CN112394532A - Preparation method and application of high-brightness unidirectional quantum guided state with adjustable purity - Google Patents

Preparation method and application of high-brightness unidirectional quantum guided state with adjustable purity Download PDF

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CN112394532A
CN112394532A CN202011363283.8A CN202011363283A CN112394532A CN 112394532 A CN112394532 A CN 112394532A CN 202011363283 A CN202011363283 A CN 202011363283A CN 112394532 A CN112394532 A CN 112394532A
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肖芽
曲慧超
韩鑫红
顾永建
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Ocean University of China
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    • G02B7/00—Mountings, adjusting means, or light-tight connections, for optical elements
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Abstract

The invention belongs to the technical field of quantum information, and discloses a preparation method and application of a high-brightness unidirectional quantum guide state with adjustable purity, wherein a quantum state is prepared by a II-type PPKTP crystal with the temperature of 30 ℃ in a two-way pump Sagnac ring; 5 paths of light after polarization maintaining enter a sagnac interferometer, and the purity coefficient of a quantum state is dynamically adjusted; 2 calcites with optical axes parallel to the | H > optical vibration direction and 1 half-wave plate with a fast axis at 22.5 degrees are utilized to form a depolarization channel, and 7 paths of light enter the depolarization channel and are completely dephased; the 7-path light and the 8-path light are coupled into the multimode optical fiber by a silver mirror, the 7-path light and the 8-path light pass through the silver mirror along the edge of the silver mirror, and the reflecting silver mirror of the 7-path light is finely adjusted to enable the 7-path light and the 8-path light to be nearly parallel, so that a two-bit unidirectional guiding state with high brightness and fidelity is obtained. The invention improves the brightness, the fidelity and the stability of the quantum state, and eliminates the error caused by the different inverse ratios of the beam splitter to the different polarized light.

Description

Preparation method and application of high-brightness unidirectional quantum guided state with adjustable purity
Technical Field
The invention belongs to the technical field of quantum information, and particularly relates to a preparation method and application of a high-brightness unidirectional quantum guided state with adjustable purity.
Background
Currently, quantum steering is a non-local quantum correlation between quantum entanglement and Bell non-locality, describing the ability of a local measurement of one particle to affect the quantum state of another particle non-locally, and the measurement results cannot be explained with a Local Hidden State (LHS) model. Unlike quantum entanglement and Bell non-locality, it also possesses a unique unidirectionality, i.e. one can successfully steer the other in a steering experiment, but not the other way around. The asymmetry greatly enriches the physical connotation of quantum non-locality, and plays an important role in constructing an asymmetric quantum communication network and unidirectional quantum secret communication. In order to realize long-distance one-way secret communication among multiple users, a high-brightness multiphoton one-way guiding state needs to be prepared. The existing unidirectional quantum guide state based on polarization coding is prepared by combining a II-type parametric down-conversion process with a depolarization channel, and the brightness and the fidelity of a light source of the method depend on the random coincidence count and the indistinguishable degree of parametric down-conversion photon pairs. The existing solution is to be improved in the following aspects:
(1) in the parametric down-conversion process, o light and e light appear on the same path at the same time, and because the spectra of the two lights are different (the bandwidth of the o light is larger), a narrow-band filter (FWMH-3 nm) is usually inserted experimentally to erase the frequency correlation of the o light and the e light, and partial photons are lost in the process.
(2) The existing scheme can only adjust the weights of a pure state part and a mixed state part in a unidirectional guiding state through an attenuation method, so that a large number of photons are lost.
(3) Furthermore, to collect photons in both the pure and mixed fractions, it is common experimentally to use a 50: 50, this process results in at least 50% loss of photons, and the inverse ratio of the o and e light transmissions of the beam splitter at 45 ° incidence has a 20% error, which reduces the fidelity of the quantum states to some extent.
In order to prepare a high-brightness light source satisfying the long-distance one-way secret communication, the technical problems in the aspects need to be solved.
Through the above analysis, the problems and defects of the prior art are as follows:
(1) in the parametric down-conversion process, o light and e light can simultaneously appear on the same path, because the spectrums of the two lights are different, the bandwidth of the o light is larger, a narrow-band filter (FWMH-3 nm) is usually inserted in experiments to erase the frequency correlation of the o light and the e light, and partial photons can be lost in the process.
