 223 to a first predetermined Rydberg state (whereby an energy difference between the first predetermined Rydberg state and a further Rydberg state corresponds with a frequency of the signal from the transmitter 210 in the first sub-band of the frequency hopping code). This initial configuration is complete prior to the start of the first sub-band of the frequency hopping code such that the Rydberg-atom based RF receiver 220 may successfully receive all signals transmitted in the first sub-band of the frequency hopping code.

[0040] During the time period of the first sub-band, the first coupling laser 225 may be referred to as the active coupling laser (as it is in the active state) and the second coupling laser 227 may be referred to as the reserved coupling laser (as it is not in the active state).

[0041] The method then enters an iterative loop initially comprising a fourth step (S107) that is performed during a time period of the current sub-band (the first sub-band of this first iteration) and before the start of the next sub-band (the second sub-band in this first iteration). In this fourth step, S107, the Rydberg-atom based RF receiver 220 prepares the reserved coupling laser (the second coupling laser 227 in this first iteration) for reception of signals transmitted in the next sub-band of the frequency hopping code. As noted above, the second coupling laser 227 is initially in an inactive state. Therefore, during this fourth step S107, the second coupling laser 227 switches to the preparation state in which it transitions from the inactive state up to (but not including) the active state. As noted above, the preparation state may also include one or more of: the second coupling laser 227 powering on, the second coupling laser 227 setting its control parameters (such as diode temperature, diode current, cavity piezo alignment, and may also comprise setting the coupling frequency of the second coupling laser 227 so as to excite electrons of the transmission medium to a predetermined Rydberg state so as to detect RF signals in the next sub-band of the frequency hopping code), the second coupling laser 227 achieving and maintaining an optical signal consistent with these control parameters, and triggering a switch of the optical switch 228 from the first coupling laser 225 to the second coupling lasers 227.

[0042] A fifth step (S109) is performed at the transition between the current sub-band and next sub-band of the frequency hopping code. In the fifth step, S109, the Rydberg-atom based RF receiver 220 completes the transition of the reserved coupling laser (the second coupling laser 227 in this first iteration) to the active state such that the reserved coupling laser is in the active state by the start of the second sub-band. In this example, this transition involves at least the completion of the switch of the optical switch 228 from the active coupling laser (the first coupling laser 225 in this first iteration) to the reserved coupling laser (the second coupling laser 227 in this first iteration). Accordingly, by the start of the second sub-band, the first coupling laser 225 is no longer in the active state (and may therefore be in either the inactive state or the preparation state) and therefore no longer excites electrons of the transmission medium 223, and the second coupling laser 227 is in the active state and therefore excites electrons of the transmission medium 223. The combination of the probe laser 221 and second coupling laser 227 excites electrons of the transmission medium 223 to a second predetermined Rydberg state (whereby an energy difference between the second predetermined Rydberg state and a further Rydberg state corresponds with a frequency of the signal from the transmitter 210 in the second sub-band of the frequency hopping code). A signal transmitted by the transmitter 210 in the second sub-band may therefore be successfully received by the Rydberg-atom based RF receiver 220.

[0043] During the time period of the second sub-band, the first coupling laser 225 may be referred to as the reserved coupling laser and the second coupling laser 227 may be referred to as the active coupling laser.

[0044] In step S1 11 , the Rydberg-atom based RF receiver 220 determines whether there is a subsequent sub-band to the current sub-band in the frequency hopping code. If not, then the Rydberg-atom based RF receiver 220 may terminate reception at the end of the current sub-band. If there are one or more subsequent sub-bands to the current subband, then the Rydberg-atom based RF receiver 220 loops back to step S107 so as to prepare the reserved coupling laser (which may currently be in an inactive state or a preparation state), during the current sub-band and before the start of the next sub-band, for reception of signals transmitted in the next sub-band of the frequency hopping code.

[0045] The Rydberg-atom based RF receiver 220 of the second wireless telecommunications network 200 enjoys benefits over the Rydberg-atom based RF receiver 120 of the first wireless telecommunications network 100. In particular, by including a reserved coupling laser that is at least partially prepared for receiving signals in the next sub-band of the frequency hopping code, the Rydberg-atom based RF receiver 220 of the second wireless telecommunications network 200 may start receiving signals in the next subband at (or relatively closer to) the point of transition to the next sub-band compared to the Rydberg-atom based RF receiver 120 of the first wireless telecommunications network 100. The Rydberg-atom based RF receiver 220 of the second wireless telecommunications network 200 may therefore successfully detect and receive signals at an earlier time relative to the Rydberg-atom based RF receiver 120 of the first wireless telecommunications network 100 or, put another way, communications between the transmitter 210 and Rydberg-atom based RF receiver 220 may use a relatively short guard interval between sub-bands of the frequency hopping code.

