Quantum illumination receiver using double homodyne detection

We address the characterization of lossy and dephasing channels in the presence of self-Kerr interaction using coherent probes. In particular, we investigate the ultimate bounds to precision in the joint estimation of loss and nonlinearity and of dephasing and nonlinearity. To this aim, we evaluate the quantum Fisher information matrix and compare the symmetric quantum Cramér–Rao bound to the bound obtained with Fisher information matrix of feasible quantum measurements, i.e., homodyne and double-homodyne detection. For lossy Kerr channels, our results show that the loss characterization is enhanced in the presence of Kerr nonlinearity, especially in the relevant limit of small losses and low input energy, whereas the estimation of nonlinearity itself is unavoidably degraded by the presence of loss. In the low energy regime, homodyne detection of a suitably optimized quadrature represents a nearly optimal measurement. The Uhlmann curvature does not vanish; therefore, loss and nonlinearity can be jointly estimated only with the addition of intrinsic quantum noise. For dephasing Kerr channels, the quantum Fisher information of the two parameters is independent of the nonlinearity, and therefore, no enhancement is observed. Homodyne detection and double-homodyne detection are suboptimal for the estimation of dephasing and nearly optimal for nonlinearity. Also in this case, the Uhlmann curvature is nonzero, proving that the parameters cannot be jointly estimated with maximum precision.

Quantum illumination is one of the most basic ways to realize the quantum radar for detection of low-reflectivity targets embedded in a thermal-noise bath. It preserves a strong advantage over the classical counterparts even though its fragile quantum entanglement is completely destroyed by a large amount of noise and losses. In this paper, we describe and analyze a discrete-variable quantum illumination system with ancillary degrees of freedom of entangled photon pairs. Comparing the error probability bound, i.e., the quantum Chernoff bound, we identify that this system indeed offers higher accuracy than the single-photon illumination and quantum illumination with single degree of freedom of entangled photon pairs. Furthermore, the utilization of ancillary degrees of freedom exponentially enhances signal-to-noise ratio over quantum illumination without ancillary degrees of freedom. It means that this system reduces the necessary amount of signal photons to accurately determine whether the object is present or absent in the target region, which is absolutely crucial to the stealthiness of the radar system.

Two-parameter estimation with single squeezed-light interferometer via double homodyne detection

We started to research quantum illumination radar and quantum imaging by utilizing high quality continuous-wave two-mode squeezed light source as a quantum entanglement resource. Two-mode squeezed light is a macroscopic quantum entangled state of the electro-magnetic field and shows strong correlation between quadrature phase amplitudes of each optical field. One of the most effective methods to generate two-mode squeezed light is combining two independent single-mode squeezed lights by using a beam splitter with relative phase of 90 degrees between each optical field. As a first stage of our work we are developing two-mode squeezed light source for exploring the possibility of quantum illumination radar and quantum imaging. In this article we introduce current development of experimental investigation of single-mode squeezed light. We utilize a sub-threshold optical parametric oscillator with bow-tie configuration which includes a periodically-polled potassium titanyl phosphate crystal as a nonlinear optical medium. We observed the noise level of squeezed quadrature −3.08±0.13 dB and anti-squeezed quadrature at 9.29±0.13 dB, respectively. We also demonstrated the remote tuning of squeezing level of the light source which leads to the technology for tuning the quantum entanglement in order to adapt to the actual environmental condition.

Quantum communication is a new and fascinating area in telecommunications that provides new ways of communication without classical counterparts. At the centre of this theory is quantum entanglement, a powerful and enigmatic property shown by some composite quantum systems. In this paper, the entanglement produced by sending of number states through a beam splitter is studied. The property entangled is the parity of the number of photons of the number states at the beam splitter's outputs. The decoherence is taken into account through the losses. The entanglement of pure and mixed states at the output of the beam splitter is calculated and some protocols for quantum teleportation for bipartite (C2 ⊗ C2) and tripartite (C2 ⊗ C2 ⊗ C2), systems are described.

The quantum-limited noise figure (NF) for a general combination of linear optical elements is obtained in a compact form. The entries in this expression are obtained from the classical input–output transmission coefficient of the signal, and of the various transmission coefficients from the noise inputs to the output. The same result is also obtained by means of a semiclassical derivation. The linear elements can be: population-inversion amplifiers, parametric amplifiers and/or wavelength converters, beam splitters, lossy fibers, etc. They form a network with a simple directed graph (no loops). We identify a class of networks for which Friis’ formula for concatenated elements holds. We verify the general formula for a variety of well-known cases. We also verify that the NF for an optical communication link periodically amplified by nondegenerate phase-sensitive parametric amplifiers (PSAs) is only about half of that for a similar system using degenerate PSAs.

Quantum random number generator (QRNG) allows obtaining true random bit sequences. In QRNG based on quantum nature of vacuum, optical beam splitter with two inputs and two outputs is normally used. We compare mathematical descriptions of spatial beam splitter and fiber Y-splitter in the quantum model for QRNG, based on homodyne detection. These descriptions were identical, that allows to use fiber Y-splitters in practical QRNG schemes, simplifying the setup. Also we receive relations between the input radiation and the resulting differential current in homodyne detector. We experimentally demonstrate possibility of true random bits generation by using QRNG based on homodyne detection with Y-splitter.

