[0001] The present invention relates to optical switching & routing methods & apparatus and in particular to optical switching & routing apparatus utilising quantum control mechanisms.

[0002] The invention has been developed primarily for use as an optical switching & routing methods and apparatus using quantum dynamics for control of the interaction between optical cavity modes to provide switching & routing functions, achieved through the use of an atomic system to dynamically tune the coupling between two cavities and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

Background

[0003] Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art nor that such background art is widely known or forms part of the common general knowledge in the field.

[0004] The dynamic control of the interaction between optical cavity modes is essential for the advanced functioning of photonic and quantum photonic devices such as optical delay based on the optical analog of electromagnetically induced transparency (EIT) [1 -3], all-optical switching [4] and all-optical routing [5,6]. Popular known methods to control the couplings between cavity modes involve either tuning the cavity resonance by laser-assisted carrier induced nonlinearities [1 , 7-9], or by thermal-optical effects [2]. By detuning two cavities out of resonance with each other it is possible to indirectly decouple two cavity modes which are arranged to strongly couple when on-resonance. However these tuning methods rely on media processing a small nonlinear refractive index and therefore require intense optical control fields. The interaction among cavity modes can also be controlled slowly by moving a coupling element such as a scatterer [10], or tuning the spatial gap between the cavities [2,3]. All of the current known methods to modulate the coupling between separate optical cavities suffer from various drawbacks, i.e. they are slow, may require optical nonlinearity, do not operate at the single-photon level, and/or require sophisticated physical setups.

[0005] Routing of photons plays a key role in optical communication networks and quantum information processing. All-optical switching can be achieved via the saturation of a single emitter in a cavity [1 1 ,12], but the contrast achieved is very low. The aforementioned method of tuning the resonance of a nonlinear optical cavity [7, 9, 13, 14] or the evanescent coupling between waveguides [15] with an intense laser has previously been proposed for all-optical switching and routing, but these require high pump laser powers due to the very weak optical nonlinearity. By using a high-Q cavity or a high carrier-induced nonlinearity it is possible to decrease the required pump laser intensity but this also slows down the switching speed as either the cavity exhibits a long ring down time [14] or the carrier relaxation time becomes very long [1 , 9].

[0006] Cavity quantum electrodynamics (cQED) offers a powerful toolkit to control the transmission of light through a cavity/waveguide system where the cavity resonantly interacts with an emitter or scatterer [4-6, 16]. A single scatterer strongly coupled to a one-dimensional waveguide can scatter a single photon in the waveguide into either the forward (transmission) or the backward (reflection) mode [17, 18]. This scheme has been used to propose a single- photon transistor [19, 20]. However previous cQED schemes using on-resonance interaction with a single emitter and single-photon transistor only allow the possibility of routing a single photon into either the forward path or the backward path. To date, multiport all-optical routers formed from the composition of two-port routers [15], or optical switches [9], suffer from large insertion losses, particularly when extended to provide multipath routing. Although the formalism of a Λ-type atomic scatterer (i.e. having a Lambda-system - Λ-system) comprising an internal electronic level structure with two ground states and one excited state) interacting with a cavity mode has been widely studied [18, 21 -23], using such a three-level system to dynamically control and/or modulate the coupling between cavities has yet to be realised or considered.

Summary

[0007] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0008] Disclosed herein is a new scheme for switching & routing operations, via controlling a coupling element comprising a three-level scatterer placed either within or nearby a cavity , the manipulation of which allows rapid all-optical control of cavity couplings and permits the routing of optical signals (including single photons) between multiple cavities and via these cavities into many input/output waveguides with near-perfect switching fidelity. The methods & apparatus disclosed herein are essentially different from previous methods in that it is possible to directly modulate the coherent intermode interaction strength. The methods disclosed herein relate to use of an atomic system to dynamically tune the coupling between cavities. The manipulation of the coupling element may comprise modifying the tuning of energy levels of the coupling element (and thus tuning the cavity resonance) for example by laser-assisted carrier induced nonlinearities, by evanescent coupling between waveguides or by thermal-optical effects thereby to tune the input and coupling cavities into or out of resonance as desired.

[0009] Disclosed herein are methods to control the coupling (or coherent scattering), between two optical cavity modes using a Λ-type three-level system (scatterer) which dispersively interacts with both cavity modes simultaneously in the strong coupling regime. Such dispersive coupling induces a coherent interaction between cavity modes that depends on the common detuning and the quantum state of the scatterer. To modulate the strength of this intercavity coupling, methods for controlling either the size of the common detuning or directly changing the internal quantum state of the scatterer must be developed. It is shown herein that both such routes are possible and that it is possible to modulate the interaction strength (i.e. tune the interaction strength between cavity modes) by either: (i) tuning (adjusting) the detuning via an optical Stark shift; or (ii) transferring (via stimulated Raman adiabatic passage (STIRAP) [24]) one internal ground state of the scatterer to another internal ground state, which does not interact with either cavity, to effectively turn off the inter-cavity coupling. Either proposal can rapidly switch on or off the coupling between the two cavity modes with very high (near unit), switching contrast. The switching methods disclosed herein further allow for the dynamic cancellation of scattering in a toroidal cavity if the dynamic coupling is set to be the same number of the scattering but of the opposite sign.

[0010] According to a first aspect of the invention, there is provided an optical switching system. The optical switching system may comprise a first optical input waveguide adapted to support an optical input mode. The optical switching system may further comprise an input cavity coupled to the optical input mode. The optical switching system may further comprise a first optical output waveguide adapted to support a first optical output mode. The optical switching system may further comprise a second optical waveguide output adapted to support a second optical output mode. The optical switching system may further comprise a coupling cavity adapted to be selectively coupled to the input cavity and the first and the second optical output waveguides. The optical switching system may further comprise a coupling element adapted to couple the input cavity with the coupling cavity thereby to selectively couple the optical input waveguide with a selected one of the first and second optical output waveguides. [00 1] The coupling element may be adapted to be manipulated by a control field to selectively couple the coupling cavity to the input cavity and thereby selectively couple the input waveguide to a selected one of the first and the second optical output waveguides such that the optical input mode is coupled to the selected one of the first and the second optical output waveguides to provide an optical output on the selected output waveguide.

[0012] The input cavity and the coupling cavity may be adapted to be coupled via overlap of the cavity evanescent fields with the respective waveguides.

[0013] The optical switching system may further comprise a ring resonator coupled to the input waveguide and the first and second output waveguides. The ring resonator may be adapted to support the input optical mode and the first and second output optical modes. The input optical mode and the output optical modes may comprise counter-propagating modes of the ring resonator.

