Generation and manipulation of continuous variable quantum states are the building blocks of quantum communication, quantum key distribution and quantum networks. According to the second-order nonlinear process of the periodically-poled potassium titanyl phosphate (PPKTP) crystal, we design a semi-monolithic optical parametric amplifier (OPA) cavity to generate the bright squeezed light at a wavelength of 1064 nm. With the injection of a seed beam, the squeezed state generated by the OPA has a coherent amplitude, so called bright squeezed state. The squeezing level is directly observed to be –11.6 dB when the pump power is 310 mW at an analysis frequency of 3 MHz. However, with the increase of the pump power, the purity of the squeezed state gets lower and lower due to the increased influence of the anti-squeezing quadrature component on the squeezed quadrature component in the detection process. To obtain a higher purity of the squeezed state for achieving linear optical manipulation and quantum tomography, we choose the pump power of 50 mW, the squeezing level decreases to –6 dB, and the purity of the squeezed state is 98.5% in this case. An electro-optic modulator is adopted to realize the liner manipulation of the squeezed light in the phase space. During the measurement of the bright squeezed state, all the data are taken on condition that the length of the OPA cavity and relative phase between the seed beam and the pump beam are locked by a locking loop. The direct current (DC) signal of the balanced homodyne detection (BHD) is used to accurately determine the phase corresponding to the time domain signal of the squeezed state, while the alternate current (AC) signal of the BHD is mixed with the signal generated by the function generator, after passing through a low-pass filter and a high-pass filter, the signal is then amplified by using a low-noise amplifier. A high-performance oscilloscope is finally used to simultaneously collect the signals, thus obtaining the quantum noise signal of the bright squeezed light after linear manipulation. Together with the maximum likelihood estimation algorithm, the quantum tomography, the density matrix and the Wigner function of the bright squeezed light are obtained, that is, all the information such as the photon number distribution of the quantum state is determined. Multiple iterations are taken in the maximum likelihood estimation algorithm process to eliminate the influence of the low quantum efficiency on the detection system, so that the density matrix is fitted well with the theoretical results.
Generation and manipulation of continuous variable quantum states are the building blocks of quantum communication, quantum key distribution and quantum networks. According to the second-order nonlinear process of the periodically-poled potassium titanyl phosphate (PPKTP) crystal, we design a semi-monolithic optical parametric amplifier (OPA) cavity to generate the bright squeezed light at a wavelength of 1064 nm. With the injection of a seed beam, the squeezed state generated by the OPA has a coherent amplitude, so called bright squeezed state. The squeezing level is directly observed to be –11.6 dB when the pump power is 310 mW at an analysis frequency of 3 MHz. However, with the increase of the pump power, the purity of the squeezed state gets lower and lower due to the increased influence of the anti-squeezing quadrature component on the squeezed quadrature component in the detection process. To obtain a higher purity of the squeezed state for achieving linear optical manipulation and quantum tomography, we choose the pump power of 50 mW, the squeezing level decreases to –6 dB, and the purity of the squeezed state is 98.5% in this case. An electro-optic modulator is adopted to realize the liner manipulation of the squeezed light in the phase space. During the measurement of the bright squeezed state, all the data are taken on condition that the length of the OPA cavity and relative phase between the seed beam and the pump beam are locked by a locking loop. The direct current (DC) signal of the balanced homodyne detection (BHD) is used to accurately determine the phase corresponding to the time domain signal of the squeezed state, while the alternate current (AC) signal of the BHD is mixed with the signal generated by the function generator, after passing through a low-pass filter and a high-pass filter, the signal is then amplified by using a low-noise amplifier. A high-performance oscilloscope is finally used to simultaneously collect the signals, thus obtaining the quantum noise signal of the bright squeezed light after linear manipulation. Together with the maximum likelihood estimation algorithm, the quantum tomography, the density matrix and the Wigner function of the bright squeezed light are obtained, that is, all the information such as the photon number distribution of the quantum state is determined. Multiple iterations are taken in the maximum likelihood estimation algorithm process to eliminate the influence of the low quantum efficiency on the detection system, so that the density matrix is fitted well with the theoretical results.
