21 November 2007 Minimally invasive quantum detection in Nature Physics

Quantum non-demolition

UAB, ICFO and NBI team up
to propose a new detection method of strongly correlated quantum systems.
Quantum information (QI) employs the laws of quantum mechanics, i.e. laws of microscopic world for the efficient processing of certain computational tasks that are intractable within the classical physics and modern computers. A system of ultracold trapped neutral atoms or molecules is an excellent candidate for implementing scalable quantum computing where the basic quantum bits, the so-called qubit, corresponds to two states of long-lived atoms or molecules. Despite of the progress in using atoms trapped in optical lattices to prepare and manipulate strongly correlated systems, the precise detection and experimental characterization of such states is experimentally a major challenge.

The group of Prof. Anna Sanpera (ICREA Prof. at Universitat Autònoma Barcelona) together with her colleagues from ICFO (ICREA Prof. Maciej Lewenstein) and Prof. Eugene Polzik from The Niels Bohr Institute, in Copenhagen, have proposed a very promising solution to this challenge.

They propose to detect the correlations present in a system of atoms with spin trapped in an optical lattice by shining a polarized laser beam (in a standing wave configuration) and measuring the changes and fluctuations in the polarization of the laser beam after it has crossed the atomic sample. The fluctuations of light can be measured very efficiently using the advanced method of homodyne detection and then mapped to the fluctuations of the atomic correlations. Similar methods have been pioneered in experiments of one of the members of the team (E. Polzik) and reported recently in the context of teleportation experiments in Nature.

The proposal Sanpera, Lewenstein, Polzik and their teams has another amazing aspect. Measurements in the quantum worlds are unfortunately always affecting the measured system – this is an intrinsic law of quantum mechanics. After several measurement the system usually is in a completely different state that the one prepared at the beginning, and what one measures has nothing to do with the prepared state. The new method, is not free of this disadvantage, but the influence of the measurement of the measured system is here reduced to an absolute minimum – that is why the method corresponds to what is called a “quantum non-demolition”.

The results have been published in the current issue of Nature Physics.

Intensive works toward experimental realization of the method have been started in the group of Prof. Polzik in the Niels Bohr Institute and the first results are expected soon. Other world leading experimental groups are following these developments with great care and interest, and are preparing to join these efforts.