A single photon isolator

Abstract

The embodiment of the present application discloses a single-photon isolator, including a linear waveguide, a ring microcavity and a quantum dot, wherein the transition frequency of the quantum dot is consistent with the resonance frequency of the ring microcavity. When a single photon is incident from the forward port of the linear waveguide, it is introduced into the ring microcavity, propagates in a counterclockwise whispering gallery manner in the ring microcavity, and forms right-handed circularly polarized light, which is coupled with the quantum dot and then emitted from the reverse port of the linear waveguide. When a single photon is incident from the reverse port of the linear waveguide, it is introduced into the ring microcavity, propagates in a clockwise whispering gallery manner in the ring microcavity, and forms left-handed circularly polarized light, which will not couple with the quantum dot, and is then isolated and will not be emitted from the forward port of the linear waveguide. The single-photon isolator disclosed in the embodiment of the present application does not need to add a magnetic field device for controlling atoms and a laser device for cooling in the single-photon isolator, which effectively reduces the complexity of the single-photon isolator.

Description

Technical Field

[0001] The present application relates to the technical field of optical isolators, and in particular to a single-photon isolator. Background Art

[0002] As an optical device that limits the direction of light so that light can only be transmitted in one direction, optical isolators play an important role in optical communications and optical precision measurement systems.

[0003] Currently, commonly used optical isolators mainly use the Faraday rotation effect, which is achieved by applying a strong magnetic field in a magneto-optical medium to change the polarization of the light transmitted therein. It requires a device that provides a strong magnetic field, which is large in size and not conducive to chip integration. In addition, the nonlinear optical effect of waveguides or microcavities can also be used to achieve light isolation. The nonlinear effect of waveguides or microcavities requires a very strong light intensity to excite. For a single photon, its light intensity is not enough to excite the nonlinear effect of the waveguide or microcavity. Therefore, this optical isolator can only achieve the isolation of the light beam, and cannot be used as a single-photon isolator to limit the transmission direction of a single photon.

[0004] In order to limit the transmission direction of a single photon, the interaction between cold atoms and waveguides is often used to enable single photons to be transmitted normally when incident from the forward port of the waveguide, and to be isolated when incident from the reverse port of the waveguide. In practical applications, it is necessary to use a laser device to cool the rubidium atoms in advance to form cold atoms. However, adding a laser device for cooling rubidium atoms to the single-photon isolator will greatly increase the complexity of the single-photon isolator. Summary of the invention

[0005] In order to solve the problem of high complexity of the single-photon isolator, the present application discloses a single-photon isolator through the following embodiments.

[0006] A single-photon isolator disclosed in the present application comprises: a linear waveguide, a ring microcavity and a quantum dot;

[0007] The linear waveguide and the annular microcavity are arranged in a photonic crystal layer, and the annular microcavity is located on one side of the linear waveguide;

[0008] The linear waveguide is used to introduce a single photon into the annular microcavity when the single photon is incident from the forward port of the linear waveguide, so that when the single photon propagates in the annular microcavity in a counterclockwise whispering gallery manner, right-handed circularly polarized light is formed at a specific position in the annular microcavity; and is also used to introduce a single photon into the annular microcavity when the single photon is incident from the reverse port of the linear waveguide, so that when the single photon propagates in the annular microcavity in a clockwise whispering gallery manner, left-handed circularly polarized light is formed at the specific position;

[0009] The quantum dot is disposed in the annular microcavity and is located at the specific position;

[0010] The transition frequency of the quantum dot is consistent with the resonance frequency of the annular microcavity, so that when the single photon is incident from the forward port, it couples with the right-handed circularly polarized light, and when the single photon is incident from the reverse port, it does not couple with the left-handed circularly polarized light.

[0011] Optionally, the photonic crystal layer is a gallium arsenide crystal layer, and the thickness of the photonic crystal layer is 0.22 microns;

[0012] The photonic crystal layer is provided with periodically arranged air pore columns, each of which has a radius of 0.16 microns and a height of 0.22 microns;

[0013] The arrangement period of the air pore columns in the X direction is 0.44 microns, and the arrangement period in the Y direction is 0.762 microns.

[0014] Optionally, the width of the linear waveguide is 0.442 microns;

[0015] The outer radius of the annular microcavity is 0.801 microns and the inner radius is 0.4 microns;

[0016] The shortest distance between the central axis of the linear waveguide and the outer circle of the annular microcavity is 0.5408 microns.