[0046] Furthermore, the preparation phase may be completed over a much larger time period relative to the Rydberg-atom based RF receiver 120 of the first wireless telecommunications network 100 that must switch between the first and second predetermined Rydberg state as quickly as possible at the point of transition between sub-bands. The first and second coupling lasers 225, 227 of the Rydberg-atom based RF receiver 220 are therefore at a much lower risk of thermal and / or current shock relative to the coupling laser of the Rydberg-atom based RF receiver 120, which may increase their operational lifespan.

[0047] As noted above, the first and second predetermined Rydberg states are non-adjacent Rydberg states. Therefore, the Rydberg-atom based RF receiver 220 reconfigures between a first state for reception of signals in a first sub-band of the frequency hopping code and a second state for reception of signals in a second sub-band of the frequency hopping code when those states correspond with non-adjacent Rydberg states (that is, the difference between the principal quantum number of the Rydberg-atoms in the first state and the principal quantum number of the Rydberg-atoms in the second state is greater than one). This enables the Rydberg-atom based RF receiver 220 to hop between any two sub-bands (i.e. frequencies) of the frequency-hopping code.

[0048] A first example of a Rydberg-atom based RF receiver for implementing the method of Figure 4 is shown in Figure 5. The Rydberg-atom based RF receiver 320 includes a probe laser 321 , a probe laser controller 322, a transmission medium 323, a plurality of coupling lasers 325, a plurality of coupling laser controllers 326, an optical switch 328, and a photodiode 329. The plurality of coupling lasers 325 includes an active coupling laser and one or more reserved coupling lasers (such that the plurality of coupling lasers includes at least two coupling lasers). The benefits of having more than one reserved coupling laser are discussed below. The probe laser controller 322 and plurality of coupling laser controllers 326 respectively control the probe laser 321 and plurality of coupling lasers 325, such as by setting their respective frequencies, stabilising their respective optical signals, and setting their respective control parameters. The optical switch 328 is configured to selectively permit passage of a coupling optical signal from the active coupling laser of the plurality of coupling lasers 325 to the transmission medium 323.

[0049] A second example of a Rydberg-atom based RF receiver for implementing the method of Figure 4 is shown in Figure 6. The Rydberg-atom based RF receiver 420 includes a probe laser (not shown) producing a probe optical signal that is split into a plurality of probe optical signals having distinct physical paths (that is, a distinct transmission medium, such as a vapour cell or hollow core fibre). The Rydberg-atom based RF receiver 420 also includes a plurality of coupling lasers (not shown) which are each configured for producing a coupling optical signal that overlaps with a particular probe optical signal of the plurality of probe optical signals. The plurality of coupling lasers include an active coupling laser and one or more reserved coupling lasers. The probe optical signals are directed (e.g. by a plurality of mirrors 421 ) to a photodiode 423.

[0050] As noted above, the Rydberg-atom based RF receiver 220, 320, 420 may comprise a plurality of reserved coupling lasers. The active coupling laser of the plurality of coupling lasers 325 elevates electrons (in combination with the probe laser) to a first predetermined Rydberg state and each reserved coupling laser of the plurality of coupling lasers 325 elevates electrons (in combination with the probe laser) to a respective Rydberg state. Each respective Rydberg state is non-adjacent the first predetermined Rydberg state. That is, the difference between the principal quantum number of the Rydberg-atoms in the first predetermined Rydberg state and the principal quantum number of the Rydberg-atoms in each respective Rydberg state is greater than one. One benefit of implementing a plurality of reserved coupling lasers is that, whilst the active coupling laser is configured to receive signals in the current (n-th) sub-band, a first reserved laser may be in a preparation state so as to prepare for reception of signals in the next (n+1) sub-band, and a second reserved laser may be in a preparation state so as to prepare for reception of signals in a subsequent sub-band (n+2) to the next (n+1 ) sub-band. This is beneficial when the time-period of the next (n+1 ) sub-band is shorter than a desired preparation time of a reserved coupling laser. The desired preparation time may accommodate a relatively slow preparation of the reserved coupling laser such that it is at a lower risk of thermal and / or current shock.

