CN110224820B - On-chip decoder and decoding method for polarization BB84 protocol - Google Patents
On-chip decoder and decoding method for polarization BB84 protocol Download PDFInfo
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- CN110224820B CN110224820B CN201910488792.4A CN201910488792A CN110224820B CN 110224820 B CN110224820 B CN 110224820B CN 201910488792 A CN201910488792 A CN 201910488792A CN 110224820 B CN110224820 B CN 110224820B
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Abstract
An on-chip decoder and decoding method for a polarized BB84 protocol, comprising: an input waveguide (100); a polarization beam splitting rotator (200) for splitting and rotating the received signal light into two signal lights having the same polarization direction; a phase offset modulator (300) for adjusting the phase of the input signal light; a 2 × 2 interference coupler (400) for performing hermitian operations on the received signal light; a 1 × 2 optical splitter (500) for splitting the received signal light into two signal lights of equal intensity; and an output waveguide (600) for outputting the decoded signal light. The decoder and the decoding method can realize passive demodulation of different preparation base polarization BB84 protocols, and for polarization unbalance in a fiber channel, the decoder can realize accurate compensation through on-chip regulation.
Description
Technical Field
The invention relates to the technical field of quantum communication and integrated optics, in particular to an on-chip decoder and a decoding method for a polarization BB84 protocol.
Background
The quantum cryptography is a product combining quantum mechanics and cryptography, and solves the problem of key distribution of a classical cryptography. The method utilizes the basic principle of quantum mechanics, namely the principle of inaccurate measurement and the single quantum state unclonable theorem, and ensures that data in a public channel is not necessary to worry about eavesdropping in the key distribution process. The quantum key distribution device generally recognized at present is based on the traditional discrete optical prism or optical fiber device, and has the advantages of large volume, difficult integration, high cost and no contribution to large-scale commercialization. With the development of silicon-based photonics, the functions of discrete optical devices can be gradually realized on a chip, so that the integration is convenient, and meanwhile, the mass production with large scale and low cost can be realized by utilizing a mature silicon device processing platform. Attempts have then been made to integrate the components and subsystems required for quantum key distribution devices on-chip. For the quantum key distribution of the polarization BB84 protocol, the setting of the demodulation end generally corresponds to the polarization state prepared by the emission end, i.e. the detection base of the demodulation end is consistent with the preparation base of the emission end, and the polarization BB84 protocol of different preparation bases cannot be demodulated. Meanwhile, because the birefringence effect in a general optical fiber channel can cause polarization imbalance, the conventional scheme needs to add a polarization controller in front of the demodulation end of the QKD system to perform polarization compensation, which causes additional cost and sacrifice of code rate.
Disclosure of Invention
Technical problem to be solved
Based on the technical problems, the invention provides an on-chip decoder and a decoding method for a polarization BB84 protocol, which are used for solving the problems that discrete elements in the traditional scheme are large in size and high in cost, the polarization BB84 protocol under different preparation bases cannot be demodulated, on-chip polarization compensation cannot be realized and the like in the prior art.
