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We describe an effect called Dipole Induced Transparency which enables a dipole emitter to strongly modify the cavity spectrum, even in the weak coupling regime. We then describe a method for generating entanglement and performing a full Bell measurement between two QDs using Dipole Induced Transparency. Finally, we show how DIT enables entanglement between QDs with vastly different radiative properties. The proposal is shown to be robust to cavity resonance mismatch.

We describe a method for generating entanglement between two spatially separated dipoles coupled to optical micro-cavities. The protocol works even when the dipoles have different resonant frequencies and radiative lifetimes. This method is particularly important for solid-state emitters, such as quantum dots, which suffer from large inhomogeneous broadening. We show that high fidelities can be obtained over a large dipole detuning range without significant loss of efficiency. We analyze the impact of higher order photon number states and cavity resonance mismatch on the performance of the protocol.

Coupling of photonic crystal (PC) linear three-hole defect cavities (L3) to PC waveguides is theoretically and experimentally investigated. The systems are designed to increase the overlap between the evanescent cavity field and the waveguide mode, and to operate in the linear dispersion region of the waveguide. Our simulations indicate increased coupling when the cavity is tilted by 60 degrees with respect to the waveguide axis, which we have also confirmed by experiments. We obtained up to 90% coupling efficiency into the waveguide.

A differential phase shift quantum key distribution experiment employing up-conversion single-photon detectors is presented. We achieved a 1-Mbit/s sifted key rate over a 20-km optical fiber, and distributed keys secure against general individual attacks over 75 km.

An inverse problem approach to designing photonic crystal cavities is reviewed. The theoretical and experimental work on integration of a number of photonic crystal components (cavities and waveguides) into functional circuits for quantum information processing, including nontrivial two-qubit quantum gates is also reviewed.

We calculate the dispersive properties of a single-sided cavity coupled to a single dipole. We show that when a field is resonant with the dipole and the Purcell factor exceeds the bare cavity reflection coefficient, the field experiences a phase shift relative to reflection from a bare cavity. We then show that optically Stark shifting the dipole resonance with a second field creates large Kerr nonlinearities. An approximate expression for the total number of photons needed to create a phase shift is derived.

We use photon counters to obtain the joint photon counting statistics from twin-beam non-degenerate parametric down conversion, and we demonstrate directly, and with no auxiliary assumptions, that these twin beams are nonclassical.

A differential-phase-shift quantum key distribution experiment employing up-conversion single-photon detectors is presented. We achieved a 1-Mbit/s sifted key rate over a 20-km optical fiber, and distributed keys secure against general individual attacks over 75 km.

We have demonstrated classical and quantum information processing devices enabled by strong light-matter interaction in photonic crystals, including quantum dot-photonic crystal single photon source, ultrafast photonic crystal nanocavity laser, and small quantum network on chip.

We show that a waveguide which is opaque due to interaction with a drop-filter cavity can be made transparent when the cavity is also coupled to a dipole. A transparency condition is derived.

We show that a waveguide that is normally opaque due to interaction with a drop-filter cavity can be made transparent when the drop filter is also coupled to a dipole. A transparency condition is derived between the cavity lifetime and vacuum Rabi frequency of the dipole. This condition is much weaker than strong coupling, and amounts to simply achieving large Purcell factors. Thus, we can observe transparency in the weak coupling regime. We describe how this effect can be useful for designing quantum repeaters for long distance quantum communication.

We report on single photon sources produced from photonic crystal - coupled InAs Quantum Dots (QDs). We observe large spontaneous emission rate modification of individual InAs Quantum Dots (QDs) in modified single defect cavities with large quality factor (Q). Compared to QDs in bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase by up to a factor of 8. In contrast, off-resonant QDs indicate up to five-fold rate quenching as the local density of optical states (LDOS) is diminished in the photonic crystal. In both cases we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using Finite Difference Time Domain (FDTD) simulations and find good agreement with experiment. High multiphoton suppression is achieved by resonant excitation. Finally, we discuss fabrication improvements based on FDTD analysis of already fabricated structures.

Single-photon sources rarely emit two or more photons in the same pulse, compared to a Poisson-distributed source of the same intensity, and have numerous applications in quantum information science. The quality of such a source is evaluated based on three criteria: high efficiency, small multi-photon probability, and quantum indistinguishability. We have demonstrated a single-photon source based on a quantum dot in a micropost microcavity that exhibits a large Purcell factor together with a small multi-photon probability. For a quantum dot on resonance with the cavity, the spontaneous emission rate has been increased by a factor of five, while the probability to emit two or more photons in the same pulse has been reduced to 2% compared to a Poisson-distributed source of the same intensity. The indistinguishability of emitted single photons from one of our devices has been tested through a Hong-Ou-Mandel-type two-photon interference experiment; consecutive photons emitted from such a source have been largely indistinguishable, with a mean wave-packet overlap as large as 0.81. We have also designed and demonstrated two-dimensional photonic crystal GaAs cavities containing InAs quantum dots that exhibit much higher quality factors together with much smaller mode volumes than microposts, and therefore present an ideal platform for construction of single photon sources of even higher quality.

