Quantum-enhanced hollow-core photonic crystal fiber gas sensor

Abstract

The application discloses a quantum-enhanced hollow photonic crystal fiber gas sensor, which is characterized by comprising a laser, a coarse wavelength division multiplexer, an erbium-doped fiber amplifier, a filter, a polarization controller, a polarization beam splitter, a coupler, a dispersion-shifted fiber, a fiber delay line, a fiber isolator, a single-mode fiber, a hollow photonic crystal fiber, a balanced detector and a spectrum analyzer; the application uses quantum entangled double beams generated after four-wave mixing, which has high quantum correlation, and the intensity difference quantum noise of each mode is reduced; the quantum correlation noise of the probe light and the reference light is subtracted, a noise floor lower than the shot noise limit is generated, and the signal annihilated in the quantum noise can be detected, so that the super high sensitivity measurement beyond the quantum noise limit is realized. The quantum-enhanced hollow photonic crystal fiber gas sensor has the advantages of high sensitivity, good stability, high practicability and the like.

Description

Technical Field

[0001] This invention belongs to the field of gas detection technology, and specifically relates to a quantum-enhanced hollow photonic crystal fiber gas sensor. Background Technology

[0002] Concentration detection plays a vital role in scientific research and daily life. It is well known that coal mining generates large amounts of pollutants such as carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen oxides, and soot, which are major causes of air pollution and acid rain. Monitoring the concentration of these harmful gases is therefore essential. Similarly, measurement scenarios often involve complex environments such as high temperature and pressure, and diverse compositions, where the concentration of the gas being measured may be on the order of one million molecules or less. For example, my country's ongoing space projects, including the Tiangong space station, the BeiDou navigation system, and the Chang'e series of satellites, as well as marine and deep-sea transportation technologies such as the domestically built aircraft carrier and the Jiaolong manned submersible, all require strict monitoring of trace gases inside and outside the spacecraft and launch vehicles to ensure the safety of personnel and the normal operation of equipment. Therefore, gas sensing systems are required to have high measurement sensitivity and response speed to ensure production efficiency.

[0003] Photonic crystal fibers have attracted widespread attention in recent years. Microperforated cladding provides additional degrees of freedom for tailoring the waveguide properties of fibers, and has led to the fabrication of photonic crystal fibers with unique optical properties such as endless single-mode operation, high birefringence, and high nonlinearity. Besides the flexibility in fiber manufacturing, the optical properties of photonic crystal fibers can also be improved through post-processing of commercial photonic crystal fibers. For example, this can be achieved by selectively filling air holes with liquids or gases, modifying the fiber geometry using CO2 lasers or fiber taper, and filling the fiber with quantum dots. In particular, selective filling is considered an important way to modify photonic crystal fibers to obtain unique optical properties. Some functional photonic crystal fibers, such as birefringent tunable photonic crystal fibers and bend-sensitive photonic crystal fibers, are achieved through selectively filling air holes. This invention uses hollow-core photonic crystal fibers to ensure sufficient contact between light and the gas being measured.

[0004] The basic principle of fiber optic gas sensors is to detect the absorption of light power by a gas at a specific wavelength. Gas molecules absorb light at wavelengths corresponding to their absorption spectrum, thus changing the intensity of the emitted light and enabling concentration measurement. It has the following advantages: 1) intrinsic safety; 2) resistance to high temperature and pressure, and unaffected by external influences; 3) ease of remote transmission and multiplexing; 4) resistance to electromagnetic interference and relative stability. Based on these advantages, fiber optic gas sensors are increasingly favored by researchers and are widely used in various aspects of life. For example, they are used for the strict monitoring of residues emitted during combustion processes in thermal power generation, industrial production, and automobile exhaust.

