A method for continuous-variable quantum secure direct communication

Abstract

The application provides a method for continuous variable quantum secure direct communication, and belongs to the technical field of quantum secure direct communication, and comprises the following steps: a sending end prepares a double-mode compressed entangled light beam pair S, and sends a light beam S1 to a receiving end; the sending end modulates the light beam S1 into a first Laguerre-Gaussian light beam S'1 and sends the light beam S'1 to the receiving end; the sending end selects a subset, measures the orthogonal position and the orthogonal momentum of a light beam S2 in the subset, and compares the measurement result of the light beam S1 with the receiving end to check interference; the receiving end modulates the S'1 into a second Laguerre-Gaussian light beam S''1 according to secret information, and sends the S''1 to the sending end; the sending end recombines the S2 and the S''1 into a light beam pair S' and images, and obtains the secret information according to the imaging result. The application solves the problems that the continuous variable quantum secure direct communication is prone to noise and loss in the transmission process, and the implementation and management are complex.

Description

Technical Field

[0001] This invention belongs to the field of quantum secure direct communication technology, and in particular relates to a method for continuous variable quantum secure direct communication. Background Technology

[0002] Quantum-Secure Direct Communication (QSDC) is a technology that utilizes the principles of quantum mechanics to achieve secure communication. Unlike traditional quantum key distribution (QKD), QSDC allows communicating parties to directly transmit messages without prior key establishment and management. The no-cloning theorem states that an unknown quantum state cannot be precisely copied. This means that any attempt to copy or measure a quantum state introduces a non-negligible perturbation, which can be detected by both communicating parties. The Heisenberg uncertainty principle states that the position and momentum of a particle cannot be precisely measured simultaneously; any unauthorized measurement introduces noise, thus compromising communication security. Based on the no-cloning theorem and the Heisenberg uncertainty principle, QSDC provides unconditional security, guaranteeing communication security even under quantum computer attacks.

[0003] Continuous-variable quantum secure direct communication (CV-QSDC) combines the advantages of continuous-variable quantum systems with the security of QSDC, providing a highly efficient and secure communication method. CV-QSDC allows communicating parties to directly transmit messages without the need for pre-establishing and managing keys. Utilizing continuous-variable quantum systems, it enables high-speed communication; through highly sensitive detectors and advanced signal processing techniques, it achieves low-noise communication; and based on the quantum no-cloning theorem and the Heisenberg uncertainty principle, it provides unconditional security for communication.

[0004] However, continuous variable quantum secure direct communication is susceptible to noise and loss during transmission, and its implementation and management are relatively complex, which limits the application of quantum technology. Summary of the Invention

[0005] The purpose of this invention is to provide a method for continuous variable quantum secure direct communication, in order to solve the problems that continuous variable quantum secure direct communication is susceptible to noise and loss during transmission, and is complex to implement and manage.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is a method for continuous variable quantum secure direct communication, comprising the following steps:

[0007] S1. The transmitter prepares a momentum-compressed vacuum state and a position-compressed vacuum state respectively through two optical parametric oscillators, and then prepares a dual-mode compressed entangled beam pair S through a beam splitter, including beam S1 and beam S2. The transmitter directly sends beam S1 to the receiver and retains beam S2.

[0008] S2. The transmitting end modulates the beam S1 into a first Laguerre-Gaussian beam S'1 through a liquid crystal spatial light modulator, and attenuates it to a quantum state through an attenuator before sending it to the receiving end.

[0009] S3. The transmitting end randomly selects a subset of the dual-mode compressed entangled beam pair, measures the orthogonal position and orthogonal momentum of beam S2 in the subset, and publishes the position and measurement results of the subset through the classical channel. The receiving end measures the orthogonal position and orthogonal momentum of beam S1 based on the position of the subset.

[0010] S4. The receiving end modulates S'1 into a second Laguerre-Gaussian beam S”1 according to the secret information through a reflective liquid crystal spatial light modulator, and sends it to the transmitting end through a turbulent atmospheric channel.

