Far-field superresolution of thermal sources by double homodyne or double array homodyne detection.

We propose two schemes for estimating the separation of two thermal sources via double homodyne and double array homodyne detection considering the joint measurement of conjugate quadratures of the image plane field.By using the Cramér-Rao bound, we demonstrate that the two schemes can estimate the separation well below the Rayleigh limit and have an advantage over direct imaging when the average photon number per source exceeds five.For arbitrary source strengths, double homodyne detection is superior to homodyne detection when the separation is above 25/4 σ/N s , σ is the beam width, Ns is the average photon number per source.A larger separation can be estimated better via double array homodyne detection with the superiority of flexible operation compared with other schemes. High-speed and high-efficiency detection enables the measurement schemes with potential practical applications in fluorescence microscopy, astronomy and quantum imaging.

Continuous variable spin-orbit total angular momentum entanglement on the higher-order Poincaré sphere.

Optical spin-orbit coupling is an important phenomenon and has fruitful applications. Here, we investigate the spin-orbit total angular momentum entanglement in the optical parametric downconversion process. Four pairs of entangled vector vortex modes are experimentally generated directly using a dispersion- and astigmatism-compensated single optical parametric oscillator, and for the first time, to the best of our knowledge, the spin-orbit quantum states are characterized on the quantum higher-order Poincaré sphere, and the relationship of spin-orbit total angular momentum Stokes entanglement is demonstrated. These states have potential applications in high-dimensional quantum communication and multiparameter measurement.

Manipulation of continuous variable orbital angular momentum squeezing and entanglement by pump shaping.

Spatially structured quantum states, such as orbital angular momentum (OAM) squeezing and entanglement, is currently a popular topic in quantum optics. The method of generating and manipulating spatial quantum states on demand needs to be explored. In this paper, we generated OAM mode squeezed states of -5.4 dB for the L G0+1 mode and -5.3 dB for the L G0-1 mode directly by an optical parametric oscillator (OPO) for the first time. Additionally, we demonstrated that the OAM mode squeezed and entangled states were respectively generated by manipulating the nonlinear process of the OPO by controlling the relative phase of two beams of different modes, thus making two different spatial multimode pump beams. We characterized the Laguerre-Gaussian (LG) entangled states by indirectly measuring the squeezing for the H G 10(45∘) mode and H G 10(135∘) mode, and directly measuring the entanglement between the L G0+1 and L G0-1 modes. The effective manipulation of the OAM quantum state provides a novel insight into the continuous variable quantum state generation and construction on demand for high-dimensional quantum information and quantum metrology.

Pure modulation and accurate measurement of optical beam's tilt and displacement.

We developed a tilt modulation technique of a laser beam with a wedged crystal. Combined with a phase-compensating crystal, a pure tilt modulation with a wide bandwidth (actually determined by the bandwidth of electro-optic crystals) is realized. By Fourier transformation with a lens, the tilt signal is transformed into displacement. With homodyne detection using a local oscillator of the first-order Hermite-Gauss mode (HG10) and a 4F phase-monitoring system, we measure the displacement and tilt of a laser probe beam. This technique can be used in metrology, such as Newtonian gravitational constant determination and gravitational wave detection, or the calibration of a spatial sensor, such as tilt/displacement sensors.

Long-term stable continuous variable entanglement generation in type-II non-degenerate optical parametric amplifier.

Long-term stable entanglement is a crucial aspect in the implementation of reliable quantum information processes. However, long-term continuous variable entanglement generation, especially in type-II non-degenerate optical parametric amplifier, has yet to be reported on. Here, we derive the relationship between entanglement and temperature fluctuations in the crystal of a type-II non-degenerate optical parametric amplifier, and propose a novel method for long-term stable entanglement generation by locking the temperature of the crystal. In the experiment, we obtain a 5.4 dB entanglement lasting two hours. The method holds promise in the generation of a truly usable above 10 dB entanglement and brings us closer to continuous-variable quantum information processing.

Compact sub-kilohertz low-frequency quantum light source based on four-wave mixing in cesium vapor.

Using a nondegenerate four-wave mixing (FWM) process based on a double-Λ scheme in hot cesium vapor, we demonstrate a compact diode-laser-pumped quantum light source for the generation of quantum correlated twin beams with a maximum squeezing of 6.5 dB. The squeezing is observed at a Fourier frequency in the audio band down to 0.7 kHz which, to the best of our knowledge, is the first observation of sub-kilohertz intensity-difference squeezing in an atomic system so far. A phase-matching condition is also investigated in our system, which confirms the spatial-multi-mode characteristics of the FWM process. Our compact low-frequency squeezed light source may find applications in quantum imaging, quantum metrology, and the transfer of optical squeezing onto a matter wave.

Experimental characterization of continuous-variable orbital angular momentum entanglement using Stokes-operator basis.

The continuous-variable (CV) orbital angular momentum (OAM) entanglement is very different to the traditional quadrature entanglement. The Stokes-operators directly reflect the character of OAM light. Here, we report the first direct experimental demonstration the Stokes-operator entanglement of continuous-variable OAM entanglement. Generated by transforming quadrature entanglement in the HG01 mode onto the orbital Stokes-operator basis, the entanglement is measured in the Stokes-operator basis using a self-designed detection scheme. An inseparability of I(O^2,O^3)<1 is achieved over a wide analyzing frequency of 1-10 MHz. Moreover, experimental fluctuations at 5.0 MHz are visualized using the quantum orbital Poincaré sphere representation. The OAM entanglement with Stokes-operators measurement has a promising application in certain nonlocal quantum information protocols and rotational optomechanics by interacting with nanoparticle or atoms.

