Creating and sharing knowledge for telecommunications

Deterministic realization of a universal quantum gate in a single scattering process

on 16-03-2012

... Francesco Ciccarello, Scuola Normale Superiore, Pisa

16/03/2012, 15:00
Room P4.35, Post-Graduation Building, IST.

We show that a flying particle, such as an electron or a photon, scattering along a one-dimensional waveguide from a pair of static spin-1/2 centers, such as quantum dots or superconducting qubits, can implement a CZ gate (universal for quantum computation) between them. This occurs deterministically in a single scattering event, hence with no need for any post-selection or iteration, and without demanding the flying particle to bear any internal spin. We show that an easily matched hard-wall boundary condition along with the elastic nature of the process are key to such performances. More Information..

The closest vector problem in multi-dimensional wireless channels

on 02-03-2012

... Francisco Monteiro (IT & ISCTE-IUL).

March 2, 2012, Friday, 15h.
Location: Room P4.35, Post-Graduation Building, IST.

This talk provides an overview of one of the central problems in communication engineering in the last 10 years, whose solution allows us now to reach the 1 Gbps frontier in wireless systems such as LTE Advanced and WiMax. The capacity limits set by Shannon in 1948 for digital transmission were reached in 1993 and 1995 after the discovery of turbo-codes and low-density parity-check codes. While this put an end to an era in coding theory, a new door was opened in the late 90s for wireless channels: multiple-input multiple output (MIMO). It was soon mathematically proved that increasing the number of antennas both at the transmitter and at the receiver would increase the capacity of the radio link. However, this gain comes at the expense of a much higher algorithmic complexity at the receiver side. The mathematical underlying detection problem is the closest vector problem (CVP) in a lattice. The problem had been mostly investigated in algorithmic number theory and much of the progress made in signal processing in communications came in fact from the re-discovery of algorithms known in the communities of algorithmic number theory and cryptography. The talk will describe several approximate and exact solutions to CVP, emphasising the geometric manipulation of lattices that is carried out by the most relevant algorithms: maximum likelihood detection, zero-forcing, minimum mean square error, successive detection, sphere decoding and lattice reduction. A novel approach to the problem will also be presented, which maps the problem onto a graph-based path minimisation problem. More Information..