[0011] S5. The transmitter recombines S2 and S”1 into a beam pair S' using a beam splitter and images it on a charge-coupled camera. The transmitter determines the orbital angular momentum eigenstate of the beam pair S' based on the imaging results and obtains the secret information.

[0012] Furthermore, in step S1, the preparation of beam S1 and beam S2 are represented as follows:

[0013]

[0014] In the formula, The orthogonal position of beam S1, Let S1 be the orthogonal momentum of the beam. The orthogonal position of beam S2, Let S2 be the orthogonal momentum of the beam. The orthogonal positions are those of the momentum-compressed vacuum state. The position is the orthogonal position of the compressed vacuum state. The orthogonal momentum of the momentum-compressed vacuum state. Let e ​​be the orthogonal momentum of the positionally compressed vacuum state, where e is exp (the base of the natural logarithm), and r is the compressibility coefficient (r > 0). The initial orthogonal positions of the quantum states obtained through the first optical parametric oscillator. The initial orthogonal position of the quantum state through the second optical parametric oscillator. The initial orthogonal momentum of the quantum state passing through the first optical parametric oscillator. Let be the initial orthogonal momentum of the quantum state through the second optical parametric oscillator.

[0015] Furthermore, in step S2, the first Laguerre-Gaussian beam S'1 carries in the paraxial region The orbital angular momentum is represented by |l1>, and S'1 is loaded with vortex phase exp(il1θ), where l1 is the topological charge of S'1. To reduce Planck's constant, exp is the base of the natural logarithm, i is the complex unit, and θ is the azimuth angle.

[0016] Furthermore, step S3 also includes a quantum state check. If the measurement results of the transmitting end and the receiving end in this subset satisfy the inseparability criterion, it indicates that the quantum state has not been destroyed. The inseparability criterion is:

[0017]

[0018] In the formula, Let S be the orthogonal position of beam S1 in this subset. Let S1 be the orthogonal momentum of the beam in this subset. Let S be the orthogonal position of beam S2 in this subset. Let S be the orthogonal momentum of beam S2 in this subset.

[0019] Furthermore, in step S4, the second Laguerre-Gaussian beam S”1 is loaded with a vortex phase exp(il2θ), where l2 is the topological charge of S”1, i is the complex unit, and θ is the azimuth angle. If the secret information is 0, then l2<0 is set; if the secret information is 1, then l2>0 is set.

[0020] Furthermore, in step S5, if the imaging result on the charge-coupled camera is a clockwise spiral, the transmitting end determines that the orbital angular momentum eigenstate l'2 of the beam relative to S' is >0, and the secret information is 1. If the imaging result is a counterclockwise spiral, the transmitting end determines that the orbital angular momentum eigenstate l'2 of the beam relative to S' is <0, and the secret information is 0.

[0021] The beneficial effects of this invention are:

[0022] (1) This invention provides a method for continuous variable quantum secure direct communication. It adopts continuous variable quantum technology and verifies the security of communication by detecting the orthogonal position and orthogonal momentum of quantum states. This ensures the reliability of information transmission and is compatible with classical channels such as optical fiber, radio waves, and satellite.

[0023] (2) This invention modulates the orbital angular momentum of a Laguerre-Gaussian beam by using a liquid crystal spatial light modulator and a reflective liquid crystal spatial light modulator. By altering the eigenstate of the orbital angular momentum, secret information is carried, making it easy to implement and manage. Any unauthorized measurement will introduce noise, disrupting the eigenstate of the beam's orbital angular momentum, thus being detected by both communicating parties, achieving highly secure and low-noise communication. Furthermore, the orbital angular momentum eigenstate of the Laguerre-Gaussian beam is determined through the imaging results of a charge-coupled device (CCD) camera, and the secret information is derived. Even if affected by channel noise or interference attacks, the imaging results will not be affected, demonstrating excellent security and robustness. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart of a method for continuous variable quantum secure direct communication according to the present invention;