Measurement of Stokes-operator squeezing for continuous-variable orbital angular momentum.

We demonstrate experimentally a measurement scheme for the Stokes operators for the continuous-variable squeezed states of orbital angular momentum (OAM). An OAM squeezed state is generated by coupling a dim Hermite-Gauss HG01-mode quadrature-squeezed light beam with a bright HG10-mode coherent light beam on a 98/2 beam splitter. Using an asymmetric Mach-Zehnder interferometer with an extra Dove prism in one arm, we measured the three orbital Stokes operators of the OAM squeezed states with a self-homodyne detection and finally characterized their positions and noise on the orbital Poincaré sphere.

Higher order mode entanglement in a type II optical parametric oscillator.

Nonclassical beams in high order spatial modes have attracted much interest but they exhibit much less squeezing and entanglement than the fundamental spatial modes, limiting their applications. We experimentally demonstrate the relation between pump modes and entanglement of first-order Hermite Gauss modes (HG10 entangled states) in a type II OPO and show that the maximum entanglement of high order spatial modes can be obtained by optimizing the pump spatial mode. To our knowledge, this is the first time to report this. Utilizing the optimal pump mode, the HG10 mode threshold can be reached easily without HG00 oscillation and HG10 entanglement is enhanced by 53.5% over HG00 pumping. The technique is broadly applicable to entanglement generation in high order modes.

Direct generation of spatial quadripartite continuous variable entanglement in an optical parametric oscillator.

Multipartite entanglement is used for quantum information applications, such as building multipartite quantum communications. Generally, generation of multipartite entanglement is based on a complex beam-splitter network. Here, based on the spatial freedom of light, we experimentally demonstrated spatial quadripartite continuous variable entanglement among first-order Hermite-Gaussian modes using a single type II optical parametric oscillator operating below threshold with an HG0245° pump beam. The entanglement can be scalable for larger numbers of spatial modes by changing the spatial profile of the pump beam. In addition, spatial multipartite entanglement will be useful for future spatial multichannel quantum information applications.

Experimental generation of continuous-variable hyperentanglement in an optical parametric oscillator.

We report on the generation of continuous-variable hyperentanglement of polarization and orbital angular momentum with a type II optical parametric oscillator. By compensating for the astigmatism between spatial modes, we produce an entangled pair of Hermite-Gauss beams. From correlations measurements, we verify the existence of continuous-variable hyperentanglement by the general entanglement criterion as well as by the continuous-variable version of the Peres-Horodecki criterion visualized on an equivalent Poincaré sphere.

Low-frequency phase measurement with high-frequency squeezing.

Squeezed-state enhanced audio-frequency signal measurement is crucial for some special applications, such as gravitational wave detection. But generation of squeezed state of light at such frequency is more difficult than that at megahertz-frequency. In this paper we propose an experimental scheme to measure low-frequency phase signal with high-frequency squeezing. To utilize the high-frequency sidebands of the squeezed light, a two-frequency intense laser is applied in the interferometry instead of a single-frequency laser as usual. This technique is in the reach of modern quantum optics technology.

Bright two-color tripartite entanglement with second harmonic generation.

The bright two-color tripartite entanglement is investigated in the process of type II second harmonic generation (SHG) operating above threshold. The two pump fields and the second harmonic field are proved to be entangled, and the dependence of the entanglement degree on pump parametersigma and normalized frequency Omega is also analyzed.

Transfer of intensity quantum correlation with twin beams.

We demonstrate experimentally a protocol of transferring nonclassical quantum properties using two pairs of quantum-correlated twin beams in the continuous variable regime. The intensity quantum correlation from one twin beam is transferred to two initially independent idler beams with the help of a displacement transformation. It makes two originally independent beams exhibit an intensity quantum correlation of 0.8 dB below shot-noise level.

Preparation and measurement of tunable high-power sub-Poissonian light using twin beams.

Widely frequency tunable bright sub-Poissonian field preparation has been experimentally achieved with quantum-correlated twin beams in the continuously variable regime. The noise of the sub-Poissonian field is reduced to more than 2 dB below the shot-noise level throughout the entire wavelength-tunable range of 7.4 nm. A maximum noise reduction of 45% (2.6 dB) is observed. The statistical distribution of a sub-Poissonian field is also obtained.

Experimental demonstration of unconditional entanglement swapping for continuous variables.

The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.

Application of an all-solid-state, frequency-doubled Nd:YAP laser to the generation of twin beams at 1080 nm.

A laser-diode-pumped intracavity frequency-doubled Nd:YAP/KTP laser is presented. Over 110 mw of TEM00 single-frequency output power at 540-nm wavelength was obtained. The output green laser was employed to pump a semimonolithic nondegenerate optical parametric oscillator to produce intensity quantum correlated twin beams at 1080 nm, and the maximum quantum noise squeezing of 74% (5.9dB) on the intensity difference fluctuation between the twin beams is observed. The threshold was reduced and the stability was increased significantly when compared with similar lamp-pumped systems.

Tunable continuous-wave doubly resonant optical parametric oscillator by use of a semimonolithic KTP crystal.

A temperature-tuned continuous-wave doubly resonant optical parametric oscillator (OPO) consisting of a semimonolithic KTP crystal and a concave mirror has been designed and built. Under single-axial-mode-pair operation, we obtained a combined output power of the signal and idler light fields up to 365 mW at a pump power of 680 mW. The output wavelength of the OPO can be temperature tuned by as much as 9 nm. We achieved 2.8-GHz continuous frequency tuning of the OPO by tuning the pump laser frequency.