[0014] The optical switching system may comprise a photonic crystal adapted to provide the input cavity for supporting the input mode. The photonic crystal may be further adapted to provided the coupling cavity. The coupling cavity may be adapted to selectively support at least one of the first or second output modes.

[0015] The control field may be adapted to provide near instantaneous switching of the output mode between the first and the second output waveguides. The control field may be an optical field.

[0016] The coupling element may comprise an atomic system adapted to dynamically tune the coupling between the input cavity and the coupling cavity. The atomic system may comprise an atom or group of atoms having an internal electronic level structure with two ground states and one excited state. The atomic system may comprises a rare earth or neutral atom or a group of such rare-earth or neutral atoms trapped near the input cavity.

[0017] According to a second aspect, there is provided an optical switch. The optical switch may comprise an optical input waveguide. The optical switch may further comprise an input optical cavity coupled to the optical input waveguide. The optical switch may further comprise at least one output optical cavity. The at least one output optical cavity may be adapted to be selectively coupled to the input optical cavity. The optical switch may further comprise a coupling element for selectively coupling the input optical cavity to a selected one of the at least one output optical cavity. The optical switch may further comprise a respective optical output waveguide coupled to each of the at least one output optical cavities such that, when a selected output cavity is coupled with the input optical cavity, then an optical input on the input waveguide is output on the output waveguide associated with the selected output cavity.

[0018] The coupling element may comprise a quantum dot. Alternatively, the coupling element may comprise an NV center, for example, in a nano or bulk diamond or a diamond membrane or alternate host material. Alternatively, the coupling element may comprise an atomic system adapted to dynamically tune the coupling between the input cavity and the coupling cavity. The atomic system may comprise an atom or group of atoms having an internal electronic level structure with two ground states and one excited state. The coupling element may be any structure having a Lambda system (Λ-system / Λ-type) internal electronic structure which couples strongly to an optical field. In particular arrangements the coupling element may comprise a rare earth or neutral atom or a group of such rare-earth or neutral atoms trapped near the optical resonator. The coupling element may comprise a rare earth or neutral atom, or a group of such rare-earth or neutral atoms, trapped near the input cavity.

[0019] According to a third aspect, there is provided an optical router. The optical router may comprise an optical input waveguide. The optical router may further comprise an input optical cavity coupled to the optical input waveguide. The optical router may further comprise at least two output optical cavities, each adapted to be selectively coupled to the input optical cavity. The optical router may further comprise at least two coupling elements, each associated with a respective one of the at least two output optical cavities for selectively coupling the input optical cavity to one of the at least two output optical cavities. The optical router may further comprise at least two optical output waveguide coupled to a respective one of the at least two output optical cavities such that, when a selected output cavity is coupled with the input optical cavity, then a optical input on the input waveguide is output on the output waveguide associated with the selected output cavity.

[0020] The coupling elements may each comprise a quantum dot. Alternatively, the coupling elements may each comprise an NV center in a nano diamond.

[0021] According to a fourth aspect, there is provided an optical switching method. The method ma comprise the step of providing a first optical input waveguide adapted to support an optical input mode. The method may comprise the further step of providing an input cavity coupled to the optical input mode. The method may comprise the further step of providing a first optical output waveguide adapted to support a first optical output mode. The method may comprise the further step of providing a second optical waveguide output adapted to support a second optical output mod. The method may comprise the further step of providing a coupling cavity adapted to be selectively coupled to the input cavity and the first and the second optical output waveguides. The method may comprise the further step of providing a coupling element adapted to couple the input cavity with the coupling cavity thereby to selectively couple the optical input waveguide with a selected one of the first and second optical output waveguides. The method may comprise the further step of manipulating the coupling element by a control field to selectively couple the coupling cavity to the input cavity and thereby selectively couple the input waveguide to a selected one of the first and the second optical output waveguides such that the optical input mode is coupled to the selected one of the first and the second optical output waveguides to provide an optical output on the selected output waveguide. The control field may comprise an optical field.

[0022] According to a fifth aspect of the invention, there is provided an optical switch comprising: an optical input waveguide; an input optical cavity coupled to the optical input waveguide; at least one output optical cavity adapted to be selectively coupled to the input optical cavity; coupling element for selectively coupling the input optical cavity to a selected one of the at least one output optical cavity; an optical output waveguide coupled to each of the at least one output optical cavities such that, when a selected output cavity is coupled with the input optical cavity, then an optical input on the input waveguide is output on the output waveguide associated with the selected output cavity.

[0023] According to a sixth aspect of the invention, there is provided an optical router comprising: an optical input waveguide; an input optical cavity coupled to the optical input waveguide; at least two output optical cavities, each adapted to be selectively coupled to the input optical cavity; at least two coupling elements, each associated with a respective one of the at least two output optical cavities for selectively coupling the input optical cavity to one of the at least two output optical cavities; at least two optical output waveguide coupled to a respective one of the at least two output optical cavities such that, when a selected output cavity is coupled with the input optical cavity, then a optical input on the input waveguide is output on the output waveguide associated with the selected output cavity.

[0024] According to a seventh aspect of the invention, there is provided an optical switching method comprising the steps of: providing a first optical input waveguide adapted to support an optical input mode; providing an input cavity coupled to the optical input mode; providing a first optical output waveguide adapted to support a first optical output mode; providing a second optical waveguide output adapted to support a second optical output mode; providing a coupling cavity adapted to be selectively coupled to the input cavity and the first and the second optical output waveguides; providing a coupling element adapted to couple the input cavity with the coupling cavity thereby to selectively couple the optical input waveguide with a selected one of the first and second optical output waveguides; and manipulating the coupling element by a control field to selectively couple the coupling cavity to the input cavity and thereby selectively couple the input waveguide to a selected one of the first and the second optical output waveguides such that the optical input mode is coupled to the selected one of the first and the second optical output waveguides to provide an optical output on the selected output waveguide. The control field may be an optical field.

[0025] According to an eighth aspect of the invention, there is provided an all-optical router comprising a plurality of coupling elements coupled to an input cavity and at least one of a respective plurality of output cavities, wherein each said coupling element is an atomic system adapted to dynamically tune the coupling between the input cavity and a selected output cavity thereby to route an optical signal on an input waveguide coupled to the input cavity to one or more of a plurality of output waveguides each coupled to a respective output cavity.

[0026] The all-optical router may be a switch comprising an input waveguide coupled to an input cavity, the input cavity in turn coupled to a coupling cavity by a coupling element for outputting an optical signal on the input waveguide on one of two output waveguides, wherein an output waveguide is selected for outputting the optical signal by selective tuning of the coupling between the input cavity and the coupling cavity.

[0027] In any one of the above aspects, the optical signal may comprise a single photon or a plurality of photons. The optical signal may comprise a high power optical signal, a low power optical signal or a single photon. The systems & methods of the above aspects provide for the use of an atomic system to dynamically tune the coupling between two cavities.