We observe large spontaneous emission rate modification of individual InAs quantum dots (QDs) in a 2D photonic crystal with a modified, high-$Q$ single-defect cavity. Compared to QDs in a bulk semiconductor, QDs that are resonant with the cavity show an emission rate increase of up to a factor of 8. In contrast, off-resonant QDs indicate up to fivefold rate quenching as the local density of optical states is diminished in the photonic crystal. In both cases, we demonstrate photon antibunching, showing that the structure represents an on-demand single photon source with a pulse duration from 210 ps to 8 ns. We explain the suppression of QD emission rate using finite difference time domain simulations and find good agreement with experiment.

We calculate the dispersive properties of the reflected field from a cavity coupled to a single dipole. We show that when a field is resonant with the dipole it experiences a 90 degree phase shift relative to reflection from a bare cavity if the Purcell factor exceeds the bare cavity reflectivity. We then show that optically Stark shifting the dipole with a second field can be used to achieve giant Kerr non-linearites. It is shown that currently achievable cavity lifetimes and cavity quality factors can allow a single emitter in the cavity to impose a nonlinear $\pi$ phase shift at the single photon level.

We derive a proof of security for the Differential Phase Shift Quantum Key Distribution (DPSQKD) protocol under the assumption that Eve is restricted to individual attacks. The security proof is derived by bounding the average collision probability, which leads directly to a bound on Eve's mutual information on the final key. The security proof applies to realistic sources based on pulsed coherent light. We then compare individual attacks to sequential attacks and show that individual attacks are more powerful.

Quantum cryptography is a method to exchange secret messages with unconditional security over a potentially hostile environment using single photons. Previous implementations of quantum cryptography have relied on highly attenuated laser light to approximate single photo states. Such sources are vulnerable to eavesdropping attacks based on photon splitting. Here we present an experimental demonstration of quantum cryptography using a single photon source based on Indium Arsenide quantum dots. We achieve a communication rate of 25kbits/s. This source allows secure communication over a quantum channel with up to 28dB of channel loss, as opposed to only 23dB for an attenuated laser.

A high efficiency, triggered single photon source with applications to quantum communications is discussed. The sources is formed from an InAs-based quantum dot located in the center of a micropost cavity formed from GaAs, with top and bottom GaAs/AlAs distributive Bragg reflector pairs, and lateral processing. When pumped above band into the semiconductor host, correlation measurements show a reduction in the two-photon probability to 0.14, compared to unity for a Poisson source. The external efficiency of this structure is 0.24.

Differential-phase-shift quantum key distribution based on two nonorthogonal states is described. A weak coherent pulse train is sent from Alice to Bob, in which the phase of each pulse is randomly modulated by ${0,\ensuremath{\pi}}.$ Bob measures the differential phase by a one-bit delay circuit. The system has a simple configuration without the need for an interferometer and a bright reference pulse in Alice's site, unlike the conventional QKD system based on two nonorthogonal states, and has an advantage of improved communication efficiency. The principle of the operation is successfully demonstrated in experiments.

Photon correlation measurements reveal memory effects in the optical emission of single InAs quantum dots with timescales from 10 to 800 ns. With above-band optical excitation, a long-timescale negative correlation (antibunching) is observed, while with quasi-resonant excitation, a positive correlation (blinking) is observed. A simple model based on long-lived charged states is presented that approximately explains the observed behavior, providing insight into the excitation process. Such memory effects can limit the internal efficiency of light emitters based on single quantum dots, and could also be problematic for proposed quantum-computation schemes., 8 pages, 8 figures

We employ a high quantum efficiency photon number counter to determine the photon number distribution of the output field from a parametric downconverter. The raw photocount data directly demonstrates that the source is nonclassical by forty standard deviations, and correcting for the quantum efficiency yields a direct observation of oscillations in the photon number distribution.

A quantum key distribution scheme is described, in which a time (or phase) entangled-photon pair posteriorly generated by a single-photon source and a simple beam splitter. This scheme offers an efficient entanglement-based quantum key distribution system superior to the system using a standard parametric down-converter.

The security of quantum key distribution with sub-Poisson light sources is investigated. It is shown that a quantitative analysis of the security of such sources requires only two measured values, the efficiency and second-order correlation. These two numbers represent figures of merit, which characterize the performance of such light sources. We show that sub-Poisson light sources can offer significant improvements in communication rate over Poisson light in the presence of realistic experimental imperfections. We also investigate the amount of channel loss that can be tolerated for secure communication to be possible, and show that this only depends on the second-order correlation, provided the device efficiency exceeds a critical value. If this critical efficiency is exceeded, an inefficient source can perform as well as an efficient one at sufficiently high channel losses.

