The quantum states of two laser pulses-coherent states-are never mutually orthogonal, making perfect discrimination impossible. Even so, coherent states can achieve the ultimate quantum limit for capacity of a classical channel, the Holevo capacity. Attaining this requires the receiver to make joint-detection measurements on long codeword blocks, optical implementations of which remain unknown. We report the first experimental demonstration of a joint-detection receiver, demodulating quaternary pulse-position-modulation (PPM) codewords at a word error rate of up to 40% (2.2 dB) below that attained with direct-detection, the largest error-rate improvement over the standard quantum limit reported to date. This is accomplished with a conditional nulling receiver, which uses optimized-amplitude coherent pulse nulling, single photon detection and quantum feedforward. We further show how this translates into coding complexity improvements for practical PPM systems, such as in deep-space communication. We anticipate our experiment to motivate future work towards building Holevo-capacity-achieving joint-detection receivers.One of the most important insights of quantum physics in the modern theory of optical communication is the realization that it is impossible to perfectly discriminate states of light that are not mutually orthogonal. While orthogonal quantum states of light {|ψ k }, k = 1, 2, . . ., i.e., whose inner products satisfy ψ k |ψ j = δ kj , can in principle be discriminated with zero probability of error, such states of light are hard to create [27]. An ordinary laser pulse is in a coherent state, |α (where α is a complex number denoting the mean field value). However, no two coherent states-even those in orthogonal spacetime field modes-are ever in mutually orthogonal quantum states, i.e., α|β = exp α * β − 1 2 |α| 2 + |β| 2 = 0, precluding perfect discrimination. In spite of this apparent impairment, coherent states are sufficient to attain the ultimate (Holevo) capacity of optical communication, even on a lossy channel [2], a channel on which mutually-orthogonal quantum states such as Fock states actually fail to achieve the Holevo capacity. Recently, we discovered the first Holevo-capacity achieving codes [12], and some advances on the optimal receiver measurements have been reported in recent literature [8,11,12]. However, designing and building such optical receivers remain elusive.Helstrom gave a recipe to compute the minimum achievable average probability of error to discriminate quantum states from a given ensemble [1], known as the Helstrom limit. He also found necessary and sufficient conditions on the operators describing the optimal measurement that achieves that minimum. Unfortunately, very little is known in general about how to build receivers achieving this limit using laboratory optics. The binary-hypothesis Dolinar receiver can attain it in the case of discriminating two coherent states [5]. This was demonstrated only recently, three decades after its invention [6], owing to the difficult...

Computational encryption, information-theoretic secrecy and quantum cryptography offer progressively stronger security against unauthorized decoding of messages contained in communication transmissions. However, these approaches do not ensure stealth—that the mere presence of message-bearing transmissions be undetectable. We characterize the ultimate limit of how much data can be reliably and covertly communicated over the lossy thermal-noise bosonic channel (which models various practical communication channels). We show that whenever there is some channel noise that cannot in principle be controlled by an otherwise arbitrarily powerful adversary—for example, thermal noise from blackbody radiation—the number of reliably transmissible covert bits is at most proportional to the square root of the number of orthogonal modes (the time-bandwidth product) available in the transmission interval. We demonstrate this in a proof-of-principle experiment. Our result paves the way to realizing communications that are kept covert from an all-powerful quantum adversary.

The authors report on the full implementation of a superconducting detector technology in a fiber-based quantum key distribution (QKD) link. Nanowire-based superconducting single-photon detectors (SSPDs) offer infrared single-photon detection with low dark counts, low jitter, and short recovery times. These detectors are highly promising candidates for future high key rate QKD links operating at 1550nm. The authors use twin SSPDs to perform the BB84 protocol in a 1550nm fiber-based QKD link clocked at 3.3MHz. They exchange secure key over a distance of 42.5km in telecom fiber and demonstrate that secure key can be transmitted over a total link loss exceeding 12dB.

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