#### ABSTRACT

We present the first experimental realization of the quantum illumination protocol proposed by Lloyd [Science 321, 1463 (2008)] and S. Tan *et al.* [Phys. Rev. Lett. 101, 253601 (2008)], achieved in a simple feasible experimental scheme based on photon-number correlations. A main achievement of our result is the demonstration of a strong robustness of the quantum protocol to noise and losses that challenges some widespread wisdom about quantum technologies.

Received 21 January 2013

DOI:http://dx.doi.org/10.1103/PhysRevLett.110.153603

© 2013 American Physical Society

#### ARTICLE TEXT

The properties of quantum states have disclosed the possibility of realizing tasks beyond classical limits, originating a field collectively christened quantum technology [1–7]. Among them, quantum metrology and imaging aim to improve the sensitivity and/or resolution of measurements exploiting nonclassical features, in particular, nonclassical correlations [8–12]. However, in most of the realistic scenarios, losses and noise are known to nullify the advantage of adopting quantum strategies [13]. Here, we present the first experimental realization of a quantum enhanced scheme [14,15], designed to target detection in a noisy environment, preserving a strong advantage over the classical counterparts even in presence of large amounts of noise and losses. This work, inspired by theoretical ideas elaborated in Refs. [14–17] (see also Ref. [18]), has been implemented exploiting only photon-number correlations in twin beams and, for its accessibility, it can find widespread use. Even more important, it paves the way to the real application of quantum technologies by challenging the common belief that they are limited by their fragility to noise and losses.

Our scheme for target detection is inspired by the “quantum illumination” (QI) idea [14,15], where the correlation between two beams of a bipartite nonclassical state of light is used to detect the target hidden in a noisy thermal background, which is partially reflecting one of the beams. In Refs. [15,16]it was shown that for QI realized by twin beams, like the ones produced by parametric down-conversion, there exists, in principle, an optimal reception strategy offering a significant performance gain with respect to any classical strategy. Unfortunately, this quantum optimal receiver is not yet devised, and even the theoretical proposal of a suboptimal quantum receiver [19] was very challenging from an experimental point of view, and has not been realized yet.

Our aim is to lead the QI idea to an experimental demonstration in a realistic scenario. Therefore, in our realization we consider a realistic *a priori* unknown background, and a reception strategy based on photon-counting detection and second-order correlation measurements. We demonstrate that the quantum protocol performs astonishingly better than its classical counterpart based on classically correlated light at any background noise level. More in detail, we compare quantum illumination, specifically twin beams (TWB), with classical illumination (CI) based on correlated thermal beams, that turns out to be the best possible classical strategy in this detection framework.

On the one hand, our approach, based on a specific and affordable detection strategy in the context of the current technology, cannot aim to achieve the optimal target-detection bounds of Ref. [15], based on the quantum Chernoff bound [20–22]. On the other hand, it maintains most of the appealing features of the original idea, like a huge quantum enhancement and a robustness against noise, paving the way to future practical application because of the accessible measurement technique. Our study also provides a significant example of an ancilla-assisted quantum protocol, besides the few previous realizations, e.g., Refs. [11,23–25].

In our setup (see Fig. 1) parametric down-conversion (PDC) is exploited to generate two correlated light emissions with an average number of PDC photons per spatiotemporal mode,

We measured the correlation in the photon numbers

In order to quantify the quantum resources exploited by our QI strategy, we introduce a suitable nonclassicality parameter: the generalized Cauchy-Schwarz parameter

We consider an *a priori* unknown background, meaning that it is impossible to establish a reference threshold of photocounts (usually the mean value of the background) to be compared with the possible additional mean photocounts coming from the reflected probe beam (if the target is present). Therefore, the estimation of the first order (mean values) of the photocounts’ distribution, typical of other protocols (e.g., Refs. [9,11,12]), is here not informative regarding the presence or absence of the object. We underline that this unknown-background hypothesis accounts for a “realistic” scenario where background properties can randomly change and drift with time and space.

For this reason we propose to discriminate the presence or absence of the object by distinguishing between the two corresponding values of the covariance

where “in” and “out” refer to the presence and absence of the object.

For

While the signal-to-noise ratio unavoidably decreases with the added noise for both QI and CI, the quantum enhancement parameter [

Being

According to Eq. (3), the enhancement is lower bounded by the amount of violation of the Cauchy-Schwarz inequality for the quantum state considered in the absence of the background, i.e.,

In particular, in our experiment we compared the performance of TWB with a classically correlated state with

Incidentally, since covariance is always zero (i.e.,

In Fig. 4, the theoretical prediction for

Another figure of merit that highlights the superiority of the quantum strategy versus the classical one is the the error probability in the discrimination of the presence or absence of the target (

In conclusion, we demonstrated experimentally quantum enhancement in detecting a target in a thermal radiation background. Our system shows quantum correlation [

In paradigmatic quantum enhanced schemes, often based on the experimental estimation of the first momenta of the photon-number distribution, such as quantum imaging protocol [11], the detection of small beam displacement [9] and phase estimation by interferometry [12], it is well known that losses and noise rapidly decrease the advantage of using quantum light [13,31]. This is enforced inside the generic scientific community by the common belief that the advantages of entangled and quantum state are hardly applicable in a real context, and they will remain limited to experiments in highly controlled laboratories, and/or to mere academic discussions. Our work breaks this belief by showing orders of magnitude improvements compared to CI protocol, independent of the amount of noise and losses using devices available nowadays. In summary, we believe that the photon-counting based QI protocol, for its robustness to noise and losses, has a huge potentiality to promote the usage of quantum correlated light in real environments.

#### SUPPLEMENTAL MATERIAL

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