April 2013

Abstracts of the QSIT Lunch Seminar, April 4, 2013

Realization of a photonic micro-macro entangled state

Natalia Bruno, Group of Applied Physics, University of Geneva

co-authors: A. Martin, P. Sekatski, N. Sangouard, R. T. Thew and N. Gisin

Quantum theory has historically been presented as the theory describing the microscopic world. However, the technological progress of the last decades is now allowing us to experimentally test the theory in different regimes. Many questions can be raised at this point, e.g. whether quantum mechanics applies at any scale, and what is the meaning of “macroscopic”. Probing these questions using accessible tools like linear optics is a fascinating challenge.
We experimentally demonstrate the presence of entanglement in a system involving “macroscopic” states of light. The number of photons (order of 103) is large enough to be easily seen by the naked eye (provided that the wavelength is in the visible spectrum) and the two macro components of the state can be efficiently distinguished using “classical detectors”, i.e. detectors that only resolve large photon number differences. This makes the state analogous to an optical Schrödinger cat state. Starting by generating heralded single photon entanglement between two spatially separated optical modes, we subsequently displace one of these modes towards the macroscopic domain. A final displacement back to the single photon regime allows us to measure a well-defined entanglement witness and set a lower bound on the concurrence (a measure of entanglement)  [1-3].
Our results highlight the idea that although observing macroscopic entanglement with coarse-grained measurements is very challenging, the creation of quantum macro systems can be straightforward. This suggests that quantumness is a concept that extends into our macroscopic world and provides us with renewed motivation to look for quantum effects in Nature.

References:

[1]   P. Sekatski, N. Sangouard, M. Stobińska, F. Bussières, M. Afzelius, and N. Gisin, Phys. Rev. A,  86,  060301 (2012).
[2]   N. Bruno, A. Martin, P. Sekatski, N. Sangouard, R. T. Thew, and N. Gisin, arXiv:1212.3710 [quant-ph] (2012).
[3]   A. I. Lvovsky, R. Ghobadi, C. Simon, A. Chandra, A. S. Prasad, arXiv:1212.3713 [quant-ph] (2012).

Entanglement Polytopes: Multi-Particle Entanglement from Single-Particle Information

Michael Walter, Institute for Theoretical Physics, ETH Zurich

joint work with B. Doran (ETH), D. Gross (Freiburg), and M. Christandl (ETH)

Entangled many-body states are an essential resource for quantum computing and interferometry. Unfortunately, determining the type of entanglement present in a system quickly becomes impractical using existing methods which require access to an exponential number of parameters.
We show that in the case of pure multi-particle quantum states, significant features of the global entanglement can already be extracted from local information alone. This is achieved by associating to any given class of entanglement an entanglement polytope---a geometric object which characterizes the single-particle states compatible with that class. Our results, applicable to systems of arbitrary size and statistics, give rise to local witnesses for global pure-state entanglement, thus unveiling a fundamental aspect of entangled many-body systems.

Reference:

M. Walter, B. Doran, D. Gross, M. Christandl, arXiv:1208.036 (2012)