Brief Description of the Drawings

[0028] Figures 1 (a) to 1 (d) describe the parameters for description of the optical switching/routing method disclosed herein via a Λ-type qubit interacting with a cavity; [0029] Figures 2{a) and 2(b) show graphs depicting the steady state transmission '/'„ and T _b as a function of detuning in the case of overcoupling for the optical switching/routing method of Figure 1 ;

[0030] Figures 3(a) and 3(b) show graphs depicting the numerical results of all-optical switching of the transmission T _a is in the two schemes of: (i) Stark control; (ii) STIRAP control for the optical switching/routing method of Figure 1 ;

[0031] Figures 4(a) and 4(b) show graphs depicting the switching contrast as a function of the intrinsic coupling for Stark control [Figure 4(a)] and shuffle control [Figure 4(b)] using the time averages of transmissions T _on and T _ofr of Figure 3;

[0032] Figures 5(a) to 5(d) show graphs depicting the numerical results is for an all-optical router with output ports (solid Iine501 ) and when controlled by Stark shift [Figures 5(a) and (b)] or via STIRAP pulses [Figures 5(c) and (d)]; and

[0033] Figures 6(a) and 6(b) show graphs depicting the switching contrast as a function of the intrinsic coupling for Stark control [Figure 6(a)] and shuffle control [Figure 6(b)] using the time averages of transmissions T _on and _nf f of Figure 5;

[0034] Figures 7(a) to 7(c) show various configurations of a multiport optical router as disclosed herein.

Definitions

[0035] The following definitions are provided as general definitions and should in no way limit the scope of the present invention to those terms alone, but are put forth for a better understanding of the following description.

[0036] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. For the purposes of the present invention, additional terms are defined below. [0037] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular articles "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise and thus are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" refers to one element or more than one element.

[0038] The term "about" is used herein to refer to quantities that vary by as much as 30%, preferably by as much as 20%, and more preferably by as much as 10% to a reference quantity. The use of the word 'about' to qualify a number is merely an express indication that the number is not to be construed as a precise value.

[0039] Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements.

[0040] The term "real-time" for example "displaying real-time data," refers to the display of the data without intentional delay, given the processing limitations of the system and the time required to accurately measure the data.

[0041] As used herein, the term "exemplary" is used in the sense of providing examples, as opposed to indicating quality. That is, an "exemplary embodiment" is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.

Detailed Description

[0042] Disclosed herein are schemes for optical switching & routing operations, via controlling a coupling element comprising a three-level scatterer placed within a cavity or nearby a cavity, that allows rapid all-optical control of cavity couplings and permits the routing of optical signals (including single photons) between multiple cavities and via these cavities into many input/output waveguides with near-perfect switching fidelity. The methods & apparatus disclosed herein are essentially different from previous methods in that it is possible to directly modulate the coherent intermode interaction strength. The methods disclosed herein relate to use of an atomic system to dynamically tune the coupling between the two cavities. Basic Protocol:

[0043] Referring to Figures 1 (a) to 1(d) in the drawings, it is possible to create a tunable coherent coupling between two cavity modes using a Λ-type three-level system (comprising for example a single NV center in a nano diamond or a quantum dot) with levels | 1) 101 , |2) 102 and |3) 103 with associated eigenfrequencies ω _ί for energy eigenstate \j),j e {1,2,3}. Figures 1 (a) to 1(d) depict arrangements for control of switching and routing via a Λ-type qubit interacting with a cavity. By controlling the three level system the transmission of the combined qubit-cavity system can be altered in a controllable manner.

[0044] Figure 1(a) depicts the energy level diagram of a Λ-type three-level scatter. Two cavity modes a and b drive the same transition | 1) → |3) with strength, detuning {# _α , Δ _α and {g _b , A _b }, respectively. The classical field Ω _ρ (Ω ₅ ) drives transition |1 ) <-→ |3)(|2) *→ |3» with detuning Δ _Ρ (Δ ₅ ). The field Ω ₅ shifts the energy of level |3) or in combination with Cl _p is used to swap quantum state between | 1) and \2).

[0045] Figure 1(b) depicts an arrangement for optical switching via the controlling of the transmission. The waveguide 120 overcouples to the cavity 130. Via tuning of the coherent coupling h created by the scatterer 140 (the Λ-type qubit), both forward and backward transmissions T _a and T _h can be switched on/off.

[0046] Figure 1(c) depicts an arrangement of an optical router to control the output path. Photonic crystal cavity a 150 couples to a one-dimensional waveguide 151. The coupling between two photonic crystal cavities a 150 and b 155 is controlled by a scatter such as a NV center or QD 160.

[0047] Figure 1(d) depicts an arrangement of the setup for an all-optical router. The cavity a 161 couples to waveguide 60 and can individually couple to each cavity ¾ 1 1 and 1 2 mediated by the individual scatterers 181 and 182 respectively. The field stored in cavity ¾ couples out to the associated I ^th waveguide ft. 191 / and 192 respectively.

[0048] The method includes arranging for two cavity modes a; 6, to simultaneously couple to the transition | 1) <→ |3> with the coupling strengths g _a/b . It is assumed that these two modes have identical frequencies and thus suffer identical detunings, i.e. Δ _α = A _b = Δ, and couple identically to the qubit, i.e. g _a = g _h = g. Both fields a and h induce Stark shifts on levels | 1) and |3). The value of this shift ^ _s f _aTk is given by L-/c = \a\ ² ((a ^f + b ^f )(a + b))/& . In the situation of |Δ| ² » \g\ ² (&*a), \g\ ² {b b), state |3) is negligibly populated i.e. <σ ₃₃ )~υ, where <¾ = with i,j e {1,2,3}. This Stark shift effectively yields a coherent interaction h(a ^† B + fi ^† a) between the two cavity modes with a coupling strength h = \g\ ² (a,, )/& (1 )

[0049] The coupling strength h (and resultant coupling strength including the effective coupling between the connected waveguides through the cavities and scatterer), depends both on the detuning Δ and the population (a _lt ) in state | 1). Through the use of another optical field to transfer the qubit state |1) <→ [2), the cavity coupling can effectively be switched on/off. This control optical field is a classical field and can be a large amplitude coherent laser field. The intermode coupling is on only while the three level system is in the state When the classical control optical field switches the internal state of the three level system to the state |2> the intermode coupling turns off. These two coupled cavity modes can be either two counter- propagating modes of a ring resonator (see Figure 1 (b)), or two photonic crystal cavity modes (see Figure 1 (c)). Tunable coupling enables photon routing. This can be seen in the following manner: since the interaction between two cavities can be controlled, the field energy can be selectively transferred from one cavity to another and then fed into a selected output waveguide. Using a waveguide to input/output the field from the cavity, an all-optical router can be constructed, as shown in Figure 1 (d). Tilted oriented waveguides may be used to optimally output fields in cavities b [25].