The squeezed light at 1.5 m telecommunication band has been considered as an important resource of continuous variable (CV) practical fiber-based quantum information research because it is the lowest loss in fiber. A bright phase quadrature squeezed light for continuous variable at 1.5 m is demonstrated from a semi-monolithic degenerate optical parametric amplifier (DOPA) based on a periodically poled KTiOPO4 (PPKTP) crystal. The laser source is a continuous wave (CW) single-frequency fiber laser at 1.5 m, which is sent through a ring mode cleaner (MC) as a preliminary spatial and noise filter. And then the main portion of the output from the MC is used for external-enhanced second harmonic generation to obtain a CW single-frequency low noise laser at 780 nm that acts as the pump of the DOPA. The residual light of the output from the MC at 1.5 m is used as the injected signal light of the DOPA and the local oscillator (LO) of a balanced homodyne detector (BHD) system. The DOPA is built by using a type-I PPKTP crystal and a piezo-actuated output coupler and works in double-resonance case with a threshold power of 230 mW. When the DOPA is operating in the state of amplification, the output down-conversion field should be a bright phase quadrature squeezed light, where the relative phase between the pump and the injected signal is locked to 0. A 4.7 dB bright phase quadrature squeezed light is measured by the BHD system with the pump light of 110 mW and the injected signal of 3 mW, where the relative phase between the down-conversion field and the LO is locked to 0. Our measurement is limited by the optical losses and the detection efficiency. We have taken into account the detection efficiency of 86.6%, and the actual squeezing of the squeezed light being 6.3 dB. Moreover, because it is so crucial a process for CV quantum information system that the transmission and evolution of the CV squeezed states in the fiber may reappear in all information of the quantum states in the phase space, then the bright squeezed light is detected by a BHD system in the time domain, and its Wigner quasi-probability distribution function can be reconstructed by using a quantum tomographic technique. Furthermore, the bright squeezed state at 1.5 m is an ideal source for fiber-based long-distance quantum information because of its stability and bright mean field.
The use of optical sensing for in vivo applications is compelling, since it offers the advantages of non-invasiveness, non-ionizing radiation, and real-time monitoring. However, the signal-to-noise ratio (SNR) of the optical signal deteriorates dramatically as the biological tissue increases. Although increasing laser power can improve the SNR, intense lasers can severely disturb biological processes and viability. Quantum sensing with bright squeezed light can make the measurement sensitivity break through the quantum noise limit under weak laser conditions. A bright squeezed light source is demonstrated to avoid the deterioration of SNR and biological damage, which integrates an external cavity frequency-doubled laser, a semi-monolithic standing cavity with periodically poled titanyl phosphate (PPKTP), and a balanced homodyne detector (BHD) assembled on a dedicated breadboard. With the rational design of the mechanical elements, the optical layout, and the feedback control equipment, a maximum non-classical noise reduction of −10.7 ± 0.2 dB is observed. The average squeeze of −10 ± 0.2 dB in continuous operation for 60 min is demonstrated. Finally, the intracavity loss of degenerate optical parametric amplifier (DOPA) and the initial bright squeezed light can be calculated to be 0.0021 and −15.5 ± 0.2 dB, respectively. Through the above experimental and theoretical analysis, the direction of improving bright squeeze level is pointed out.
