Discrete-variable quantum illumination with ancillary degrees of freedom

A quantum receiver is an essential element of quantum illumination (QI), which outperforms its classical counterpart, called classical illumination. However, there are only few proposals for realizable quantum receiver, which exploits nonlinear effects leading to increasing the complexity of receiver setups. To compensate this, in this paper, we design a quantum receiver with linear optical elements for Gaussian QI. Rather than exploiting nonlinear effect, our receiver consists of a 50:50 beam splitter and homodyne detection. Using double homodyne detection after the 50:50 beam splitter, we analyze the performance of the QI in different regimes of target reflectivity, source power, and noise level. We show that our receiver has better signal-to-noise ratio and is more robust against noise than the existing simple-structured receivers.

Quantum illumination can utilize entangled light to detect the low-reflectivity target that is hidden in a bright thermal background. This technique is applied to the detection of an object in the curved spacetime of the Earth, in order to explore how the curvature of spacetime affects quantum illumination. It is found that the spatial quantum illumination with entangled state transmitter outperforms that with coherent-state transmitter in the near-Earth curved spacetime. Moreover, either the quantum illumination system or the coherent-state system is employed, and gravity can enhance the spacetime target detection by reducing the thermal signal at the receiver. Besides, our model in principle can be applied to microwave quantum illumination and thus provides, to some degree, a theoretical foundation for the upcoming spatial quantum radar technologies.

The authors consider a quantum radar which operates on the quantum illumination principle. The authors’ attention is focused on its function of target detection in a noisy environment. The role of the optical parametric amplifier (OPA) in detection is first examined by the authors, and a dual‐OPA design for more flexible combination of optimised gains is proposed, resulting in a detector substantially improved in its performance from the normally used 1‐OPA design. Then, the use of the entanglement information in the covariance matrix (CM) between the returned signal and idler beams for detection is considered, and a technique to extract such information is proposed. By employing some statistical relationships between positive definite matrices, the authors come up with a new target detection method. Numerical experiments confirm the superior detection performance of the CM detectors compared to that of the OPA detectors.

Quantum two-mode squeezed (QTMS) radar, inspired by quantum illumination but without joint measurements in the receiver, has shown promise in target detection. However, the current prototype of quantum radar, using full-microwave superconducting components, faces challenges in achieving long-range performance. In this study, we propose the integration of an array of Josephson parametric amplifiers (JPAs) in a dilution refrigerator to enhance the performance of QTMS quantum radar. Through the evaluation of signal-to-noise ratio (SNR) and receiver operating characteristic (ROC) metrics, we demonstrate the effectiveness of this enhancing strategy. Our results indicate that the presence of an array of two JPAs significantly improves both the SNR and the detection probability compared to a single JPA configuration, thereby boosting the overall performance of QTMS radar. The key factor of this improvement is the cross-correlation between JPAs, which has a notable impact on the analytical outcomes of the quantum radar. This research provides valuable insights to engineers who want to optimize the design of advanced quantum QTMS radar systems.

In quantum illumination, the signal mode of light, entangled with an idler mode, is dispatched towards a suspected object bathed in thermal noise and the returning mode, along with the stored idler mode, is measured to determine the presence or absence of the object. In this process, entanglement is destroyed but its benefits in the form of classical correlations and enlarged Hilbert space survive. Here, we propose the use of probe state hyperentangled in two degrees of freedom - polarization and frequency, to achieve a significant 12dB performance improvement in error probability exponent over the best known quantum illumination procedure in the low noise regime. We present a simple receiver model using four optical parametric amplifiers (OPA) that exploits hyperentanglement in the probe state to match the performance of the feed-forward sum-frequency generator (FF-SFG) receiver in the high noise regime. By replacing each OPA in the proposed model with a FF-SFG receiver, further 3dB improvement in the performance of a lone FF-SFG receiver can be seen.