[0005] Conventional fiber optic gas concentration sensors are often susceptible to noise interference; even minute light intensities can be masked by noise, making it impossible to detect minute gas concentrations. This invention utilizes a conjugate beam generated by a quantum entanglement source to detect gas concentration. Because quantum conjugate beams exhibit high quantum correlation, balanced detection of the two beams can overcome the quantum noise limit in terms of sensitivity. Therefore, the quantum-enhanced hollow-core photonic crystal fiber gas sensor proposed in this invention possesses extremely high sensitivity, a simple structure, and high practical value. Summary of the Invention

[0006] To overcome the problem that ordinary fiber optic gas sensors are subject to noise interference, which prevents further improvement in measurement accuracy, this invention proposes a quantum-enhanced hollow-core photonic crystal fiber optic gas sensor. This sensor can break through the quantum noise limit, has extremely high sensitivity, a simple structure, and high practical value.

[0007] The sensing method adopted by this invention to solve the technical problem is as follows:

[0008] Step (1) Generation of the fiber quantum entanglement source: The laser emitted by the fiber femtosecond laser is split into pump light and signal light by a coarse wavelength division multiplexer. The two beams are coupled into a dispersion-shifted fiber through a coupler and, through a four-wave mixing effect, generate signal light and idler light with two frequencies of . The photon annihilates, simultaneously generating two frequencies. and The new photon satisfies the following equation:

[0009] (1)

[0010] in The frequency of the signal light, The frequency of idler light, Let be the frequency of the pump wave. For this process to proceed, the phase matching condition must be satisfied:

[0011] (2)

[0012] In the four-wave mixing process, the signal light and the idler light satisfy energy conservation. The Hamiltonian in the single-space mode case is:

[0013] (3)

[0014] Where k i Represents the spatial pattern of the field. These are nonlinear coefficients. It is the amplitude of the signal field. It is the amplitude of the idle frequency field. It is the amplitude of the pump field. Here, C is the reduced Planck constant, and C is the fundamental amplitude. The equation of motion can be described as follows: ; The time-varying operator can be solved from the above equation:

[0015] (4)

[0016] (5)

[0017] in , ,because It has a much larger magnitude than the signal and idler beams, so it is a constant. Thus, the signal and idler beams in the same spatial mode have quantum correlation intensities. These quantum correlations manifest as a low noise floor when measuring the intensity difference between beams, which normalizes to the shot noise limit without loss.

[0018] (6)

[0019] in , is the photon number difference operator.

[0020] Within the constraint that each coherent region is described by a single spatial mode, the Hamiltonian consists of multiple concurrent nonlinearities:

[0021] (7)

[0022] It has been shown that equation (7) leads to a reduction in quantum noise for multiple modes in both the time and spatial domains. The coherent regions are actually independent, and if the coherence in the far field tends to be zero, it can be considered as a spatial mode in equation (7), i.e. If the coherent regions contained within the beam do not interfere with each other in the detection plane, the condition is effectively satisfied, and if each pair is isolated and the intensity difference is measured, the reduction in quantum noise will approach equation (6).

[0023] The signal light and idler light generated after four-wave mixing are conjugate and highly quantum correlated, significantly reducing intensity difference quantum noise and obtaining the desired fiber quantum entanglement source. The signal light serves as the reference light, and the idler light as the probe light.

[0024] Step (2) Gas sensing: The reference light generated after the light is mixed by four waves enters one port of the balanced detector through the fiber delay line, while the signal light enters the hollow core photonic crystal fiber with side perforation after passing through the fiber isolator, and finally enters the other port of the balanced detector.

[0025] Because the environment contains a certain concentration of the gas to be measured, the gas diffusing into the hollow-core photonic crystal fiber absorbs the signal light of a wavelength corresponding to its absorption spectrum, causing a change in the intensity of the signal light. The principle is as follows:

[0026] Molecular spectral absorption theory is the foundation of gas detection technology. According to quantum theory, the energy level structure of molecules is discrete rather than continuous. This determines that the absorption spectrum of molecules is a discrete spectrum rather than a continuous spectrum; specific molecular structures can only absorb light of specific wavelengths. According to quantum theory, the energy of light is expressed in discrete packets, which can be represented as:

[0027] (8)

[0028] Where E represents the energy of a photon, h is Planck's constant, v is the photon frequency, and c is the speed of light. Let λ be the wavelength of light. We can see that the energy of a photon is related to its wavelength. When the energy of a photon of a certain wavelength is exactly equal to the energy difference between the energy levels of a molecule, the electrons of that molecule will absorb the light energy of that wavelength and jump from the lower energy level to the higher energy level. This is characterized by the absorption specificity of light of a fixed wavelength, that is, gas molecules will absorb light at a specific wavelength corresponding to their energy level structure.

