Entangled Beams Research Articles

Generation and homodyne detection of continuous-variable entangled optical beams with a large wavelength difference

We present a scheme for generating and homodyne detecting of continuous-variable entanglement of bright optical beams with a large wavelength difference by utilizing an optical parametric oscillator (OPO) and an optical parametric amplifier (OPA) simultaneously. Entangled optical beams at 0.8 and 1.5 \ensuremath{\mu}m are generated from the OPA; the seed beams injected in the OPA as well as the local oscillators at the two wavelengths needed for homodyne detection are provided by the OPO. The entangler is a ring resonator involving a second-order nonlinear crystal that is pumped from two opposite directions. In one direction the pump power is above the oscillation threshold and the optical nonlinear resonator operates as an OPO. In the other direction the pump power is below the threshold and it operates as a phase-sensitive frequency nondegenerate optical parametric amplifier. Our scheme combines the advantages of both OPO and OPA quantum optical devices and opens another avenue for preparation and homodyne detection of high quality bright entangled light with a large wavelength difference.

Absolute calibration of photodiodes with bright twin beams

We present a scheme based on the use of bright entangled twin beams for the calibration of analog photodiodes, providing an extension of the schemes for calibrating photon-counting detectors with photon pairs. Previous proposals for the calibration of analog photodiodes with entangled beams are based on a stimulated parametric process operating in the low-gain regime. We start by showing that in those proposals it is necessary to know the exact value of the gain as any error in estimating it will lead to an error in the calibration of the quantum efficiency. We then introduce a technique that is independent of the gain, thus eliminating the uncertainty introduced from errors in estimating the gain or experimental fluctuations of parameters that affect the gain. A proof of principle implementation of this technique using bright twin beams obtained from a four-wave mixing process in rubidium vapor shows its viability for the calibration of analog photodiodes.

连续变量频率可调谐纠缠态光场的实验产生

连续变量频率可调谐纠缠态光场的实验产生

Experimental generation of frequency-tunable entangled optical beams with continuous variables

Experimental generation of frequency-tunable entangled optical beams with continuous variables

连续变量频率可调谐纠缠态光场的实验产生

Frequency tunable continuous variable (CV) entangled optical beams are experimentally demonstrated from a non-degenerate optical parametric oscillator working above the threshold. The measured correlation variances of amplitude and phase quadratures are 3.2 and 1.5 dB, respectively, below the corresponding shot noise level (SNL) in the tuning range of 580 GHz (2.25 nm). The frequency tuning is realized by simply controlling the temperature of the nonlinear crystal.

Gaussian-state quantum-illumination receivers for target detection

The signal half of an entangled twin beam, generated using spontaneous parametric downconversion, interrogates a region of space that is suspected of containing a target and has high loss and high (entanglement-breaking) background noise. A joint measurement is performed on the returned light and the idler beam that was retained at the transmitter. An optimal quantum receiver, whose implementation is not yet known, was shown to achieve 6 dB gain in the error-probability exponent relative to that achieved with a single coherent-state (classical) laser transmitter and the optimum receiver. We present two structured optical receivers that achieve up to 3 dB gain in the error exponent over that attained with the classical sensor. These are designs of quantum-optical sensors for target detection, which can be readily implemented in a proof-of-concept experiment, that appreciably outperform the best classical sensor in the low-signal-brightness, high-loss, and high-noise operating regime.

Entangling Light in its Spatial Degrees of Freedom with Four‐Wave Mixing in an Atomic Vapor

The entanglement properties of two beams of light can reside in subtle correlations that exist in the unavoidable quantum fluctuations of their amplitudes and phases. Recent advances in the generation of nonclassical light with four-wave mixing in an atomic vapor have permitted the production and the observation of entanglement that is localized in almost arbitrary transverse regions of a pair of beams. These multi-spatial-mode entangled beams may prove useful for an array of applications ranging from noise-free imaging and improved position sensing to quantum information processing.

Optimal continuous variable telecloning with bright entangled beams

We address continuous variable 1 → 2 telecloning based on three-mode entangled states of the radiation field. After reviewing existing protocols using phase-space formalism we suggest a novel scheme which allows telecloning with optimal fidelity also in the high energy regime. Being not limited by energy con- straints our scheme is suitable to be implemented with feasible three-mode entan- glement sources.

Entanglement and quantum correlations in the optical parametric oscillator above threshold

We describe several recent experimental studies in the above-threshold optical parametric oscillator (OPO). Seventeen years after the original prediction, entanglement of the twin beams was experimentally demonstrated in 2005. It was followed by the prediction that the OPO should directly produce tripartite pump-signal-idler entangled beams. We also present experimental quantum correlations among these three fields.

Macroscopic three-mode squeezed and fully inseparable entangled beams from triply coupled intracavity Kerr nonlinearities

The generation of macroscopic and spatially separated three-mode entangled light for triply coupled ${\ensuremath{\chi}}^{(3)}$ Kerr coupler inside a pumped optical cavity is investigated. With the linearized analysis for small perturbation around the steady state of the pumped cavity modes, the linearized Hamiltonian and the corresponding master equation are obtained. It is found that the bright three-mode squeezing and full inseparable entanglement can be achieved both inside and outside the cavity.

