Patent ID: 12248011

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure provide a Rydberg-atom based electromagnetic field receiver in which overlapping probe and coupling lasers interact with a set of Rubidium-85 atoms only after a particular time period has elapsed since that set of Rubidium-85 atoms' preceding interaction with the overlapping probe and coupling lasers. It is shown, below, that such a receiver is able to respond to a change in state (that is, from a first state in which an electromagnetic signal is incident at the receiver to a second state in which there is no electromagnetic signal incident at the receiver, or vice versa) more quickly than prior art Rydberg-atom based electromagnetic receivers, thereby allowing higher data rate communications.

A first embodiment of a wireless telecommunications network100of the present disclosure will now be described. As shown inFIG.2, the wireless telecommunications network100includes a first wireless transmitter110and a Radio-Frequency (RF) receiver120. The first wireless transmitter110is configured to transmit wireless signals at a frequency of 2.8 GHz.FIG.3illustrates the RF receiver120in more detail. The RF receiver120is a Rydberg-atom based RF receiver and includes a vapor cell121, a first dichroic mirror122a, a second dichroic mirror122b, a third dichroic mirror122c, a probe laser123, a coupling laser125, a rotating mirror126, a photodetector127, a set of mirrors128, a first lens129aand a second lens129b. The vapor cell121contains a vapor of alkali metal (in this embodiment, Rubidium-85).

In this embodiment, the probe laser123produces a probe signal (illustrated as a solid arrowed line) that is reflected off the first dichroic mirror122a, the rotating mirror126, and a mirror of the set of mirrors128. The probe signal is then focused by the first lens129aso as to pass through the vapor cell121. Following passage of the vapor cell121, the probe signal is passes through the second lens129b, is reflected off the third dichroic mirror122cand enters the photodetector127. The coupling laser125produces a coupling signal (illustrated as a dashed arrowed line) that is reflected off the second dichroic mirror122b, the rotating mirror126, and several mirrors of the set of mirrors128. The coupling signal is then focused by the second lens129bso as to pass though the vapor cell121. The probe signal and coupling signal are overlapping and counter-propagating through the vapor cell121. The coupling signal may terminate at a filter (not shown) following passage of the vapor cell121.

The RF receiver120also has a controller124for controlling a frequency of the probe signal (i.e. to adjust and/or stabilize the frequency of the probe signal), for controlling a frequency of the coupling signal (i.e. to adjust and/or stabilize the frequency of the coupling signal), for controlling an intensity of the probe laser123(i.e. the transmission power and/or cross-sectional area of the probe laser123), and for controlling an intensity of the coupling laser125(i.e. the transmission power and/or cross-sectional area of the coupling laser125).

The RF receiver120may be configured so as to detect a wireless signal incident upon the vapor cell121. This is achieved by configuring the frequency of the probe signal to correlate with the transition of an electron of a Rubidium-85 atom from a ground state to a first excited state, and by configuring the frequency of the coupling signal to correlate with the transition of an electron of a Rubidium-85 atom from the first excited state to a predetermined Rydberg state. The predetermined Rydberg state is selected (based on the specific frequencies of the probe and coupling signals) so that a wireless signal of a particular frequency incident upon the vapor cell121excites electrons from the predetermined Rydberg state to another Rydberg state, causing a detectable change in the Electromagnetically Induced Transparency (EIT) signal at the photodetector127. In this example, the controller124configures the probe laser123and coupling laser125so that the vapor cell121detects wireless signals at a frequency of 2.8 GHz. Thus, in this example, the probe signal and coupling signal are selected so as to excite electrons to a first predetermined Rydberg state having the 90thprincipal quantum number energy state (i.e. the probe signal has a wavelength of 780 nm and the coupling signal has a wavelength of 479.380 nm) such that wireless signals transmitted by the wireless transmitter110at 2.8 GHz that pass through the vapor cell121will excite electrons from the predetermined Rydberg state to another Rydberg state, causing an EIT signal in the monitored probe signal at the photodetector127.

The EIT signal at the photodetector127may be processed to demodulate data from the incoming signal. That is, a change in the EIT signal from a calibrated baseline above a certain threshold may be considered a first bit (e.g. a bit of value ‘1’), whereas no change in the EIT signal from the calibrated baseline (or a change less than the threshold) may be considered a second bit (e.g. a bit of value ‘0’). The data rate of the RF detector127is thus limited based on the response time for the EIT signal to change between these states. This response time is discussed in more detail below.