This paper presents novel quantum key distribution (QKD) schemes that use differential phases of sequential pulses as an information carrier. Alice sends a photon in a linear superposition state of three temporal slots, in which each amplitude is phase-modulated by {0,π}. Bob measures the phase difference of an asymmetric interferometer, and tells Alice time-instance at which the photon was counted. From this time information and her modulation data, Alice knows which detector counted the photon in Bob s site. Then, the two parties create a secret key. The scheme is suitable for fiber transmission systems and offers key creation efficiency higher than conventional phase-encoding QKD. The above scheme utilizes fully non-orthogonal four states. Differential phase shift QKD utilizing two non-orthogonal states is also presented. Alice sends a coherent pulse train with less than one photon per pulse, which is phase modulated by {0,π} for each pulse, and Bob measures the pulse train by a one-bit delay circuit. The system has a simple configuration such that Alice has no interferometer and sends no intense reference pulse, unlike to conventional QKD scheme using two non-orthogonal states, and also has an advantage of high key creation efficiency.

A novel quantum cryptography scheme is proposed, in which a single photon is prepared in a linear superposition state of three basis kets. A photon split to three pulses is sent from Alice to Bob, where the phase difference between sequential two pulses carries bit information. Bob measures the phase difference by passive differential phase detection. This scheme is suitable for fiber transmission systems and offers a key creation efficiency higher than conventional fiber-based BB84.

The emerging discipline of quantum plasmonics, which promises new applications and approaches in quantum information systems, formed the topic of a recent OSA Incubator Meeting.

Security of the Ekert protocol is proven against individual attacks where an eavesdropper is allowed to share any density matrix with the two communicating parties. The density matrix spans all of the photon number states of both receivers, as well as a probe state of arbitrary dimensionality belonging to the eavesdropper. Using this general eavesdropping strategy, we show that the Shannon information on the final key, after error correction and privacy amplification, can be made exponentially small. This is done by finding a bound on the eavesdropper's average collision probability. We find that the average collision probability for the Ekert protocol is the same as that of the BB84 protocol for single photons, indicating that there is no analog in the Ekert protocol to photon splitting attacks. We then compare the communication rate of both protocols as a function of distance, and show that the Ekert protocol has potential for much longer communication distances, up to 170km, in the presence of realistic detector dark counts and channel loss. Finally, we propose a slightly more complicated scheme based on entanglement swapping that can lead to even longer distances of communication. The limiting factor in this new scheme is the fiber loss, which imposes very slow communication rates at longer distances., Comment: 28 pages

Using the process of spontaneous parametric down conversion in a novel two-crystal geometry, one can generate a source of polarization-entangled photon pairs which is orders of magnitude brighter than previous sources. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source intensity, we obtained a 242-$\sigma$ violation of Bell's inequalities in less than three minutes., Comment: 4 pages, 5 encapsulated Postscript figures. To appear in Physical Review A (Rapid Communication)

We demonstrate spontaneous emission rate enhancement and saturable absorption of cadmium selenide colloidal quantum dots coupled to a nanobeam photonic crystal cavity. We perform time-resolved lifetime measurements and observe an average enhancement of 4.6 for the spontaneous emission rate of quantum dots located at the cavity as compared to those located on an unpatterned surface. We also demonstrate that the cavity linewidth narrows with increasing pump intensity due to quantum dot saturable absorption.

Describes a computational simulator for use in cardiac imaging using tagged magnetic resonance imaging. The simulator incorporates a 13-parameter model of left-ventricular motion due to Arts et al. (1992) and applies it to a confocal prolate spherical shell, resembling the shape of the left ventricle. Using parameters determined in other work, our model can be made to assume a configuration representing one of 60 phases in the cardiac cycle. In this paper we determine the inverse motion map analytically, allowing pointwise correspondences to be made between two points at any two times. Using this mathematical relationship, we simulate the (tagged) magnetic resonance imaging process using a standard (tagged) spin-echo imaging equation. Image sequences can be synthesized at arbitrary orientations at any phase. We currently synthesize a SPAMM tag pattern with arbitrary spatial frequency, but other patterns can be readily incorporated. To accommodate two-dimensional motion estimation algorithms, we have created a two-dimensional simulator which restricts the three-dimensional motion to two dimensions. In either two or three dimensions, a true motion is output so that motion estimation algorithms can be compared against the truth. We conclude with a simple demonstration of the performance of the simulator.