(II) technical scheme
In a first aspect, the present invention provides an on-chip decoder for a polarized BB84 protocol, comprising:
an input waveguide 100 for inputting signal light to be decoded;
the polarization beam splitting rotator 200 is used for splitting and rotating the signal light to be decoded into two beams of signal light with the same polarization direction;
the phase offset modulator 300 includes a first phase offset modulator 301, a second phase offset modulator 302, and a third phase offset modulator 303, and is configured to adjust a phase of the signal light received by the phase offset modulator, where the first phase offset modulator 301 is configured to perform phase adjustment on one of the two signal lights output by the polarization beam splitting rotator 200 and having the same polarization direction;
the 2 × 2 interference coupler 400 includes a first interference coupler 401, a second interference coupler 402, and a third interference coupler 403, configured to perform hermitian operation on received signal light, where the first interference coupler 401 is configured to perform hermitian operation on the signal light after phase adjustment of the first phase offset modulator 301 and another signal light of the two signal lights with the same polarization direction output by the polarization beam splitting rotator 200, transmit one signal light after phase adjustment to the second interference coupler 402 after phase adjustment of the second phase offset modulator 302, directly transmit the other signal light after phase adjustment to the second interference coupler 402, and perform hermitian operation on the two signal lights received by the second interference coupler 402;
a 1 × 2 optical splitter 500, including a first optical splitter 501 and a second optical splitter 502, configured to split the received signal light into two signal lights with equal intensity, where the first optical splitter 501 receives one signal light after hermitian operation of the second interference coupler 402 and splits the signal light into two signal lights with equal intensity, and the second optical splitter 502 receives the other signal light after hermitian operation of the second interference coupler 402 and splits the signal light into two signal lights with equal intensity;
the output waveguide 600 includes a first output waveguide 601, a second output waveguide 602, a third output waveguide 603, and a fourth output waveguide 604, where the first output waveguide 601 is configured to output one of the two signal lights split by the first optical splitter 501, the fourth output waveguide 604 is configured to output one of the two signal lights split by the second optical splitter 502, the other of the two signal lights split by the first optical splitter 501 is output to the third interference coupler 403 after being phase-adjusted by the third phase bias modulator 303, the other of the two signal lights split by the second optical splitter 502 is directly output to the third interference coupler 403, and the third interference coupler 403 performs hermitian operation on the two signal lights received by the third interference coupler 403 and outputs the two signal lights through the second output waveguide 602 and the third output waveguide 603.
Optionally, the input waveguide 100, the polarization beam splitter rotator 200, the phase offset modulator 300, the 2 × 2 interference coupler 400, the 1 × 2 optical beam splitter 500, and the output waveguide 600 are made of silicon materials, and are processed by a process compatible with a microelectronic process, so as to realize on-chip integration.
Optionally, the polarization beam splitting rotator 200 is an adiabatic asymmetric directional coupler, one side of which has an input port and the other side of which has two output ports, the input port receives the signal light to be decoded, the signal light to be decoded is first decomposed into two beams of signal light with orthogonal polarization directions, and then the polarization direction of one of the two beams of signal light with orthogonal polarization directions is rotated to be the same as the polarization direction of the other beam of signal light, so as to obtain the two beams of signal light with the same polarization directions.
Alternatively, the phase offset modulator 300 employs a thermo-optically tuned phase modulator.
Alternatively, the 2 × 2 interference coupler 400 employs a 2 × 2 multimode interference coupler; the 1 x 2 optical splitter 500 employs a 1 x 2 multimode interference coupler.
Alternatively, the input waveguide 100 employs fundamental mode transmission of transverse electric field mode and fundamental mode transmission of transverse magnetic field mode, and the output waveguide 600 employs fundamental mode transmission of transverse electric field mode.
Alternatively, the on-chip decoder may decode four polarization quantum states at any two sets of non-orthogonal bases that satisfy the polarization BB84 protocol.
Alternatively, the signal light input to the waveguide 100 may be polarization-compensated by adjusting the first phase bias modulator 301, the second phase bias modulator 302, and the third phase bias modulator 303.
Another aspect of the present invention provides a decoding method, including:
s1, inputting signal light to be decoded through the input waveguide 100, wherein the signal light to be decoded is quantum state signal light transmitted by the polarization BB84 protocol;
s2, the signal light to be decoded is decomposed and rotated into two signal lights with the same polarization direction through the polarization beam splitter rotator 200;
s3, the phase of the received signal light is adjusted by the phase offset modulator 300, and hermitian operation is performed on the phase-adjusted signal light by the 2 × 2 interference coupler 400, so as to obtain a decoded signal light meeting the decoding requirement of the polarization BB84 protocol, and the decoded signal light is output through the output waveguide 600.