In this embodiment, the rotating mirror126is configured as a regular dodecagon that rotates about its central axis. As the rotating mirror126rotates, the angle of reflection of the probe signal and the angle of reflection of the coupling signal changes. The rotating mirror126is configured (together with the first and second dichroic mirrors122a,122b, the set of mirrors128and the first and second lens129a,129b) such that, as these angles of reflection change, the probe signal and coupling signal pass through a different region of the vapor cell121(and remain overlapping and counter-propagating). In other words, the interaction region (in which the overlapping probe and coupling signals interact with the Rubidium-85 atoms) changes as the rotating mirror126rotates. Furthermore, these components of the RF detector120are configured so that, as the point of incidence of the probe signal and coupling signal moves from one vertex of the rotating mirror126to an adjacent vertex of the rotating mirror126, the interaction region of the vapor cell121changes from a first side of the vapor cell121to an opposing side of the vapor cell121. Subsequently, as the rotating mirror126rotates further such that the probe signal and coupling signal are incident on the adjacent edge of the rotating mirror126, then the interaction region of the vapor cell121switches back to the first side of the vapor cell121.

The RF detector120of this first embodiment therefore differs from prior art Rydberg-atom based RF detectors in that, in the prior art, only a single region of the vapor cell interacted with the probe and coupling lasers whilst all other regions of the vapor cell did not interact with the probe and coupling lasers. Therefore, substantially the same set of Rubidium-85 atoms (being those in that single region of the vapor cell) interacted with the probe and coupling lasers. In this first embodiment, each region of the vapor cell121has two conditions, 1) a non-interacting condition in which the probe and coupling signals do not pass through that region, and 2) an interacting condition in which the probe and coupling signals do pass through that region and interact with the Rubidium-85 atoms that are within that region. In this embodiment, as the overlapping probe and coupling lasers continually move through the vapor cell121(from a first side to the opposing side) and move back to the first side, then each region iterates through being in the non-interacting condition and in the interacting condition. There is a time period between each region being in an interacting condition of a first iteration and being in an interacting condition in a second iteration (equal to the time that region is in the intervening non-interacting condition). This time period allows electrons of the Rubidium-85 atoms in that region to decay to the ground state (that is, the state of these electrons of the Rubidium-85 atoms when in the non-interacting condition), before being re-excited again (once they enter the interacting condition in the subsequent second iteration and interact again with the probe and coupling signals). It is shown, below, that this arrangement reduces the response time of the EIT signal compared to the prior art Rydberg-atom based RF detector.

A first embodiment of a method of the present disclosure will now be described with reference toFIG.4. This first embodiment relates to the RF detector120of the first embodiment, described above. In S101, a coupling laser125power value is configured so that Ωc2(in which Ωcis the Rabi frequency of the coupling signal, which is the product of the E-field amplitude and the transition dipole moment divided by the reduced Planck constant) is less than Γ1Γ2(in which Γ1and Γ2are the transition rates out of the first excited state and predetermined Rydberg state respectively). In S103, the RF detector120is operated so that the probe and coupling signals successively pass through regions of the vapor cell121, wherein successive passages of the probe and coupling signals through a single region of the vapor cell121only occur after a time period equal to or greater than the time period for the Rubidium-85 atoms in that region to return to the ground state. In S105, data is demodulated from the EIT signal at the photodetector127. This data may then be processed (locally or by another entity).

The following description is a theoretical analysis of the RF detector120of the first embodiment to facilitate understanding of its technical benefit of achieving faster EIT signal response times (and thus greater data rates) compared to the conventional Rydberg-atom based RF receiver. This theoretical analysis assumes a ladder-configuration for realizing the EIT effect, but the skilled person will understand that other configurations (such as Vee and Lambda) are equally applicable. This theoretical analysis also assumes that an intensity of the probe laser123is relatively weak (compared to the intensity of the coupling laser125), such that the absorbance of the probe signal at the photodetector127is not a function of the intensity of the probe laser123. The absorbance, A, of the atomic medium of Rubidium-85 atoms in the vapor cell121is described by the imaginary part of the complex electric susceptibility as
A=(2N|12|2/ε0h̆)(Γ2Γ3+Ωr2)/[Γ1(Γ2Γ3+Ωr2)+Γ3Ωc2]  (1)
In which:N is the number of atoms engaged in the interaction,|12| is the transition dipole moment of the first transition (between the ground state and the first excited state),ε0is the electric permittivity of free space,h̆ is the reduced Planck constant,Γ1, Γ2and Γ3are the transition rates out of the first excited state, the predetermined Rydberg state, and the adjacent Rydberg state respectively, andΩrand Ωcare the Rabi frequencies of the RF and coupling signals, which is the product of the E-field amplitude and the relevant transition dipole moment divided by the reduced Planck constant.

