Quantum illumination is a paradigm for target detection that employs

quantum entanglement Quantum entanglement is the phenomenon where the quantum state of each Subatomic particle, particle in a group cannot be described independently of the state of the others, even when the particles are separated by a large distance. The topic o ...

between a signal electromagnetic mode and an idler electromagnetic mode, as well as joint measurement of these modes. The signal mode is propagated toward a region of space, and it is either lost or reflected, depending on whether a target is absent or present, respectively. In principle, quantum illumination can be beneficial even if the original entanglement is completely destroyed by a lossy and noisy environment.

Introduction

Many

quantum information Quantum information is the information of the state of a quantum system. It is the basic entity of study in quantum information theory, and can be manipulated using quantum information processing techniques. Quantum information refers to both t ...

applications, such as

quantum teleportation Quantum teleportation is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from on ...

quantum error correction Quantum error correction (QEC) is a set of techniques used in quantum computing to protect quantum information from errors due to decoherence and other quantum noise. Quantum error correction is theorised as essential to achieve fault tolerant ...

, and

superdense coding In quantum information theory, superdense coding (also referred to as ''dense coding'') is a quantum communication protocol to communicate a number of classical bits of information by only transmitting a smaller number of qubits, under the ass ...

, rely on entanglement. However, entanglement is a fragile quantum property between particles and can be easily destroyed by loss and noise arising from interaction with the environment, leading to

quantum decoherence Quantum decoherence is the loss of quantum coherence. It involves generally a loss of information of a system to its environment. Quantum decoherence has been studied to understand how quantum systems convert to systems that can be expla ...

. Some showed that, even though entanglement itself may not survive, the residual correlation between the two initially-entangled systems remains much higher than any initial classical states can provide. This implies that the use of entanglement should not be dismissed in entanglement-breaking scenarios. Quantum illumination takes advantage of this stronger-than-classical residual correlations between two systems to achieve a performance enhancement over all schemes based on transmitting classical states with comparable power levels. Quantum illumination has been proven to be particularly useful in very noisy environments.

History

Theory

The concept of quantum illumination was introduced by

Seth Lloyd Seth Lloyd (born August 2, 1960) is a professor of mechanical engineering and physics at the Massachusetts Institute of Technology. His research area is the interplay of information with complex systems, especially quantum systems. He has perfor ...

and collaborators at

MIT The Massachusetts Institute of Technology (MIT) is a private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of modern technology and sc ...

in 2008. This included a discrete-variable version and a continuous-variable version developed in collaboration with

Jeffrey Shapiro Jeffrey H. Shapiro is a Julius A. Stratton Professor of electrical engineering and computer science at the Massachusetts Institute of Technology and the former director of the Research Laboratory of Electronics. He made seminal contributions to u ...

, Stefano Pirandola, Saikat Guha and others, the latter version being based on Gaussian states. The basic setup of quantum illumination is target detection. Here the sender prepares two entangled systems, called signal and idler. The idler is retained while the signal is sent to probe the presence of a low-reflectivity object in a region with bright background noise. The reflection from the object is then combined with the retained idler system in a joint quantum measurement providing two possible outcomes: object present or object absent. More precisely, the probing process is repeated many times so that many pairs of signal-idler systems are collected at the receiver for the joint quantum detection. The advantage of the scheme is evident at low energies where the mean number of photons in each signal system is very low (of the order of one photon or less). In this case, at fixed low energy, the probability of success in detecting a target has a remarkable improvement with respect to classical detection schemes, where entanglement is not used and signal systems are prepared in coherent states (technically, there is a 6 dB improvement in the error exponent ). A key feature of quantum illumination is that the entanglement between the idler system and the reflected signal system is completely lost in the process. However, the residual quantum correlations between these two systems (idler-reflected signal) remain so strong that they could only be created by the presence of entanglement in the initial systems (idler-signal). Because the reflected signal is quantum-correlated with the retained idler system, it can be distinguished among all the uncorrelated background thermal photons that are also received by the detector. Because of this quantum labeling of the systems, the detection of quantum illumination is very efficient. In 2015, an international collaboration coordinated by Stefano Pirandola extended the protocol of quantum illumination to the microwave frequencies, thus providing the first theoretical prototype of

quantum radar In physics, a quantum (: quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a property can be "quantized" is referred to as "the hypothesis of quantization". This me ...

. The original proposal from was analyzed in the

Bayesian Thomas Bayes ( ; c. 1701 – 1761) was an English statistician, philosopher, and Presbyterian minister. Bayesian ( or ) may be either any of a range of concepts and approaches that relate to statistical methods based on Bayes' theorem Bayes ...

setting of

hypothesis testing A statistical hypothesis test is a method of statistical inference used to decide whether the data provide sufficient evidence to reject a particular hypothesis. A statistical hypothesis test typically involves a calculation of a test statistic. T ...

, in which prior probabilities are assigned to the hypotheses that the target is absent or present. In 2017, a research paper analyzed quantum illumination in the Neyman-Pearson or asymmetric setting of hypothesis testing, which is a setting of interest in

applications. It was found that the performance gains of quantum illumination are even greater than those from. In 2017, an optimum receiver design was proposed by Quntao Zhuang, Zheshen Zhang, and

. Quantum illumination has also been extended to the scenario of target fading. In 2020, the ultimate limits for quantum illumination, allowing for an arbitrary number of optical modes entangled with a quantum memory were derived by Ranjith Nair and Mile Gu for all levels of background noise. The results also showed that the 6 dB improvement cannot be surpassed - and is only achievable for very large background noise.

Related work on secure communication

In 2009, a secure communication scheme based on quantum illumination was proposed. This scheme is a variant of the quantum cryptographic protocols based on continuous variables and two-way quantum communication introduced by Stefano Pirandola,

and collaborators in 2008.

Experiment

In 2013, Lopaeva ''et al.'' exploited photon number correlations, instead of entanglement, in a sub-optimal target detection experiment. To illustrate the benefit of

, in 2013 Zhang ''et al.'' reported a secure communication experiment based on quantum illumination and demonstrated for the first time that entanglement can enable a substantial performance advantage in the presence of

. In 2015, Zhang ''et al.'' applied quantum illumination in sensing and showed that employing entanglement can yield a higher

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...

than the optimal classical scheme can provide, even though the highly lossy and noisy environment completely destroys the initial entanglement. This sensing experiment thus proved the original theoretical proposals of quantum illumination. The first experimental effort to perform microwave quantum illumination was based on using Josephson parametric amplifier and a digital receiver. As applied to imaging, in 2019 England ''et al.'' demonstrated this principle by imaging through noise in a scanning configuration. The first full-field imaging system based on quantum illumination that uses spatially-entangled photon pairs for imaging in the presence of noise and losses was reported in a two successive publications in 2019 and 2020 by two research groups from the University of Glasgow.

Applications

Potential applications of quantum illumination include target detection in high background noise environments, but also ultra-sensitive

biological imaging Biological imaging may refer to any imaging technique used in biology. Typical examples include: * Bioluminescence imaging, a technique for studying laboratory animals using luminescent protein * Calcium imaging, determining the calcium status of ...

and sensing, and

secure communication Secure communication is when two entities are communicating and do not want a third party to listen in. For this to be the case, the entities need to communicate in a way that is unsusceptible to eavesdropping or interception. Secure communication ...

Media reporting

Several news articles on quantum illumination have appeared in popular science media, with the goal of elucidating the concept of quantum illumination in less technical terms.

References

{{quantum computing Quantum information science