[0026] Figure 2 This is a schematic diagram of the encoding principle of orbital angular momentum in Embodiment 1 of the present invention;

[0027] Figure 3 This is a schematic diagram of the intensity distribution of the Laguerre-Gaussian beam in Embodiment 2 of the present invention;

[0028] Figure 4 This is a schematic diagram of the phase distribution of the Laguerre-Gaussian beam in Embodiment 2 of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] like Figure 1 As shown, this invention provides a method for continuous-variable quantum-secure direct communication, comprising the following steps:

[0031] S1. The transmitting end Bob prepares a momentum-compressed vacuum state through the first optical parametric oscillator OPO1 and a position-compressed vacuum state through the second optical parametric oscillator OPO2, as follows:

[0032]

[0033] In the formula, The orthogonal positions are those of the momentum-compressed vacuum state. Let e ​​be the orthogonal position of the positional compression vacuum state, e be exp (the base of the natural logarithm), and r be the compression coefficient (r > 0). To determine the initial orthogonal positions of the quantum states through OPO1, To determine the initial orthogonal positions of the quantum states of OPO2, The orthogonal momentum of the momentum-compressed vacuum state. The orthogonal momentum of the position-compressed vacuum state. The initial orthogonal momentum of the quantum state through OPO1, The initial orthogonal momentum of the quantum state through OPO2;

[0034] Bob balanced the momentum-compressed vacuum state and the position-compressed vacuum state using a beam splitter (BS) to prepare a two-mode compressed entangled beam pair S, including beam S1 and beam S2, denoted as:

[0035]

[0036] In the formula, The orthogonal position of beam S1, Let S1 be the orthogonal momentum of the beam. The orthogonal position of beam S2, Let S2 be the orthogonal momentum of the beam S2;

[0037] Bob sends beam S1 to the receiver Alice, while retaining beam S2.

[0038] S2. Bob modulates beam S1 using a liquid crystal spatial light modulator (LC-SLM) into a first Laguerre-Gaussian beam S'1 carrying orbital angular momentum. Its amplitude distribution exhibits a vortex phase structure, carrying orbital angular momentum in the paraxial region. The orbital angular momentum is represented by |l1>, and S'1 is loaded with a vortex phase phase exp(il1θ), where exp is the base of the natural logarithm, and l1 is the topological charge of the Bob-modulated first Laguerre-Gaussian beam S'1, which is an arbitrary integer. To reduce Planck's constant, i is the complex unit, and θ is the azimuth angle;

[0039] Bob then attenuates the first Laguerre-Gaussian beam S'1 to a quantum state via an attenuator (Att.) and sends it to Alice through a turbulent atmospheric channel (AT). The turbulent atmospheric channel has a transmittance of T and over-noise ∈, which is introduced by possible interference from other sources.

[0040] S3, Bob randomly selects a subset of the dual-mode compressed entangled beam pair S, determines the position of this subset, and measures the orthogonal position of beam S2 within this subset. and orthogonal momentum The location and measurement results of this subset are published through the turbulent atmospheric channel, and Alice measures the orthogonal position of beam S1 at the same subset location. and orthogonal momentum And perform a quantum state check (CE). If the measurement result satisfies the inseparability criterion, that is:

[0041]

[0042] This indicates that the quantum state has not been destroyed. Bob compares the measurement results with Alice's measurement results and calculates the key rate. He sets the threshold to 0. If the key rate is higher than the threshold, he continues to the next step. If the key rate is lower than the threshold, it indicates that the communication is being eavesdropped on, and Bob abandons the communication.

[0043] S4. Alice sends beam S'1 into a reflective liquid crystal spatial light modulator (RLC-SLM) to modulate it into a second Laguerre-Gaussian beam S”1, and loads a vortex phase exp(il2θ), where l2 is the topological charge of beam S”1 and can be any integer. Alice modulates beam S”1 according to the secret information. If the secret information is 0, then l2<0 is set; if the secret information is 1, then l2>0 is set.

