Short answer: Quantum illumination uses pairs of entangled photons, one sent at a target and one kept, to detect a weakly reflecting object in a bright thermal background. Theory predicts an advantage over any classical approach of the same transmitted energy. Laboratory experiments from 2020 and 2023 at microwave frequencies report a measured advantage in a superconducting circuit and a free-space test with a room-temperature object at 1 m. These are small laboratory demonstrations, not working radar systems.
The idea
A 2015 Physical Review Letters paper describes quantum illumination as a quantum-optical sensing technique in which an entangled source improves detection of a low-reflectivity object immersed in a bright thermal background. It argues the microwave region suits target detection better than the optical, because of the naturally occurring bright thermal background there. The authors propose a system in which an electro-optomechanical converter entangles microwave signal and optical idler fields. The microwave signal probes the target region while the optical idler is kept at the source. Returned microwave radiation is phase conjugated and upconverted to the optical domain, then combined with the idler in a joint measurement. They show the error probability of this proposed microwave quantum-illumination system, or quantum radar, is lower than that of any classical microwave radar of equal transmitted energy.
A free-space experiment
A 2020 Science Advances paper says quantum illumination uses entangled signal-idler photon pairs to boost detection of low-reflectivity objects in environments with bright thermal noise, with an advantage most evident at low signal powers. It mentions possible uses such as noninvasive biomedical scanning or low-power short-range radar. The authors generate entangled microwave fields to illuminate a room-temperature object 1 m away in free space. A digital phase-conjugate receiver using linear quadrature measurements outperforms a symmetric classical noise radar under the same conditions, even though the signal path breaks the entanglement. Starting from the experimental data, they simulate perfect idler photon number detection and find a quantum advantage over the corresponding classical benchmark. The authors describe their results as highlighting both opportunities and challenges on the way to a first room-temperature application of microwave quantum circuits. The paper adds that at room temperature there are about a thousand thermal quanta per mode at a few gigahertz, and that the protocol probes the target with less than one entangled photon per mode.
A superconducting-circuit radar
A 2023 Nature Physics paper starts from the goal of showing a quantum technology outperforming the best possible classical implementation. A quantum radar improves detection of a target in a noisy environment by using quantum correlations between two modes, probe and idler. The predicted enhancement is less sensitive to loss than most quantum metrology applications and is supposed to improve with additional noise. The authors demonstrate a superconducting circuit implementing a microwave quantum radar that provides more than 20 percent better performance than any possible classical radar. The scheme jointly measures entangled probe and idler microwave states after the probe has reflected from the target and mixed with thermal noise. By storing the idler in a resonator, they reduce the harm from idler loss. Across a wide range of parameters, they find the purity of the initial entangled state is the main limiting factor and must be considered in any practical application.
What this does not tell you
The 2023 result is a circuit experiment, and the 2020 result compares against a specific classical noise radar with a simulated improvement. Neither shows a radar that works at useful range, and the papers name state purity and loss as limits.
FAQ
What is quantum illumination?
A sensing method that uses entangled photon pairs to detect weakly reflecting objects in thermal noise.
Why microwaves?
A 2015 paper says the naturally bright microwave thermal background makes target detection there a good fit.
How big was the measured advantage?
The 2023 circuit experiment reports more than 20 percent better performance than any possible classical radar.
What limits it?
The purity of the initial entangled state, according to the 2023 paper.
Is there a working quantum radar?
These papers describe laboratory experiments, not deployed systems.
All field notes