Squeezed state of light is an important resource of optical measuerments below the shot noise limit and has been used to improve measurement sensitivity in many areas such as gravitational wave detection, especially in audio frequency region. Compared with the high-frequency squeezed states, the generation of the low-frequency squeezed states is more difficult, because it is limited by several technical noise sources. In this paper we report the observation of more than 2 dB of vacuum squeezing at 1064 nm in the gravitational-wave detection band down to 3 kHz with a double-resonant optical parametric oscillator (OPO). The OPO has a configuration of linear cavity consisting of an input coupling mirror with a transmission of 11% at 532 nm and an output coupling mirror with the transmission of 12% at 1064 nm. The nonlinear materials in the OPO is type-I periodically poled potassium titanyl phosphate (PPKTP) crystal which is chosen for this experiment due to its higher nonlinearity, broader phase matching temperature, and smaller photo-thermal effect. The OPO is pumped by the light of 532 nm from Nd: YVO4/KTP solid-state laser of maximum optical power 3 W. To avoid various noise coupled from the seed beam, the OPO is seeded by vacuum fluctuations instead of coherent field at the fundamental wavelength (1064 nm). A Pound-Drever-Hall (PDH) locking scheme is used to lock the OPO cavity length with the signal derived from the reflected pump beam, so as to lock the pump field and also lock the fundamental field. To make both the pump and seed beams resonant simultaneously, the temperature of the PPKTP is carefully adjusted. The squeezed state can be detected on a homodyne detection by interfering it with the local oscillator (LO) and detected by a balanced detector with two photodiodes (EXT500 T) but having the same quantum efficiency of 86% at 1064 nm. The subsequent electronic noise is analyzed with a low-frequency spectrum analyzer, which shows that the audio noise sources from lab enviroment, locking quality, escape efficiency, propagation loss, homodyne efficiency and detection efficiency have effect on the squeezing pruced by an OPO.
We demonstrate the methods for increasing the observed squeezing level in the generation of squeezed states of light at 1064 nm with periodically poled KTiOPO4 (PPKTP) crystal. We analyze the technical limits to the reduction of noise in the squeezed quadrature theoretically, including the intra-cavity loss of the optical parametric amplifier (OPA) cavity, the normalized amplitude, the detection frequency, and the mode matching on the balanced homodyning stage. To observe a high degree of squeezing, we improve the quality of mode matching into the OPA cavity and the mode cleaning (MC) cavity experimentally. By optimizing mode matching of the light into cavities, the finesse of the cavities would be higher in practice and the desirable spatial mode can be realized. As the intensity noise of the laser light reach the shot noise limit above the frequency of 1 MHz, which is sufficient for our experiment, so the MC cavity is incorporated to filter higher-order transverse modes of the local oscillator (LO) beam for the balanced homodyne detection. The experimental and theoretical results show that it is essential to optimize the mode matching efficiency to generate and detect high degree of squeezing otherwise an inefficiency mode matching will throw away the squeezing and transform the squeezed field into a vacuum field.
The squeezed light field is a kind of important continuous variable quantum resource.It has wide applications in precision measurement and quantum information processing.Quantum storage is the foundations of quantum repeater and long distance quantum communication,and alkali metal atoms are an ideal quantum storage medium due to long ground state coherent time. With the rapid development of quantum storage technology in atomic medium,the preparation of the squeezed light which resonates with alkali metal atoms has become one of the research hotspots in the field of quantum information.In this paper,we report the generation of squeezed vacuum at 795 nm (resonant on the rubidium D1 transition line) by using an optical parametric oscillation based on a periodically poled KTiOPO4 crystal. The generated squeezed light field is detected by a balanced homodyne detector,and the squeezing of-3 dB and anti-squeezing of 5.8 dB are observed at a pump power of 45 mW.By using a maximum likelihood estimation,the density matrix of the squeezed light field is reconstructed.The time-domain signals from the balanced homodyne detector are collected to acquire the noise distribution of the squeezed light under different phase angles.The likelihood function is established for the measured quadrature components.An identity matrix is chosen as an initial density matrix,and the density matrix of the squeezed field is obtained through an iterative algorithm.The diagonal elements of the density matrix denote the photon number distribution,which