[0044] Because vortex light has mirror properties, after n reflections, the change in the orbital angular momentum eigenstate is as follows:

[0045] |l>→|(-1) n l>

[0046] In the formula, n is the number of reflections, and l is the topological charge, which is an arbitrary integer.

[0047] The orbital angular momentum eigenstate change after passing through the reflective liquid crystal spatial light modulator is as follows:

[0048]

[0049] After modulation, Alice sends beam S”1 to Bob through the turbulent atmospheric channel;

[0050] S5. Bob recombines beams S2 and S”1 into beam pair S’ using a beam splitter and images them on a charge-coupled device (CCD). Based on the imaging result, Bob determines the orbital angular momentum eigenstate |l’2> of beam pair S’. If the imaging result on the CCD is a clockwise spiral, Bob determines that the orbital angular momentum eigenstate |l’2>0 of beam pair S’, and the secret information is 1. If the imaging result is a counterclockwise spiral, Bob determines that the orbital angular momentum eigenstate |l’2<0 of beam pair S’, and the secret information is 0.

[0051] Example 1

[0052] The method for continuous variable quantum secure direct communication described in this invention is used for communication, with l1=2 and l2=5.

[0053] like Figure 2 The diagram shown illustrates the orbital angular momentum encoding principle of Embodiment 1 of the present invention. Bob uses a liquid crystal spatial light modulator to encode the orbital angular momentum of a Gaussian beam, generating vortex light, which carries... A unit orbital angular momentum, with orbital angular momentum eigenstate |2>, is transmitted through an atmospheric turbulence channel and modulated by a reflective liquid crystal spatial light modulator, at which point the vortex light increases. The orbital angular momentum is 1 unit, and the orbital angular momentum eigenstate is |3>. After mirror reflection, the orbital angular momentum of the vortex light becomes negative. Therefore, the final orbital angular momentum eigenstate returning to Bob's position is denoted as |-(l1+l2)>, which is |-5>.

[0054] Example 2

[0055] The method for continuous variable quantum secure direct communication described in this invention is used for communication, with l2 = 1, -1, 3, -3.

[0056] like Figure 3 The image shows the intensity distribution of the second Laguerre-Gaussian beam S”1 in Embodiment 2 of the present invention. The first row represents the intensity distribution of the beam S”1 generated by Alice, the second row represents the intensity distribution of S”1 after passing through the turbulent atmospheric channel, and the third row represents the intensity distribution of S”1 after being interfered with by the interferer Eve. From the second row, it can be seen that due to the influence of atmospheric turbulence, the intensity distribution of the beam will have some random fluctuations, but overall, most of the intensity is still retained. From the third row, it can be seen that the interference from Eve further increases the randomness of the beam intensity distribution, but the core intensity of the beam still exists, indicating that the present invention has excellent safety.

[0057] like Figure 4 The diagram shows the phase distribution of the second Laguerre-Gaussian beam S”1 in Embodiment 2 of this invention. The first row represents the phase distribution of beam S”1 generated by Alice, the second row represents the phase distribution of S”1 after passing through the turbulent atmospheric channel, and the third row represents the phase distribution of S”1 after being interfered with by Eve. The second row shows that the phase distribution of the beam exhibits random fluctuations due to atmospheric turbulence. The third row shows that Eve's interference further increases the fluctuations in the beam's phase distribution, but a clear spiral characteristic is still observable. This indicates that external influences do not interfere with the invention's ability to derive secret information based on the spiral direction, demonstrating excellent robustness.