Waveguide all-optical router:

[0050] Referring to the general setup shown in Figure 1 (d), the router output is chosen amongst the ports ct _out 163 and a plurality of ports (e.g. output ports #^191 and 192). After going to the frame defined by the unitary transformation 0 - exp ^~i _in t ^' a - i∑i + £ ₃ σ ₃₃ + (e ₃ - ω _1η σ + (e ₃ — ω^σ^ ] t}, and making the rotating-wave approximation RWA), the dynamics of the system can be written in the form:

with

H = H _in + H ₀ + H _c + H _sc + H _c (3a) fio = - Σι + Δ, _η ) a® + Δ® 8g ] + A _ra § ^† a +∑,(Δ _ιη + δ,^Β, (3c)

H _cc =∑) h ₍ ⁽ ₎ ¹⁾ (a ^† h ₎ + bia) (3d)

H _Sc = ( _a a + ?bi) e® + H. c. (3e)

H _c = Ω® (Οβ + Ω®(.)3¾ + H. c. (3f) where Q is any operator within the enlarged system of scatterer and modes and {. ) denotes the quantum average value of variables. tQ _q/r describes the decoherence of the scatterer or cavities. For Stark control, Ω^(ί) are defined as the Stark fields and Cl _p ^{ (t) = 0. To swap the quantum ground states of the scatterer, both are set to be nonzero and finite to form a pair of STIRAP pulses. To distinguish from the Stark fields, Ω® © and o! Ct) in the

STIRAP case are denoted by S _p ^} (t) and Ω®ζϊ) respectively. In this case, H _in represents the driving of the cavity mode 6 with resonant frequency ω , via the input field _ln of frequency ω _ίη through the waveguide, where a _in corresponds to coherent amplitude of the input field and k _ex describes the extrinsic loss due to coupling of the modes to the waveguides. H ₀ describes the self-energy and is the free Hamiltonian of the cavity mode a, and output modes b where modes S, have resonant frequencies Relative to the incoming drive the a mode is detuned by Δ _ίη = ω _α - ω _1η , and the b ^w mode by Δ _; „ + S _t with δ _ι — ω^' - ω _α . H ₀ also includes the free energy of the two ground states of all of the /-scatterers with the detunings Δ^ = -

- ω _α and = — — , where is the frequency of the classical control field 110 shown in Figure 1{a) between the states |2) <→ |3), in the -th scatterer. The detuning between the mode />; and the /-th scatterer is given by Δ _¾ ⁽⁾ = εψ - e ^l> - where the eigenenergy of state \j) of /-th scatter is denoted with j e {1,2,3}. The excited state of the scatterers are assumed to be negligibly occupied and are not included in H ₀ . H _cc is the intrinsic coupling and describes the intrinsic cross-coupling among the cavity modes a and b with small rates due to e.g. evanescent coupling, Rayleigh, Brillouin, and Raman scattering. H _sc is the scatterer coupling and describes the coherent coupling between the cavities modes 3, ¾ and scatterers via the| l) «→ |3>, transition with coupling strengths g _a and g^. H _c represents the external controls and describes the coherent control used to induce Stark shifts or swap the scatterer's quantum state between | 1) and|2>. For tuning method (A) or Stark control, the classical fields required by method (B) are omitted [set as Ω _ρ ' (ί) and Cl^(t) to be nonzero and finite in amplitude and duration to form a pair of STIRAP pulses. To distinguish between the two tuning methods, for (A) Stark control, the control fields are denoted as [Ω^ί ]; and for (B) shuffling control, the control fields are denoted as [fi^Ct), H _j ^ Ct)]. Each classical control field only drives one scatterer and for the Stark control: has an oscillation, with the Rabi frequency [ω^, Ω®] driving the |2) <→ |3>, transition, while for the state

|3> (|2) <→ |3» transition of the -th scatterer. Both and Δ^ are assumed to be red-detuned, while the control fields are

+ & _in - Δρ·*. In this case generation of any Raman transition involving the cavity mode can be avoided when the controlling Stark or STIRAP fields are applied.

[0051] The decay of the scatterer and cavities can be modelled via the Linblad master equations (Eqn. 4(a) and 4(b)) assuming that the excited state | 3)j of I ^th scatter decays to the ground state \j) ( ^' e{l,2}) at the rate γ^), and decay rates κ _Α for cavity modes A(A e {abi})[h = 1] are also assumed. Q _q = - (Qd ) - (a Q)} (4a) Q _Q = ^Y {2(A QA†} - {Q A) - (A Q)} (4b)

[0052] The decay rate κ _Α of each cavity consists of two contributions, κ _Α - κ + for cavity a and κ _Α = represents the intrinsic loss in cavity α(ί>,), while describes loss due to coupling of modes a{bd to waveguides.

[0053] The overlap of the cavity evanescent fields with the waveguides leads to a coupling that is dependent on their gap, which is normally fixed. Only the cavity mode a couples to the input waveguide with strength K _CX . There is also bare cross talk coupling between the cavities i.e. in the absence of any scatterers. The a mode couples to the I ^th cavity S ₍ with strength ft® . Both these strengths can be adjusted by engineering the spacing between the waveguides or cavities. However, for the scatterer-mediated modulation to be fast, the bare intercavtiy crosstalk coupling must be much smaller than the coupling by each cavity to the scatterer. To achieve small cross talk is difficult, however a means for achieving this is discussed below. [0054] The goal is to optically control the effective couplings h _t = g _a gl _i <¾ } /A ⁽ _fl \ via the application of classical coherent fields which are selectively applied to implement one of the above-mentioned tuning methods: (A) Stark tuning is implemented by shifting the transition frequency of I ^th scatter or (B) Shuffle tuning is implemented by implementing STiRAP shuffling of the scattered internal population [24],

Input-Output Relations:

[0055] According to the input-output relation of an optical cavity [5, 26-28], the output field operators for the a and b _t cavities are given in terms of the input and intracavity field operators as o-out = -α _ίη + J2K~ a t) (5a)

where [A ^† ( to the decay rate from cavity bj due to coupling to the output p® _r . The coherent amplitudes of the input fields are given by <§ _in } = a _in and (6®) = 0. The transmission amplitudes are defined here as t _a = a _out /a _jn and t ₁ = b® _u /a _in . Therefore, the corresponding transmission coefficients are T _a (T _j ) = |t _a | ² (|t _] | ² ) for a coherent input a _in .

