Quantum entanglement is a key ingredient in quantum information processing. In practice, entanglement can be easily destroyed by environment noise. Quantum illumination(QI) can benefit from entanglement in target detection even it is under these entanglement destroying noise.
The aim of quantum illumination is to detect low reflective target which is embedded in a bright background thermal bath. Half of a pair of entangled optical beams is sent out to interrogate the target region. Then the returned and the retained signal beam are used to decide the presence or absence of the object. Even though the fragile entanglement is easily destroyed by the bright thermal noise, quantum-illumination protocol still has remarkable advantage over classical probe protocol. Several experiments have realized the practical protocol in quantum sensing. Recently, Weedbrook et al. show that quantum discord exhibits the role of preserving the benefits of quantum illumination while entanglement is broken.
It is advantageous to operate the frequency of the signal which interrogates the object in microwave region. But so far there is no efficient way to detect single photon in microwave region while in the visible light frequency region, ultrasensitive detection of single photon have been achieved. Such that the detection of microwave signals via the detection of their entangled optical signals is a more efficient way. Optomechanical systems could be a good candidate to realize the entanglement of microwave and optical fields by using mechanical motion.
In this paper, we propose a novel realization of quantum illumination in weak coupling regime based on multimode optomechanical systems. The transmitter is an optomechanical system consisting of a two-mode microwave cavity coupling with a two-mode optical cavity via a mechanical resonator. In this system, the output microwave signal and the optical signal of the two cavities are entangled. The receiver is a similar optomechanical system which converts the reflected microwave signal into optical signal. The retained optical mode of the transmitter and the optical mode of the receiver are then sent to the photon-detectors to make a joint measurement.
We use a two-mode and off-resonant(with frequency detuning δ) process to minimize the mechanical thermal noise. Consequently, our method can achieve a significant reduction of error probability than classical system of the same transmitted energy. Our method works in the weak many-photon coupling regime where rotating-wave approximation works well. Moreover, we optimize the delay time of the microwave signal’s filter function. This makes it possible for our method to work with a finite bandwidth of signals. In this case, we can still achieve an improvement of signal-to-noise ratio by 48% than that of classical detection given that signals-bandwidth σ which equals cavities-bandwidth κ.
We propose to realize microwave quantum illumination in weak coupling regime based on multimode optomechanical systems. In our proposal the multimode together with a frequency-mismatch process could reduce mechanical thermal noise. Therefore, we achieve a significant reduction of error probability than conventional detector in weak coupling regime. Moreover, we optimize the signal-to-noise ratio for limited bandwidth by tuning the delay time of entangled wave-packets.