Optionally, S3 includes:
one of the two signal lights with the same polarization direction output by the polarization beam splitting rotator 200 is input into the first interference coupler 401 after being subjected to phase adjustment through the first phase offset modulator 301, the other signal light is directly input into the first interference coupler 401, and the first interference coupler 401 performs hermitian operation on the input signal light;
one beam of signal light after Hermite operation of the first interference coupler 401 is subjected to phase adjustment through the second phase bias modulator 302 and then is input into the second interference coupler 402, the other beam of signal light is directly input into the second interference coupler 402, and the second interference coupler 402 is used for Hermite operation on the input signal light;
the first optical beam splitter 501 receives one beam of signal light after Hermite operation of the second interference coupler 402 and splits the signal light into two beams of signal light with equal intensity, and the second optical beam splitter 502 receives the other beam of signal light after Hermite operation of the second interference coupler 402 and splits the signal light into two beams of signal light with equal intensity;
one of the two signal lights split by the first optical splitter 501 is output through the first output waveguide 601, and one of the two signal lights split by the second optical splitter 502 is output through the fourth output waveguide 604;
the other of the two signal lights split by the first optical splitter 501 is output to the third interference coupler 403 after being phase-adjusted by the third phase shift modulator 303, the other of the two signal lights split by the second optical splitter 502 is directly output to the third interference coupler 403, and the third interference coupler 403 performs hermitian operation on the two signal lights received by the third interference coupler 403 and outputs the two signal lights through the second output waveguide 602 and the third output waveguide 603.
(III) advantageous effects
The invention provides an on-chip decoder and a decoding method for a polarization BB84 protocol, wherein the whole device is processed by adopting a process compatible with a microelectronic process, and large-scale mass production with integration and low cost can be realized. The device realizes the large-scale on-chip phase offset regulation and control by utilizing the thermo-optic effect of the silicon material, and realizes the demodulation function of the polarization BB84 protocol under different preparation bases by combining other logic devices and phase debugging on the basis; meanwhile, for the polarization unbalance of the quantum state signal light in a channel, the decoder can carry out on-chip compensation through corresponding regulation and control to realize accurate passive demodulation, and the extra cost and code rate sacrifice caused by an off-chip polarization controller are reduced.
Drawings
Fig. 1 schematically illustrates a decoder structure for the polarization BB84 protocol and a schematic diagram of an external probe according to an embodiment of the present disclosure;
fig. 2 schematically illustrates a composition diagram of quantum states of a polarization BB84 protocol and an orientation diagram thereof in a two-dimensional hilbert space, in accordance with an embodiment of the disclosure;
fig. 3 schematically shows an orientation schematic diagram of four quantum states in a two-dimensional hilbert space and quantum state schematic diagrams of three typical BB84 protocols, which satisfy the BB84 protocol according to an embodiment of the disclosure;
fig. 4 schematically shows a quantum state signal light evolution diagram of a decoder for polarization BB84 protocol in an operating state according to an embodiment of the present disclosure;
fig. 5 schematically illustrates a hermitian operation diagram of quantum-state signal light under the decoding method for the polarization BB84 protocol according to the embodiment of the present disclosure;
fig. 6 schematically shows a decoded signal light diagram at the output waveguide for the decoding method of the polarization BB84 protocol according to an embodiment of the disclosure.
[ reference numerals ]
100-input waveguide
200-polarization beam splitting rotator
300-phase offset modulator
301-first phase offset modulator 302-second phase offset modulator
303-third phase offset modulator
400-2 x 2 interference coupler
401-first interference coupler 402-second interference coupler
403-third interference coupler
500-1X 2 optical beam splitter
501-first beam splitter 502-second beam splitter
600-output waveguide
601-first output waveguide 602-second output waveguide
603-third output waveguide 604-fourth output waveguide
700-probe
701-first external probe 702-second external probe
703-third external probe 704-fourth external probe
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.
In a first aspect, the present invention provides an on-chip decoder for the polarized BB84 protocol, see fig. 1, comprising:
an input waveguide 100 for inputting signal light to be decoded.
Specifically, the input waveguide 100 is used for inputting signal light to be decoded, and in the embodiment of the present invention, the signal light is quantum-state signal light sent by a polarization BB84 protocol.
The polarization beam splitter rotator 200 is configured to split and rotate the signal light into two signal lights with the same polarization direction.
Specifically, in the decoder according to the embodiment of the present invention, the polarization beam splitter rotator 200 is an adiabatic asymmetric directional coupler, one side of which has one input port and the other side of which has two output ports, the input port receives the signal light transmitted by the waveguide 100, and then decomposes the signal light into two signal lights with orthogonal polarization directions, and then rotates the polarization direction of one of the two signal lights with orthogonal polarization directions to be the same as the polarization direction of the other signal light, so as to obtain two signal lights with the same polarization direction, and output the two signal lights to the first interference coupler 401.