All-Optical Switch:

[0056] Both optical switching and routing rely on the realization of coupling between cavities & and Compared with an optical router with multiple ports, it is much easier to realize an optical switch. In the following description, an all-optical switch is first described, and this is then extended to the case of an optical router. In the former, only one b mode and one scatterer is present. Two specific arrangements of the all-optical switch system setup are depicted in Figure 1 (b) and Figure 1(c) respectively depicting a ring resonator setup and a photonic crystal setup. Figure 1 (c) depicts a photonic crystal switch with multiple outputs (i.e. an all-optical router as discussed in greater detail below).

[0057] To present a transparent picture showing how to create the coherent interaction between cavity modes a and b, the excited state |3> of the scatterer is adiabatically eliminated to obtain a reduced Hamiltonian H _red . To do this, the last term H _c . in Eq. 3(a) is dropped and it is assumed that g _a = g„ = g and δ _α = ω ₆ = ω, \δ\ « (ω ₆ + ω _α ). Applying the RWA, the reduced Hamiltonian then takes the form

#r«j = I Δ _ίη -— σ-n I a ^T a + I A _in - yo _u I 6 ^T h

(ho - ^-S!^ f-i+a + a+S)† i^2k _ex (a _ln &^ - α·„α), (6) with an effective coherent coupling (¾ _e/ > = {h ₀ - ^ _j -ffn ) ~ (¾> - ^j-) <ffu> + ¾2>-

[0058] For |Δ[ » |,g| ² (a ^† a), [g| ² <S ^† £>, the population in states \3) is negligible. Therefore, throughout the investigation discussed below, the state |3) is assumed to be adiabatically eliminated and is negligibly populated. For the sake of simplicity the intrinsic scattering is also neglected i.e. coupling ft ₀ = 0. Thus h _eff = - γ . The same intrinsic decay rate - κ — K\ and the same external coupling κ _βχ = κ^. are assumed as well. Under these assumptions, κ _α = κ ₆ . In numerical simulations, the assumption (O _s O _a/iJ ) ~ {0 _s )(d _a/h ) for a coherent input is also made, where O _s ; O _a and O _h are operators related to the scatterer, mode a and mode b, respectively.

[0059] The resulting semiclassical equations of motion for the mean values of the observables are valid when the scatterers are weakly driven by the cavity modes and also excited by a coherent input field. This approximation has been widely used in the study of Cavity-QED systems [5, 29, 30]. To obtain Eqn.6 from Eqn. 3, optical routing is enabled by modulating the coupling strength between two cavity modes, thus allowing the directed transmission of an incoming signal through the coupled-cavity system and out to an exit waveguide. To study the routing, we only one switching or routing node that consists of two cavity modes needs to be considered, the atomic scatterer, the input signal, and the associated classical control fields. The control fields serve to control the dynamics of the scattering either via (a) Stark tuning via a rapid tuning of the transition energy of the scatterer through the application of an intense Stark pulse or via (b) shuffling by turning on or off the intercavity coupling by transferring the scatterer's internal atomic state to an internal state that does not couple to either cavity.

[0060] In the setup as depicted in Figure 1(A), the arrangements of classical and quantum fields are far from two-photon resonance, and thus any Raman transitions induced by these fields between the two ground states of the scatterer are greatly suppressed. Thus, the term c Eq. (3f) can be neglected since it is due only to the classical control fields. As only a single node is considered, the index = 1) in Eq. (3) is dropped and replace

( , dl, bl _l aW, ag, ag) by (Δ _α , Δ,, 5, h _a , g _b b , σ _ι σ ₂₂ , σ ₁₃ ).

[0061] To proceed, it is assumed that g _a = g _b = g and that ω _α - ω„, which gives δ - 0. In the dispersive coupling regime, the population of the excited state |3) is negligible. It is possible therefore to adiabatically eliminate this excited state from the original Hamiltonian H' = H = H _c and derive an effective reduced Hamiltonian H _red - H and H _c are given by Eqn. (3). Using ~ = i[H', Q] and applying the rotating-wave approximation, we obtain:

⁽ π = -ί(Δ - Δ _[π ) ₁₃ + ig{ff _3:i - σ„)(α + b) (8)

[0062] Note that the detuning in is introduced because of the external driving of the cavity, which is independent of the scatterer. This detuning causes the operators {σ^, , b) to oscillate at frequency A _in . The oscillation can be elimi-nated from the equation by replacing (σ^, α, δ) by

[0063] Since varies slowly and the population in [3> is small, it is reasonable to assume 0 [53-55]. This assumption gives ¾s ^¾ f (<¾3 - ¾ι)(α + δ) (9)

[0064] Substituting Eq. (9) into the cavity-scatterer interaction Hamiltonian H _sc Eqn. (3e), we obtain gives

¾c - - δ ₁₁ (8*α + Β*Β)- & ₁₁ (α*Β + Β*&) (10)

[0065] Here, we have dropped the terms from H _n that only associate with 8 ₁ or <¾ ₂ , which commute with H _red , and also drop the terms associated with small σ ₃₃ . The effective Hamiltonian ¾ _c has also been corrected by dividing by 2. The necessity of this renormalization is verified by numerically comparing the Raman transition of a full three-level Λ-type system and its effective two-level counterpart when the excited state is eliminated. This correction has also been justified by other works [54,55]. After substituting Eq. (10) into Eq. (3), the reduced Hamiltonian given by Eq. (6) above is obtained. Steady State Solution

[0066] In the following discussion, expressions for the transmission, T _a , where light is routed out of the exit waveguide connected to cavity a, and T _b , where light is routed out of the exit waveguide connected to cavity b, are derived. Setting h = ^- <σ _η ) the detuning is redefined as Δ\ _η = Δ,„ - h. The steady state transmission is then calculated using the reduced Hamiltonian Eq. 6 with the assumptions of a maximum coupling h _max = 8κ _; and a minimum coupling h _min = 0 such that 0 < h≤ h _max . Such maximum coupling strength is easy to realize, using the current experimental technology. Note that h = 0 is achievable in practice only if h ₀ is zero for the shuffling control or a small h ₀ is cancelled by™- <t7 _1:L ) in h _eft for the Stark control. In the steady state, the transmission amplitudes t _a and t _b are given by:

(7b)

[0067] The corresponding transmission coefficients are T _a = \t _a \ ^z and 1), = \t _b \ ^z . When the transmission of signal is high, the state of channel is "on" and the transmission is denoted by T _on . On the contrary, T _off indicates a low-level output. The performance in switching the output on or off can be evaluated using the switching contrast:

SC— {T _on — T _n ff)/(r _on + T ₀ f .