(2) The weight of the pure state part and the complete mixed state part in the unidirectional guiding state can be adjusted only by an attenuation method in the prior art, so that a large amount of photons are lost.
(3) To collect photons in both the pure and fully mixed fractions, a 50: 50, this process results in at least 50% loss of photons, and the inverse ratio of the o and e light transmissions of the beam splitter at 45 ° incidence has a 20% error, which reduces the fidelity of the quantum states to some extent.
The difficulty in solving the above problems and defects is: the existing preparation method is difficult to achieve two indexes of high brightness and high fidelity at the same time. To increase the brightness of the light source, the most straightforward approach is to pump the nonlinear crystal with a higher power laser. However, the parametric down-conversion process can not only produce single photon pairs, but also accompany the production of multi-photon pairs. Although the coincidence counting rate of two photons can be improved by increasing the pumping power of the laser, the coincidence counting rate of multiple photons is inevitably increased, so that the coincidence counting rate becomes important noise of a two-photon entanglement source, and the fidelity of a light source is reduced. In order to improve the fidelity of the entanglement source, it is necessary to eliminate the distinguishability of the photon pair and simultaneously achieve the indistinguishability in the aspects of spatial mode, polarization information, time information and the like, so that the filtering is performed by using a narrow-band filter and the like. In general, the narrower the pass band of the filter, the better the identity of the photon pair and the higher the fidelity, but the more photons are lost at this time. In addition, in order to collect multiple paths of light into the same single-mode fiber common beam splitter for beam combination, half of photons are lost in the process, the differentiability of photon polarization information is increased, and the fidelity of a light source is reduced. Most importantly, the purity factor cannot be adjusted without light intensity loss, and improvement is needed from the principle aspect.
The significance of solving the problems and the defects is as follows: there are two problems to be solved urgently in the technical field of quantum information: increasing the number of user communications and extending the safe communication distance requires the preparation of a high-brightness and high-fidelity multiphoton entanglement source. The scheme can greatly improve the brightness and the fidelity of the two-photon entanglement source during the pumping of low-power laser. On the basic research level, the high-quality quantum light source can solve two key problems of low multiphoton coincidence counting and poor interference visibility, thereby creating conditions for preparing a multiphoton entanglement source with high brightness and high fidelity; on the application level, the high-brightness light source can prolong the communication distance, which provides a basic guarantee for long-distance quantum secret communication.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a preparation method and application of a high-brightness unidirectional quantum guided state with adjustable purity, and particularly relates to a preparation method of a high-brightness and high-fidelity unidirectional quantum guided state with adjustable purity.
The invention is realized in such a way that a preparation method of a high-brightness unidirectional quantum guided state with adjustable purity comprises the following steps:
step one, preparing cos theta | H by bidirectional pumping of II-type PPKTP crystal with the temperature set to 30 ℃ in a Sagnac ringe>1|V0>2+sinθ|Vo>1|He>2Quantum state of (1), wherein | He>1|V0>2And | Vo>1|He>2Is adjusted by changing the angle theta of the half-wave plate.
Step two, compensating time walk-off of photons with different polarizations caused by different group velocities by tilting a half-wave plate HWP with an optical axis at 0 degree; the degree of spatial coincidence of the converted light in the parametric under clockwise and counterclockwise pumping is controlled by adjusting the interference visibility of the Sagnac loop.
Step three, carrying out coherent superposition on the 1 path photon and the 2 path photon on the polarization beam splitter, and removing the frequency correlation of the o light and the e light; 2 interference filters with the wavelength of 3nm are used for filtering the pump light; by adjusting the position of the triangular prism mounted on the one-dimensional micrometer displacement stage, the 1-path photon and the 2-path photon are completely indistinguishable in time and space.
Step four, enabling the 5 paths of light after polarization maintaining to enter a sagnac interferometer, and dynamically adjusting the purity coefficient of a quantum state; 2 calcite with optical axis parallel to | H > optical vibration direction and 1 half-wave plate with fast axis at 22.5 ° are used to form a depolarization channel, and 7 paths of light enter the depolarization channel and then become a complete mixed state.