The phase offset modulator 300 includes a first phase offset modulator 301, a second phase offset modulator 302, and a third phase offset modulator 303, and is configured to adjust the phase of the input signal light. Further, by adjusting the first phase bias modulator 301, the second phase bias modulator 302, and the third phase bias modulator 303, the signal light input to the input waveguide 100 can be polarization-compensated.
Specifically, in the decoder according to the embodiment of the present invention, the first phase offset modulator 301, the second phase offset modulator 302, and the third phase offset modulator 303 adopt thermo-optic tuned phase modulators, and the first phase offset modulator 301 is disposed between the polarization beam splitting rotator 200 and the first interference coupler 401, and is configured to perform phase adjustment on one of two signal lights split by the polarization beam splitting rotator 200; the second phase bias modulator 302 is arranged between the first interference coupler 401 and the second interference coupler 402, and is used for adjusting the phase of one of the two beams of signal light after hermitian operation of the first interference coupler 401; the third phase shift modulator 303 is disposed between the first optical splitter 501 and the third interference coupler 403, and is configured to perform phase adjustment on one of the two signal lights split by the first optical splitter 501.
The 2 × 2 interference coupler 400 includes a first interference coupler 401, a second interference coupler 402, and a third interference coupler 403, and is configured to perform hermitian operation on received signal light.
The 1 × 2 optical splitter 500 includes a first optical splitter 501 and a second optical splitter 502, and is configured to split the received signal light into two signal lights with equal intensities. The first optical beam splitter 501 receives one beam of signal light after hermitian operation of the second interference coupler 402 and splits the signal light into two beams of signal light with equal intensity, and the second optical beam splitter 502 receives the other beam of signal light after hermitian operation of the second interference coupler 402 and splits the signal light into two beams of signal light with equal intensity;
specifically, in the decoder according to the embodiment of the present invention, the first interference coupler 401, the second interference coupler 402, and the third interference coupler 403 employ 2 × 2 multimode interference couplers (2 × 2MMI), and the first optical beam splitter 501 and the second optical beam splitter 502 employ 1 × 2 multimode interference couplers (1 × 2 MMI); the first interference coupler 401 is configured to perform hermitian operation on the signal light subjected to phase adjustment by the first phase offset modulator 301 and another signal light split by the polarization beam splitting rotator 200, perform phase adjustment on one signal light subjected to phase adjustment by the second phase offset modulator 302, transmit the signal light to the second interference coupler 402, and directly transmit the other signal light subjected to phase adjustment to the second interference coupler 402 to perform hermitian operation; the first optical beam splitter 501 receives one beam of signal light after Hermite operation of the second interference coupler 402 and splits the signal light into two beams of signal light with equal intensity, and the second optical beam splitter 502 receives the other beam of signal light after Hermite operation of the second interference coupler 402 and splits the signal light into two beams of signal light with equal intensity; the other of the two signal lights split by the first optical splitter 501 is phase-adjusted by the third phase shift modulator 303 and then output to the third interference coupler 403, the other of the two signal lights split by the second optical splitter 502 is directly output to the third interference coupler 403, and the third interference coupler 403 performs hermitian operation on the two signal lights received by the third interference coupler 403.
The output waveguide 600 includes a first output waveguide 601, a second output waveguide 602, a third output waveguide 603, and a fourth output waveguide 604.
Specifically, the first output waveguide 601 outputs one of the two signal lights split by the first optical splitter 501, the fourth output waveguide 604 outputs one of the two signal lights split by the second optical splitter 502, and the second output waveguide 602 and the third output waveguide 603 are configured to output two signal lights after hermitian operation by the third interference coupler 403.
The output waveguide 600 is connected to the external detector 700, specifically, the first output waveguide 601 is connected to the first external detector 701, the second output waveguide 602 is connected to the second external detector 702, the third output waveguide 603 is connected to the third external detector 703, and the fourth output waveguide 604 is connected to the fourth external detector 704.
In the decoder according to the embodiment of the present invention, the input waveguide 100, the polarization beam splitter rotator 200, the phase bias modulator 300, the 2 × 2 interference coupler 400, the 1 × 2 optical beam splitter 500, and the output waveguide 600 are made of silicon materials, that is, the decoder is manufactured on a silicon substrate by a process compatible with a general microelectronic process, so as to realize on-chip integration. The input waveguide 100 adopts a transverse electric field mode fundamental mode and a transverse magnetic field mode fundamental mode for transmission, and the output waveguide 600 adopts a transverse electric field mode fundamental mode for transmission.