[0051] A further benefit of implementing a plurality of reserved coupling lasers is that the Rydberg-atom based RF receiver 220, 320, 420 has a greater chance of successfully decoding an RF signal in the next sub-band when the frequency hopping code is not known. That is, in some scenarios, the Rydberg-atom based RF receiver 220, 320, 420 may have no knowledge or only partial knowledge of the frequency hopping code. This may occur due to the frequency hopping code not being communicated to the Rydbergatom based RF receiver 220, 320, 420 or in part communicated to the Rydberg-atom based RF receiver 220, 320, 420. In these scenarios, the Rydberg-atom based RF receiver 220, 320, 420 may implement the following techniques to improve a likelihood of successfully receiving an RF signal in a sub-band of the frequency hopping code.

[0052] A first scenario will be described for a Rydberg-atom based RF receiver 220, 320, 420 having two coupling lasers, such that when receiving a signal in a current sub-band the Rydberg-atom based RF receiver has one active coupling laser and one reserved coupling laser. This scenario will initially be described so as to illustrate the benefits of a Rydberg-atom based RF receiver 220, 320, 420 having multiple reserved coupling lasers, described later in this description. When the next sub-band of the frequency hopping code is not known, then the Rydberg-atom based RF receiver may, during the current sub-band and before the start of the next sub-band, predict a next sub-band and enter the preparation state so as to be configured to receive RF signals in that predicted next sub-band. The predicted next sub-band may be determined using one or more of the following techniques. In a first technique, the Rydberg-atom based RF receiver 220, 320, 420 predicts the next sub-band based on a relationship with the current sub-band and / or one or more previous sub-bands. For example, the Rydberg-atom based RF receiver 220, 320, 420 may store a relationship indicating that the next sub-band will be plus or minus n sub-bands relative to the current sub-band, in which n is an integer and each sub-band is identified by an integer identifier.

[0053] In a second technique, which is applicable when the Rydberg-atom based RF receiver 220, 320, 420 does not store a relationship as described above in the first technique, the Rydberg-atom based RF receiver 220, 320, 420 predicts the next sub-band by calculating a probability for one or more candidate next sub-bands, and the predicted next sub-band is the candidate sub-band with the highest probability. These probabilities may be calculated as function of one or more of the following:

[0054] • A preference of the Rydberg-atom based RF receiver 220, 320, 420 for a particular sub-band, potentially reflecting the characteristics of the network (or any network node in the network), such as communication or physical characteristics of the network. For example, if the network is operating according to a particular communications protocol, then sub-bands within the frequency range(s) of that protocol are given relatively high priority to sub-bands outside that(those) frequency range(s);

[0055] • The settling time of each candidate next sub-band. That is, there may be different settling times for the reserved coupling laser to reconfigure from its current configuration in the current sub-band (which may be the configuration when it was last in its active state) to each candidate next sub-band. Therefore, the probability of each candidate next sub-band may be a function of its settling time for the current configuration of the reserved coupling laser. This function may be based on an assumption that the frequency hopping code would be more likely to transition between configurations that have a lower settling time, so the probability of each candidate next sub-band may be inversely proportional to the settling time from the current configuration of the reserved coupling laser to that candidate next sub-band. This probability may also be based on the different settling times for the reserved coupling laser to reconfigure from the candidate next sub-band to a candidate subsequent sub-band (that is, the sub-band after the next sub-band), such that the next sub-band is one that launches the reserved coupling laser to the most likely subsequent sub-band; and

[0056] • The number of frequencies of the candidate next sub-band, such that a candidate sub-band covering more frequencies will have a greater probability than a candidate sub-band covering fewer frequencies.

[0057] The second technique may be implemented by defining a two-dimensional matrix, in which a first dimension of the matrix represents a current sub-band of the Rydberg-atom based RF receiver 220, 320, 420 and a second dimension of the matrix represents each candidate next sub-band, with each matrix element representing a corresponding probability. The matrix may be populated with initial values for each probability (e.g. based on an initial calculation by the operator) and subsequently updated as more data is available to determine the probabilities. This data may be specific to the Rydbergatom based RF receiver 220, 320, 420, and / or relating to any other Rydberg-atom based RF receiver 220, 320, 420 in the network. Accordingly, a device-specific matrix may be determined and stored at the / each Rydberg-atom based RF receiver 220, 320, 420, which may then be used live by the / each Rydberg-atom based RF receiver 220, 320, 420. Alternatively, a network-wide matrix may be determined and communicated to the / each Rydberg-atom based RF receiver 220, 320, 420, which may then perform local optimisation before live use.