[0068] In comparison with the situation of critical coupling for h - 0, the spectral window for switching is broad and flat in the overcoupled regime as discussed herein, the bandwidth being determined by h _max . This wide bandwidth promises a fast switching speed. For a large coupling h = h _max , almost all of the input field is reflected (in setup Fig. 1 (b)) or transmitted to another waveguide through cavity b (in setup Fig. 1(c)). The straight through transmission T _a is flat and vanishing off" state) but T _b is large, about 0:8 ("on" state). On the contrary, for h = h _min , the incident field exits mainly from the straight through output port a _out (T _a = 0:8), whereas T _b = 0. In contrast to previous works, for the case of an "off" state in both output ports the transmission is vanishingly small in the systems disclosed herein. This indicated a significant advantage of the methods & systems disclosed herein: switching contrasts of nearly unity. All-Optical Routing:

[0069] An all-optical router can be realized using the setup shown in Fig, 1 (d) where many cavities couple to the main cavity mode a. The coupling strength ft _; is individually modulated by the scatterer. Each cavity couples out to a unique output waveguide which forms an output port jS^ _f . Thus the input field _in can be routed into various output waveguides via the intermediate cavities & and

Time-Dependent Control:

[0070] To verify the above analysis and study the temporal switching behaviour, the master equation, Eqn. 2 is solved numerically. Since the population of the excited state |3> is negligible throughout the protocol the decay of the scatterer is neglected, i.e. γ _ΐ3 = y ₃₂ = 0.

Throughought the modelling below, the assumptions δ = 0, Δ = 800 g = 80 ις, and h ₀ = 0 are made, yielding an effective coherent coupling h ₀ = 8κ _; when and A _in == 8¾. For a cavity with a moderately good intrinsic quality factor Q ₀ > 10 ⁶ . the coupling strength g is required to be larger the 8 GHz. Such strong coupling strength can be realized in quantum dot- cavity systems. The change in transmission due to a small static coupling e.g. between cavities is negligible (not shown here) and in addition this static coupling can be cancelled via the Stark control according to Eqn. 6 if ^J ^- <a ₁₁ )=/i ₀ and <σ ₂₂ )~0.

[0071] The controlling optical fields are assumed to be a train of pulses, either on Ci _s (Stark field) or pulse pairs Ω _ρ and Ω _ρ (STIRAP fields for shuffle control). For Stark control, the Stark field for each pulse is chosen to be ~ ^arctan - arctan (' ^ ^**)]. where the Stark field with amplitude of il ₀ is blue-detuned with respect to the transition |3) <→ |3), is the delay(width) of the pulse while τ is a parameter characterizing the rise orfall time of the pulse. For the Stark control protocol, the values Ω ₀ = 3200κ,· and τ = lO ^"3 * ^"1 are chosen. These parameter values are chosen so as to be strong enough to provide rapid switching but not strong enough to heat up the three level system. Since for Stark control the population in states |2) and |3) are negligible, the detuning Δ _? can be much smaller than Ω ₀ , A _d = Ω _ο /10 in order to provide a large Stark shift Cl /A _s = 3.2 x 10 ⁴ γ ₀ . Another method to control the intermode coupling is to swap the population between the two ground states. In a particular arrangement, the technology of STIRAP is used, which is robust against noise in the fields. To avoid the disturbance from the cavity modes, the fields are blue-detuned again but on two-photon resonance. The STIRAP pulses Ω _ρ and Ω ₅ have the same profile but different widths and delays and are given by H _{p s} (t) = where Ω.' ₀ is the amplitude, re characterizes the width of the pulses and r _{p s} the delay. Control fields with 10 ^_3 o ¹ which operate much faster than the ring-down time of cavity, effect nearly instantaneous turn on/off of the intermode coupling.

Numerical Results:

[0072] In the following discussion, all-optical switching of the transmission T _a is demonstrated in the two schemes: (A) Stark control; (B) shuffle control. The initial state of the scatterer is taken to be | 1>, i.e.<a _l ) = 1. Referring to the numerical results shown in Figure 3, both schemes yield a short initial burst in transmission which is due to a short starting process of system, K _t t < 0.2. Because the cavity is empty the forward transmission T _a is large during this period according to the input-ouput relation Eq. 5. To demonstrate an example of switching behaviour, the output when K _t t > 1 is considered. Figures 3(a) and 3(b) depict the numerical results of all-optical switching of the transmission T _a in the two schemes of quantum state transfer via: (a) Stark control; and (b) STIRAP control. Broken lines 303a and 303b indicate the normalized controlling fields [Stark field for 3(A) and STIRAP field for 3(B)]. The STIRAP fields are offset for clarity. Solid lines 301a and 301 b depict the transmission T„. Δ, = -320κ;, Ω ₀ = 10& _s for 3(A), and A _p = Δ ₅ = -200κ; , ' ₀ = 100κ;,τ ₃ - τ _ρ = 0.025κ _ί ~ ¹ for 3(B). The switching time is about 0.1 , ^{" 1} . From the results shown in Figures 3(A) and 3(B), it is noticed that the output closely follows the controlling Stark field (dashed line 303a in Figure 3(A)), while the shuffle (STIRAP) control pulses toggles the transmission output on and off as seen in Figure 3(B) (solid lines 301 ). The 1/e switching time is short, about 0.1 κ ^"1 , and the transmission remains constant, about 0.8, for both schemes in the steady state. These numerical results agree closely with the analysis given in Eq. 7 and Figure 2. In Figure 3(B) the modulation of the optical transmission (solid line 301 b) of the coupled system by switching the ground state of the three level system between | 1) <→ |2) is shown. The broken curves 303a and 303b indicate the classical control optical field used to switch the internal state of the three level system while solid curves 301 a and 301 b depict the changing optical transmission coefficient (switching).

[0073] In the Stark control protocol, a Stark field fi _s .(t) is used to switch on or off the induced coherent coupling between cavity modes & and b. The amplitude il ₀ of the applied Stark field is 3200 ₍ . This value can be reduced if the detuning A _s is reduced. If the intrinsic quality factor Qo of cavity exceeds ~10 ^fi , corresponding to a total quality factor Q > 10 ^s because of the overcoupling to waveguides, then the coupling strength and control pulse can be reduced g < 10 GHz and Ω ₀ < 320 GHz corresponding to an intensity of / ~ 2 x 10 ^s W/cm ² if the dipole moment of scatterer is typically d ~ 3 x 10 ^-29 C-m. Such strong coupling has been achieved in current experiments. For fi _s = 0 the coherent interaction h is maximum h _max . As a result the straight through output is "off" and the transmission T _a ~ 0. When Ω ₅ is applied, the induced Stark shift is large enough to switch off h. This vanishing small h _min leads to T _a = 0.8 when the system reaches steady state. The numerical simulations described herein show that the transmission T _a can be turned off at its peaks before it reaches the steady states. Thus it is possible to encode information more densely within the same duration.