The decoder of the embodiment of the invention can decode four polarization quantum states under any two groups of non-orthogonal bases meeting the polarization BB84 protocol.
In a second aspect, an embodiment of the present invention further provides a decoding method for a polarization BB84 protocol, which can perform passive demodulation on a polarization BB84 protocol under different preparation bases, and the following describes the demodulation method in detail by taking passive demodulation on a polarization BB84 protocol under different preparation bases as an example. The method comprises the following steps:
s1, inputting signal light to be decoded through the input waveguide 100, wherein the signal light to be decoded is quantum state signal light transmitted by the polarization BB84 protocol;
specifically, the quantum state transmitted by the polarization BB84 protocol is composed of two pulses (TE0 pulse and TM0 pulse) with orthogonal polarization directions, the basic structure and the orientation thereof in the two-dimensional hilbert space are shown in fig. 2, and for the quantum state with coordinates (θ, Φ) in the two-dimensional hilbert space
It represents TE0 pulse intensity as cos2(theta/2) TM0 pulse intensity is sin2(θ/2), and the phase difference of the TM0 pulse relative to the TE0 pulse is φ, expressed as follows using quantum mechanical operators:
(θ，φ)：
the quantum state orthogonal to the (theta, phi) quantum state in the two-dimensional Hilbert space is its point of symmetry (pi-theta, phi + pi) about the center of the sphere, using quantum mechanical operators
Is represented as follows:
(π-θ，φ+π)：
for the polarization BB84 protocol, the encoding end needs to prepare two groups of different quantum state signal lights, as shown in FIG. 3, the two quantum states in each group are orthogonal to each other, i.e. the two quantum states are orthogonal to each other
And
is orthogonal,
And
orthogonal, the two sets of quantum states are not orthogonal. In the two-dimensional hilbert space, the four quantum state signal lights are displayed as the quartering points on the circle passing through the center of sphere, so that the four quantum state signal lights input to the waveguide 100 can be represented as:
(θ1，φ1)：
(π-θ1，φ1+π)：
(θ2，φ2)：
(π-θ2，φ2+π)：
s2, the signal light to be decoded is decomposed and rotated into two signal lights with the same polarization direction by the polarization beam splitter rotator 200.
Specifically, as shown in fig. 4, the polarization beam splitter rotator 200 first splits the signal light to be decoded into two signal lights with orthogonal polarization directions, and then rotates the polarization direction of one of the two signal lights with orthogonal polarization directions to be the same as the polarization direction of the other signal light, so as to obtain two signal lights with the same polarization directions. To this end, information of the TE0 pulse in the polarized quantum state signal light in the input waveguide 100 is transferred to the upper input end of the first interference coupler 401, and the TM0 pulse is rotated to the TE0 pulse and information is transferred to the lower input end of the first interference coupler 401.
S3, the phase of the received signal light is adjusted by the phase offset modulator 300, and hermitian operation is performed on the phase-adjusted signal light by the 2 × 2 interference coupler 400, so as to obtain a decoded signal light meeting the decoding requirement of the polarization BB84 protocol, and the decoded signal light is output through the output waveguide 600.
For the polarization BB84 protocol, passive decoding needs to realize two kinds of equal probability different demodulation, the first demodulation can make an accurate response to the first group of quantum state signal light prepared by the encoding end, and make a random response to the second group of quantum state signal light; the second demodulation can respond to the second group of quantum state signal light prepared by the encoding end accurately and respond to the first group of quantum state signal light randomly. The decoder disclosed in the embodiment of the present invention can passively demodulate any four kinds of quantum state signal lights (as described in step S1) satisfying the polarization BB84 protocol.