[0029] When the signal light passes through the gas to be tested, the light energy is attenuated to varying degrees due to the absorption effect of the gas. The light intensity of the signal light before and after absorption by the gas to be tested satisfies Beer-Lambert's law, which can be derived from Maxwell's equations and the damped vibration equation of electrons under the action of the incident light wave's electric field.

[0030] (9)

[0031] The above equation illustrates the relationship between the outgoing light intensity I and the incident light intensity and the volume fraction of the gas when a beam of parallel light passes through a gas. Here, L represents the gas absorption coefficient, i.e., the absorption line shape of the gas at a certain frequency; C represents the absorption path length, which is the length through which the light beam passes in the hollow-core photonic crystal fiber; and D represents the gas concentration. The hollow-core photonic crystal fiber uses a side-hole drilled with a laser. The gas to be measured diffuses into the hollow-core photonic crystal fiber through this side-hole. When the signal light passes through the hollow-core photonic crystal fiber, due to Lambertian absorption, the gas absorbs light of the corresponding wavelength to the signal light, causing a change in the intensity of the emitted probe light. The probe light and the reference light are simultaneously received by a balanced detector for differential detection. Since the differential signal intensity is proportional to the gas concentration, the concentration of the gas in the environment can be measured.

[0032] Because the quantum entangled source has high quantum correlation, each mode exhibits reduced quantum noise, generating a noise floor below the shot noise limit, making signals that were originally buried under quantum noise detectable, thereby achieving ultra-high sensitivity measurement that breaks through the quantum noise limit.

[0033] The apparatus adopted by the present invention to solve the technical problem:

[0034] This invention comprises four modules: an optical fiber quantum entanglement source module, an optical fiber transmission module, an optical fiber sensing module, and a detection and analysis module;

[0035] The fiber quantum entanglement source module consists of a femtosecond laser, a coarse wavelength division multiplexer (CWDM), an erbium-doped fiber amplifier (EDFA), a filter, a fiber polarization controller (FPC), a fiber polarization beam splitter (FPBS), a coupler, and a dispersion-shifted fiber (DSF). The coarse wavelength division multiplexer includes first and second coarse wavelength division multiplexers, the filter includes first and second filters, the polarization controller includes first, second, third, and fourth polarization controllers, and the polarization beam splitter includes first and second polarization beam splitters. The laser is connected to the input of the first coarse wavelength division multiplexer (CWDM). One port of the output of the first CWDM is connected to the input of the erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to the input of the first filter. The other end of the first filter is connected to one end of the first polarization controller. The other end of the first polarization controller is connected to one end of the first polarization beamsplitter. The other end of the first polarization beamsplitter is connected to one end of the second polarization controller. The other end of the second polarization controller is connected to one end of the two ports of the coupler. The other port of the first CWDM is connected to the input of the second filter. The output of the second filter is connected to one end of the third polarization controller. The other end of the third polarization controller is connected to one end of the second polarization beamsplitter. The other end of the second polarization beamsplitter is connected to one end of the fourth polarization controller. The other end of the fourth polarization controller is connected to the other end of the two ports of the coupler. One end of one port of the coupler is connected to one end of the dispersion-shifting fiber. The other end of the dispersion-shifting fiber is connected to the input of the second CWDM.

[0036] The fiber optic transmission module consists of a fiber delay line and a fiber isolator. The fiber isolator ensures that the probe light propagates in the forward direction, while the fiber delay line allows the probe light and reference light to arrive at the balanced detector simultaneously. One port of the output of the second coarse wavelength division multiplexer is connected to the fiber delay line, the other end of the fiber delay line is connected to one port of the input of the balanced detector, and the other port of the output of the second coarse wavelength division multiplexer is connected to one end of the fiber isolator.

