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Quantum key distribution using gaussian-modulated coherent states


Quantum continuous variables1 are being explored2,3,4,5,6,7,8,9,10,11,12,13,14 as an alternative means to implement quantum key distribution, which is usually based on single photon counting15. The former approach is potentially advantageous because it should enable higher key distribution rates. Here we propose and experimentally demonstrate a quantum key distribution protocol based on the transmission of gaussian-modulated coherent states (consisting of laser pulses containing a few hundred photons) and shot-noise-limited homodyne detection; squeezed or entangled beams are not required13. Complete secret key extraction is achieved using a reverse reconciliation14 technique followed by privacy amplification. The reverse reconciliation technique is in principle secure for any value of the line transmission, against gaussian individual attacks based on entanglement and quantum memories. Our table-top experiment yields a net key transmission rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per second for a line with losses of 3.1 dB. We anticipate that the scheme should remain effective for lines with higher losses, particularly because the present limitations are essentially technical, so that significant margin for improvement is available on both the hardware and software.

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Figure 1: Experimental set-up.
Figure 2: Bob's measured quadrature as a function of the amplitude sent by Alice (in Bob's measurement basis) for a burst of 60,000 pulses.
Figure 3: Channel equivalent noise χline as a function of line transmission G.
Figure 4: Values of IBA, IBE and IAE as a function of the line transmission G for V ≈ 40. Here, IBA is given by equation (4a), including all transmission and detection noises for evaluating VB and (VB|A)coh.

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The contributions of J. Gao to the early stages of the experiment, and of K. Nguyen to the software development, are acknowledged. We thank S. Iblisdir for discussions, and Th. Debuisschert for the loan of the 780 nm integrated modulator. This work was supported by the EU programme IST/FET/QIPC (projects “QUICOV” and “EQUIP”), the French programmes ACI Photonique and ASTRE, and by the Belgian programme ARC.

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Correspondence to Philippe Grangier.

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Grosshans, F., Van Assche, G., Wenger, J. et al. Quantum key distribution using gaussian-modulated coherent states. Nature 421, 238–241 (2003).

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