And step five, coupling 7 paths of light and 8 paths of light into the multimode optical fiber by using a silver mirror, allowing the 7 paths of light and the 8 paths of light to pass along the edge of the silver mirror, and finely adjusting a reflective silver mirror of the 7 paths of light to enable the 7 paths of light and the 8 paths of light to be nearly parallel, thus preparing the two-bit unidirectional guiding state with high brightness and fidelity.
Further, in the step one, the expression of the whole preparation process a in the entangled state includes:
Figure BDA0002804637130000041
further, in step four, the step of entering the 5 paths of polarization-maintaining light into the sagnac interferometer and dynamically adjusting the purity coefficient of the quantum state includes:
(1) after polarization maintaining light passes through the polarizing beam splitter PBS, horizontally polarized light | H > is in the counterclockwise direction (dotted line), and vertically polarized light | V > is in the clockwise direction (solid line);
(2) after light in two directions passes through a half-wave plate with the optical axis forming a with the horizontal direction, | H > → cos | H > + sin α | V >, | V > → -sin α | H > + cos α | V >;
(3) after the second PBS coincidence, the quantum states of the 5-way and 8-way channels can be written as cos θ | Ho>5|He>8+sinθ|Vo>5|Ve>8In agreement with the initial state, the weight is cos2α；
(4) The quantum states of the 5-way and 7-way can be written as cos θ | Ho>5|He>7-sinθ|Vo>5|Ve>7Weight is sin2α。
Therefore, the weight of the 7-path light and the 8-path light can be arbitrarily controlled without losing the total light intensity by adjusting the angle alpha of the half-wave plate.
Further, in step four, the depolarization channel is formed by 2 calcites with optical axes parallel to the | H > optical vibration direction and 1 half-wave plate with a fast axis at 22.5 °, and 7 paths of light enter the depolarization channel and are completely depolarized, including:
(1) after the parametrically down-converted light passes through a 3nm interference filter, the coherence length is L ═ lambda2The optical fiber is characterized by comprising a parameter light source, a light source, an interference filter and a light source, wherein the parameter light source is used for generating parameter light;
(2) the optical path difference of the parametric light introduced after passing through calcite crystals with the thickness of d is delta d ═ nn-ne)·d，nnAnd neThe main refractive indices of o-light and e-light, respectively.
Further, the thickness of the first block of calcite crystals PC1 satisfies Δ d1> L; the thickness of the second block of calcite crystals PC2 satisfies Δ d2>2 · Δ d.
Further, in the fifth step, assuming that the coupling efficiency of the 7-path and the 8-path are the same, the prepared two-bit unidirectional guiding state with high brightness and fidelity is as follows:
Figure BDA0002804637130000051
wherein, | ψ (θ)>＝cosθ|Ho>A|He>R+sinθ|Vo>A|Ve>R，
Figure BDA0002804637130000052
p＝cos2a。
Further, the parameters of the quantum state can be controlled by adjusting the included angle (theta, alpha) between the fast axis of the half-wave plate and the horizontal polarization direction.
Further, the preparation method of the high-brightness unidirectional quantum guided state with adjustable purity further comprises the following steps:
and (3) combining a quantum state chromatography device d consisting of a quarter wave plate QWP, a half wave plate and a polarization beam splitter, and checking the quality of the prepared state by using a state chromatography method.
Another object of the present invention is to provide a method for constructing an asymmetric quantum communication network, which uses the method for preparing a high-brightness unidirectional quantum guided state with adjustable purity.
Another object of the present invention is to provide a unidirectional quantum secure communication method using the method for preparing high-brightness unidirectional quantum guided state with adjustable purity.
By combining all the technical schemes, the invention has the advantages and positive effects that: the invention provides a method for coherent superposition of parametric down-conversion light on polarization beam splitting, which removes the frequency distinguishability caused by different frequency bandwidths of o light and e light, thereby improving the fidelity of quantum state; a sagnac loop device is added, so that the controllable adjustment of the pure state coefficient from 0 to 1 is realized under the condition of no light intensity loss; the two light wiping edges are utilized to replace the original beam combiner of the beam splitter, the light source brightness can be improved by about 1.8 times, and the method can eliminate the errors caused by different inverse ratios of the beam splitter to different polarized lights. In addition, due to the stable geometric structure of the Sagnac ring, the light source can be ensured to have better stability.