[0037] The fiber optic sensing module consists of a single-mode fiber (SMF) and a side-perforated hollow-core photonic crystal fiber (HC-PCF). In this module, the gas to be measured diffuses into the hollow-core photonic crystal fiber through the side air holes. The gas absorbs part of the probe light, thus changing the intensity of the probe light. The other end of the fiber optic isolator is connected to one end of the first single-mode fiber, the other end of the first single-mode fiber is fused to one end of the side-perforated hollow-core photonic crystal fiber, and the other end of the side-perforated hollow-core photonic crystal fiber is fused to the second single-mode fiber.

[0038] The detection and analysis module consists of a balanced detector and a spectrum analyzer. This module performs differential processing on the obtained probe light and reference light; the intensity of the differential signal is proportional to the gas concentration. By measuring the intensity of the differential signal, the concentration of the gas can be obtained. The other end of the second single-mode fiber is connected to the other port of the balanced detector's input, and the output of the balanced detector is connected to the spectrum analyzer.

[0039] The beneficial effects of this invention are as follows:

[0040] This invention enables the detection of signals previously obscured by quantum noise. Quantum noise originates directly from the Heisenberg uncertainty principle and comprises two main noise sources, with photon shot noise being the primary one. Traditional optical methods cannot overcome the shot noise limit; however, quantum enhancement enables precise measurement.

[0041] The probe light and reference light used in this invention exhibit strong quantum correlation in intensity, meaning the quantum noise level of their intensity difference is 3.1 dB lower than the shot noise limit (10.4 dB after loss correction). This allows the extremely small changes in gas concentration, even when the intensity difference is below the shot noise limit, to be detected after the probe light is input into the balanced detector through a side-perforated hollow photonic crystal fiber. This enables ultra-high sensitivity measurements that break through the quantum noise limit.

[0042] The hollow-core photonic crystal fiber used in this invention has an air-hole core and a cladding composed of periodically arranged air columns forming a two-dimensional photonic crystal structure. This structure allows a mode of a specific wavelength within the photonic bandgap of the cladding to propagate within the air-hole core region, achieving over 95% light confinement within the air core. Light propagation within the air core allows for sufficient contact with the gas. Furthermore, the length of the hollow-core photonic crystal fiber can be increased, extending the operating distance and thus improving the sensor's sensitivity. The hollow-core photonic crystal fiber used in this invention employs laser-drilled holes on its sides, enabling faster diffusion of gas molecules and significantly shortening the measurement time. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of a quantum-enhanced hollow photonic crystal fiber gas concentration sensor.

[0044] Figure 2 The spectral absorption of acetylene gas near 1520 nm is shown in the HITRAN database. Detailed Implementation

[0045] The present invention will now be further described with reference to the accompanying drawings.

[0046] like Figure 1 As shown, the quantum-enhanced hollow-core photonic crystal fiber gas sensor includes a laser 1, a first coarse wavelength division multiplexer 2, an erbium-doped fiber amplifier 3, a first filter 4, a first polarization controller 5, a first polarization beam splitter 6, a second polarization controller 7, a second filter 8, a third polarization controller 9, a second polarization beam splitter 10, a fourth polarization controller 11, a coupler 12, a dispersion-shifting fiber 13, a second coarse wavelength division multiplexer 14, a fiber delay line 15, a fiber isolator 16, a first single-mode fiber 17, a side-drilled hollow-core photonic crystal fiber 18, a second single-mode fiber 19, a balanced detector 20, and a spectrum analyzer 21.

