Weak squeezed vacuum light, especially resonant to the atomic transition, plays an important role in quantum storage and generation of various quantum sources. However, the general homodyne detection (HD) cannot determine weak squeezing due to the low signal to noise ratio and the limited resolution of the HD system. Here we provide an alternative method based on photon statistics measurement to determine the weak squeezing of the squeezed vacuum light generated from an optical parametric oscillator working far below the threshold. The approach is established the relationship between the squeezing parameter and the second-order degree of coherence. The theoretical analysis agrees well with the experiment results. The advantage of this method is that it provides a feasible and reliable experimental measure to determine the weak squeezing with high precision and the measurement is independent on the detection efficiency. This method can be used to measure other quantum features for various quantum states with extremely weak non-classicality.
Weak squeezed vacuum light, especially resonant to the atomic transition, plays an important role in quantum storage and generation of various quantum sources. However, the general homodyne detection (HD) cannot determine weak squeezing due to the low signal to noise ratio and the limited resolution of the HD system. Here we provide an alternative method based on photon statistics measurement to determine the weak squeezing of the squeezed vacuum light generated from an optical parametric oscillator working far below the threshold. The approach is established the relationship between the squeezing parameter and the second-order degree of coherence. The theoretical analysis agrees well with the experiment results. The advantage of this method is that it provides a feasible and reliable experimental measure to determine the weak squeezing with high precision and the measurement is independent on the detection efficiency. This method can be used to measure other quantum features for various quantum states with extremely weak non-classicality.
Photon statistics of the weak squeezed vacuum field from an optical parametric oscillatior (OPO) is investigated both in theory and by experiment . The strong photon bunching effect of weak squeezed vacuum field, which can be used as an important source in quantum optics and quantum measurement, was demonstrated. We have experimentally produced the weak squeezed vacuum at the center wavelength of D2 line of the cesium atom by the OPO operating far below the threshold . The second-order correlation function was measured based on the Hanbury-Brown-Twiss scheme, and the result is consistent with theoretical analysis.
The squeezed state, as an important quantum resource, has great potential applications in quantum computing, quantum communication and precision measurement. In the noncritically squeezed light theory, the predicted noncritically squeezed light can be generated by breaking the spontaneous rotational symmetry occurring in a degenerate optical parametric oscillator (DOPO) pumped above threshold. The reliability of this kind of squeezing is crucially important, as its quantum performance is robust to the pump power in experiment. However, the detected squeezing degrades rapidly in detection, because the squeezed mode orientation diffuses slowly, resulting in a small mode mismatch during the homodyne detection. In this paper, we propose an experimentally feasible scheme to detect noncritically squeezing reliable by employing the spatial mode swapping technic. Theoretically, the dynamic fluctuation aroused by random mode rotation in the squeezing detection can be compensated for perfectly, and 3 dB squeezing can be achieved robustly even with additional vacuum noise. Our scheme makes an important step forward for the experimental generation of noncritically squeezed light.
In this paper, the squeezed quantum state is generated using an optical parametric oscillator via a spontaneous parametric down conversion technique to investigate squeezed states with quantum noise in one quadrature below the standard quantum limit at the expense of the other. The setup involves four main parts: generation of Nd-YAG second harmonic via a ring resonator, squeezed cavity with a nonlinear crystal inside to generate the squeezed state, Pound-Drever-Hall technique to stabilize the laser in the squeezed cavity and balanced homodyne receiver with high efficiency to detect the squeezed state. A comparison in error probability is addressed between the quantum coherent classical and the quantum squeezed non-classical state in the presence of thermal noise and the dissipation. It is found that with extremely low number of photons, the squeezed state is robust against channel noise.
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.
A method for measuring the transmittivity of optical samples by using squeezed--vacuum radiation is illustrated. A squeezed vacuum field generated by a below--threshold optical parametric oscillator is propagated through a nondispersive medium and detected by a homodyne apparatus. The variance of the detected quadrature is used for measuring the transmittivity. With this method it is drastically reduced the number of photons passing through the sample during the measurement interval. The results of some tests are reported.
A longstanding goal in quantum optics has been to realize a photon-number-resolving detector that efficiently counts the number of photons in an optical field. This goal has been largely met with the development of transition edge sensors which can count up to roughly 20 photons with efficiencies over 95%. This thesis presents three experiments that employ these detectors to characterize and prepare quantum states of light. Firstly, we develop a weak-field homodyne detector. By replacing the photodiodes conventionally used in homodyne detection with transition edge sensors, we experimentally implement a versatile measurement device that can tune between photon counting and quadrature measurements. We study the transition between these complementary measurement regimes and determine the minimum local oscillator strength needed to perform quadrature measurements. Secondly, we use the weak-field homodyne detector as a quantum state engineering tool. We propose a scheme to prepare a wide range of definite parity states, including two- and four-component Schrödinger cat states of arbitrary size with nearly perfect fidelity. Thirdly, we perform optical interferometry using quantum states of light with the aim of surpassing the maximal precision achievable with classical light, i.e. the shot-noise limit. We propose and experimentally implement a scheme that uses high-gain squeezed vacuum sources and transition edge sensors to prepare loss-tolerant entangled states containing up to 8 photons. While our achieved precision does not unconditionally (i.e. without post-selecting on certain measurement trials) surpass the shot-noise limit, our results do demonstrate the robustness of these entangled states to loss despite their size.