[0017] Optionally, the resonance frequency of the annular microcavity and the transition frequency of the quantum dot are both 2π×244.5 THz.

[0018] Optionally, the quantum dots are indium arsenide quantum dots;

[0019] The quantum dot is a two-level system that is coupled only to the right-handed circularly polarized light.

[0020] Optionally, the forward port is located at the left end of the linear waveguide, and the reverse port is located at the right end of the linear waveguide.

[0021] The embodiment of the present application discloses a single-photon isolator, including a linear waveguide, a ring microcavity and a quantum dot, wherein the transition frequency of the quantum dot is consistent with the resonance frequency of the ring microcavity. When a single photon is incident from the forward port of the linear waveguide, it is introduced into the ring microcavity, propagates in a counterclockwise whispering gallery manner in the ring microcavity, and forms right-handed circularly polarized light, which is coupled with the quantum dot and then emitted from the reverse port of the linear waveguide. When a single photon is incident from the reverse port of the linear waveguide, it is introduced into the ring microcavity, propagates in a clockwise whispering gallery manner in the ring microcavity, and forms left-handed circularly polarized light, which will not couple with the quantum dot, and is then isolated and will not be emitted from the forward port of the linear waveguide. The single-photon isolator disclosed in the embodiment of the present application realizes the limitation of the transmission direction of the single photon through the interaction between the quantum dot and the photon, without the need to use cold atoms, so there is no need to add a laser device for cooling the rubidium atoms in the single-photon isolator, which effectively reduces the complexity of the single-photon isolator. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solution of the present application, the drawings required for use in the embodiments are briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0023] Figure 1 A schematic diagram of the principle structure of a single-photon isolator disclosed in an embodiment of the present application;

[0024] Figure 2 A top view of the principle structure of a single-photon isolator disclosed in an embodiment of the present application;

[0025] Figure 3 This is a schematic diagram of the single-photon transmittance of a single-photon isolator disclosed in an embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to solve the problem of high complexity of the single-photon isolator, the present application discloses a single-photon isolator through the following embodiments.

[0027] See also Figure 1 As shown, a single-photon isolator disclosed in an embodiment of the present application includes: a linear waveguide, a ring microcavity and a quantum dot.

[0028] The linear waveguide and the annular microcavity are arranged in a photonic crystal layer, and the annular microcavity is located at one side of the linear waveguide.

[0029] In one implementation, the photonic crystal layer is a gallium arsenide (GaAs) photonic crystal layer, and the thickness of the photonic crystal layer is 0.22 microns. The photonic crystal layer is provided with periodically arranged air pore columns, each of which has a radius of 0.16 microns and a height of 0.22 microns. The arrangement period of the air pore columns in the X direction is 0.44 microns, that is, the distance between the centers of two adjacent air pore columns in the X direction is 0.44 microns. The arrangement period of the air pore columns in the Y direction is 0.762 microns, that is, the distance between the centers of two adjacent air pore columns in the Y direction is 0.762 microns.

[0030] A linear waveguide is formed by introducing a linear defect in the gallium arsenide photonic crystal layer, and an annular microcavity is formed by introducing an annular linear defect. The width of the linear waveguide is 0.442 microns, and the length is the same as the photonic crystal layer. The outer radius of the annular microcavity is 0.801 microns, and the inner radius is 0.4 microns. The closest distance between the central axis of the linear waveguide and the outer circle of the annular microcavity is 0.5408 microns.

[0031] See also Figure 2 As shown, in one implementation, the left end (end A) of the linear waveguide is set as a forward port, and the right end (end B) of the linear waveguide is set as a reverse port. In this case, a single photon propagates in the forward direction when it propagates from left to right in the linear waveguide, and propagates in the reverse direction when it propagates from right to left.

[0032] When a single photon is incident from the forward port of the linear waveguide, the linear waveguide is used to introduce the single photon into the annular microcavity, so that the single photon propagates in the annular microcavity in a counterclockwise whispering gallery (CCW) manner; when the single photon is incident from the reverse port of the linear waveguide, the linear waveguide is used to introduce the single photon into the annular microcavity, so that the single photon propagates in the annular microcavity in a clockwise whispering gallery (CW) manner.