This steady-state response can be theoretically reproduced using a mechanical model of three coupled masses connected by springs, where the first mass has the charge of an electron and is subject to a driving force due to the probe laser of amplitude Epand angular frequency ωp. This mechanical model can be described by the following coupled differential equations:
(d2x1/dt2)+Γ1(dx1/dt)+ωp2x1−ωpΩc2x2=−(2Epω|12|2/eh̆)exp(iωpt)  (2)
(d2x2/dt2)+Γ2(dx2/dt)−ωpx1+ωp2x2−=0  (3)
ωpΩr2x3(d2x3/dt2)+Γ3(dx3/dt)−ωpx2+ωp2x3=0  (4)
In which:Subscripts 1, 2 and 3 represent a particular mass,xiis a displacement of mass i from its equilibrium position, andωpis the angular frequency of probe laser,
Assuming all three masses move as xi=Xiexp(iωt), in which Xiis the maximum displacement of mass i, then equations 2, 3 and 4 can be solved for X1as:
X1=i(2Ep|12|2/eh̆)(Γ2Γ3+Ωr2)/{Γ1Γ2Γ3+Ωr2)X1+Γ3Ωc2}  (5)
Electric susceptibility, χ, is a dimensionless quantity of a medium (that is, the medium of Rubidium-85 atoms in the vapor cell121, in the first embodiment above) which is used to relate the electric polarization P to the electric field E as:
P=NeX1=ε0χEp(6)
Therefore,
A=Im(χ)=Im(NeX1/ε0Ep)=(2N|12|2/ε0h̆)(Γ2Γ3+Ωr2)/{Γ1Γ2Γ3+Ωr2)X1+Γ3Ωc2}  (7)
Which is the same result as equation 1.

The following analysis illustrates the response time in two separate scenarios for a Rydberg-atom based RF receiver which undergoes a change in the EIT signal from a first state to a second state.

In a first scenario, the probe laser123and coupling laser125of the RF detector120are powered such that the probe signal and coupling signal pass through a region of the vapor cell. For the purposes of this first scenario only, the rotating mirror126is not rotating such that the probe signal and coupling signal pass through a single region of the vapor cell121. In other words, that region of the vapor cell121is in the interacting condition (such that Rubidium-85 atoms within that region interact with the probe and coupling signals) and does not switch to the non-interacting condition, and all other regions of the vapor cell are in the non-interacting condition and do not switch to the interacting condition. In this configuration, the Rubidium-85 atoms within the region of the vapor cell121that are in the interacting condition are constantly excited by both the probe and coupling signals. These atoms therefore experience the EIT effect such that their electrons are elevated to a predetermined Rydberg state (having an adjacent Rydberg state whereby the energy difference between the predetermined Rydberg state and another Rydberg state is equal to the energy of the wireless signal to be detected).

This first scenario can be analyzed, using the mechanical model described above, to determine a response time for the EIT signal when there is a change in presence of a wireless signal at the vapor cell121(i.e. from a state where there is no wireless signal incident at the vapor cell121to a state where there is a wireless signal incident at the vapor cell121, or from a state where there is a wireless signal incident at the vapor cell121to a state where there is no wireless signal incident at the vapor cell121).