[0047] The laser 1 in the fiber quantum entanglement source module is connected to the input port 201 of the first coarse wavelength division multiplexer 2. The output port 202 of the first coarse wavelength division multiplexer 2 is connected to the input of the erbium-doped fiber amplifier 3. The output of the erbium-doped fiber amplifier 3 is connected to the input of the first filter 4. The other end of the first filter 4 is connected to one end of the first polarization controller 5. The other end of the first polarization controller 5 is connected to one end of the first polarization beam splitter 6. The other end of the first polarization beam splitter 6 is connected to one end of the second polarization controller 7. The other end of the second polarization controller 7 is connected to one end 1202 of the two ports of the coupler 12. Another output port 203 of the coarse wavelength division multiplexer 2 is connected to the input port of the second filter 8. The output port of the second filter 8 is connected to one end of the third polarization controller 9. The other end of the third polarization controller 9 is connected to one end of the second polarization beam splitter 10. The other end of the second polarization beam splitter 10 is connected to one end of the fourth polarization controller 11. The other end of the fourth polarization controller 11 is connected to the other end 1203 of the two ports of the coupler 12. One end 1201 of one port of the third coupler 12 is connected to one end of the dispersion-shifting fiber 13. The other end of the dispersion-shifting fiber 13 is connected to the input port 1401 of the second coarse wavelength division multiplexer 14.

[0048] The output port 1402 of the second coarse wavelength division multiplexer 14 in the optical fiber transmission module is connected to the optical fiber delay line 15, the other end of the optical fiber delay line 15 is connected to the input port 2002 of the balanced detector 20, and the other output port 1403 of the second coarse wavelength division multiplexer 14 is connected to one end of the optical fiber isolator 16.

[0049] The other end of the fiber optic isolator 16 in the fiber optic sensing module is connected to one end of the first single-mode fiber 17. The other end of the first single-mode fiber 17 is fused to one end of the side-perforated hollow photonic crystal fiber 18. The other end of the side-perforated hollow photonic crystal fiber 18 is fused to the second single-mode fiber 19.

[0050] The other end of the second single-mode fiber 19 in the detection and analysis module is connected to the other port 2003 of the input end of the balanced detector 20, and the output end 2001 of the balanced detector 20 is connected to the spectrum analyzer 21.

[0051] The working principle of this invention is as follows:

[0052] The light emitted by laser 1 in the fiber quantum entanglement source module is split into pump light and signal light by a first coarse wavelength division multiplexer 2. The pump light is amplified by erbium-doped fiber amplifier 3 to obtain the required pump power, and then filtered by a first filter 4. The polarization and power of the pump light are controlled by a first polarization controller 5, a first polarization beam splitter 6, and a second polarization controller 7. The pump light is then input to one end 1202 of the two ports of coupler 12. The signal light is filtered by a second filter 8. The intensity and polarization of this signal light are controlled by a third polarization controller 9, a second polarization beam splitter 10, and a fourth polarization controller 11. The signal light is then input to the other end 1203 of the two ports of coupler 12. The coupled light is input from one end 1201 of the third coupler 12 to dispersion-shifted fiber 13 to generate a four-wave mixing effect and obtain entangled dual beams. The other end of dispersion-shifted fiber 13 is connected to the input end 1401 of a second coarse wavelength division multiplexer 14 to filter out the pump light and separate the probe light and reference light.

[0053] In the optical fiber transmission module, the reference light is output from the output terminal 1402 of the second coarse wavelength division multiplexer 14 to the optical fiber delay line 15. The optical fiber delay line 15 allows the probe light and the reference light to arrive at the balanced detector 20 simultaneously. The probe light is output from the output terminal 1403 of the second coarse wavelength division multiplexer 14, and the forward light is allowed to pass through while the reverse light is isolated by the subsequent optical fiber isolator 16.

[0054] In the fiber optic sensing module: the probe light passes through a hollow photonic crystal fiber 22 with side holes filled with the gas to be measured, and due to the absorption of the probe light by the gas, a probe light with varying intensity is obtained.

[0055] The detection and analysis module: The detection light is input from the single-mode fiber 19 to port 2003 of the balanced detector 20. The entangled dual beams are processed by the balanced detector 20 and then input to the spectrum analyzer 21 for analysis.

