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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)
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)
In this paper, we present a proof-of-concept experimental demonstration of the secret key quantum cryptographic scheme. A tabletop communication link was set up in the free-space channel using ordinary lasers as transmitters, which emit coherent states of light, and quantum-limited direct detection was employed in the receivers. In the secret key scheme,(More)
The classical-information capacity of lossy bosonic channels is studied, with emphasis on the far-field free space channel. 1 Classical capacity A prominent landmark in the extension of Shannon information theory to the quantum domain is the realization that any particular physical system can store only a finite amount of information. As a consequence, the(More)