[0033] When a single photon propagates in a ring microcavity, it will be subject to strong lateral confinement. In this case, a chiral circularly polarized light field distribution will be formed in the ring microcavity. By observing the circularly polarized light intensity difference C, the circularly polarized light field distribution in the ring microcavity can be monitored. When C=0, the light field is linearly polarized light, when C>0, C is the right-handed circularly polarized light field intensity, and when C<0, C is the left-handed circularly polarized light field intensity. By monitoring the circularly polarized light field distribution in the ring microcavity, it can be found that when a single photon is incident from the forward port of the linear waveguide and coupled into the ring microcavity, right-handed circularly polarized light is formed at a specific position in the cavity; when a single photon is incident from the reverse port of the linear waveguide and coupled into the ring microcavity, left-handed circularly polarized light is formed at the same position in the cavity.

[0034] In the embodiment of the present application, the quantum dot is arranged at a specific position in the annular microcavity. When the propagation direction of light is opposite, since the circular polarized light formed here is in the opposite direction, the coupling strength between the quantum dot and the light field here is different. Wherein, the quantum dot is an indium arsenide quantum dot.

[0035] Generally, quantum dots can couple with both right-handed circularly polarized light and left-handed circularly polarized light. In order to make quantum dots couple only with right-handed circularly polarized light in forward propagation, in the embodiment of the present application, the optical Stark effect is used to initialize the quantum dots in advance.

[0036] Since the circularly polarized laser acts on the quantum dot, the transition energy level can be shifted, and the amount of shift can be controlled by the frequency and intensity of the loaded circularly polarized light. Therefore, by loading the quantum dot with an appropriate intensity of frequency Ω S The right-handed circularly polarized light is used to initialize the quantum dots, causing the energy level to shift by 2Δ OSE The subsequent quantum dot transition frequency is consistent with the resonance frequency of the annular microcavity. At this time, the right-handed circularly polarized light in the annular microcavity can couple with the quantum dot, while the transition frequency of the left-handed circularly polarized light is greatly detuned from the resonance frequency of the microcavity and does not couple. Therefore, the initialized quantum dot disclosed in the embodiment of the present application is a two-level system that is only coupled with the cavity field of the right-handed circularly polarized light.

[0037] When light is incident from one port into a linear waveguide, the transmission spectrum of the light can be obtained by monitoring the transmittance of the light at another port. There are many depressions in the transmission spectrum, and the light of the wavelength corresponding to the depression meets the resonance condition of the annular microcavity, so that the light is coupled from the waveguide into the cavity and then decays to zero in the space outside the cavity, so the transmittance is small. When the critical coupling is met, the transmittance is zero. By observing the transmission spectrum, it can be found that the light of the wavelength corresponding to the resonance point has a wavelength of 1.227μm, which meets the critical coupling. Therefore, in the embodiment of the present application, the resonant wavelength of the annular microcavity is set to 1.227 microns, and correspondingly, the resonant frequency of the annular microcavity is 2π×244.5THz. The transition frequency of the quantum dot after initialization is consistent with the resonant frequency of the annular microcavity, that is, the transition frequency of the quantum dot after initialization is 2π×244.5THz.

[0038] When the single photon is incident from the forward port, the quantum dot couples with the right-handed circularly polarized light, so that the single photon is transmitted normally and emitted from the reverse port. When the single photon is incident from the reverse port, the quantum dot does not couple with the left-handed circularly polarized light, so that the single photon is isolated and does not emit from the forward port.

[0039] The embodiment of the present application discloses a single-photon isolator, including a linear waveguide, a ring microcavity and a quantum dot, wherein the transition frequency of the quantum dot is consistent with the resonance frequency of the ring microcavity. When a single photon is incident from the forward port of the linear waveguide, it is introduced into the ring microcavity, propagates in a counterclockwise whispering gallery manner in the ring microcavity, and forms right-handed circularly polarized light, which is coupled with the quantum dot and then emitted from the reverse port of the linear waveguide. When a single photon is incident from the reverse port of the linear waveguide, it is introduced into the ring microcavity, propagates in a clockwise whispering gallery manner in the ring microcavity, and forms left-handed circularly polarized light, which will not couple with the quantum dot, and is then isolated and will not be emitted from the forward port of the linear waveguide. The single-photon isolator disclosed in the embodiment of the present application realizes the limitation of the transmission direction of the single photon through the interaction between the quantum dot and the photon, without the need to use cold atoms, so there is no need to add a laser device for cooling the rubidium atoms in the single-photon isolator, which effectively reduces the complexity of the single-photon isolator.