Quantum illumination leverages entanglement to surpass classical target detection, even in high-noise environments. Remarkably, its quantum advantage persists despite entanglement degradation caused by environmental decoherence. A central challenge lies in designing optimal receivers to exploit this advantage, with the correlation-to-displacement (C→D) conversion module emerging as a promising candidate. However, practical implementation remains technically demanding, particularly in realizing a low-noise programmable mode selector. In this work, we propose a scheme for the C→D module that incorporates a cavity-enhanced quantum pulse gate for programmable mode processing, achieving performance close to the theoretical optimum. This integrated framework paves the way for the realization of practical quantum illumination systems.

Quantum radar based on the mechanism of quantum physics and the quantum information is a new sensing technology that was developed to solve technology bottleneck of classic radar. Especially, The Quantum illumination based on entanglement light beam could overcome noise and information loss. In this paper, the principle of quantum radar is introduced in detail, and the quantum illumination radar scheme using entangled photons is discussed. Then, target detection method based on entangled photons is given, and simulation results are showed. Finally, it is demonstrated by experiments that the performance of entangled photon detection system is superior to non-entangled photon detection system in a noisy and lossy environment, and the targets appear to look bigger when looked with low photon number.

Chapter 7 - Quantum of radar theory

Most current microwave quantum illumination techniques rely on hybrid quantum systems to detect the presence of targets. However, real-world radar tasks are considerably more intricate than this simplistic model. Accurately determining physical attributes such as object speed, position, and azimuth is also essential. In this study, we explore azimuth detection using a quantum illumination approach based on a cavity-optomagnonics system and analyze the accuracy of azimuth detection in this framework. Our results indicate that this approach significantly outperforms classical microwave radar in azimuth detection within the parameters of current existing experiments. Additionally, we investigate the impact of Kerr nonlinearity of the YIG sphere on azimuth detection accuracy, revealing a clear improvement with the incorporation of Kerr nonlinearity.

A recent series of papers has shown that a symmetrical quasi-classical (SQC) windowing procedure applied to the Meyer-Miller (MM) classical vibronic Hamiltonian provides a very good treatment of electronically nonadiabatic processes in a variety of benchmark model systems, including systems that exhibit strong quantum coherence effects and some which other approximate approaches have difficulty in describing correctly. In this paper, a different classical electronic Hamiltonian for the treatment of electronically nonadiabatic processes is proposed (and "quantized" via the SQC windowing approach), which maps the dynamics of F coupled electronic states to a set of F spin-(1)/2 degrees of freedom (DOF), similar to the Fermionic spin model described by Miller and White (J. Chem. Phys. 1986, 84, 5059). It is noted that this spin-mapping (SM) Hamiltonian is an exact Hamiltonian if treated as a quantum mechanical (QM) operator-and thus QM'ly equivalent to the MM Hamiltonian-but that an analytically distinct classical analogue is obtained by replacing the QM spin-operators with their classical counterparts. Due to their analytic differences, a practical comparison is then made between the MM and SM Hamiltonians (when quantized with the SQC technique) by applying the latter to many of the same benchmark test problems successfully treated in our recent work with the SQC/MM model. We find that for every benchmark problem the MM model provides (slightly) better agreement with the correct quantum nonadiabatic transition probabilities than does the new SM model. This is despite the fact that one might expect, a priori, a more natural description of electronic state populations (occupied versus unoccupied) to be provided by DOF with only two states, i.e., spin-(1)/2 DOF, rather than by harmonic oscillator DOF which have an infinite manifold of states (though only two of these are ever occupied).

This paper addresses the problem of target detection against a background of Gaussian clutter by using frequency snapshots with reduced degrees of freedom (DOF). We derive the optimal detector and detection performance under the Neyman-Pearson criterion for general frequency snapshot selection with arbitrary DOF. When the clutter statistics are unknown, we use a uniformly random frequency snapshot selection method and show how the DOF employed affects the detection performance. When the clutter return follows a stationary Gaussian distribution with slowly varying power spectral density, the optimal selection is derived. When the clutter is composed of reflected versions of the transmitted waveforms, a greedy-based method for selecting the frequency snapshots is presented. Numerical experiments show that a receiver with reduced DOF can lead to detection performance which is very close to that of the receiver with full DOF.