[0058] The calculation of the probabilities may also be determined or at least assisted by machine learning techniques. Training data for the calculation of probabilities may therefore be obtained from the / each Rydberg-atom based RF receiver 220, 320, 420 in the network indicating historical sequences of sub-bands. The machine learning techniques may employ genetic algorithms to rapidly mutate, appraise and select new matrices for assessment against the historical training data. Furthermore, a neural network algorithm may be employed to optimise the matrix as a function of binary cross entropy (in the event the Rydberg-atom based RF receiver 220, 320, 420 uses one reserved coupling laser) or categorical cross entropy (in the event the Rydberg-atom based RF receiver 220, 320, 420 uses multiple reserved coupling lasers).

[0059] The Rydberg-atom based RF receiver 220, 320, 420 having a single reserved coupling laser may therefore address the problem of no / partial knowledge of the frequency hopping code by configuring the reserved coupling laser in the preparation state for the predicted next sub-band. In doing so, it is more likely that the Rydberg-atom based RF receiver 220, 320, 420 will successfully decode signals received in the next sub-band.

[0060] As noted above, the Rydberg-atom based RF receiver 220, 320, 420 may include a plurality of reserved coupling lasers. This further increases the likelihood that the Rydberg-atom based RF receiver 220, 320, 420 will successfully decode signals received in the next sub-band as each reserved coupling laser of the plurality of reserved coupling lasers may be in a preparation state for a distinct candidate next sub-band of the plurality of candidate next sub-bands (e.g. in order of probability). That is, if the Rydberg-atom based RF receiver 220, 320, 420 has n reserved coupling lasers, then each reserved coupling laser may be configured in the preparation state for receiving one of the n candidate sub-bands having the highest respective probabilities.

[0061] The probability of each candidate sub-band when using a plurality of reserved coupling lasers may be calculated as described above for the case of a single reserved coupling laser. However, the probabilities may also be based on the capability, count and configuration of the plurality of reserved coupling lasers such that the plurality of reserved coupling lasers operate co-operatively to cover the maximum number of frequencies. Furthermore, the probability matrix described above may be determined as a three- dimensional matrix, in which the third dimension represents the number of reserved coupling lasers.

[0062] In the above examples, the Rydberg-atom based RF receiver may use counterpropagating probe optical signals and coupling optical signals such that the Doppler shift effect can be ignored. However, this is non-essential and other techniques to mitigate Doppler shift may be implemented (such as by cooling the apparatus).

[0063] It is also non-essential that the detector is configured to detect electromagnetic fields in the RF band of the electromagnetic spectrum. That is, the receiver may be configured to receive signals in any part of the electromagnetic spectrum by suitable configuration of the probe laser, coupling lasers and / or transmission medium (e.g. by using an atomic medium with particular energy states and / or by selecting appropriate probe and coupling frequencies). Furthermore, the EIT effect may be realised by any suitable excitation scheme, such as ladder, Vee and lambda. The Rydberg-atom based EM receiver may also be configured to receive data modulated in any form, such as amplitude, frequency and phase.

[0064] As shown in Figure 7, a method of controlling a Rydberg-atom based electromagnetic field receiver may be defined as: during a time period of a first sub-band: in step S201 , configuring the receiver such that the first coupling signal is in the active state, wherein the coupling frequency of the first coupling signal is configured so as to excite electrons of the transmission medium to the first predetermined Rydberg state such that an electromagnetic field at a first frequency in the first sub-band incident at the transmission medium causes a detectable change in the probe signal, and, in step S203, configuring the receiver such that the second coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the second coupling signal comprises configuring the coupling frequency of the second coupling signal to that required to excite electrons of the transmission medium to a second predetermined Rydberg state such that an electromagnetic field at a second frequency incident at the transmission medium causes a detectable change in the probe signal; and during the time-period of the second sub-band: in step S205, configuring the receiver such that the second coupling signal of the plurality of coupling signals is in the active state.

[0065] The skilled person will understand that any combination of features is possible within the scope of the invention, as claimed.

