Date: THURSDAY 25th January

Who: Sean Barrett

Seminar type: Research Seminar

Time: 12 Midday

Where: Interaction Room

Abstract:
Single photon interference effects are of fundamental interest, and are central to many protocols for quantum information processing (QIP). In particular, effects such as photon bunching and ‘which-path’ erasure lie at the heart of many schemes for linear optics quantum computing, and hybrid matter-light QIP. Conventional folklore has that, in order to observe these effects, the photons in question must be identical, in particular that they have the same frequency. Recently, it has been shown that it is possible to observe interference between photons of different frequencies, provided one has photodetectors with sufficiently high temporal resolution. However, this places severe technical constraints on both the sources of these photons, and the detectors.

In this work, we propose a way of observing single photon interference effects between photons of different frequencies, which works even with low resolution detectors. The method is based on carefully controlling the temporal mode shape of the photons, and is roughly analogous to the stroboscopic principle that underlies temporal aliasing, the ‘waggon wheel effect’, and the optical frequency comb that was the subject of the 2005 Nobel prize in physics. The method could allow a much broader range of single photon emitters and matter qubit systems to be used in QIP.

Date: Friday 12th January 2007

Who: Andrew Scott

Seminar type: Research Seminar

Time: 12 Midday

Where: Interaction Room

Abstract:
We introduce a class of informationally complete positive-operator-valued measures which are, in analogy with a tight frame, “as close as possible”
to orthonormal bases for the space of quantum states. These measures are distinguished by an exceptionally simple state-reconstruction formula which allows “painless” quantum state tomography. Complete sets of mutually unbiased bases and symmetric informationally complete positive-operator-valued measures are both members of this class, the latter being the unique minimal rank-one members. Recast as ensembles of pure quantum states, the rank-one members are in fact equivalent to weighted 2-designs in complex projective space. These measures are shown to be optimal for (measurement-based) quantum cloning and quantum state tomography.