Quantum steering, as a quantum association in a different form from quantum entanglement, is a core resource of quantum information technology. The method has important functions in quantum information processing processes such as quantum communication, quantum calculation, precision measurement and the like, for example, an asymmetric safe communication network is constructed, the operation speed of a computer is obviously improved (exponential improvement), and precision measurement exceeding the classical limit is realized. The preparation method of the high-brightness unidirectional quantum guide state with adjustable purity can improve the brightness of a light source by several times, can realize the arbitrary adjustment of the purity coefficient from 0 to 1 under the condition of no light intensity loss in principle, and improves the fidelity and the stability of the light source to a certain degree.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments of the present invention will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flow chart of a method for preparing a high brightness unidirectional quantum guided state with adjustable purity according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of a method for preparing a high-brightness unidirectional quantum guided state with adjustable purity according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of a depolarized channel provided by an embodiment of the present invention.
Fig. 4(a) is a schematic real part of the maximum entangled state of the reconstructed density matrix provided by the embodiment of the present invention.
Fig. 4(b) is an imaginary schematic diagram of a maximum entanglement state of the reconstructed density matrix provided by the embodiment of the invention.
Fig. 4(c) is a schematic diagram of the real part of the maximum mixture state of the reconstructed density matrix provided by the embodiment of the present invention.
Fig. 4(d) is an imaginary part schematic diagram of a maximum mixture state of the reconstructed density matrix provided by the embodiment of the present invention.
Fig. 5(a) is a schematic diagram of a variation relationship between the fidelity and the entanglement of the P-1 quantum state and θ according to an embodiment of the present invention.
Fig. 5(b) is a schematic diagram of the variation of the fidelity and the entanglement of the quantum state θ ═ Pi/4 with P according to the embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Aiming at the problems in the prior art, the invention provides a preparation method and application of a high-brightness unidirectional quantum guided state with adjustable purity, and the invention is described in detail below with reference to the accompanying drawings.
As shown in fig. 1, the method for preparing a high-brightness unidirectional quantum guided state with adjustable purity according to an embodiment of the present invention includes the following steps:
and S101, preparing a quantum state by bidirectionally pumping the II-type PPKTP crystal with the temperature of 30 ℃ in the Sagnac ring.
S102, compensating time walk-off of photons with different polarizations due to different group velocities by tilting a half-wave plate with an optical axis positioned at 0 degree; the degree of spatial coincidence of the converted light in the parametric under clockwise and counterclockwise pumping is controlled by adjusting the interference visibility of the Sagnac loop.
S103, carrying out coherent superposition on the 1 path photon and the 2 path photon on the polarization beam splitter, and removing the frequency correlation of the o light and the e light; 2 interference filters with the wavelength of 3nm are used for filtering the pump light; by adjusting the position of the triangular prism mounted on the one-dimensional micrometer displacement stage, the 1-path photon and the 2-path photon are completely indistinguishable in time and space.
S104, enabling the 5 paths of light after polarization maintaining to enter a sagnac interferometer, and dynamically adjusting the purity coefficient of a quantum state; 2 calcite with optical axis parallel to | H > optical vibration direction and 1 half-wave plate with fast axis at 22.5 DEG are used for forming a depolarization channel, and 7 paths of light enter the depolarization channel and are completely decohered.
And S105, coupling the 7 paths of light and the 8 paths of light into the multimode optical fiber by using a silver mirror, allowing the 7 paths of light and the 8 paths of light to pass along the edge of the silver mirror, and finely adjusting a reflective silver mirror of the 7 paths of light to enable the 7 paths of light and the 8 paths of light to be approximately parallel, so that a high-brightness and high-fidelity two-bit unidirectional guiding state can be prepared.
The person skilled in the art can also use other steps to implement the method for preparing the high-brightness unidirectional quantum guided state with adjustable purity according to the present invention, and the method for preparing the high-brightness unidirectional quantum guided state with adjustable purity according to the present invention shown in fig. 1 is only one specific example.
