Quantum Optics with Cold Atomic Ensembles

Cold atomic ensembles are currently one of the most advanced systems for the quantum control of light matter interaction at the single excitation level.  Single collective spin excitations (so called super atoms) can be created in a heralded fashion and efficiently transferred into a single photon field, thanks to a collective interference between all the emitters. We are investigating various protocols to create, manipulate and transfer to light single collective spin excitations.

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Research Details

Manipulating single collective spin excitations

rephasing_webSingle collective spin excitations (also known as spin waves) are objects relying on the concept of quantum superposition. They consist of an ensemble of atoms with a single spin excitation delocalized over the whole ensemble. This forms an entangled state of many particles which is sometimes also known as a superatom. Apart from their fundamental interest, spin waves are also essential in the field of quantum information because they are the common form in which quantum bits are stored in ensemble based photonic quantum memories. Their good point, is that they can be efficiently converted into single photons, thanks to constructive interference between all the atomic emitters.

In this experiment, we controlled the phase of single spin waves using a magnetic gradient. The obtained results represent an example of the wave-particle duality applied to these objects. In addition, we showed that this manipulation can enable the operation of a photon pair source quantum memory in a temporally multimode fashion. This was demonstrated by creating photon – spin wave pairs in two different temporal modes, and subsequently converting the excitations into other photonic modes also separated in time.

B. Albrecht, P. Farrera, G. Heinze, M. Cristiani and H. de Riedmatten Phys. Rev. Lett. 115, 160501 (2015)

Frequency conversion of non-classical light emitted by a cold atomic quantum memory

QFCOptical quantum memories usually emit photons in the visible-near IR region of the electromagnetic spectrum. Translating back and forth the wavelength of these photons towards the telecom window around 1550nm can be useful for two reasons. First, light at telecom wavelengths exhibits the lowest possible losses in optical fibers and second, it allows to connect quantum memories of different atomic species, dealing with light at different wavelengths.

In this experiment we changed the frequency of the photons emitted by a quantum memory based on laser-cooled Rubidium atoms from 780nm to 1552nm. The conversion process applied difference frequency generation, and took place in a non-linear waveguide. Thanks to the high conversion efficiency and the low noise of the quantum frequency conversion device, we managed to show that the non-classical behaviour of the light emitted by the quantum memory is preserved after the frequency conversion of the photons. This is an important step towards the connection of different quantum memories through telecommunication channels.

B. Albrecht, P. Farrera, X. Fernandez Gonzalvo, M. Cristiani and H. de Riedmatten Nature Commun. 5,3376 (2014)



  • ERC starting grant