At time t<0, a wireless signal is incident at the vapor cell121. The probe signal may therefore be absorbed (as electrons move from the predetermined Rydberg state to the adjacent Rydberg state and then to the ground state). Assuming the wireless signal is incident at the vapor cell121for sufficient time for the system to reach a steady state, it can be shown that:
x1=X1(Ωr2,Ωc2)exp(iωpt)  (8)
x2=X2(Ωr2,Ωc2)exp(iωpt)  (9)
x3=X3(Ωr2,Ωc2)exp(iωpt)  (10)
At time t=0, there is no wireless signal incident at the vapor cell121. The probe signal is therefore absorbed less as there is no longer a path for electrons to move from the predetermined Rydberg state to the ground state (that is, the step from the predetermined Rydberg state to the adjacent Rydberg state is no longer available). The EIT effect is therefore restored. At time t>0, it can be shown that:
x1=X1(0,Ωc2)exp(iωpt)+D1exp(−αt)  (11)
x2=X2(0,Ωc2)exp(iωpt)+D2exp(−αt)  (12)
x3=X3(0,Ωc2)exp(iωpt)+D3exp(−αt)  (13)
In which Direpresents a magnitude of the response of mass i to the incident wireless signal, which has a complex decay rate u. At time t=0, it can be shown that:
D1=X1(Ωr2,Ωc2)−X1(0,Ωc2)  (14)
D2=X2(Ωr2,Ωc2)−X2(0,Ωc2)  (15)
D3=X3(Ωr2,Ωc2)−X3(0,Ωc2)  (16)
Applying equation 2 and separating out the transient behavior, it can be shown that:
(α2−Γ1α+ωp2)D1−ωpΩc2D2=0  (17)
It can be further shown that:
iΓ1D1=Ωc2D2(18)
Thus, the only physical solution for the complex decay term is:
α=Γ1+iωp(19)
This analysis shows that, when both the probe signal and coupling signal are constantly interacting with Rubidium-85 atoms in the region of the vapor cell121, the transparency of the probe signal (and thus the EIT signal) will react to a change in presence of a wireless signal at the vapor cell121with a dynamic response having a decay rate of exp(−Γ1t) with a sinusoidal variety of 2ωp. The response time of the EIT signal in this first scenario is therefore limited only by Γ1, the transition rate out of the first excited state of the Rubidium-85 atom.

A second scenario will now be analyzed. In this second scenario, the probe signal is present in all regions of the vapor cell121whilst the coupling signal successively passes through each region of the vapor cell121. As the intensity of the probe laser is relatively weak (compared to the coupling signal), such that absorbance of the probe signal at the photodetector is not a function of the intensity of the probe laser, then there is no difference in the response of the EIT signal between such a configuration and that of the RF detector120described above (in which the probe signal also successively passes through each region of the vapor cell121at the same time as the coupling laser125). This second scenario can also be analyzed, using the mechanical model described above, to determine a response time for the EIT signal when there is a change in presence of a wireless signal at the vapor cell121(i.e. from a state where there is no wireless signal incident at the vapor cell121to a state where is a wireless signal incident at the vapor cell121, or from a state where there is a wireless signal incident at the vapor cell121to a state where there is no wireless signal incident at the vapor cell121).

