## Friday, August 27, 2010

### ECDL with frequency modulation saturation spectroscopy laser lock

We have built an "ECDL laser" in the lab and are locking it succesfully to the potassium D2-line using "frequency modulation spectroscopy".

## Thursday, August 12, 2010

### An entangled-light-emitting diode

C. L. Salter, R. M. Stevenson, I. Farrer, C. A. Nicoll, D. A. Ritchie & A. J. Shields

An optical quantum computer, powerful enough to solve problems so far intractable using conventional digital logic, requires a large number of entangled photons1, 2. At present, entangled-light sources are optically driven with lasers3, 4, 5, 6, 7, which are impractical for quantum computing owing to the bulk and complexity of the optics required for large-scale applications. Parametric down-conversion is the most widely used source of entangled light, and has been used to implement non-destructive quantum logic gates8, 9. However, these sources are Poissonian4, 5 and probabilistically emit zero or multiple entangled photon pairs in most cycles, fundamentally limiting the success probability of quantum computational operations. These complications can be overcome by using an electrically driven on-demand source of entangled photon pairs10, but so far such a source has not been produced. Here we report the realization of an electrically driven source of entangled photon pairs, consisting of a quantum dot embedded in a semiconductor light-emitting diode (LED) structure. We show that the device emits entangled photon pairs under d.c. and a.c. injection, the latter achieving an entanglement fidelity of up to 0.82. Entangled light with such high fidelity is sufficient for application in quantum relays11, in core components of quantum computing such as teleportation12, 13, 14, and in entanglement swapping15, 16. The a.c. operation of the entangled-light-emitting diode (ELED) indicates its potential function as an on-demand source without the need for a complicated laser driving system; consequently, the ELED is at present the best source on which to base future scalable quantum information applications17.

## Monday, August 9, 2010

### Thermalization of photons

Jan Klaers1, Frank Vewinger1 & Martin Weitz1

Bose–Einstein condensation1, the macroscopic accumulation of bosonic particles in the energetic ground state below a critical temperature, has been demonstrated in several physical systems2, 3, 4, 5, 6, 7, 8. The perhaps best known example of a bosonic gas, blackbody radiation9, however exhibits no Bose–Einstein condensation at low temperatures10. Instead of collectively occupying the lowest energy mode, the photons disappear in the cavity walls when the temperature is lowered—corresponding to a vanishing chemical potential. Here we report on evidence for a thermalized two-dimensional photon gas with a freely adjustable chemical potential. Our experiment is based on a dye-filled optical microresonator, acting as a ‘white wall’ box for photons. Thermalization is achieved in a photon-number-conserving way by photon scattering off the dye molecules, and the cavity mirrors provide both an effective photon mass and a confining potential—key prerequisites for the Bose–Einstein condensation of photons. As a striking example of the unusual system properties, we demonstrate a yet unobserved light concentration effect into the centre of the confining potential, an effect with prospects for increasing the efficiency of diffuse solar light collection11.

### Deterministic entanglement of two neutral atoms via Rydberg blockade

X. L. Zhang, L. Isenhower, A. T. Gill, T. G. Walker, M. Saffman

We demonstrate the first deterministic entanglement of two individually addressed neutral atoms using a Rydberg blockade mediated controlled-NOT gate. Parity oscillation measurements reveal an entanglement fidelity of $F=0.58\pm0.04$, which is above the entanglement threshold of $F=0.5$, without any correction for atom loss, and $F=0.71\pm0.05$ after correcting for background collisional losses. The fidelity results are shown to be in good agreement with a detailed error model.