Frontiers of quantum physics

Abstract:

Practical implementations of quantum cryptography use attenuated laser pulses as the signal source rather than single photons. The channels used to transmit are also lossy. Here we give a simple derivation of two beamsplitting attacks on quantum cryptographic systems using laser pulses, either coherent or mixed states with any mean photon number. We also give a simple derivation of a photon-number splitting attack, the most advanced, both in terms of performance and technology required. We find bounds on the maximum disturbance for a given mean photon number and observed channel transmission efficiency for which a secret key can be distilled. We start by reviewing two incoherent attacks that can be used on single photon quantum cryptographic systems. These results are then adapted to systems that use laser pulses and lossy channels. © 2003 Taylor & Francis Group, LLC.

Abstract:

A variable length quantum data compression scheme was outlined. By looking at quantum data compression in the second quantization framework, one can generate variable length codes in a natural and efficient manner without having the significant memory overhead common to other variable length schemes. By sending the total length of the transmitted signal through a classical channel enables to compress and decompress with perfect fidelity for any number of qubits.

Abstract:

We present an idealized model involving interacting quantum dots that can support both the dynamical and geometrical forms of quantum computation. We show that by employing a structure similar to the one used in the Aharonov-Bohm effect we can construct a topological two-qubit phase-gate that is to a large degree independent of the exact values of the control parameters and therefore resilient to control errors. The main components of the set-up are realizable with present technology.