At time t<0, the probe laser123is powered, the coupling laser125is unpowered, and a wireless signal is incident at the vapor cell121. The probe signal therefore interacts with Rubidium-85 atoms in the region of the vapor cell121but there is no coupling signal in the region of the vapor cell121for the Rubidium-85 atoms to interact with. Electrons of the Rubidum-85 atoms in this region are therefore excited from the ground state to the first excited state but are not excited to the predetermined Rydberg state. There are therefore no Rubidium-85 atoms for the wireless signal to interact with. As the transition from the first excited state to the ground state is permitted (i.e. not forbidden), then electrons oscillate between the ground state and the first excited state, absorbing the probe signal such that the vapor cell121is opaque to the probe signal. Accordingly, at time t<0, it can be shown that:
x1=X1(Ωr2,0)exp(iωpt)  (20)
x2=X2(Ωr2,0)exp(iωpt)  (21)
x3=X3(Ωr2,0)exp(iωpt)  (22)
In a second state of this second scenario (at time t=0), the probe laser123and coupling laser125are powered and a wireless signal is incident at the vapor cell121. Thus, both the probe and coupling signals pass through the region of the vapor cell121. Accordingly, the Rubdium-85 atoms of vapor cell121may interact with both the probe and coupling signals so that the EIT effect is restored. At time t>0, it can be shown that:
x1=X1(Ωr2,Ωc2)exp(iωpt)+C1exp(−αt)  (23)
x2=X2(Ωr2,Ωc2)exp(iωpt)+C2exp(−αt)  (24)
x3=X3(Ωr2,Ωc2)exp(iωpt)+C3exp(−αt)  (25)
In which Cirepresents a magnitude of the response of mass i to the incident wireless signal, which has a complex decay rate u. At time t=0, it can be shown that:
C1=X1(Ωr2,0)−X1(Ωr2,Ωc2)  (26)
C2=X2(Ωr2,0)−X2(Ωr2,Ωc2)  (27)
C3=X3(Ωr2,0)−X3(Ωr2,Ωc2)  (28)
Applying equation 2 and separating out the transient behavior, it can be shown that:
(α2−Γ1α+ωp2)C1−ωpΩc2C2=0  (29)
It can be further shown that:
Γ3C1=i(Γ2Γ3+Ωr2)C2(30)
So,
(α2−Γ1α+ωp2)i(Γ2Γ3+Ωr2)−Γ3ωpΩc2=0  (31)
Considering just the real part β of α=β+iγ, which represents the exponential decay of the dynamic response, it can be shown that:
2β=Γ1+(Γ12Γ2/Ωc2)  (32)
Accordingly, when both the probe signal and wireless signal are present in the region of the vapor cell121, the transparency of the probe signal (and thus the EIT signal) will react to a change in intensity of the coupling signal in the region of the vapor cell121with a dynamic response that is a function of the coupling power (in addition to the transition rates out of the first excited state and predetermined Rydberg state). This property can be exploited so as to reduce the response time of the EIT signal. That is, by configuring the RF detector so that each region transitions from a non-interacting condition (which has not been subject to the coupling signal for at least sufficient time for electrons of the Rubidum-85 atoms of that region to reach a steady-state in this non-interacting condition) to an interacting condition (where there is a coupling signal), and using a coupling power value so that Ωc2is less than Γ1Γ2, then the response time of the EIT signal will be less than the response time had the region been constantly excited by the coupling signal.

It is further noted that the modulation depth (that is, the difference in magnitude of the EIT signal when there is a wireless signal incident at the vapor cell121and when there is no wireless signal incident at the vapor cell121) will be sub-optimal. That is, modulation depth is optimized when Ωc2equals Γ1Γ2. Therefore, in selecting a coupling laser power value such that Ωc2is less than Γ1Γ2, the modulation depth is necessarily sub-optimal.FIG.5is a graph of the relationship between modulation depth and the decay rate (in units of Fi). This graph that the modulation depth is maximized when Ωc2equals Γ1Γ2(represented by the vertical dashed line), which corresponds with a decay rate of Γ1(represented by the horizontal dashed line). As the value of Ωc2decreases from Γ1Γ2(i.e. to the left hand side of the vertical dashed line), the decay rate increases. Thus, although the modulation depth is compromised, the decay rate (and therefore the EIT signal's response time) is improved. The coupling laser power value must therefore be selected such that Ωc2is less than Γ1Γ2but the modulation depth on receipt of a wireless signal is greater than a background noise (which will be based on the strength of the wireless signal at the RF detector120and the background noise level).

A second embodiment of a wireless telecommunications network200will now be described with reference toFIG.6. The wireless telecommunications network200includes a wireless transmitter210and an RF receiver220. In this embodiment, the wireless transmitter210is configured to transmit wireless signals at a frequency of 2.8 GHz. As shown inFIG.7, the RF receiver220is a Rydberg-atom based RF receiver and includes a vapor cell221, a probe laser223, a first beam splitter224a, a second beam splitter224b, a coupling laser225, a first photodetector227a, a second photodetector227b, a first dichroic mirror228aand a second dichroic mirror228b. The RF receiver220also includes a processor229for processing the received signals of the first and second photodetectors227a,227b. The vapor cell221contains a vapor of alkali metal (in this embodiment, Rubidium-85).

The RF receiver220also has a controller222for controlling a frequency of the probe signal (i.e. to adjust and/or stabilize the frequency of the probe signal), for controlling a frequency of the coupling signal (i.e. to adjust and/or stabilize the frequency of the coupling signal), for controlling an intensity of the probe laser223(i.e. the transmission power and/or cross-sectional area of the probe laser223), and for controlling an intensity of the coupling laser225(i.e. the transmission power and/or cross-sectional area of the coupling laser225).