[0074] In the shuffle control protocol, the STIRAP pulse fields are used to swap the population of the scatterer between states | 1) and |2). The scatterer is initially prepared in i.e., (σ _η > = l, and the transmission T _a is negligible. When the population is swept to |2>, the coherent coupling h vanishes because of < _η > = 0, and subsequently the system yields a large transmission T _a = 0.8. Each pair of STIRAP fields encodes one bit information into the output a _aut and toggles the output on or off. In comparison with the Stark control protocol, an important advantage of the shuffle control protocol is that the applied fields are much weaker, about i¾ = IOOKJ.

[0075] It is important in practice to look into the performance of devices under the situation of nonzero intrinsic coupling h _u . The switching contrast is used in Figure 4 to show the robustness of devices disclosed herein against small h ₀ . It can be seen that the switching contrast decreases slowly as h ₀ increases. For a reasonably small coupling h ₀ < 2κ _£ , SC > 0.8 for the Stark control and SC > 2 /3 for the shuffling control. If we apply an optimal control scheme (dashed lines 403a and 403b), then the switching contrast can be larger than 0.8 for the two protocols.

[0076] Figures 5(a) to 5(d) show graphs depicting the numerical results obtained from the above-described models of an all-optical router with output ports (solid lines 501a -501 (d)] and (dashed lines 503a -503d) controlled by Stark control [Figures 5(a) and 5(b)] or via shuffling control with STIRAP pulses [Figures 5(c) and (d)]. Initially (ffn(O)} = 1 in the Stark control protocol, but (<¾ ₂ (0)) = 1 in the shuffling control. Parameters and the profiles for the control fields fi^" ¹ and ¾° are the same as in Figure 3. The solid lines 501 a and 501c and dashed lines 503a and 503c in Figures 5(a) and 5(c) respectively indicate the field applied to control the scatter 1 and 2, respectively. The solid lines 501 b and 501 d in Figures 5(b) and 5(d) are for the output β^ ⁽ _ν but the dashed lines 503a and 503b are for output β^. In Figure 5(c), solid lines in 501c represents the fields Ω® , and for while the broken lines in 503c are the control fields for In the cases depicted in Figures 5(a) to 5(b), the output a _aut is switched off. The profile of the controlling laser pulses are the same as in Fig. 3 but the delays are different. The scatterers are individually controlled by the corresponding laser pulse trains. In the Stark control scatterers always stay in state [ 1>, i.e. (σ ₁ ) = 1 but strong Stark fields are applied to eliminate the effective coupling h. In STIRAP control all scatterers are initially populated in state |2). Thus the ports β ₀ ⁽ are initially isolated from the input field. As a result, all β ports are initially "off". The coupling A for each scatterer is sequentially switched on, to h _max , when the Stark field is turned off (see Fig. 5(a)) (Stark control) or the scatter is swept into state | 1) (see Fig. 5(c)) (STIRAP control). Therefore the input field is routed to either waveguide ftC^ «= 0.8) or β ₂ (Τ ₂ ~ 0.8), which means either output turns "on". As shown in Figures 5(b) and 5(d), the binary optical information "10 0" and "0101" are encoded into ports and ?o„ _t , respectively. Unlike the demultiplexer-type router [9], the output of each port is similar because the energy of the input light is only transferred to the port that is switched on. This setup promises a small insertion loss of 20% independent of the number of output ports. If two output ports simultaneously turn "on", the light energy will be evenly fed into two ports.

[0077] Using the quantum switching methods and systems disclosed herein, it is possible to control not only the forward transmission but also the output β _οια from waveguide coupled to the cavity mode b as shown in Figures 2(b) and 2(c). In contrast to previously published works, the methods and systems disclosed herein can selectively route photons to many different output ports. One of possible setups for an all-optical router is illustrated in Figure 1 (d). Here the photons can be selectively sent out to ports a _aut or β^ _{ . Unlike Ref. [9], which demultiplexed the total field energy into several ports and then controlled the output of each port, each output port in the methods and systems disclosed herein withdraws light separately from a common cavity. So the switching fidelity of each output in the systems disclosed herein is independent of the number of ports.

[0078] The optical routers described herein are also robust against the small /¾, as shown in Figure 6 which depicts the switching contrast as a function of the intrinsic coupling for (A) Stark control and (B) shuffle control. Solid lines 601a and 60 b indicate the output β^, while dashed lines 602a and 602b are for the output β^ in each Figure. The time averages of T _on and T _of f in Figure 5 are used to evaluate the switching contrast. In Figure 6A, T _on is evaluated from / t calculated from

K _L t— 2.5 to 3 ?g„ _t and from K _t t = 3.5 to 4 for β^. The fixed internal coupling K _ex = hmax + kf is applied in all cases. As can be seen, the outputs /¾ _{t t} and / ^ _t decrease slightly in both protocols. For example, for h ₀ - 2κ _; , the switching contrast of is still 0.84, and that of slightly decreases to 0.75 in the Stark control, while it can remain 0.85 in the shuffling control. Such a level of switching contrast allows for routing quality larger than 0.75 in optical communications up to an intrinsic coupling of h ₀ =

Implementation:

[0079] The implementation of the methods and systems disclosed herein requires a strong coupling between a three-level Λ-type solid-state quantum system and a single photon in a "good" optical cavity. A coupling strength of GHz is already available in quantum dot-cavity systems [4, 31 , 32], in NV center-cavity systems [33] and Bose-Einstein condensate-cavity systems [34]. The deep strong coupling regime of g = 80y ₀ requires that the cavity has an intrinsic quality factor Q > 10 ⁶ but a total quality factor Q > 10 ^s . This requisite can be met using either photonic crystal cavities [9, 35-37] or toroidal cavities [38, 39]. If the state of the art technique can combine strong coupling [34] and a high-Q cavity [38], the rate g/K _t can reach 10 ⁴ [40]. The A-type scatter can be a single nitrogen-vacancy (NV) center in nanodiamond at low temperature [41 -43], quantum dot [44] or rare-earth ion-doped crystals [45,46]. Therefore, the methods and systems disclosed herein for all-optical switching/routing can be realized on a chip in various kinds of systems using current experimental techniques.

