Quantum illumination

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Quantum illumination is a paradigm that uses quantum entanglement beneficially even if the original entanglement is completely destroyed by a lossy and noisy environment.[1][2]


Many quantum information applications, such as quantum teleportation,[3] quantum error correction, and superdense coding, 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. Entanglement is therefore considered very hard to use in lossy and noisy environment.

Lloyd, Shapiro and collaborators 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 is particularly useful in extremely lossy and noisy situations.



The concept of quantum illumination was first introduced by Seth Lloyd and collaborators at MIT in 2008.[4] An experimentally feasible scheme for quantum illumination can be realized using Gaussian states[5] as demonstrated by Jeffrey Shapiro and collaborators[6] from the same institute.

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 6dB improvement in the error exponent [6]). 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 2009, a secure communication scheme based on quantum illumination[7] 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, Seth Lloyd and collaborators[8] in 2008. In 2015, an international collaboration coordinated by Stefano Pirandola [9][10] extended the protocol of quantum illumination to the microwave frequencies, thus providing the first theoretical prototype of quantum radar. In 2017, the optimum receiver design was proposed by Quntao Zhuang, Zheshen Zhang, and Jeffrey Shapiro[11]. Quantum illumination has also been extended to the scenario of target fading[12].


In 2013, Lopaeva et al. exploited photon number correlations, instead of entanglement, in a sub-optimal target detection experiment.[13] To illustrate the benefit of quantum entanglement, 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 quantum decoherence.[14] In 2015, Zhang et al. applied quantum illumination in sensing and showed that employing entanglement can yield a higher signal-to-noise ratio than the optimal classical scheme can provide, even though the highly lossy and noisy environment completely destroys the initial entanglement.[15][16] This sensing experiment thus proved the original theoretical proposals of quantum illumination.


Potential applications of quantum illumination include target detection in high background noise environments, but also ultra-sensitive biological imaging and sensing, and secure communication.


  1. ^ Quantum Communications: Broken quantum links still work, Nature 499, 129 (2013) ([1])
  2. ^ Fragility of entanglement no bar to quantum secrets, New Scientist July 17, 2013([2])
  3. ^ C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, A. Peres, W. K. Wootters, Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels, Phys. Rev. Lett. 70, 1895-1899 (1993) (online)
  4. ^ Seth Lloyd, Enhanced Sensitivity of Photodetection via Quantum Illumination, Science 321, 1463-1465 (2008) ([3])]); Si-Hui Tan, Baris I. Erkmen, Vittorio Giovannetti, Saikat Guha, Seth Lloyd, Lorenzo Maccone, Stefano Pirandola, and Jeffrey H. Shapiro, Quantum Illumination with Gaussian States, Phys. Rev. Lett. 101, 253601 (2008)([4])
  5. ^ Christian Weedbrook, Stefano Pirandola, Raul Garcia-Patron, Nicolas J. Cerf, Timothy C. Ralph, Jeffrey H. Shapiro, and Seth Lloyd, Gaussian Quantum Information, Rev. Mod. Phys. 84, 621 (2012) ([5])
  6. ^ a b Si-Hui Tan, Baris I. Erkmen, Vittorio Giovannetti, Saikat Guha, Seth Lloyd, Lorenzo Maccone, Stefano Pirandola, and Jeffrey H. Shapiro, Quantum Illumination with Gaussian States, Phys. Rev. Lett. 101, 253601 (2008)([6])
  7. ^ Jeffrey H. Shapiro, Defeating passive eavesdropping using quantum illumination, Phys. Rev. A 80, 022320 (2009) ([7])
  8. ^ Stefano Pirandola, Stefano Mancini, Samuel L. Braunstein, and Seth Lloyd, Continuous-variable quantum cryptography using two-way quantum communication, Nat. Phys. 4, 726-730 (2008)([8])
  9. ^ Shabir Barzanjeh, Saikat Guha, Christian Weedbrook, David Vitali, Jeffrey H. Shapiro, and Stefano Pirandola, Microwave Quantum Illumination, Phys. Rev. Lett. 114, 080503 (2015)([9])
  10. ^ Quantum Mechanics Could Improve Radar, Physics 8, 18 (2015)([10])
  11. ^ Zhuang, Quntao; Zhang, Zheshen; Shapiro, Jeffrey H. (2017-01-27). "Optimum Mixed-State Discrimination for Noisy Entanglement-Enhanced Sensing". Physical Review Letters. 118 (4): 040801. arXiv:1609.01968. Bibcode:2017PhRvL.118d0801Z. doi:10.1103/PhysRevLett.118.040801.
  12. ^ Zhuang, Quntao; Zhang, Zheshen; Shapiro, Jeffrey H. (2017-08-15). "Quantum illumination for enhanced detection of Rayleigh-fading targets". Physical Review A. 96 (2): 020302. arXiv:1706.05561. Bibcode:2017PhRvA..96b0302Z. doi:10.1103/PhysRevA.96.020302.
  13. ^ E. D. Lopaeva, I. Ruo Berchera, I. P. Degiovanni, S. Olivares, G. Brida, and M. Genovese, Experimental Realization of Quantum Illumination, Phys. Rev. Lett. 110, 153603 (2013)([11])
  14. ^ Zheshen Zhang, Maria Tengner, Tian Zhong, Franco N.C. Wong, and Jeffrey H. Shapiro, Entanglement's benefit survives an entanglement-breaking channel, Phys. Rev. Lett. 111, 010501 (2013) ([12])
  15. ^ Zheshen Zhang, Sara Mouradian, Franco N.C. Wong, and Jeffrey H. Shapiro, Entanglement-enhanced sensing in a lossy and noisy environment, Phys. Rev. Lett. 114, 110506 (2015) ([13])
  16. ^ Quantum sensor's advantages survive entanglement breakdown, MIT News, 9 March (2015), ([14])