The schematic diagram of the preparation method of the purity-adjustable high-brightness unidirectional quantum guided state provided by the embodiment of the invention is shown in fig. 2; in the figure, (a) is the preparation of the entangled state; (b) controlling the purity coefficient; (c) is a depolarized channel; (d) is a state chromatography device.
The technical solution of the present invention is further described with reference to the following examples.
Example 1
The preparation method of the high-brightness unidirectional quantum guided state with adjustable purity, provided by the invention, comprises the following complete steps:
(1) cos theta | H is prepared by bi-directionally pumping a type II PPKTP crystal with the temperature in a Sagnac ring set to 30 DEG Ce>1|V0>2+sinθ|Vo>1|He>2Quantum state of (1), wherein | He>1|V0>2And | Vo>1|He>2Can be adjusted by changing the angle theta of the half-wave plate HWP. To improve the fidelity of the entangled state, it is necessary to ensure that the photons are completely indistinguishable in time, space, and frequency domain. In the scheme of the invention, the time walk-off of photons with different polarizations caused by different group velocities can be compensated by tilting the half-wave plate with the optical axis positioned at 0 degrees; the spatial coincidence degree of the converted light under the parameters during clockwise and counterclockwise pumping is controlled by adjusting the interference visibility of the Sagnac ring; coherent superposition is carried out on the 1 path of photons and the 2 path of photons on the polarization beam splitting, so that o light or e light is detected at the same output end, and the frequency correlation of the o light and the e light is removed; 2 interference filters with the wavelength of 3nm are used for filtering the pump light; by adjusting the three ridges arranged on the one-dimensional micrometer displacement tableThe position of the mirror is such that the 1-way and 2-way photons are completely indistinguishable in time and space. The whole process (a) for preparing the entangled state can be expressed by the following formula:
Figure BDA0002804637130000081
in the past experiment, the indistinguishability of time and space can be realized, the indistinguishability of frequency is realized by a narrow-band filter, in the scheme of the invention, o light is collected in 5 paths, and e light is collected in 6 paths, so that the indistinguishability of the photon frequency at the same output end is ensured in principle.
(2) And 5 paths of light after polarization maintaining enter a sagnac interferometer for dynamically adjusting the purity coefficient of the quantum state. The specific principle is that after polarization-maintaining light passes through a polarizing beam splitter PBS, horizontally polarized light | H>Light | V vertically polarized in the counter-clockwise direction (dotted line)>In the counterclockwise direction (solid line); after light in two directions passes through a half-wave plate with the optical axis forming alpha with the horizontal direction, | H>→-cosα|H>+sinα|V)，|V>→-sinα|H>+cosα|V>(ii) a After the second PBS coincidence, the quantum states of the 5-way and 8-way channels can be written as cos θ | Ho>5|He>8+sinθ|Vo>5|Ve>8In agreement with the initial state, the weight is cos2α; the quantum states of the 5-way and 7-way can be written as cos θ | Ho>5|He>7-sinθ|Vo>5|Ve>7Weight is sin2α. Therefore, the weight of the 7-path light and the 8-path light can be arbitrarily controlled without losing the total light intensity by adjusting the angle alpha of the half-wave plate.