[0056] Since the hollow-core photonic crystal fiber is filled with the gas to be tested, the gas to be tested will absorb the probe light, thereby changing the intensity of the probe light. By performing differential processing on the probe light and the reference light, since the intensity of the differential signal is proportional to the concentration of the gas signal, the concentration of the gas in the environment can be obtained.

[0057] Furthermore, because the present invention uses a quantum entangled source generated after four-wave mixing, it is a correlated dual beam with entangled spatial modes and has high quantum correlation. Each mode exhibits reduced intensity difference quantum noise, and the quantum correlation noise between the probe light and the reference light is subtracted, generating a noise floor below the shot noise limit. This allows signals that were originally buried under quantum noise to be detected, thereby achieving ultra-high sensitivity measurement that breaks through the quantum noise limit.

[0058] The key technologies enabling this device to realize a quantum-enhanced hollow photonic crystal fiber gas sensor include:

[0059] 1. The four-wave mixing process corresponds to the annihilation of two pump photons, simultaneously generating two signal photons and an idler photon of different frequencies. In this parametric process, net energy and momentum are conserved. Parametric processes require the selection of specific frequencies and fiber parameters to satisfy phase-matching conditions. The signal and reference beams generated by four-wave mixing are quantum entangled sources with high quantum correlation. Detection techniques rely on non-degenerate four-wave mixing to generate a dual-beam array with entangled spatial modes, producing a noise floor below the shot noise limit, thereby enabling high-precision measurements.

[0060] 2. Wavelength selection for the coarse wavelength division multiplexer: After four-wave mixing, the signal light, idler light, and remaining pump light are obtained. The signal light is used as the probe light to measure changes in parameters, the idler light is used as the reference light for comparison with the signal light, and the remaining pump light will affect our experiment. Therefore, we need to select a suitable coarse wavelength division multiplexer to separate the three lights.

[0061] 3. The probe light and reference light generated by four-wave mixing become entangled with each other. The wavelength of the probe light matches the gas absorption limit, and the gas molecules will absorb the probe light, thereby changing the intensity of the probe light.

[0062] In a specific embodiment of this invention, the absorption peak of acetylene molecules near 1520 nm was selected as the absorption spectral line. A mode-locked fiber laser was used, from which a 40 MHz 150 fs pulse was extracted, with a center wavelength of 1560 nm. A coarse wavelength division multiplexer was used to separate the pump light and seed light; the pump light had a center wavelength of 1552.5 nm, and the seed signal had a center wavelength of 1569.8 nm. To achieve the required pump power, the pump pulse was amplified by an erbium-doped fiber amplifier (EDFA), and the light was purified using a bandpass filter. The polarization and power of the pump light and idler light were controlled by a fiber polarization controller and a fiber polarization beam splitter. A 90 / 10 fiber coupler was used to couple 90% of the pump pulses and 10% of the signal pulses into a 300m dispersion-shifted fiber, generating four-wave mixing to obtain signal and reference light. To effectively separate the signal, idler, and remaining pump light, a four-channel coarse wavelength division multiplexer (CWDM) was placed after the DSF, with center wavelengths of 1511, 1531, 1551, and 1571 nm and a 1dB bandwidth of 16 nm. The signal light wavelength was 1571 nm, the idler wavelength was 1531 nm, and the remaining pump light wavelength was 1552 nm. The idler light was used as the probe light, coupled via a single-mode fiber into a side-drilled hollow-core photonic crystal fiber, and then connected to a balanced detector. The signal light was used as the reference light, fed through a fiber delay line into the balanced detector. Finally, the results were analyzed using a spectrum analyzer. The hollow-core photonic crystal fiber with side-drilled holes is 20m long. A femtosecond laser is used to drill the holes on the side, and the diameter of the holes is 5 mm. The number of apertures was 100, with a spacing of 20 cm between apertures; the fiber delay line length was 20 m; and the gas diffusion time was 30 minutes. The spectrum analyzer settings were as follows: resolution bandwidth: 10 kHz; display bandwidth: 100 Hz; scan time: 2 s; average sampling times: 20. The resulting noise floor was 4.0 ± 0.1 dB lower than that of the classical readout light at the same optical power. Under most concentration limitations, the signal-to-noise ratio was determined by the reduction in available quantum noise in the readout light, meaning that we directly observed an increase in the signal-to-noise ratio, thus enabling us to measure gas concentration values ​​previously obscured by shot noise.