We propose a nearly deterministic scheme for generating a six-photon polarization-entangled cluster state via the weak cross-Kerr nonlinearity combined with linear optics elements such as polarization beam splitters (PBSs), beam splitters, etc. Besides, by using the six-photon cluster state as the quantum channel, we propose a scheme for quantum state sharing (QSTS) of an unknown two-photon polarization state based on the weak cross-Kerr medias and linear optics elements. With the help of X quadrature homodyne measurements, our two schemes can be realized in an almost deterministic way.

We present some different hyperentanglement concentration protocols (hyper-ECPs) for nonlocal N-photon systems in partially polarization-spatial hyperentangled states with known parameters, resorting to linear optical elements only, including those for hyperentangled Greenberger---Horne---Zeilinger-class states and the ones for hyperentangled cluster-class states. Our hyper-ECPs have some interesting features. First, they require only one copy of nonlocal N-photon systems and do not resort to ancillary photons. Second, they work with linear optical elements, neither Bell-state measurement nor two-qubit entangling gates. Third, they have the maximal success probability with only a round of entanglement concentration, not repeating the concentration process some times. Fourth, they resort to some polarizing beam splitters and wave plates, not unbalanced beam splitters, which make them more convenient in experiment.

We proposed a passive continuous-variable quantum key distribution (CVQKD) using a locally generated local oscillator (LLO) at reception. Different from the LLO-based Gaussian-modulated coherent-state quantum key distribution, our protocol is implemented by taking advantage of a multimode thermal source, beam splitters, optical attenuators, and homodyne detectors rather than the amplitude and phase modulators, which could be a cost-effective solution for LLO-based CVQKD. We adopt a general LLO noise model to analyze the performance of our protocol. That is to say, our consideration for noise in the proposed scheme contains the excess noise caused by passive state preparation, the phase noise, and the photon-leakage noise from the reference path. Because our protocol waives the necessity of the use of amplitude and phase modulators, there is no need to bear the effect of modulation noise caused by finite dynamics. Simulation results show that the improvement of the mode-overlap coefficient can suppress the excess noise caused by passive state preparation and thus enhance the performance of our protocol. Because lower relative phase drift and the photon-leakage noise can be obtained by using the multiplexing technique, we can still achieve reasonable performance of our protocol with the mode-overlap coefficient values which are very close to the experimental reference. Furthermore, we perform the finite-size analysis for the proposed protocol, which is more practical than that achieved in the asymptotic limit. This work confirms the feasibility of passive CVQKD implemented in the LLO configuration within the metropolitan area.

Homodyne detection is the most effective detection scheme employed in quantum optics to characterize quantum states. It is based on mixing at a beam splitter the signal to be measured with a coherent state, called the “local oscillator,” and on evaluating the difference of the photocurrents of two photodiodes measuring the outputs of the beam splitter. If the local oscillator is much more intense than the field to be measured, the homodyne signal is proportional to the signal-field quadratures. If the local oscillator is less intense, the photodiodes can be replaced with photon-number-resolving detectors, which have a smaller dynamics but can measure the light statistics. The resulting new homodyne-like detector acquires a hybrid nature, being it capable of yielding information on both the particle-like (statistics) and wave-like (phase) properties of light signals. The scheme has been tested in the measurement of the quadratures of coherent states, bracket states and phase-averaged coherent states at different intensities of the local oscillator.

Two independent experiments demonstrate that quantum entanglement that has been lost in decoherence processes can be recovered. For the first time such ’entanglement distillation’ has been achieved for states of light that are entangled in continuous variables, which should help to increase the distance over which quantum information can be distributed. The distribution of entangled states of light over long distances is a major challenge in the field of quantum information. Optical losses, phase diffusion and mixing with thermal states lead to decoherence and destroy the non-classical states after some finite transmission-line length. Quantum repeater protocols1,2, which combine quantum memory3, entanglement distillation4,5 and entanglement swapping6, were proposed to overcome this problem. Here we report on the experimental demonstration of entanglement distillation in the continuous-variable regime7,8,9. Entangled states were first disturbed by random phase fluctuations and then distilled and purified using interference on beam splitters and homodyne detection. Measurements of covariance matrices clearly indicate a regained strength of entanglement and purity of the distilled states. In contrast to previous demonstrations of entanglement distillation in the complementary discrete-variable regime10,11, our scheme achieved the actual preparation of the distilled states, which might therefore be used to improve the quality of downstream applications such as quantum teleportation12.

Equivalence of lossless beam splitter and nondegenerate parametric amplifier in conditional measurement

We analyze the linear optical realization of number-sum Bell measurement and number-state manipulation by taking into account the realistic experimental situation, specifically imperfectness of single-photon detector. The present scheme for number-state manipulation is based on the number-sum Bell measurement, which is implemented with linear optical elements, i.e., beam splitters, phase shifters and zero-one-photon detectors. Squeezed vacuum states and coherent states are used as optical sources. The linear optical Bell state detector is formulated quantum theoretically with a probability operator measure. Then, the fidelity of manipulation and preparation of number-states, particularly for qubits and qutrits, is evaluated in terms of the quantum efficiency and dark count of single-photon detector. It is found that a high fidelity is achievable with small enough squeezing parameters and coherent state amplitudes.

We examine various manipulations of photon number states which can be implemented by teleportation technique with number sum measurement. The preparations of the Einstein-Podolsky-Rosen resources as well as the number sum measurement resulting in projection to certain Bell state may be done conditionally with linear optical elements, i.e., beam splitters, phase shifters and zero-one-photon detectors. Squeezed vacuum states are used as primary entanglement resource, while single-photon sources are not required.