Continuous variable quantum communication with bright entangled optical beams

In this paper, we briefly introduce the basic concepts and protocols of continuous variable quantum communication, and then summarize the experimental researches accomplished by our group in this field. The main features of quantum communication systems used in our experiments are: (1) The bright entangled optical beams with the anticorrelated amplitude quadratures and the correlated phase quadratures that serve as the entanglement resources and (2) The Bell-state direct detection systems are utilized in the measurements of quantum entanglement and transmitted signals instead of the usually balanced homodyne detectors.

Bright entanglement in the intracavity nonlinear coupler

We show that the intracavity Kerr nonlinear coupler is a potential source of bright continuous variable entangled light beams which are tunable and spatially separated. We use a linearized fluctuation analysis to calculate the necessary correlations in regimes where it is valid. This means that we are treating regimes where the system exhibits Gaussian statistics so that well-known criteria are both necessary and sufficient to demonstrate entanglement. This system may be realized with integrated optics and thus provides a potentially rugged and stable source of bright entangled beams.

Frequency conversion of an entangled state

The quantum characteristics of sum-frequency process in an optical cavity with an input signal optical beam, which is a half of entangled optical beams, are analyzed. The calculated results show that the quantum properties of the signal beam can be maintained after its frequency is conversed during the intracavity nonlinear optical interaction. The frequency-conversed output signal beam is still in an entangled state with the retained other half of initial entangled beams. The resultant quantum correlation spectra and the parametric dependences of the correlations on the initial squeezing factor, the optical losses and the pump power of the sum-frequency cavity are calculated. The proposed system for the frequency conversion of entangled state can be used in quantum communication network and the calculated results can provide direct references for the design of experimental systems.

The influence of mode mismatch on correlation measurement in a Bell state direct detector

The influence of mode mismatch on the quantum correlation measurement of phase quadratures of entangled state light beams in a Bell state direct detector is quantitatively discussed. The experimental measurements and theoretical calculation are in good agreement.

Ghost imaging schemes: fast and broadband

In ghost imaging schemes information about an object is extracted by measuring the correlation between a beam that passed the object and a reference beam. We present a spatial averaging technique that substantially improves the imaging bandwidth of such schemes, which implies that information about high-frequency Fourier components can be observed in the reconstructed diffraction pattern. In the many-photon regime the averaging can be done in parallel and we show that this leads to a much faster convergence of the correlations. We also consider the reconstruction of the object image, and discuss the differences between a pixel-like detector and a bucket detector in the object arm. Finally, it is shown how to non-locally make spatial filtering of a reconstructed image. The results are presented using entangled beams created by parametric down-conversion, but they are general and can be extended also to the important case of using classically correlated thermal-like beams.

Correlated imaging with macroscopic fields

AbstractWe demonstrate analytically that correlated imaging in the regime of large photon number can be implemented using both a classical and an entangled source. The classically correlated beams are obtained by dividing thermal radiation by a beam splitter while entangled beams are obtained through parametric down‐conversion. By treating the two system in parallel, a formal analogy is outlined between the two cases showing that classical beams can mimic qualitatively all the imaging features of the entangled beams. We also propose a method to improve substantially the imaging performances of correlated imaging which is based on homodyne detection combined with spatial averages.

Testing the entanglement of intense beams produced by a non-degenerate optical parametric oscillator

We propose a direct measurement of the quadrature correlations of signal and idler beams in a non-degenerate optical parametric oscillator operating above threshold. We investigate the experimental limits where quantum correlations can be observed, fulfilling an inseparability criterion for defining them as intense entangled beams. The use of optical cavities to access quadrature noise in this situation is studied, and its advantages over homodyne detection are discussed. We also consider the application of this entanglement and the quadrature noise measurement technique to quantum cryptography.

Sub-shot-noise phase quadrature measurement of intense light beams.

We present a setup for performing sub-shot-noise measurements of the phase quadrature of intense pulsed light without the use of a separate local oscillator. A Mach-Zehnder interferometer with an unbalanced arm length is used to detect the fluctuations of the phase quadrature at a single sideband frequency. With this setup, the nonseparability of a pair of quadrature-entangled beams is demonstrated experimentally.

Teleportation of continuous-variable polarization states

This paper discusses methods for the optical teleportation of continuous variable polarisation states. We show that using two pairs of entangled beams, generated using four squeezed beams, perfect teleportation of optical polarisation states can be performed. Restricting ourselves to 3 squeezed beams, we demonstrate that polarisation state teleportation can still exceed the classical limit. The 3-squeezer schemes involve either the use of quantum non-demolition measurement or biased entanglement generated from a single squeezed beam. We analyse the efficacies of these schemes in terms of fidelity, signal transfer coefficients and quantum correlations.

Remote state preparation and teleportation in phase space

Continuous variable remote state preparation and teleportation are analyzed using Wigner functions in phase space. We suggest a remote squeezed state preparation scheme between two parties sharing an entangled twin beam, where homodyne detection on one beam is used as a conditional source of squeezing for the other beam. The scheme works also with noisy measurements, and provide squeezing if the homodyne quantum efficiency is larger than 50%. Phase space approach is shown to provide a convenient framework to describe teleportation as a generalized conditional measurement, and to evaluate relevant degrading effects, such the finite amount of entanglement, the losses along the line, and the nonunit quantum efficiency at the sender location.