This PDF file contains the front matter associated with SPIE Proceedings Volume 7611, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The object is to summarise our understanding of the negative nitrogen-vacancy center in diamond and also to highlight difficulties with current models.

The understanding of the coherence properties of photons emitted from negatively charged nitrogen-vacancy (NV) centers in diamond is essential for the success of quantum information applications based on indistinguishable photons. Here we study both the polarization of photons emitted from and the linewidth of photons absorbed by single NV centers as a function of temperature T. We find that for T < 100 K the main dephasing mechanism contributing to the linewidth broadening is phonon-mediated population transfer between the two excited orbital states. The observed T⁵ temperature dependence of the population transfer rate and linewidth is experimental evidence of a dynamic Jahn-Teller effect in the excited states.

We introduce new figures of merit (FOM's) for resonant optical materials used in recording, storage, and processing of optically encoded information using coherent optical transients. The goal is to account for maximum coherence storage time as well as for efficiency of the light matter interaction quantified using the ratio between the rate of dephasing and the rate of spontaneous radiative decay. Highest FOM values are achieved when the dephasing rate approaches the fundamental limit set by spontaneous emission under the condition that the total transition oscillator strength is concentrated between a single pair of energy levels. In this case, the information (both classical and quantum) can be transferred from the radiation field to the storage medium and back at the fastest possible rate, while the loss of optically prepared coherence is minimized. We analyze FOM's of some of the most promising rare-earth-doped crystals at cryogenic temperatures and show that the homogeneous line width may approach the radiative limit in some cases even when the peak cross section remains below the fundamental limit.

The ability to manipulate the spin states of charges confined in quantum dots (QDs) is essential for the realization of a quantum computer based on such spins. Here, we present experimentally realized electron spin qubit gates in a single self-assembled InAs QD using a combination of picosecond optical pulses, spin precession about an external DC magnetic field and optically generated geometric phases. Arbitrary unitary operations on the electron spin qubit may be constructed using a combination of optical pulses and either spin precession or the optically generated geometric phases.

Spin-based quantum computing and magnetic resonance techniques rely on the ability to measure the coherence time, T₂, of a spin system. We report on the experimental implementation of all-optical spin echo to determine the T₂ time of a semiconductor electron-spin system. We use three ultrafast optical pulses to rotate spins an arbitrary angle and measure an echo signal as the time between pulses is lengthened. Unlike previous spin-echo techniques using microwaves, ultrafast optical pulses allow clean T₂ measurements of systems with dephasing times (T ^*₂ ) fast in comparison to the timescale for microwave control. We measure a 7 μs coherence time, which is similar to previous measurements in quantum dots and indicates that nuclear spin diffusion is the primary mechanism for decoherence. This demonstration is a critical step towards optical, dynamic decoupling, which can eliminate fast decoherence and can be integrated into quantum computer architectures based on opticallycontrolled spin qubits.

The phase coherence of a physical qubit is essential for quantum information processing, motivating fast control methods to preserve that phase. Ultrafast optical techniques allow complete spin control to be performed on a much faster timescale than microwave or electrical control (ps vs. ns at best). Using our ultrafast control techniques, we demonstrate our experimental approach towards a spin echo sequence on the spin of a single electron confined in a semiconductor quantum dot (QD), increasing the observed decoherence time of a single QD electron spin from nanoseconds to several microseconds. The ratio of the observed decoherence time to the demonstrated single-qubit gate time exceeds 10⁵, suggesting strong promise for future quantum information processors.

In this paper, we review some recent cavity quantum electrodynamic (CQED) experiments with single quantum dot exciton coupled to photonic crystal cavities, performed in our group. We show how the coupled quantum-dot/ cavity system can be used to modulate light with at a very fundamental level with very low power and discuss some applications of these low power modulators.

Semiconductor nanostructures such as quantum dots (QDs) have offered unique opportunities to investigate quantum optical effects in solid-state systems. These include quantum interference, Rabi oscillations, as well as photon antibunching, and were previously observable only in isolated atoms or ions. In addition, QDs can be integrated into optical microcavities, making them attractive for applications in quantum information processing and high efficiency quantum light sources. Despite much progress towards these goals, one area that was little explored is coherent control of such solid-state quantum emitters in cavities. The main technical hurdle lies in overcoming the laser background scattering. By using a sample structure in which QDs are embedded in a planar Fabry-Perot cavity and by using an orthogonal excitation geometry, we have achieved a nearly complete elimination of laser background scattering. This in turn allows us to show resonantly controlled light emission of quantum dots in the cavity including (a) Rabi flopping using pulse control, (b) direct observation of Mollow triplets in the frequency domain, and (c) simultaneously measured first-order and second order photon-photon correlations.

Single InAs quantum dots can be used to control the transmission function of photonic crystal cavities, as we have already shown for systems that operate both in strong and weak coupling regime. Here we present our most recent work on devices where the cavity is connected in a micron-scale optical network via multiple photonic crystal waveguides terminated with input and output optical couplers. This architecture allows for multiple signal and control beams to be coupled simultaneously in the cavity via distinct ports. The devices are equipped with two input ports where the waveguides are terminated with input grating-couplers that allow for coupling into the waveguide from an out-of-plane direction. A third waveguide coupled to the cavity is terminated with a different kind of grating out-coupler that allows for improved directional scattering of the light transmitted through the cavity. We have already shown in previous experiments with a single cavity with coupled quantum dots, that this system acts as a highly nonlinear medium that enables all optical switching at powers down to the single photon level. In our most recent experiments we take significant steps towards demonstrating that this switching can be done in integrated structures, as needed for optical signal processing devices for both classical and quantum information science.

The single photon character of nonclassical states of light that can be generated using photon blockade is analyzed for time domain operation. We show that improved single photon statistics (single photon around 85% with a multi-photon of 8%) can be obtained by adequately choosing the parameters (mainly amplitude and pulseduration) of the driving laser pulses. An alternative method, where the system is driven via a continuous wave laser and the frequency of the dipole is controlled (e.g. electrically) at very fast timescales is presented. We also show that this non-classical state performs better than a weak coherent pulse, when applied to BB84 quantum cryptography protocol.

We show, by using parity detection, that the phase estimate obtained from the linear error propagation formula, indeed saturates the lower bound set by maximizing quantum Fisher information for several of the well-known states, proposed for the Heisenberg limited interferometry. Our results show that one can achieve the quantum Cram´er-Rao bound, by simply counting the number of photons which is not very far from actual realization.