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The classical capacity of the lossy bosonic channel is calculated exactly. It is shown that its Holevo information is not superadditive, and that a coherent-state encoding achieves capacity. The capacity of far-field, free-space optical communications is given as an example.
Strengthening a result of Yuen-Kennedy-Lax and Holevo (YKLH), we relax the (redundant) standard conditions for optimality of minimum-error quantum measurements while retaining sufficiency. This result is made robust by giving quantitative bounds on non-optimality when the simplified conditions fail to hold. These conditions serendipitously appeared as an… (More)
We demonstrate theoretically and experimentally that secure communication using intermediate-energy (mesoscopic) coherent states is possible. Our scheme is different from previous quantum cryptographic schemes in that a short secret key is explicitly used and in which quantum noise hides both the bit and the key. This encryption scheme allows optical… (More)
We demonstrate high-speed (250 Mbps) data encryption over 25 km of telecommunication fiber by use of coherent states. For the parameter values used in the experiment, the demonstration is secure against individual ciphertext-only eavesdropping attacks near the transmitter with ideal detection equipment. Whereas other quantum-cryptographic schemes require… (More)
A new cryptographic tool, anonymous quantum key technique, is introduced that leads to unconditionally secure key distribution and encryption schemes that can be readily implemented experimentally in a realistic environment. If quantum memory is available, the technique would have many features of public-key cryptography; an identification protocol that… (More)
Quantum-mechanical calculations of the mean-square fluctuation spectra in optical homodyning and heterodyning are made for arbitrary input and local-oscillator quantum states. In addition to the unavoidable quantum fluctuations, it is shown that excess noise from the local oscillator always affects homodyning and, when it is broadband, also heterodyning.… (More)
—The fundamental security and efficiency considerations for fresh key generation will be described. It is shown that the attacker's optimal probability of finding the generated key is an indispensable measure of security and that this probability limits the possibility of privacy amplification and the amount of fresh key that can be generated. A new… (More)