on 27-11-2015
Leonardo Novo (Physics of Information Group, IT & IST, ULisbon)
27/11/2015, 14:00
Room P9, Mathematics Building, IST
There is great interest in designing photonic devices capable of disorder-resistant transport and information processing. In this work we propose to exploit 3D integrated photonic circuits in order to realize 2D discrete-time quantum walks in a background synthetic gauge field, for both the single and many walker case. The gauge fields are generated by introducing the appropriate phase shifts between waveguides. Polarization-independent phase shifts lead to an Abelian or magnetic field, a case we describe in detail. We find that, in the disordered case, the magnetic field enhances transport due to the presence of topologically protected chiral edge states which do not localize. Polarization-dependent phase shifts lead to effective non-Abelian gauge fields, which could be adopted to realize Rashba-like quantum walks with spin-orbit coupling. Our work introduces a flexible platform for the experimental study of multi-particle quantum walks in the presence o f synthetic gauge fields, which paves the way towards topologically robust transport of many-body states of photons.
Quantum Computation and Information Seminar
http://math.tecnico.ulisboa.pt/seminars/qci/index.php.en?action=next
Support: Phys-Info (IT), SQIG (IT), CFIF and CAMGSD, with support from FCT, FEDER and EU FP7, namely via the Doctoral Programme in the Physics and Mathematics of Information (DP-PMI), and projects PEst-OE/EEI/LA0008/2013, QuSim, QUTE-EUROPE (GA 600788), Landauer (318287) and PAPETS (323901).
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on 06-11-2015
Eugene Polzik, Niels Bohr Institute, University of Copenhagen
06/11/2015, 14:00
Room P9, Mathematics Building, IST
Continuous observation of an oscillator results in quantum back-action, which limits the knowledge acquired by the measurement. A careful balance between the information obtained and the back-action disturbance leads to the standard quantum limit of precision. This limit can be surpassed by a measurement with strength modulated at twice the oscillator frequency, resulting in a squeezed state of the oscillator motion, as proposed decades ago by Braginsky and colleagues. We have recently implemented such a measurement experimentally using a collective spin of an atomic ensemble precessing in a magnetic field as an oscillator [1]. The oscillator initially prepared nearly in the ground state is stroboscopically coupled to an optical mode of a cavity. A measurement of the output light results in a 2.2 dB squeezed state of the oscillator. The demonstrated spin-squeezed state of 108 atoms with an angular spin variance of 8 10-10 rad2 allowed us to achieve 150 femtoT/ Hz magnetic field sensitivity.
An even more striking notion is the back-action evasion in both quadratures for the continuous measurement on a mechanical oscillator entangled with the atomic spin oscillator [2]. Such measurement does not violate quantum mechanics, but still provides a way to detect disturbances on the oscillator trajectory which are way below those set by the standard Heisenberg uncertainty bound by using a negative mass reference system [3]. Experimental demonstration of this ABATE (Atomic Back-action Eater) principle will be discussed.
Support: Phys-Info (IT), SQIG (IT), CFIF and CAMGSD, with support from FCT, FEDER and EU FP7, namely via the Doctoral Programme in the Physics and Mathematics of Information (DP-PMI), and projects PEst-OE/EEI/LA0008/2013, QuSim, QUTE-EUROPE (GA 600788), Landauer (318287) and PAPETS (323901).
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