The correspondence principle plays a fundamental role in quantum mechanics, which naturally leads us to inquire whether it is possible to find or determine close classical analogs of quantum states in phase space—a common meeting point to both classical and quantum density statistical descriptors. Here, this issue is tackled by investigating the behavior of classical analogs arising upon the removal of all interference traits displayed by the Wigner distribution functions associated with a given pure quantum state. Accordingly, the dynamical evolution of the linear and von Neumann entropies is numerically computed for a continuous-variable bipartite system, and compared with the corresponding classical counterparts, in the case of two quartic oscillators nonlinearly coupled under regular and chaos conditions. Three quantum states for the full system are considered: a Gaussian state, a cat state, and a Bell-type state. By comparing the quantum and classical entropy values, and particularly their trends, it is shown that, instead of entanglement production, such entropies rather provide us with information on the system (either quantum or classical) delocalization. This gradual loss of information translates into an increase in both the quantum and the classical realms, directly connected to the increase in the correlations between both parties’ degrees of freedom which, in the quantum case, is commonly related to the production of entanglement.

Target detection for synthetic aperture radar (SAR) images has great influence on the successive discrimination based on the target regions. However, as a pixel-based method, the traditional constant false alarm rate (CFAR) detection could not work well for the ship target detection problem of multiple ship targets with different sizes in a SAR image, which is referred to as the multiscale situation. Moreover, it needs to use the clustering method on the pixel-level detection results to obtain the accurate target regions, which may merge two or more different targets into a target region. In this letter, a modified CFAR based on object proposals is proposed. We use the object proposal generator to generate a small set of object proposals with different sizes, and then use the proposal-based CFAR detector, where the extracted object proposals are regarded as the guard windows instead of setting fixed guard window, to detect the true positive object proposals. By introducing the object proposals as the variable guard windows in the CFAR detector, the proposed algorithm could gain good detection performance in the multiscale situation, since the missed detection resulting from the big differences between the sizes of the fixed guard window and ship targets can be avoided. Meanwhile, the proposed method can directly obtain the accurate target regions. The effectiveness of the proposed algorithm is verified using the measured SAR data.

To meet the urgent demand of low-observable moving target detection in complex environments, a novel method of Frequency Diverse Array (FDA) radar signal processing method based on Space-Rang-Doppler Focusing (SRDF) is proposed in this paper. The current development status of the FDA radar, the design of the array structure, beamforming, and joint estimation of distance and angle are systematically reviewed. The extra degrees of freedom provided by FDA radar are fully utilizsed, which include the Degrees Of Freedom (DOFs) of the transmitted waveform, the location of array elements, correlation of beam azimuth and distance, and the long dwell time, which are also the DOFs in joint spatial (angle), distance, and frequency (Doppler) dimensions. Simulation results show that the proposed method has the potential of improving target detection and parameter estimation for weak moving targets in complex environments and has broad application prospects in clutter and interference suppression, moving target refinement, etc..

In the quantum world, a single particle can have various degrees of freedom to encode quantum information. Controlling multiple degrees of freedom simultaneously is necessary to describe a particle fully and, therefore, to use it more efficiently. Here we introduce the transverse waveguide-mode degree of freedom to quantum photonic integrated circuits, and demonstrate the coherent conversion of a photonic quantum state between path, polarization and transverse waveguide-mode degrees of freedom on a single chip. The preservation of quantum coherence in these conversion processes is proven by single-photon and two-photon quantum interference using a fibre beam splitter or on-chip beam splitters. These results provide us with the ability to control and convert multiple degrees of freedom of photons for quantum photonic integrated circuit-based quantum information process.