What Is Quantum Radar?

Quantum radar is a class of sensing and detection systems that exploit quantum mechanical properties, particularly entanglement and quantum correlations, to detect and characterize targets in ways that offer advantages over classical radar of equivalent transmitted power. The concept draws on quantum information theory, quantum optics, and microwave engineering, and its most studied form is quantum illumination, a protocol in which a signal photon entangled with a retained idler photon is sent toward a target while the idler is stored at the receiver. Target detection proceeds by measuring correlations between the returned signal and the stored idler, a process that can outperform classical matched-filter detection even after loss and thermal noise have destroyed the initial entanglement. Research on the theoretical and engineering basis for these systems is reviewed in the arXiv overview of quantum illumination and quantum radar.

The physical basis for quantum radar emerged from work by Seth Lloyd in 2008, who showed that entanglement between signal and idler modes provides a 6 dB improvement in error probability exponent over the best classical protocol using the same transmitted energy, in the regime of low signal reflectivity and high background thermal noise. This advantage is particularly relevant in the microwave band, where blackbody thermal photons are abundant at room temperature and therefore constitute a substantial noise floor for conventional radar.

Quantum Illumination Protocol

In a quantum illumination protocol, an entangled photon pair is produced from a source such as a Josephson parametric amplifier for microwave frequencies or a nonlinear crystal for optical frequencies. The signal mode is transmitted toward the target region, and the idler mode is stored in a quantum memory or delay line. After the round-trip time, the receiver performs a joint measurement on the returned field and the stored idler. The key insight is that even though thermal noise and propagation loss eliminate all entanglement between the returned mode and the idler, residual quantum correlations in the joint state still allow a receiver that exploits these correlations to achieve a lower error rate than a classical receiver. The IEEE journal treatment of this approach is documented in the IEEE Xplore article on the quantum illumination story, which covers both the theoretical framework and early experimental demonstrations.

Signal Detection and Performance

The performance improvement of quantum illumination is measured in terms of the error probability exponent: for low reflectivity targets in high thermal noise, the quantum protocol achieves an error exponent four times that of the optimal classical coherent-state protocol at the same transmitted energy. Practical realization of this gain requires a joint receiver that coherently combines the returned signal and the stored idler, which is technically demanding; simpler receivers such as optical parametric amplifiers or phase-conjugate receivers capture part of the advantage. At microwave frequencies, where most practical radar operates, generating and detecting single-mode entanglement requires cryogenic Josephson parametric amplifiers, which complicates fieldable implementations. An engineering assessment of these practical constraints is provided in the arXiv engineering perspective on quantum radar.

Quantum radar also offers a degree of resilience against spoofing: because an adversary cannot replicate the quantum correlations between the signal and the idler held by the operator, attempted replay or deceptive return signals are distinguishable in principle through Bell inequality tests.

Applications

Quantum radar has applications in a range of fields, including:

• Low-reflectivity target detection in high-noise electromagnetic environments
• Covert sensing where low transmitted power reduces adversarial detection
• Integrated sensing and communication systems combining quantum signal processing
• Biomedical imaging at low radiation exposure using entangled photon sources