Specifically, in the decoder disclosed in the embodiment of the present invention, the polarization beam splitter rotator 200 receives quantum-state signal light input by the input waveguide 100 and splits and rotates the quantum-state signal light into two beams of signal light with the same polarization direction, wherein one beam of signal light is transmitted to the first interference coupler 401 after being subjected to phase adjustment by the first phase bias modulator 301, and the other beam of signal light is directly transmitted to the first interference coupler 401 for hermitian operation; the first interference coupler 401 performs phase adjustment on one beam of signal light after operation through the second phase offset modulator 302, and then transmits the signal light to the second interference coupler 402, and directly transmits the other beam of signal light after operation to the second interference coupler 402; the second interference coupler 402 performs hermitian operation on the two received signal lights, transmits one signal light after operation to the first optical beam splitter 501 and splits the signal light into two signal lights with equal intensity, transmits the other signal light after operation to the second optical beam splitter 502 and splits the signal light into two signal lights with equal intensity; the first optical splitter 501 and the second optical splitter 502 each include an upper output end and a lower output end, the signal lights at the upper output end of the first optical splitter 501 and the lower output end of the second optical splitter 502 jointly enter the first group of demodulation components a (as shown by the solid line box in fig. 1), and the signal lights at the lower output end of the first optical splitter 501 and the upper output end of the second optical splitter 502 jointly enter the second group of demodulation components B (as shown by the solid line box in fig. 1). The first group of demodulation components a includes an output waveguide 601 and an output waveguide 604, and are configured to output signal light at the lower output end of the first optical splitter 501 and the upper output end of the second optical splitter 502, the second group of demodulation components B includes a third phase offset modulator 303, a third interference coupler 403, and output waveguides 602 and 603, the signal light at the lower output end of the first optical splitter 501 is subjected to phase adjustment by the third phase offset modulator 303, is transmitted to the third interference coupler 403, and the signal light at the upper output end of the second optical splitter 502 is directly transmitted to the third interference coupler 403 for hermitian operation, and the two signal lights after hermitian operation are output by the output waveguides 602 and 603. Since the first optical splitter 501 and the second optical splitter 502 each split the input signal light into two signal lights having equal intensity, the demodulation probabilities of two different sets of demodulation components are the same.
For the first group of demodulation modules a, as shown in fig. 1 and 4, information of TE0 pulse in quantum state signal light is transferred to the upper input end of the first interference coupler 401, TM0 pulse is rotated to TE0 pulse and information is transferred to the lower input end of the first interference coupler 401, and the phases of the first phase bias modulator 301 and the second phase bias modulator 302 are set to γ, respectively1＝π-φ1And gamma2＝θ1Then the equivalent Hermite operations and the joint Hermite operations L of the first phase bias modulator 301, the second phase bias modulator 302, the first interference coupler 401 and the second interference coupler 4021Comprises the following steps:
under operation of L1:
the above results show that the quantum state is obtained after Hermite's operation L1
And
will rotate to two extreme points of the hilbert space as shown in fig. 5, which shows that the two signal lights output by the second coupler 402 will generate the signal lights as shown in fig. 6, i.e. quantum state lights, at the output waveguides 601, 604 after entering the first group of demodulation components a through the first optical splitter 501 and the second optical splitter 502, respectively
Will respond accurately, quantum states, at the first external detector 701 as shown in fig. 1
Will respond accurately at the fourth external detector 704 due to the four quantum states
And
is a quartering point on a circle passing through the center of the sphere, so the first external detector 701 and the fourth external detector 704 are for quantum states
And
the response of (a) is random.
For the second group of demodulation components B, after Hermite operation L1, quantum state
And
will rotate to two extreme points of the Hilbert space as shown in FIG. 5, then the quantum state
And
must go to the equator of the hubert space, and will not be recorded as:
at this time, the phase γ of the third phase bias modulator 303 is set2＝σ2+ π/2, the equivalent Hermite's calculation L of the third phase-shift modulator 303 and the third interference coupler 4032Comprises the following steps:
under operation of L2:
the above results show that the quantum state is obtained after Hermite's operation L2
And
will rotate to two extreme points of the hilbert space as shown in fig. 5, which shows that the two signal lights output from the second coupler 402 will generate the signal lights as shown in fig. 6, i.e. the quantum state lights, at the output waveguides 602 and 603 after entering the second group of demodulation components B through the first optical splitter 501 and the second optical splitter 502, respectively
Will be outside the second position as shown in figure 1Accurate response, quantum state, at photodetector 702
Will respond accurately at the third external detector 703 due to the four quantum states
And
is a quartering point on a circle passing through the center of the sphere, and thus the second external detector 702 and the third external detector 703 are for quantum states
And
the response of (a) is random.
