Principal Investigator: Professor Eugene Polzik
The Project
Quantum memories and Interfaces: Room temperature vapours
This project is aimed at demonstrating quantum memories based on
room-temperature atomic vapours and dispersive light-atoms interactions with a
quantum feedback.
Quantum teleportation is an important ingredient in distributed quantum
networks, and can also serve as an elementary operation in quantum computers.
The generic protocol of quantum teleportation is as follows. First, a pair of
entangled objects is created and shared by two parties, Alice and Bob. This step
establishes a quantum link between them. Next, Alice receives an object to be
teleported and performs a joint measurement on this object and her entangled
object (a Bell measurement). In this way she does not obtain any information
about the unknown state. Rather, this information is now distributed between the
measurement result and the quantum mechanical correlations in the entangled
pair. The result of the measurement is communicated via a classical
communication channel to Bob, who uses it to perform local operations on his
entangled object, thus completing the process of teleportation. Both the
classical and the quantum channels are crucial for the protocol since at no
point in the protocol information about the unknown state is entirely in one of
them. The teleportation distance is set by the distance by which the entangled
pair can be created.
Quantum teleportation between light and matter has been demonstrated by the
Copenhagen University team. It involves the dispersive off-resonant Faraday
interaction of a pulse of light with a spin polarized atomic ensemble placed in
a magnetic field at Bobs location. This interaction produces multimode
entanglement between light and atoms. The entangled pulse of light then travels
to Alice where it is mixed with the pulse to be teleported on a 50/50 beam
splitter. Bell measurements are then performed in the outputs of the beam
splitter and classical feedback signals sent to Bob who uses them to rotate the
atomic spins, thus completing the teleportation protocol (see also figure). The
protocol was implemented using few-photon coherent pulses of light and an
ensemble of Cs atoms in a paraffin coated cell at room temperature.
Quantum memories and Interfaces: Cold Atoms
The aim of this project is to develop an interface between light and
ensembles of cold caesium atoms in order to demonstrate so-called spin-squeezing
on the clock transition, and to store and recall light states with
higher-than-classical fidelity. The atomic player in this project is a laser
cooled sample of caesium atoms trapped in the focus of a strong laser beam. The
atoms are prepared in such away that each atom can be described as a quantum
mechanical superposition of only two internal states: the so-called clock
states.
The internal quantum state of the cold trapped atoms is manipulated using a
microwave field with a frequency in resonance with the clock transition and the
quantum state is detected using dispersive interaction with laser light.
Depending on the atomic state, the probing laser light will see a positive or
negative phase shift. This phase shift is measured using a Mach-Zehnder
interferometer, where the atomic ensemble is located in one of two separated
arms.
If all the atoms of the atomic ensemble are prepared independently in an equal
superposition of the clock states a measurement of the pseudo spin projection
will be subjected to so-called projection noise. This noise is a result of the
atomic system being quantum mechanical. It can be shown that the mere of fact of
looking at the atoms with the dispersive probing will generate a non-classical
spin-squeezed state conditioned on the measurement outcome. The atomic spin
projection will be defined beyond the projection noise limit for an ensemble of
individual atoms. This is a result of the probe light introducing non-classical
correlations (entanglement) in the ensemble.
The non-classical atomic states produced from the quantum non-demolition
interaction described above will serve as a starting point for quantum state
engineering.
List of Publications
QAP
J. S. Neergaard-Nielsen, B. Melholt Nielsen, C. Hettich et al, Generation of a superposition of odd photon number states for quantum information networks., Phys. Rev. Lett. 97 083604 (2006)
J. F. Sherson, H. Krauter, R. K. Olsson et al, Quantum teleportation between light and matter., Nature 443 557 (2006)
P. G. Petrov et al., Nondestructive interferometric characterization of an optical dipole trap, Phys. Rev. A 75 033803 (2007) quant-ph/0610107
P. J. Windpassinger, D. Oblak, U. B. Hoff et al, Inhomogeneous light shift effects on atomic quantum state evolution in non-destructive measurements, New J. Phys. 10 053032 (2008)
P. J. Windpassinger, D. Oblak, P. G. Petrov et al, Nondestructive Probing of Rabi Oscillations on the Cesium Clock Transition near the Standard Quantum Limit, Phys. Rev. Lett. 100 103601 (2008)
Related work
S. R. de Echaniz, M. W. Mitchell, M. Kubasik, M. Koschorreck, H. Crepaz, J. Eschner and E. S. Polzik, Conditions for spin squeezing in a cold87Rb ensemble, J. Opt. B 7 S548 (2005)
K. Hammerer, E. S. Polzik, J. I. Cirac., Teleportation and spin squeezing utilizing multimode entanglement of light with atoms., Phys. Rev. A 72 052313 (2005)
K. Hammerer, M. M. Wolf, E. S. Polzik, J. I. Cirac,, Quantum benchmark for storage and transmission of coherent states, Phys. Rev. Lett. 94 150503 (2005) quant-ph/0409109
D. V. Kupriyanov, O. Mishina, I. M. Sokolov et al, Multimode entanglement of light and atomic ensembles via off-resonant coherent forward scattering., Phys. Rev. A 71 032348 (2005)
J. Sherson, J. Fiuráek, K. Mølmer et al, Quantum storage and retrieval of a light qubit with atomic ensembles., Phys. Rev. A 74 011802 (2006)