[0063] The foregoing has shown and described the basic principles and main features of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A quantum-enhanced gas sensing method based on hollow-core photonic crystal fiber, characterized in that... Includes the following steps: Step (1) Generation of the fiber quantum entanglement source: The laser emitted by the fiber femtosecond laser is split into pump light and signal light by a coarse wavelength division multiplexer. The two beams are coupled into a dispersion-shifted fiber through a coupler and, through a four-wave mixing effect, generate signal light and idler light with two frequencies of . The photon annihilates, simultaneously generating two frequencies. and The new photon satisfies the following equation: （1） in The frequency of the signal light, The frequency of idler light, For the pump wave frequency to occur, the phase matching condition must be met: （2） In the four-wave mixing process, the signal light and the idler light satisfy energy conservation. The Hamiltonian in the single-space mode case is: （3） Where k i Represents the spatial pattern of the field. These are nonlinear coefficients. It is the amplitude of the signal field. It is the amplitude of the idle frequency field. It is the amplitude of the pump field. The reduced Planck constant is used, and the equation of motion can be described as follows: ; The time-varying operator can be solved from the above equation: (4) (5) in , Because the pump light is significantly larger in magnitude than the signal light and idler light, This is a constant, so that the signal and idler beams in the same spatial mode have quantum correlation intensities. These quantum correlations manifest as a low noise floor when measuring the intensity difference between beams, which is normalized to the shot noise limit without loss. (6) in , is the photon number difference operator; Within the constraint that each coherent region is described by a single spatial mode, the Hamiltonian consists of multiple concurrent nonlinearities: （7） Equation (7) leads to a reduction in quantum noise for multiple modes in the time and spatial domains. The coherent regions are actually independent. If the coherence in the far field tends to be zero, it is considered as the spatial mode in equation (7), i.e. If the coherent regions contained within the beam do not interfere with each other in the detection plane, the condition is effectively satisfied. If each pair is isolated and the intensity difference is measured, the quantum noise reduction will approach equation (6). The signal light and idler light generated after four-wave mixing are conjugate and have high quantum correlation. Their intensity difference quantum noise is greatly reduced, and the desired fiber quantum entanglement source is obtained, in which the signal light is used as the reference light and the idler light is used as the probe light. Step (2) Gas sensing: The reference light generated after the light is mixed by four waves enters one port of the balanced detector through the fiber delay line, while the signal light enters the hollow core photonic crystal fiber with side perforation after passing through the fiber isolator, and finally enters the other port of the balanced detector. Because the environment contains a certain concentration of the gas to be measured, the gas diffuses into the hollow-core photonic crystal fiber and absorbs the signal light of the wavelength corresponding to its absorption spectrum, causing a change in the intensity of the signal light. The principle is as follows: Molecular spectral absorption theory is the foundation of gas detection technology. According to quantum theory, the energy level structure of molecules is discrete rather than continuous. This determines that the absorption spectrum of molecules is a discrete spectrum rather than a continuous spectrum. Specific molecular structures can only absorb light of specific wavelengths. According to quantum theory, the energy of light is distributed in discrete packets, which can be expressed as: （8） Where E represents the energy of a photon, h is Planck's constant, v is the photon frequency, and c is the speed of light. As we can see from the wavelength of light, the energy of a photon is related to its wavelength. When the energy of a photon of a certain wavelength is exactly equal to the energy difference between the energy levels of a certain molecule, the electrons of that molecule will absorb the light energy of that wavelength and jump from the lower energy level to the higher energy level. This is manifested as the absorption specificity of light of a fixed wavelength, that is, gas molecules will absorb light at a specific wavelength corresponding to their energy level structure. When the signal light passes through the gas to be tested, the light energy is attenuated to varying degrees due to the absorption effect of the gas. The light intensity before and after absorption by the gas to be tested satisfies Beer-Lambert's law, which can be derived from Maxwell's equations and the damped vibration equation of electrons under the action of the incident light wave's electric field. （9） The above equation illustrates the relationship between the outgoing light intensity I and the incident light intensity and the volume fraction of the gas when a beam of parallel light passes through a gas. Here, L is the gas absorption coefficient, which is the absorption line shape of a gas at a certain frequency; C is the absorption path length, which is the length through which the light beam passes in the hollow-core photonic crystal fiber; and C is the gas concentration. The hollow-core photonic crystal fiber uses a laser with holes punched on the side. The gas to be measured diffuses into the hollow-core photonic crystal fiber through the air holes on the side. When the signal light passes through the hollow-core photonic crystal fiber, due to Lambertian absorption, the gas absorbs light of the wavelength corresponding to the signal light, which causes a change in the intensity of the emitted probe light. The probe light and the reference light are simultaneously received by the balanced detector for differential detection. Since the differential signal intensity is proportional to the gas concentration, the concentration of gas in the environment can be measured. Because the quantum entangled source has high quantum correlation, each mode exhibits reduced quantum noise, generating a noise floor below the shot noise limit, making signals that were originally buried under quantum noise detectable, thereby achieving ultra-high sensitivity measurement that breaks through the quantum noise limit.