In summary, the results of the four detectors in the above arrangement completely satisfy the decoding requirements of the current polarization BB84 protocol, that is, the passive demodulation function for any four quantum states satisfying the polarization BB84 protocol is realized.
More generally, the quantum states of three typical polarization BB84 protocols are shown in fig. 3, which includes:
(θ＝0，φ＝0)：
(θ＝π，φ＝0)：
(θ＝π/2，φ＝0)：
(θ＝π/2，φ＝π)：
(θ＝π/2，φ＝π/2)：
(θ＝π/2，φ＝3π/2)：
polarization BB84 protocol 1: the encoding end is provided with |0 >, |1 >, | + > and | - > four quantum states;
polarization BB84 protocol 2: the encoding end prepares |0 >, |1 >, | + i > and | -i > four quantum states;
polarization BB84 protocol 3: the encoding end is prepared with four quantum states of | + >, | - >, | + i > and | -i >;
the decoder disclosed in the embodiment of the present invention sets the decoding settings for the above 3 typical polarization BB84 protocols as shown in table 1 below:
| γ1(501) | γ2(502) | γ3(503) | type of decoding |
| 0° | 0° | 90° | |0＞，|1＞，|+＞，|-＞ |
| 0° | 0° | 0° | |0＞，|1＞，|+i＞，|-i＞ |
| 0° | 90° | 0° | |+＞，|-＞，|+i＞，|-i＞ |
In addition, for the polarization imbalance generated by quantum state signals in channel transmission, the decoder disclosed by the embodiment of the invention can carry out on-chip compensation through corresponding regulation and control to realize accurate passive demodulation.
When quantum state
And
when polarization imbalance occurs:
only the first phase modulator 301 and the second phase modulator 302 need to be adjusted accordingly:
γ1：
γ2：
after adjustment, for
And
the change in (c) is noted as:
then, the third phase bias modulator 303 is adjusted accordingly:
γ3：
on-chip polarization compensation can be carried out on the quantum state, accurate passive demodulation is realized again, and extra cost and code rate sacrifice caused by off-chip devices are reduced.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. An on-chip decoder for a polarized BB84 protocol, comprising:
an input waveguide (100) for inputting signal light to be decoded;
the polarization beam splitting rotator (200) is used for splitting the signal light to be decoded and rotating the signal light to be decoded into two beams of signal light with the same polarization direction;
a phase bias modulator (300) including a first phase bias modulator (301), a second phase bias modulator (302), and a third phase bias modulator (303) for adjusting the phase of the signal light received by the phase bias modulator, wherein the first phase bias modulator (301) is configured to perform phase adjustment on one of the two signal lights with the same polarization direction output by the polarization beam splitter rotator (200), and the signal light input by the input waveguide (100) can be subjected to polarization compensation by adjusting the first phase bias modulator (301), the second phase bias modulator (302), and the third phase bias modulator (303);
a 2 × 2 interference coupler (400) including a first interference coupler (401), a second interference coupler (402), and a third interference coupler (403) configured to perform hermitian operation on received signal light, wherein the first interference coupler (401) is configured to perform hermitian operation on the signal light after phase adjustment by the first phase offset modulator (301) and the other one of the two signal lights with the same polarization direction output by the polarization beam splitting rotator (200), transmit one of the signal lights after phase adjustment to the second interference coupler (402) through the second phase offset modulator (302), directly transmit the other signal light after phase adjustment to the second interference coupler (402), and perform hermitian operation on the two signal lights received by the second interference coupler (402);
a 1 × 2 optical splitter (500) including a first optical splitter (501) and a second optical splitter (502) for splitting the received signal light into two signal lights with equal intensity, wherein the first optical splitter (501) receives one signal light after hermitian operation of the second interference coupler (402) and splits the signal light into two signal lights with equal intensity, and the second optical splitter (502) receives the other signal light after hermitian operation of the second interference coupler (402) and splits the signal light into two signal lights with equal intensity;
an output waveguide (600) comprising a first output waveguide (601), a second output waveguide (602), a third output waveguide (603), and a fourth output waveguide (604), wherein the first output waveguide (601) is configured to output one of the two signal lights split by the first optical splitter (501), the fourth output waveguide (604) is configured to output one of the two signal lights split by the second optical splitter (502), the other of the two signal lights split by the first optical splitter (501) is output to the third interference coupler (403) after being phase-adjusted by the third phase offset modulator (303), the other of the two signal lights split by the second optical splitter (502) is directly output to the third interference coupler (403), and the third interference coupler (403) performs hermite operation on the two signal lights received by the third interference coupler (403), output through the second output waveguide (602) and a third output waveguide (603);
the input waveguide (100), the polarization beam splitting rotator (200), the phase bias modulator (300), the 2 x 2 interference coupler (400), the 1 x 2 optical beam splitter (500) and the output waveguide (600) are made of silicon materials, and are processed by a process compatible with a microelectronic process to realize on-chip integration;
the on-chip decoder can decode four polarization quantum states under any two sets of non-orthogonal bases satisfying the polarization BB84 protocol.
