Entanglement is essential to many quantum information applications, but it is easily destroyed by quantum decoherence arising from interaction with the environment. We report the first experimental demonstration of an entanglement-based protocol that is resilient to loss and noise which destroy entanglement. Specifically, despite channel noise 8.3 dB beyond the threshold for entanglement breaking, eavesdropping-immune communication is achieved between Alice and Bob when an entangled source is used, but no such immunity is obtainable when their source is classical. The results prove that entanglement can be utilized beneficially in lossy and noisy situations, i.e., in practical scenarios.PACS numbers: 42.50.Dv, 03.67.Hk Entanglement is essential to many quantum information applications [1][2][3][4][5][6][7][8][9][10][11], but it is easily destroyed. Quantum illumination (QI) [12][13][14][15] is a radically different entanglement-based paradigm for bosonic channels: it thrives on entanglement-breaking loss and noise. For a given transmitter power, an initially entangled state's nonclassical correlation produces a classical state at the output of an entanglement-breaking channel whose correlation can greatly exceed what any classical input of the same power can yield through that channel. This suggests that bosonic entanglement can be utilized advantageously in practical situations where it does not survive.First proposed to increase the signal-to-noise ratio (SNR) for detecting a weakly-reflecting target in the presence of strong background noise [12][13][14], quantum illumination was later shown, theoretically, to enable high data-rate classical communication that is immune to passive eavesdropping [15]. In the latter application, Alice and Bob use an entangled-state input for their data transfer. Eve, however, has no access to Alice's retained portion of the entangled state, so her eavesdropping performance is that of a classical-state input. The resulting disparity between Alice and Eve's performance-in bit-error rate (BER) and information received per transmitted bit-guarantees Alice and Bob's communication security. In this Letter we report the first experimental demonstration of QI's passive-eavesdropping immunity. Aside from its relevance to secure communication, our experiment represents the first time that bosonic entanglement has yielded a strong performance benefit over an entanglement-breaking channel. Thus it implies that the use of entanglement should not be dismissed for environments in which it will be destroyed. Moreover, unlike the recent experiment [16] reporting the target-detection advantage of photon-pair correlations, our eavesdroppingimmune QI protocol requires an initial state that is entangled. Also, our communication protocol uses only one pulse to decode a bit, whereas target detection in [16] depends on the accumulation of enough data to accurately estimate a covariance.Our QI communication experiment is shown schematically in Fig. 1. Alice prepares maximally-entangled signal and i...

We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasi-phase matched bulk crystals for the use in quantum communication sources. We show that, by careful design, one can attain well defined modes close to the fundamental mode of optical fibers and obtain high coupling efficiencies also for bulk crystals, these being more easily aligned than crystal waveguides. We distinguish between singles coupling, γs and γi, conditional coincidence, µ i|s , and pair coupling, γc, and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal pumped by a 532 nm laser creating photon pairs at 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation L/zR to be ≈ 1 to 2 for the pump mode and ≈ 2 to 3 for the fiber-modes, where L is the crystal length and zR is the Rayleigh-range of the mode-profile. These results are independent on L. By showing that the single-mode bandwidth decreases ∝ 1/L, we can therefore design the source to produce and couple narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means both less chromatic dispersion for long propagation distances in fibers, and that telecom Bragg gratings can be utilized to compensate for broadened photon packets-a vital problem for time-multiplexed qubits. Longer crystals also yield an increase in fiber photon flux ∝ √ L, and so, assuming correct focusing, we can only see advantages using long crystals.

We use two perpendicular crystals of periodically-poled KTP to directly generate polarization-entangled photon pairs, the majority of which are emitted into a single Gaussian spatial mode. The signal and idler photons have wavelengths of 810 nm and 1550 nm, respectively, and the photon-pair generation rate is 1.2x107 sec-1 for a pump power of 62 mW. The apparatus is compact, flexible, and easily to use.

We have experimentally demonstrated a decoy-state quantum key distribution scheme (QKD) with a heralded single-photon source based on parametric down-conversion. We used a one-way Bennett-Brassard 1984 protocol with a four states and one-detector phase-coding scheme, which is immune to recently proposed time-shift attacks, photon-number splitting attacks, and can also be proven to be secure against Trojan horse attacks and any other standard individual or coherent attacks. In principle, the setup can tolerate the highest losses or it can give the highest secure key generation rate under fixed losses compared with other practical schemes. This makes it a quite promising candidate for future quantum key distribution systems.

We present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 × 10 6 s -1 THz -1 mW -1 after coupling to single-mode fiber. The quantum channel at 1555 nm and the synchronization signal gating the single photon detector are multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This implementation makes quantum communication applications compatible with current high-speed optical networks.