[0040] According to the single-photon isolator disclosed in the above embodiment, after numerical simulation using electromagnetic simulation software based on the finite-difference time-domain method, the transmittance of single photons in the positive and negative propagation directions is obtained, such as Figure 3 As shown, the curve represented by the solid line represents the transmittance of a single photon propagating in the forward direction, and the curve represented by the dotted line represents the transmittance of a single photon propagating in the reverse direction. It can be seen from the figure that when the incident frequency of the single photon is consistent with the resonance frequency of the annular microcavity, that is, at the resonance point Δ=0, the transmittance of a single photon propagating in the forward direction approaches 1, and the transmittance of a single photon propagating in the reverse direction approaches 0. In other words, a single photon can be transmitted in the forward direction but not in the reverse direction. Therefore, the single-photon isolator disclosed in the embodiment of the present application can achieve a single-photon isolation effect. And, from Figure 3 It can be concluded that the isolation working bandwidth of the single-photon isolator is about 1.4κ i ≈2π×34.2 GHz. Compared with the single-photon isolator in the prior art with a very narrow working bandwidth (usually only tens of megahertz (MHz)), the single-photon isolator disclosed in the embodiment of the present application has the advantage of a very wide working bandwidth.

[0041] In addition, the single-photon isolator disclosed above does not require an external magnetic field, and single-photon isolation can be achieved by only using optical field control. In summary, the single-photon isolator disclosed in the embodiment of the present application has the advantages of simple structure, being easy to integrate into a chip, being able to achieve all-optical optical isolation without a magnetic field, and having a wide working bandwidth.

[0042] The present application is described in detail above in conjunction with specific implementation methods and exemplary examples, but these descriptions cannot be understood as limiting the present application. Those skilled in the art understand that, without departing from the spirit and scope of the present application, a variety of equivalent replacements, modifications or improvements can be made to the technical solution of the present application and its implementation methods, all of which fall within the scope of the present application. The scope of protection of the present application shall be subject to the attached claims.

Claims

1. A single photon isolator, characterized in that: include: Linear waveguides, ring microcavities, and quantum dots; The linear waveguide and the annular microcavity are arranged in a photonic crystal layer, and the annular microcavity is located on one side of the linear waveguide; The linear waveguide is used to introduce a single photon into the annular microcavity when the single photon is incident from the forward port of the linear waveguide, so that when the single photon propagates in the annular microcavity in a counterclockwise whispering gallery manner, right-handed circularly polarized light is formed at a specific position in the annular microcavity; It is also used to introduce the single photon into the annular microcavity when the single photon is incident from the reverse port of the linear waveguide, so that when the single photon propagates in the annular microcavity in a clockwise whispering gallery manner, left-handed circularly polarized light is formed at the specific position; The quantum dot is disposed in the annular microcavity and is located at the specific position; The transition frequency of the quantum dot is consistent with the resonance frequency of the annular microcavity, so as to couple with the right-handed circularly polarized light when the single photon is incident from the forward port, and not couple with the left-handed circularly polarized light when the single photon is incident from the reverse port; The resonance frequency of the annular microcavity and the transition frequency of the quantum dots are both 2π×244.5 THz; the quantum dots are indium arsenide quantum dots; the quantum dots are two-level systems that only couple with the right-handed circularly polarized light; the forward port is located at the left end of the linear waveguide, and the reverse port is located at the right end of the linear waveguide.

2. The single photon isolator according to claim 1, characterized in that: The photonic crystal layer is a gallium arsenide crystal layer, and the thickness of the photonic crystal layer is 0.22 microns; The photonic crystal layer is provided with periodically arranged air pore columns, each of which has a radius of 0.16 microns and a height of 0.22 microns.

3. The single photon isolator according to claim 1, characterized in that: The width of the linear waveguide is 0.442 microns; The outer radius of the annular microcavity is 0.801 microns and the inner radius is 0.4 microns; The shortest distance between the central axis of the linear waveguide and the outer circle of the annular microcavity is 0.5408 microns.

Images