2. The on-chip decoder according to claim 1, wherein the polarization beam splitter rotator (200) is an adiabatic asymmetric directional coupler, one side of which has an input port and the other side of which has two output ports, the input port receives the signal light to be decoded, the signal light to be decoded is first decomposed into two beams of signal light with orthogonal polarization directions, and then the polarization direction of one of the two beams of signal light with orthogonal polarization directions is rotated to be the same as the polarization direction of the other beam of signal light, so as to obtain the two beams of signal light with the same polarization directions.
3. The on-chip decoder of claim 1, the phase offset modulator (300) employing a thermo-optically tuned phase modulator.
4. The on-chip decoder of claim 1, said 2 x 2 interference coupler (400) employing a 2 x 2 multimode interference coupler; the 1 × 2 optical splitter (500) employs a 1 × 2 multimode interference coupler.
5. The on-chip decoder of claim 1, the input waveguide (100) employs transverse electric field mode fundamental mode and transverse magnetic field mode fundamental mode transmission, and the output waveguide (600) employs transverse electric field mode fundamental mode transmission.
6. A decoding method for an on-chip decoder for the polarized BB84 protocol according to any of claims 1 to 5, comprising:
s1, inputting signal light to be decoded through the input waveguide (100), wherein the signal light to be decoded is quantum state signal light transmitted by a polarization BB84 protocol;
s2, the signal light to be decoded is decomposed and rotated into two signal lights with the same polarization direction through the polarization beam splitting rotator (200);
and S3, adjusting the phase of the received signal light through the phase offset modulator (300), and performing Hermitian operation on the phase-adjusted signal light through the 2 x 2 interference coupler (400) to obtain decoded signal light meeting the decoding requirement of the polarization BB84 protocol, and outputting the decoded signal light through the output waveguide (600).
7. The decoding method of claim 6, wherein S3 comprises:
one of the two beams of signal light with the same polarization direction output by the polarization beam splitting rotator (200) is input into the first interference coupler (401) after being subjected to phase adjustment through the first phase offset modulator (301), the other beam of signal light is directly input into the first interference coupler (401), and the first interference coupler (401) performs hermitian operation on the input signal light;
one beam of signal light after Hermite operation of the first interference coupler (401) is input into the second interference coupler (402) after being subjected to phase adjustment through the second phase bias modulator (302), the other beam of signal light is directly input into the second interference coupler (402), and the second interference coupler (402) performs Hermite operation on the input signal light;
the first optical beam splitter (501) receives one beam of signal light after Hermite operation of the second interference coupler (402) and splits the signal light into two beams of signal light with equal intensity, and the second optical beam splitter (502) receives the other beam of signal light after Hermite operation of the second interference coupler (402) and splits the other beam of signal light into two beams of signal light with equal intensity;
outputting one of the two signal lights split by the first optical splitter (501) through the first output waveguide (601), and outputting one of the two signal lights split by the second optical splitter (502) through the fourth output waveguide (604);
and the other of the two signal lights split by the first optical splitter (501) is output to the third interference coupler (403) after being subjected to phase adjustment by the third phase offset modulator (303), the other of the two signal lights split by the second optical splitter (502) is directly output to the third interference coupler (403), and the third interference coupler (403) performs hermite operation on the two signal lights received by the third interference coupler and outputs the two signal lights through the second output waveguide (602) and the third output waveguide (603).
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