2. An apparatus for implementing the method of claim 1, characterized in that... It comprises four modules: a laser, a coarse wavelength division multiplexer, an erbium-doped fiber amplifier, a filter, a polarization controller, a polarization beam splitter, a coupler, a dispersion-shifting fiber, a fiber delay line, a fiber isolator, a single-mode fiber, a hollow-core photonic crystal fiber, a balanced detector, and a spectrum analyzer; the fiber quantum entanglement source module, the fiber transmission module, the fiber sensing module, and the detection and analysis module. The coarse wavelength division multiplexer includes first and second coarse wavelength division multiplexers; the filter includes first and second filters; the polarization controller includes first, second, third, and fourth polarization controllers; and the polarization beam splitter includes first and second polarization beam splitters. The laser in the fiber quantum entanglement source module is connected to the input of the first coarse wavelength division multiplexer. One port of the output of the first coarse wavelength division multiplexer is connected to the input of the erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to the input of the first filter. The other port of the first filter is connected to one end of the first polarization controller. The other end of the first polarization controller is connected to one end of the first polarization beamsplitter. The other end of the first polarization beamsplitter is connected to one end of the second polarization controller. The other end of the second polarization controller is connected to one end of the two ports of the coupler. The other port of the output of the first coarse wavelength division multiplexer is connected to the input of the second filter. The output of the second filter is connected to one end of the third polarization controller. The other end of the third polarization controller is connected to one end of the second polarization beamsplitter. The other end of the second polarization beamsplitter is connected to one end of the fourth polarization controller. The other end of the fourth polarization controller is connected to the other end of the two ports of the coupler. One end of one port of the coupler is connected to one end of the dispersion-shifting fiber. The other end of the dispersion-shifting fiber is connected to the input of the second coarse wavelength division multiplexer. One port of the output of the second coarse wavelength division multiplexer in the optical fiber transmission module is connected to the optical fiber delay line, the other end of the optical fiber delay line is connected to one port of the input of the balanced detector, and the other port of the output of the second coarse wavelength division multiplexer is connected to one end of the optical fiber isolator. The other end of the fiber optic isolator in the fiber optic sensing module is connected to one end of the first single-mode fiber, the other end of the first single-mode fiber is fused to one end of the hollow photonic crystal fiber with side perforation, and the other end of the hollow photonic crystal fiber with side perforation is fused to the second single-mode fiber. The other end of the second single-mode fiber in the detection and analysis module is connected to the other port of the input end of the balanced detector, and the output end of the balanced detector is connected to the spectrum analyzer.

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