[0058] The various embodiments in this specification are described in a related manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions of the method embodiments.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. A method of continuous-variable quantum secure direct communication, characterized in that, The method comprises the following steps: S1, the sending end prepares a momentum squeezed vacuum state and a position squeezed vacuum state through two optical parametric oscillators respectively, and then prepares a two-mode squeezed entangled light beam pair S including a light beam S1 and a light beam S2 through a beam splitter, and the sending end directly sends the light beam S1 to the receiving end and retains the light beam S2; S2, the sending end modulates the light beam S1 into a first Laguerre-Gaussian light beam S1' through a liquid crystal spatial light modulator, and attenuates the light beam S1' into a quantum state through an attenuator and sends the light beam S1' to the receiving end; S3, the sending end randomly selects a subset of the two-mode squeezed entangled light beam pair, measures the orthogonal position and the orthogonal momentum of the light beam S2 in the subset, and publishes the position and the measurement result of the subset through a classical channel, and the receiving end measures the orthogonal position and the orthogonal momentum of the light beam S1 according to the position of the subset; S4, the receiving end modulates S1' into a second Laguerre-Gaussian light beam S1'' through a reflective liquid crystal spatial light modulator according to secret information, and sends S1'' to the sending end through a turbulent atmosphere channel; S5, the sending end recombines S2 and S1'' into a light beam pair S' through a beam splitter, and images the light beam pair S' on a charge coupled camera, and the sending end judges the orbital angular momentum eigenstate of the light beam pair S' according to the imaging result, and obtains the secret information; In step S2, the first Laguerre-Gaussian beam S1' carries one unit of orbital angular momentum in the paraxial region, the orbital angular momentum eigenstate is denoted as |l1>, and S1' is loaded with a vortex phase exp(il1θ), where l1 is the topological charge of S1', h is the reduced Planck constant, exp is the base of the natural logarithm, i is the complex unit, and θ is the azimuthal angle. In step S3, a quantum state check is further included, and if the measurement results of the sending end and the receiving end in the subset satisfy the inseparable criterion, it indicates that the quantum state is not damaged, and the inseparable criterion is: wherein is the orthogonal position of the light beam S1 at this subset, is the orthogonal momentum of the light beam S1 at this subset, is the orthogonal position of the light beam S2 at this subset, is the orthogonal momentum of the light beam S2 at this subset.

2. The method of continuous-variable quantum secure direct communication according to claim 1, wherein, In step S1, the light beam S1 and the light beam S2 are represented as: wherein is the quadrature position of the optical beam S1, is the quadrature momentum of the optical beam S1, is the quadrature position of the optical beam S2, is the quadrature momentum of the optical beam S2, is the quadrature position of the momentum squeezed vacuum state, is the quadrature position of the position squeezed vacuum state, is the quadrature momentum of the momentum squeezed vacuum state, is the quadrature momentum of the position squeezed vacuum state, e is exp, the base of the natural logarithm, r is the squeezing factor, r > 0, is the initial quadrature position of the quantum state through the first optical parametric oscillator, is the initial quadrature position of the quantum state through the second optical parametric oscillator, is the initial quadrature momentum of the quantum state through the first optical parametric oscillator, is the initial quadrature momentum of the quantum state through the second optical parametric oscillator.

3. The method of continuous-variable quantum secure direct communication according to claim 1, wherein, In step S4, the second Laguerre-Gaussian light beam S1'' is loaded with a vortex phase exp(il2θ), wherein l2 is the topological charge of S1'', i is a complex unit, and θ is an azimuth angle, if the secret information is 0, l2 is set to be less than 0, and if the secret information is 1, l2 is set to be greater than 0.

4. The method of continuous-variable quantum secure direct communication according to claim 1, wherein, In step S5, if the imaging result on the charge coupled camera is a clockwise spiral, the sending end judges that the orbital angular momentum eigenstate l'2 of the light beam pair S' is greater than 0, and obtains that the secret information is 1, and if the imaging result is a counterclockwise spiral, the sending end judges that the orbital angular momentum eigenstate l'2 of the light beam pair S' is less than 0, and obtains that the secret information is 0.

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