The RF receiver220may be configured so as to detect a wireless signal incident upon the vapor cell221. This is achieved by configuring the probe laser223of the RF receiver220to transmit a probe signal (illustrated by the solid arrowed line) in a first path that passes through a first region of the vapor cell221to excite electrons of the Rubidium-85 atoms of the first region of the vapor cell221(in which the first path is created by splitting the probe signal at the first beam splitter224a), by configuring the probe laser223of the RF receiver220to transmit a probe signal in a second path that passes through a second region to excite electrons of the Rubidium-85 atoms of the second region of the vapor cell221(in which the second path is also created by splitting the probe signal at the first beam splitter224a), by configuring the coupling laser225of the RF receiver220to transmit a coupling signal (illustrated by the dashed arrowed line) in a first path that passes through the first region of vapor cell221(counter-propagating and overlapping the first path of the probe signal) to excite electrons of the Rubidium-85 atoms of the first region of the vapor cell221(in which the first path of the coupling signal is created by splitting the coupling signal at the second beam splitter224b), and by configuring the coupling laser225of the RF receiver220to transmit a coupling signal in a second path that passes through the second region of vapor cell221(counter-propagating and overlapping the second path of the probe signal) to excite electrons of the Rubidium-85 atoms of the second region of the vapor cell221(in which the second path of the coupling signal is also created by splitting the coupling signal at the second beam splitter224b). Following passage of the first region, the first path of the probe signal is directed towards the first photodetector227aby the first dichroic mirror228aand, following passage of the second region, the second path of the probe signal is directed towards the second photodetector227bby the second dichroic mirror228b. The frequency of the probe signal is set to correlate with the transition of an electron of a Rubidium-85 atom from a ground state to a first excited state, and the frequency of the coupling signal is set to correlate with the transition of an electron of a Rubidium-85 atom from the first excited state to a predetermined Rydberg state. The predetermined Rydberg state is selected (based on the specific frequencies of the probe and coupling signals) so that a wireless signal of a particular frequency incident upon the first or second region of the vapor cell221excites electrons from the predetermined Rydberg state to another Rydberg state, causing a detectable change in the EIT signal at the first or second photodetector227a,227brespectively.

The RF receiver220is therefore configured to receive of a plurality of spatially-multiplexed data streams. In this embodiment, communications are further multiplexed in the time domain to enable communication of a plurality of time-and-space multiplexed data streams. Accordingly, the first wireless transmitter210and RF receiver220are further configured for time-division multiplexing by communicating according to a time frame structure in which a first data stream is communicated during a first subset of timeslots of the time frame structure and a second data stream is communicated during a second subset of timeslots of the time frame structure. The RF receiver220may therefore be configured so that the processor229receives and demultiplexes signals at the first photodetector227aaccording to the time frame structure so that a first data stream is demultiplexed as data received during each timeslot of the first set of timeslots of the time frame structure and receives and demultiplexes signals at the second photodetector227baccording to the time frame structure so that a second data stream is demultiplexed as data received during each timeslot of the second set of timeslots of the time frame structure.

In this example, the controller222configures the probe laser223and coupling laser225so that the first and second regions detect wireless signals at a frequency of 2.8 GHz. Thus, in this example, the probe signal and coupling signal are selected so as to excite electrons to a predetermined Rydberg state having the 90thprincipal quantum number energy state (i.e. the probe signal has a wavelength of 780 nm and the coupling signal has a wavelength of 479.380 nm) such that wireless signals transmitted by the wireless transmitter210at 2.8 GHz that pass through the first or second region of the vapor cell221will excite electrons from the predetermined Rydberg state to an adjacent Rydberg state, causing an EIT signal in the monitored probe signal at the first or second photodetector227a,227b.

The RF detector220of this second embodiment therefore implements space-and-time division multiplexing in which each spatial region is configured to detect a wireless signal of the same frequency. This may be achieved by directing the coupling signal to the first path only (and thus to the first region of the vapor cell221only) during the first subset of timeslots and directing the coupling signal to the second path only (and thus to the second region of the vapor cell221only) during the second subset of timeslots. The RF detector220may therefore be configured such that the time difference between the coupling signal being directed along a particular path and subsequently being directed along that same path again is greater than the time for electrons of the Rubidium-85 atoms of the region of the vapor cell121contained in that path to return to the 1stexcited state (that is, the state the electrons are excited to by the probe signal in the non-interacting condition). In other words, the time difference between adjacent timeslots of the subset of timeslots allocated to a particular path is greater than the time for electrons of the Rubidium-85 atoms of the region of the vapor cell121contained in that path to return to the 1stexcited state. Furthermore, the coupling laser225power is set such that Ωc2is less than Γ1Γ2. Accordingly, this second embodiment also enjoys the benefit of the response time of the EIT signal being reduced relative to the scenario where the coupling signal is constantly directed to all paths.