(3)2 optical axes parallel to | H>Calcite in the optical vibration direction and 1 half-wave plate with the fast axis at 22.5 degrees form a depolarization channel, 7 paths of light enter the depolarization channel and are completely decohered, and the specific principle is shown in fig. 3. After the parametrically down-converted light passes through a 3nm interference filter, the coherence length is L ═ lambda2And/Δ λ, where λ is the center wavelength and Δ λ is the full width at half maximum of the parametric light after passing through the interference filter. Parametric lightThe optical path difference introduced after the calcite crystal with the thickness of d is delta d ═ (n)n-ne)·d，nnAnd neThe main refractive indices of o-light and e-light, respectively. In order to form complete decoherence, the thickness of the first calcite crystal PC1 is such that Δ d1 is satisfied>L; the thickness of the second calcite crystal PC2 is required to satisfy delta d2>2·Δd。
In the schematic diagram of the depolarized channel shown in figure 3,
Figure BDA0002804637130000091
(4) a silver mirror is used for coupling 7 paths of light and 8 paths of light into the multimode optical fiber at the same time, in order to improve the coupling efficiency of the multimode optical fiber as much as possible, the 7 paths of light and the 8 paths of light are wiped on the edge of the silver mirror to pass through, and the reflecting silver mirror of the 7 paths of light is finely adjusted to enable the 7 paths of light and the 8 paths of light to be nearly parallel, at the moment, the coupling efficiency of the two paths of light can reach about 90 percent (at least 50 percent of light intensity is lost when the two paths of light are combined through a beam splitter originally). At this time, because the optical path difference between the 7-path light and the 8-path light is larger than the coherence time of the parametric light and smaller than the coincidence time window (generally 3ns), the number of photons collected by the multimode fiber is equal to the probability superposition between the number of pure photons (7-path) and the number of mixed photons (8-path).
(5) Therefore, the device can be used for preparing a two-bit unidirectional guiding state with high brightness and fidelity
Figure BDA0002804637130000092
Wherein | ψ (θ)>＝cosθ|Ho>A|He>R+sinθ|Vo>A|Ve>R，
Figure BDA0002804637130000093
p＝cos2α (assuming equal coupling efficiency for the 7-way and 8-way paths). The parameters of the quantum state can be controlled simply by adjusting the angle (theta, alpha) of the fast axis of the half-wave plate to the horizontal polarization direction. Combining Quarter Wave Plate (QWP), half wave plate and polarization beam splittingThe quantum state chromatography device (d) consists of a device, and the quality of the preparation state can be checked by using a state chromatography method.
Example 2
The 405nm continuous laser pump 2cm PPKTP (the temperature is set at 30 ℃), the coincidence window is set to be 3ns, and the brightness of the light source is about 50,000 calls/s/mW. In the experiment, the coupling efficiency of 7 paths is 91.2%, the coupling efficiency of 8 paths is about 89%, and the interference visibility of the Sagnac loop is 99.2%. In order to verify the performance of the quantum state prepared by the experimental device, a series of quantum states are prepared for quantum state chromatography. FIG. 4 shows the maximum entanglement
Figure BDA0002804637130000101
And maximum mixing state
Figure BDA0002804637130000102
The reconstructed density matrix of (1).
In addition, the invention further calculates Concurrence and fidelity of the quantum state, and the figure 5 shows the change relation of the fidelity and the entanglement of the quantum state with P and theta. It can be seen from the figure that the measured entanglement degree is consistent with the theoretical entanglement degree under different conditions, and the fidelity of the quantum state is basically stabilized to be more than 99%. The experimental error comes from the dithering of photon number, the invention uses the Monte Carlo method to carry out fitting, and the experimental error is in the magnitude of 10^ (-4).
The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.

Claims (10)

1. A preparation method of a high-brightness unidirectional quantum guided state with adjustable purity is characterized by comprising the following steps:
II type PPKTP crystal with temperature set to 30 ℃ in Sagnac ring through bidirectional pumpingPreparation of cos θ | He>1|V0>2+sinθ|Vo>1|He>2Quantum state of (1), wherein | He>1|V0>2And | Vo>1|He>2The weight of (a) is adjusted by changing the angle theta of the half-wave plate;
compensating for the temporal walk-off of differently polarized photons due to different group velocities by tilting the half-wave plate HWP with its optical axis at 0 °; the spatial coincidence degree of the converted light under the parameters during clockwise and counterclockwise pumping is controlled by adjusting the interference visibility of the Sagnac ring;
coherent superposition is carried out on the 1 path of photons and the 2 path of photons on the polarization beam splitting, and the frequency correlation of the o light and the e light is removed; 2 interference filters with the wavelength of 3nm are used for filtering pumping light and other background noises; the positions of the triple prisms arranged on the one-dimensional micrometer displacement platform are adjusted, so that the 1-path photon and the 2-path photon are completely indistinguishable in time and space;
5 paths of light after polarization maintaining enter a sagnac interferometer, and the purity coefficient of a quantum state is dynamically adjusted; 2 calcites with optical axes parallel to the | H > optical vibration direction and 1 half-wave plate with a fast axis at 22.5 degrees are utilized to form a depolarization channel, and 7 paths of light enter the depolarization channel and are completely dephased;
a silver mirror is used for coupling 7 paths of light and 8 paths of light into the multimode optical fiber at the same time, the 7 paths of light and the 8 paths of light pass through the edge of the silver mirror, and the reflecting silver mirror of the 7 paths of light is finely adjusted to enable the 7 paths of light and the 8 paths of light to be nearly parallel, so that a two-bit unidirectional guiding state with high brightness and fidelity can be prepared.
