Henk Snijders

Homodyne detection of coherence and phase shift of a quantum dot in a cavity.

A homodyne measurement technique is demonstrated that enables direct observation of the coherence and phase of light that passed through a coupled quantum dot (QD)-microcavity system, which in turn enables clear identification of coherent and incoherent QD transitions. As an example, we study the effect of power-induced decoherence, where the QD transition saturates and incoherent emission from the excited state dominates at higher power. Further, we show that the same technique allows measurement of the quantum phase shift induced by a single QD in the cavity, which is strongly enhanced by cavity quantum electrodynamics effects.

Observation of the Unconventional Photon Blockade

We observe the unconventional photon blockade effect in quantum dot cavity QED, which, in contrast to the conventional photon blockade, operates in the weak coupling regime. A single quantum dot transition is simultaneously coupled to two orthogonally polarized optical cavity modes, and by careful tuning of the input and output state of polarization, the unconventional photon blockade effect is observed. We find a minimum second-order correlation g^{(2)}(0)≈0.37, which corresponds to g^{(2)}(0)≈0.005 when corrected for detector jitter, and observe the expected polarization dependency and photon bunching and antibunching; close by in parameter space, which indicates the abrupt change from phase to amplitude squeezing.H.J. Snijders,1 J. A. Frey,2 J. Norman,3 H. Flayac,4 V. Savona,4 A. C. Gossard,3 J. E. Bowers,3 M. P. van Exter,1 D. Bouwmeester,1, 2 and W. Löffler1 1Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands 2Department of Physics, University of California, Santa Barbara, California 93106, USA 3Department of Electrical & Computer Engineering, University of California, Santa Barbara, California 93106, USA 4Institute of Physics iPHYS, École Polytechnique Fédérale de Lausanne EPFL, CH-1015 Lausanne, Switzerland

Purification of a single-photon nonlinearity

Single photon nonlinearities based on a semiconductor quantum dot in an optical microcavity are a promising candidate for integrated optical quantum information processing nodes. In practice, however, the finite quantum dot lifetime and cavity-quantum dot coupling lead to reduced fidelity. Here we show that, with a nearly polarization degenerate microcavity in the weak coupling regime, polarization pre- and postselection can be used to restore high fidelity. The two orthogonally polarized transmission amplitudes interfere at the output polarizer; for special polarization angles, which depend only on the device cooperativity, this enables cancellation of light that did not interact with the quantum dot. With this, we can transform incident coherent light into a stream of strongly correlated photons with a second-order correlation value up to 40, larger than previous experimental results, even in the strong-coupling regime. This purification technique might also be useful to improve the fidelity of quantum dot based logic gates.

Monitoring the formation of oxide apertures in micropillar cavities

An imaging technique is presented that enables monitoring of the wet thermal oxidation of a thin AlAs layer embedded between two distributed Bragg reflector mirrors in a micropillar. After oxidation we confirm by white light reflection spectroscopy that high quality optical modes confined to a small volume have been formed. The combination of these two optical techniques provides a reliable and efficient way of producing oxide apertured micropillar cavities for which the wet thermal oxidation is a critical fabrication step.

Fine tuning of micropillar cavity modes through repetitive oxidations

Repetitive wet thermal oxidations of a tapered oxide aperture in a micropillar structure are demonstrated. After each oxidation step the confined optical modes are analyzed at room temperature. Three regimes are identified. First, the optical confinement increases when the aperture oxidizes toward the center. Then, the cavity modes shift by more than 30 nm when the taper starts to oxidize through the center, leading to a decrease in the optical path length. Finally, the resonance frequency levels off when the aperture is oxidized all the way through the micropillar, but confined optical modes with a high quality factor remain. This repetitive oxidation technique therefore enables precise control of the optical cavity volume or wavelength.

Polarization tuning for high-fidelity fiber-coupled single photon sources

The performance of single photon sources based on single quantum dot emitters coupled to microcavities is analyzed with respect to different conditions of polarization. Electro-optic tuning is shown as a method to tune microcavities with distributed Bragg reflector mirrors into polarization degeneracy. Typically, for large cavity polarization splitting, excitation in the linearly polarized cavity modes is the only viable method for resonantly driving a single photon source. However, polarization degenerate cavities allow for arbitrary polarization conditions. A semi-classical model is used to analyze the performance of single photon sources under different polarization conditions. Further, the effect of residual cavity polarization splitting is analyzed under pulsed excitation.

A fiber coupled source of identical single photons

High quality sources of single and identical (indistinguishable) photons are essential for photonic quantum information processing including boson sampling, cluster-state quantum computing, and optical quantum repeaters. To enable the use of several single photon sources in parallel and to allow the implementation of complex photonic networks, it is essential that the single photon source can be integrated easily and efficiently with optical fiber technology. Here we show a first all-solid-state InGaAs quantum dot based, integrated cavity-QED enhanced, single photon source. The source is operated by resonant optical pumping through an excitation fiber, and the single photons are collected in the fiber attached to the transmission channel of the cavity.