A second embodiment of a method of the present disclosure will now be described with reference toFIG.8. This second embodiment of a method relates to the second embodiment of the wireless telecommunications network200described above and shown inFIG.6. In this second embodiment, the wireless transmitter210is transmitting wireless signals at 2.8 GHz. Wireless signals from the wireless transmitter210are incident upon the vapor cell221of the RF detector220.

In S201, the RF detector220is operated so that the probe and coupling signals pass through the vapor cell221and so that the time period between adjacent iterations of the interacting condition for each region of the vapor cell is equal to or greater than the time period for electrons of the Rubidium-85 atoms in that region to return to the 1stexcited state. Furthermore, a coupling laser125power value is configured so that Ωc2is less than Γ1Γ2. In S203of this second embodiment, the processor229receives signals at the first photodetector227aand demodulates these signals into a first data stream. These signals received at the first photodetector227aare those transmitted by the wireless transmitter210during each timeslot of the first set of timeslots and incident upon the first region of the vapor cell221of the RF detector220. In S205, the processor229receives signals at the second photodetector227band demodulates these signals into a second data stream. These signals received at the second photodetector227bare those transmitted by the wireless transmitter210during each timeslot of the second set of timeslots and incident upon the second region of the vapor cell221of the RF detector220. The first and second data streams may then be further processed (locally or by another entity).

In the above embodiments, the vapor cell220is static and the probe and coupling signals are manipulated so as to pass through different regions of the vapor cell220(and thus excite a different set of Rubidium-85 atoms). However, this is non-essential and there are alternative implementations which enable different sets of Rubidium-85 atoms to interact with the probe and coupling lasers. For example, the probe and coupling signals may be static, and a vapor of Rubidium-85 atoms may be configured to flow through the probe and coupling signals (such as by flowing along a glass tube). A path of the flowing vapor may be configured in a loop such that a section of the loop passes through the probe and coupling signals whilst the remainder of the loop does not pass through the probe and coupling signals.

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 vapor cell. That is, the atomic medium may be part of a hollow-core fiber segment of an optical fiber. For example, a hollow core fiber implementation of the second embodiment above may include counter propagating probe and coupling lasers that are respectively split into a plurality of optical fiber cores wherein the probe signal of each optical fiber core is directed towards a photodetector using a circulator. The plurality of optical fiber cores may be a plurality of optical fibers each having a single core or may be a plurality of cores of a single fiber. The plurality of optical fiber cores may comprise, for example, 64 cores.

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 Cesium or Strontium.

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, Cesium 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 skilled person will understand that it is non-essential that the non-interacting condition of each region of the atomic medium excludes both the probe and coupling signals. As illustrated by the second embodiment above, the probe signal may pass through a region when that region is in the non-interacting condition. In other words, in the non-interacting condition, the region is not subject to the coupling signal but may be subject to the probe signal and, in the interacting condition, the region is subject to both the probe and coupling signals. The skilled person would understand, following review of the first and second embodiments above, that these differing implementations alter the time period between iterations of the interacting condition for any particular region. That is, in the first embodiment, a region was not subject to the probe signal during the non-interacting condition and, as such, the time period was that required for electrons of the Rubidium-85 atoms of that region to return to the ground state (the state for electrons that are not being excited). In the second embodiment, a region was subject to the probe signal during the non-interacting condition and, as such, the time period was that required for electrons of the Rubdium-85 atoms of that region to return to the 1stexcited state (the state the probe signal excites electrons to). Thus, stated generally, the time period between adjacent iterations of the interacting condition is that required for electrons of the Rubidium-85 atoms of that region to return to their state of the non-interacting condition.

Furthermore, it is non-essential that the power of the coupling laser is controlled so that Ωc2is less than Γ1Γ2. Alternatively or additionally, the cross-sectional area of the coupling laser may be controlled so that Ωc2is less than Γ1Γ2.

The skilled person will understand that any combination of features is possible within the scope of the disclosure, as claimed.