2. The method of claim 1, wherein the expression of the whole entangled state preparation process a includes:
Figure FDA0002804637120000011
3. the method of claim 1, wherein the step of passing the 5-path polarization-maintaining light into a sagnac interferometer to dynamically adjust the purity factor of the quantum state comprises:
(1) after polarization maintaining light passes through the polarizing beam splitter PBS, horizontally polarized light | H > is in the counterclockwise direction (dotted line), and vertically polarized light | V > is in the clockwise direction (solid line);
(2) after light in two directions passes through a half-wave plate with the optical axis forming alpha with the horizontal direction, | H > → cos α | H > + sin α | V >, | V > → -sin α | H > + cos α | V >;
(3) after the second PBS coincidence, the quantum states of the 5-way and 8-way channels can be written as cos θ | Ho>5|He>8+sinθ|Vo>5|Ve>8In agreement with the initial state, the weight is cos2α；
(4) The quantum states of the 5-way and 7-way can be written as cos θ | Ho>5|He>7-sinθ|Vo>5|Ve>7Weight is sin2α；
Therefore, the weight of the 7-path light and the 8-path light can be arbitrarily controlled without losing the total light intensity by adjusting the alpha of the half-wave plate.
4. The method according to claim 1, wherein the method for preparing the high-brightness unidirectional quantum guided mode with tunable purity comprises the steps of forming a depolarization channel by using 2 calcite with optical axis parallel to the | H > optical vibration direction and 1 half-wave plate with fast axis at 22.5 °, and completely decoherently after 7 paths of light enter the depolarization channel, comprising:
(1) after the parametrically down-converted light passes through a 3nm interference filter, the coherence length is L ═ lambda2The optical fiber is characterized by comprising a parameter light source, a light source, an interference filter and a light source, wherein the parameter light source is used for generating parameter light;
(2) the optical path difference of the parametric light introduced after passing through calcite crystals with the thickness of d is delta d ═ no-ne)·d，noAnd neThe main refractive indices of o-light and e-light, respectively.
5. The method of claim 4, wherein the thickness of the first calcite crystal PC1 is Δ d1> L; the thickness of the second block of calcite crystals PC2 satisfies Δ d2>2 · Δ d.
6. The method according to claim 1, wherein the coupling efficiency of 7-path and 8-path is as high as 90% or more, and the prepared high-brightness and fidelity two-bit unidirectional quantum guided state is:
Figure FDA0002804637120000031
wherein, | ψ (θ)>＝cosθ|Ho>A|He>B+sinθ|Vo>A|Ve>B，
Figure FDA0002804637120000032
p＝cos2α。
7. The method according to claim 1, wherein the parameters of the quantum state are controlled by adjusting the angle (θ, α) between the fast axis of the half-wave plate and the horizontal polarization direction.
8. The method of claim 1, wherein the method of preparing the adjustable-purity high-brightness quantum guided state further comprises: and (3) combining a quantum state chromatography device d consisting of a quarter wave plate QWP, a half wave plate and a polarization beam splitter, and checking the quality of the prepared state by using a state chromatography method.
9. A method for constructing an asymmetric quantum communication network, which is characterized in that the method for constructing an asymmetric quantum communication network uses the method for preparing a high-brightness unidirectional quantum guide state with adjustable purity according to any one of claims 1 to 8.
10. A unidirectional quantum secure communication method, wherein the unidirectional quantum secure communication method uses the method for preparing the high-brightness unidirectional quantum guided state with adjustable purity according to any one of claims 1 to 8.
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