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)
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)
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)
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 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)