**Towards unambiguous quantum state discrimination in an optical memory**

Towards unambiguous quantum state discrimination in an optical memory
Ahdiyeh Delfan*, Cecilia La Mela, Wolfgang Tittel
Institute for Quantum Information Science, University of Calgary,
2500 University Drive, NW Calgary, Alberta, Canada, T2N 1N4
The possibility to store and recall information encoded into quantum states of light is at the heart of many
applications of quantum information processing. For example such a quantum memory is a basic component of a
quantum repeater [1] which allows realization of quantum cryptography [2] over distances longer than 100km. A
recently proposed, photon echo related approach to quantum state storage in atomic ensembles employs controlled
reversible inhomogeneous broadening (CRIB) [3]. Beyond storage, a modified version of CRIB allows quantum
state manipulations including single qubit rotations [4, 5]. As the implementation of CRIB is currently still
challenging [6], we investigate quantum state transformation using stimulated photon echoes with the example of a
POVM (positive operator valued measure) [7].
Nonorthogonal state discrimination based on POVMs has become an interesting problem in quantum information
processing from a fundamental [7] as well as applied [8] point of view. In this kind of measurement a set of
nonorthogonal states is mapped onto a set of orthogonal ones by a non unitary transformation.
Experimental realization of POVMs is challenging as it is usually based on an optical interferometeric set up, where
stabilization is the main problem. We propose a novel implementation of POVMs that relies on quantum state
rotations based on stimulated photon echoes, which has been shown to allow a very robust implementation of
interferometeric, standard projection measurements [9]. Considering the preservation of the relative phase and
amplitude of the optical pulses, the desired interference is achieved by properly choosing the amplitude, phase and
the timing of multiple read pulses.
We will present simulations based on numerically solving Maxwell Bloch equations in an inhomogenously
broadened medium, and discuss the current experimental status.
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[2]N. Gisin et al,Rev. Mod. Phys,74,145(2002)
[3]S. A. Moiseev and S. Kroll, Phys Rev Lett. 87, 173601(2001)
[4]See also contributions by S. A. Moiseev et al.
[5]See also contributions by M. Underwood et al.
[6]A. L. Alexander et al, Phys. Rev.Lett.96, 043602(2006)
[7]Y. Sun et al, Phys. Rev A.64, 022311(2001)
[8]V. Scarani et al, Phys. Rev.Lett.92, 057901(2004)
[9]M. U. Staudt et al, Phys. Rev. Lett. 98, 113601 (2007)
* adelfan@