includes not only even photon number states but also odd photon number states.The occurrence of odd photon number states mainly comes from the system losses and the imperfect quantum efficiency of detector.The Wigner function in phase space is calculated through the density matrix,and the maximum value of the Wigner function is 0.309.The standard deviation of the squeezed component is 64.4% of that of the vacuum state,corresponding to the squeezing degree of-3.8 dB.The standard deviation of the anti-squeezing component is 1.64 times that of the vacuum state,corresponding to the anti-squeezing degree of 4.3 dB.We theoretically calculate the photon number distribution and the Wigner function of the vacuum squeezed field,and compare the results obtained by theoretical calculation with those obtained by maximum likelihood reconstruction.The probability of vacuum state|0 obtained by maximum likelihood reconstruction is greater,and the probability of photon number state|n(n=1,2,) is smaller than the corresponding theoretical calculation results.From the theoretical calculation,the maximum value of Wigner function is 0.231,and the short axis and long axis of noise range deduced from the contours of the Wigner function are larger than the results from the maximum likelihood reconstruction.The possible reasons for the discrepancy are as follows. 1) The phase scanning is nonuniform during the measurement of the quadrature components.2) The low-frequency electronic noise is not completely filtered out in the datum acquisition process.3) The datum points of measured quadrature components are not enough.In conclusion,we produce a vacuum squeezed field of 795 nm,and obtain the photon number distribution and the Wigner function in phase space through maximum likelihood estimation and theoretical calculation,respectively.This work will provide an experimental basis for generating the Schrodinger cat state.
Squeezed states are important sources in quantum physics, which have potential applications in fields such as quantum teleportation, quantum information networks, quantum memory, and quantum metrology and precise measurements. For our interest, the squeezed vacuum will be used in the quantum-enhanced optical atomic magnetometers, filling the vacuum port of the probe beam to improve measurement sensitivity. Based on the sub-threshold optical parametric oscillator (OPO) with PPKTP crystal, the squeezed vacuum at rubidium D1 line of 795 nm is obtained. In our work, we investigate the noise sources in an OPO system. By carefully controlling the classical noise source, the squeezing band extends to the analysis frequency of 2.6 kHz. The flat squeezing trace is 2.8 dB below the shot noise limit. In our work, we focus on the difference between the squeezing results at the analysis frequency of kilohertz regime at two different wavelengths, 1064 nm and 795 nm. The difference mainly comes from the absorption of 795 nm laser and its second harmonic at 397.5 nm in crystal (397.5 nm laser is at the edge of transparent window of PPKTP crystal that has an absorption index much higher than at other wavelength). The absorption induced nonlinear loss and thermal instability greatly affect the squeezing results, which is discussed in our work. Squeezing level at 795 nm is worse than at 1064 nm due to the above-mentioned factors. Noise coupling to the detection system limits the squeezing band. In the audio frequency band, squeezing is easily submerged in roll-up noises and the measured squeezing level is limited. Two factors limit the obtained squeezing:the technical noise induced in the detection and the squeezing degradation by the noise coupling of the control beams. In experiment, we carefully control the classical noise at analytical frequency of kilohertz by means of a vacuum-injected OPO, a counter-propagating cavity locking beam with orthogonal polarization, low noise homodyne detector, stable experimental system and quantum noise locking method for squeezing phase locking. Firstly, to preclude the classical noise from coupling the laser source, we use the vacuum injected OPO. A signal beam helps optimize the parametric gain and is blocked in the squeezing measurement process. In order to maintain the OPO, a counter-propagating beam with orthogonal polarization is used for locking the cavity. Then, a low noise balanced homodyne detector with a common-mode rejection ratio of 45 dB helps improve the audio frequency detection. Finally, the quantum noise locking provides a method to lock the relative phase between the coherent beam and the squeezed vacuum field. With the combination of these technical improvements, a squeezed vacuum of 2.8 dB is obtained at the analysis frequency of 2.6-100 kHz. The obtained squeezing level is mainly limited by the relatively large loss in OPO, which is induced by ultra-violet absorption in PPKTP crystal. The generated squeezed field is used to reduce the polarization noise of probe beam in an optical magnetometer, in order to increase detection sensitivity.