[0080] Another important requisite is to effectively couple the scatterer to two cavities simultaneously but greatly suppress the intrinsic coupling or the natural cross-talk coupling between the modes (coupling without the scatterer present). This issue has been solved in recent state-of-the-art experiments. If the two degenerate modes are considered to be counterpropagating modes in a toroidal cavity, it is possible to make toroidal cavities with a negligible intrinsic scattering (2h ₀ < κ ₍ + K _CX ) using existing technology [38, 39, 47], If quantum dots or nanodiamonds are inserted into a cavity, their geometric profiles cause additional scattering or cross talk between modes in the cavity. This cross talk is unwanted - the desire is to only have cross coupling mediated by the dipole coupling to the internal states of the scatterer. The geometrical scattering rate caused by a nanoparticle decreases quickly ( x r ³ ) as the size (radius r) of particle decreases [10]. Therefore, the effects of geometric scattering can be neglected for a scatterer with r<10 nm. Experiments have demonstrated that the scattering of a toroidal cavity embedding a nanoparticle only causes negligible broadening of the linewidth of the cavity mode, even with a Q factor Q > 10 ⁸ [38] much larger than that which is required for the presently disclosed systems. [0081] Rather than use degenerate modes in a toroidal cavity, it is preferable to use two spatially separated cavities, e.g., two photonic crystal cavities, and it is also desirable to decouple these cavities from each other if their mode fields are orthogonally polarized in the ^Λ Ex E _x and E _y in planes [48], respectively. In Ref. [48], spatially overlapping one-dimensional (1 D) photonic crystal cavities that are individually tunable and that are engineered to have very little cross-talk coupling was demonstrated experimentally. In this arrangement, by positioning the scatterer at the spatial crossing point of the two 1 D photonic crystal (PC) cavities and arranging that the dipole moment of the nanoscatterer is oriented along the direction of E _x + E _y , it is possible to couple the scatterer to each cavity mode with little intrinsic cross talk between the cavity modes. This configuration of orthogonal polarized cavities can be extended to multiport optical routers as discussed in the Example below.

[0082] The following discussion analyses the energy costs for the systems and methods for optical switching/routing discussed above. Assuming a typical transition dipole moment of d = 3.0 x 10 ^~29 C ^■ m and a refractive index of n ~ 3, the required electric field E is about 10 ⁶ V/m which corresponds to an intensity of / = 2 x 10 ⁵ W/cm ² required to achieve the strong Stark field Ω ₀ = 320 GHz. Due to the large dipole moment [49] the intensity required to drive a quantum dot can be lower [44]. Classical binary information can be encoded at 100 MHz rates. To neglect the creation of any cavity excitations the Stark pulse energy can be as low as 2 pj/bit if the field is tightly focused to 1 μιη ² . Optical control of nanoscale scatterers like NV centers or quantum dots but can also avoid exciting the optical cavity. Therefore the required energy cost can be reduced to 24 fj/bit if the fields are focussed into a nano sized area of approximately 5 x 80 nm ² using plasmons [50-52]. If the scatterers are driven via the excitation of another cavity mode [4], the laser power incident into the waveguide can be 50 nW (refer to Supplementary information of [4]). This energy cost is comparable to recent work using InGaAsP materials [9]. More interestingly proposed STIRAP scheme discussed above, where photons are routed via the coherent control of the ground state populations, requires vastly lower control powers than the STARK control scheme. The intensity (/ - 200W7cm ² ), of the STIRAP fields can be three orders lower than the intensity required for the Stark fields (Ω ₀ = 3200 γ ₀ ), thus indicating that the STIRAP protocol will be far more economical to control from a practical viewpoint.

Example

[0083] The present example illustrates a configuration for the above-described all-optical router with one forward input-output port and two cross output ports. This configuration uses either the 1 D nanobeams [48] or 1 D or 2D photonic crystal cavities [56, 57] in a planar configuration. The possible realisations of a three-port optical router are shown in Figures 7(a) and 7(b). These designs are used to show the main idea of how to suppress the intrinsic coupling between cavities and allow construction of multiport devices, but are not meant to be a detailed study of the optimal configuration.

[0084] Three possible structures for a multiport optical router are envisaged. In each, the light is always incident into cavity 1 and is routed into the β output ports [not shown in Figure 7(b)] mediated by the associated cavities 2 and 3. The polarisation of the electric field of a cavity mode is perpendicular to the 1 D cavity axis [48,56-60]. Therefore, it is possible to engineer the orientation of the polarization of the cavity mode. In the structures depicted in Figures 7(a) and 5(b), cavity 1 is y-polarized, but the other two cavities are polarized along the x-axis. In this configuration, cavities 1 , 2, and 3 decouple from each other, and their couplings can be only mediated by the scatterers. Unlike the structures of Figures 7(a) or 7(b), in the side-by-side configuration depicted in Figure 7(c) [61], cavity 1 can be z -polarized if it is thick in the z-direction [58,60], while cavities 2 and 3 are y-polarised. There, three arrangements can suppress the intrinsic coupling between cavities but allow interactions that are only mediated by the scatterers. Since the mode volume of a nanobeam nanocavity or a PC nanocavity is very small, the cavity-scatterer interaction can still be strong enough, even if two nanocavities (e.g., cavity 1 and cavity 2 or 3) is spatially separated. So, a multiport optical router can be experimentally realized using the geometry structures shown in Figures 7(a), 7(b) and 7(c).

[0085] In conclusion, a protocol to dynamically control the coupling between two cavity modes is presented. Using this protocol, all-optical switching and routing are demonstrated using numerical simulations. The wide bandwidth of transmission promises a short switching time and dense encoding capability. Because the photonic output while in the off state vanishes, a unit switching contrast is obtained. The output of the router is high and independent of the number of ports. If two scatterers are entangled in their ground states, the methods discussed herein will be able to create entangled coherent output fields.

[0086] As would be appreciated by the skilled addressee, the above disclosure presents protocols to dynamically control the coupling between two cavity modes. Using these protocols all-optical switching and routing are demonstrated using numerical simulations. The wide bandwidth of transmission promises a short switching time and dense encoding capability. Because the output of "off" state vanishes, a unity switching contrast is obtained. The output of the router is high and independent of the number of ports. If two scatterers are entangled in their ground states, the methods and systems disclosed herein would further enable the creation of entangled coherent output fields.

[0087] It will be appreciated that the methods, systems & apparatus described/illustrated above at least substantially provide a systems & methods for optical switching & routing utilising quantum control mechanisms enabling all-optical switching & routing functions.

[0088] The methods, systems & apparatus described herein, and/or shown in the drawings, are presented by way of example only and are not limiting as to the scope of the invention. Unless otherwise specifically stated, individual aspects and components of the methods, systems or apparatus may be modified, or may have been substituted therefore known equivalents, or as yet unknown substitutes such as may be developed in the future or such as may be found to be acceptable substitutes in the future. The methods, systems or apparatus may also be modified for a variety of applications while remaining within the scope and spirit of the claimed invention, since the range of potential applications is great, and since it is intended that the presently disclosed methods, systems & apparatus be adaptable to many such variations.

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