Claims

CLAIMS1 . A Rydberg-atom based electromagnetic field receiver for a wireless telecommunications network, the wireless telecommunications network comprising a transmitter configured to transmit an electromagnetic field in a sequence of sub-bands of a frequency band, the Rydberg-atom based electromagnetic field receiver comprising: one or more transmission media; one or more probe signals, each probe signal having a probe frequency and being configured to pass through a transmission medium of the one or more transmission media, wherein the probe frequency is configured to excite electrons of the respective transmission medium from a first state to a second state; a plurality of coupling signals, each coupling signal of the plurality of coupling signals having a preparation state and an active state, wherein: in the active state, each coupling signal is configured to pass through a transmission medium of the one or more transmission media and overlap a probe signal of the one or more probe signals so as to induce an Electromagnetically Induced Transparency, EIT, effect in the transmission medium, and in the preparation state, each coupling signal transitions from an inactive state to the active state; and a controller configured to control the receiver such that, in a time period of a first sub-band of the sequence of sub-bands: a first coupling signal of the plurality of coupling signals is in the active state and has a coupling frequency so as to excite electrons of the transmission medium to a first predetermined Rydberg state such that an electromagnetic field at a first frequency in the first sub-band incident at the transmission medium causes a detectable change in the probe signal, and a second coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the second coupling signal comprises configuring a coupling frequency of the second coupling signal to that required to excite electrons of the transmission medium to a second predetermined Rydberg state such that an electromagnetic field at a second frequency incident at the transmission medium causes a detectable change in the probe signal,and, in a time period of a second sub-band of the sequence of subbands, the second sub-band succeeding the first sub-band and comprising the second frequency: the second coupling signal of the plurality of coupling signals is in the active state; and the Rydberg-atom based electromagnetic field receiver further comprising a processor configured to predict the second frequency of the second sub-band.

2. A Rydberg-atom based electromagnetic field receiver as claimed in Claim 1 , wherein a transition of the second coupling signal from the preparation state to the active state is contemporaneous with a transition between the first sub-band and the second sub-band.

3. A Rydberg-atom based electromagnetic field receiver as claimed in any one of the preceding claims, wherein, in the time period of the first sub-band: a third coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the third coupling signal comprises configuring a coupling frequency of the third coupling signal to that required to excite electrons of the transmission medium to a third predetermined Rydberg state such that an electromagnetic field at a third frequency incident at the transmission medium causes a detectable change in the probe signal.

4. A Rydberg-atom based electromagnetic field receiver as claimed in Claim 3, wherein the processor is further configured to predict the third frequency, wherein the second and third frequencies are both candidate frequencies of the second sub-band.

5. A Rydberg-atom based electromagnetic field receiver as claimed in any one of the preceding claims, wherein the or each prediction is based on one or more of a group comprising: a relationship with the first frequency of the first sub-band, a characteristic of the wireless telecommunications network, a settling time of the candidate frequency of the second sub-band, and a frequency range of the candidate frequency of the second sub-band.

6. A system comprising:a transmitter; and a Rydberg-atom based electromagnetic field receiver as claimed in any one of the preceding claims, wherein: the transmitter is configured to transmit the electromagnetic field at the first frequency during the time period of the first sub-band and to transmit the electromagnetic field at the second frequency during the time period of the second subband.

7. A method of controlling a Rydberg-atom based electromagnetic field receiver as claimed in any one of Claims 1 to 5, the method comprising the steps of: during the time period of the first sub-band: configuring the receiver such that the first coupling signal is in the active state, wherein the coupling frequency of the first coupling signal is configured so as to excite electrons of the transmission medium to the first predetermined Rydberg state such that an electromagnetic field at the first frequency in the first sub-band incident at the transmission medium causes a detectable change in the probe signal, configuring the receiver such that the second coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the second coupling signal comprises configuring the coupling frequency of the second coupling signal to that required to excite electrons of the transmission medium to a second predetermined Rydberg state such that an electromagnetic field at the second frequency incident at the transmission medium causes a detectable change in the probe signal, and predicting the second frequency of the second sub-band; and during the time-period of the second sub-band: configuring the receiver such that the second coupling signal of the plurality of coupling signals is in the active state.

8. A method as claimed in Claim 7, further comprising the step of: during the time-period of the first sub-band: configuring the receiver such that a third coupling signal of the plurality of coupling signals is in the preparation state, wherein the transition from the inactive state to the active state of the third coupling signal comprisesconfiguring the coupling frequency of the third coupling signal to that required to excite electrons of the transmission medium to a third predetermined Rydberg state such that an electromagnetic field at a third frequency incident at the transmission medium causes a detectable change in the probe signal.

9. A method as claimed in Claim 8, further comprising the step of: predicting the third frequency, wherein the second and third frequencies are both candidate frequencies of the second sub-band.

10. A computer program comprising instructions which, when the program is executed by a Rydberg-atom based electromagnetic field receiver as claimed in any one of Claims 1 to 5, cause the Rydberg-atom based electromagnetic field receiver to carry out the steps of any one of Claims 7 to 9.

11. A computer readable carrier medium comprising the computer program of Claim10.

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