QUROPE Quantum Information Processing and Communication in Europe

J.P. Cotter (P5 IMPERIAL), poster, Measuring energy differences by BEC interferometry on a chip

R. Nyman (P5 IMPERIAL), seminar, An integrated photonic atom chip and minimally destructive detection of magnetically trapped atoms

R. Nyman (P5 IMPERIAL), colloquium, An integrated photonic atom chip and minimally destructive detection of magnetically trapped atoms

R. Nyman (P5 IMPERIAL), seminar, An integrated photonic atom chip and minimally destructive detection of magnetically trapped atoms

R. Nyman (P5 IMPERIAL), seminar, An integrated photonic atom chip and minimally destructive detection of magnetically trapped atoms

We present direct UV-written waveguides and Bragg gratings operating at 780 nm. By combining two gratings into a Fabry-Perot cavity we have devised and implemented a novel and practical method of measuring the group delay of Bragg gratings.

We investigate the use of integrated, microfabricated photonic-atomic junctions for quantum information processing applications. The coupling between atoms and light is enhanced by using microscopic optics without the need for cavity enhancement. Qubits that are collectively encoded in hyperfine states of small ensembles of optically trapped atoms, coupled via the Rydberg blockade mechanism, seem a particularly promising implementation. Fast and high-fidelity gate operations, efficient readout, long coherence times and large numbers of qubits are all possible.

We present a technique for atomic density measurements by the off-resonant phase shift induced on a two-frequency, coherently synthesized light beam. We have used this scheme to measure the column density of a magnetically trapped atom cloud and to monitor oscillations of the cloud in real time by making over a hundred non-destructive local density measurements. For measurements using pulses of 104–105 photons lasting ~10 μs, the precision is limited by statistics of the photons and the photodiode avalanche.

We investigate the operation of pyramidal magneto-optical traps (MOTs) microfabricated in silicon. Measurements of the loading and loss rates give insight into the role of the nearby surface in the MOT dynamics. Studies of the fluorescence versus laser frequency and intensity allow us to develop a simple theory of operation. The number of $^{85}$Rb atoms trapped in the pyramid is approximately $L^6$, where $L \lesssim 6$ is the size in mm. This follows quite naturally from the relation between capture velocity and size and differs from the $L^{3.6}$ often used to describe larger MOTs.

Atom chips are a promising candidate for a scalable architecture for quantum information processing provided a universal set of gates can be implemented with high fidelity. The difficult part in achieving universality is the entangling two-qubit gate. We consider a Rydberg phase gate for two atoms trapped on a chip and employ optimal control theory to find the shortest gate that still yields a reasonable gate error. Our parameters correspond to a situation where the Rydberg blockade regime is not yet reached.