We demonstrate large cavity-enhanced optical Stark shifts for a single quantum dot (QD) coupled to a photonic crystal cavity. A maximum Stark shift of 20 GHz is observed for a QD detuned by 104 GHz from the cavity mode. These Stark shifts are attained with extremely low cavity field energies of only ten photons. The changes in the QD emission wavelength are monitored via nonresonant transfer between the QD and cavity mode. Experimental results are compared to theoretical predictions based on the solution to the full master equation and found to be in excellent agreement.

We apply magnetic fields of up to 7 T to an indium arsenide quantum dot (QD) strongly coupled to a photonic crystal cavity. The field lifts the degeneracy of QD exciton spin states, and tune their emission energy by a combination of diamagnetic and Zeeman energy shifts. We use magnetic field tuning to shift the energies of the two exciton spin states to be selectively on resonance with the cavity. Strong coupling between the cavity and both states is observed. Magnetic field tuning enables energy shifts as large as 0.83 meV without significant degradation of the QD-cavity coupling strength.

We demonstrate reversible tuning of a photonic crystal cavity resonance using a thin photochromic film composed of spiropyran and polymethylmethacrylate that serves as a photosensitive cladding layer. Exposure of spiropyran to ultraviolet light results in smooth redshift of the cavity resonance that can be reversed by exposure to visible wavelength light. We achieve a reversible resonance shift of up to 2.7 nm, which can be performed locally on individual cavities. The resonance shift over multiple successive UV and visible light exposures is studied to determine the repeatability of the photochromic film.

This paper introduces the current state of the development of single photon sources that will play a central role in future quantum information systems. By optically pumping a system consisting of a semiconductor single quantum dot confined in a monolithic microcavity, it is possible to produce a single photon pulse stream at the Fourier transform limit with high efficiency and with a high repetition speed. It is expected that this technique will not only prove to be useful for BB84 quantum cryptography using single photons, but will also find applications in other fields such as BBM92 quantum cryptography using entangled photon pairs, quantum teleportation, quantum repeaters, and linear optical quantum computers.

Quantum cryptography generates unbreakable cryptographic codes by encoding information using single photons, which until now have relied on highly attenuated lasers as sources1,2. But these sources can create pulses that contain more than one photon, making them vulnerable to eavesdropping by photon splitting3,4. Here we present an experimental demonstration of quantum cryptography that uses a photon turnstile device, which is more reliable for delivering photons one at a time. This device allows completely secure communication in circumstances under which this would be impossible with an attenuated laser.

Preselected single photon sources are positioned and immobilized to nanometer precision using flow control and local polymerization. This technique could find important applications in integration of single photon sources with nanophotonic structures for quantum devices.

We demonstrate fast (up to 20 GHz), low power (5 $\mu W$) modulation of photonic crystal (PC) cavities in GaAs containing InAs quantum dots. Rapid modulation through blue-shifting of the cavity resonance is achieved via free carrier injection by an above-band picosecond laser pulse. Slow tuning by several linewidths due to laser-induced heating is also demonstrated.

Photonic crystal (PC) cavities enable localization of light into volumes (V) below a cubic optical wavelength (smaller than any other types of optical resonators) with high quality (Q) factors [1]. This permits a strong interaction of light and matter, which is relevant for construction of classical light sources with improved properties (e.g., low threshold lasers) and of nonclassical light sources (such as single and entangled photon sources), which are crucial pieces of hardware of quantum information processing systems. This talk will cover some of our recent experimental results on quantum and classical devices enabled by such interaction [2-4], as well as our work on designing such devices and circuits efficiently [5-7]. We have demonstrated a spontaneous emission rate enhancement by a factor of 8 and suppression by a factor of 5 for a single self-assembled InAs/GaAs quantum dot (QD) embedded in a GaAs photonic crystal cavity and on- and off-resonance with the cavity mode, respectively [2]. A strong localization of optical field in such a nanocavity (experimental Q-factor of 5000 and mode volume below a cubic optical wavelength) with a quantum dot embedded inside is of importance for building single photon sources with improved efficiency, photon indistinguishability, and repetition rate. We have demonstrated a single photon source on demand based on the pulsed excitation of a single quantum dot in such a nanocavity, with pulse duration between 200 ps and 8ns and with a small multi-photon probability (as small as 5% compared to an attenuated laser of the same intensity) [2]. In addition, we have shown that colloidal PbS quantum dots coupled to AlGaAs photonic crystal cavities can be used as an alternative to self-assembled InAs/GaAs quantum dots for construction of cheap and reusable quantum and classical light emitters [3]. We have also demonstrated an improved classical light source - laser, based on coupling of a large number (81) of photonic crystal nanocavities inside a two dimensional array. Such a laser exhibits a low lasing threshold (similar to 2.5 mW), operates in a single mode, produces large output powers (greater than 12 mu W, which two orders of magnitude larger than a single nanocavity laser), and can be directly modulated as speeds greater than 100 GHz [4]. An inverse problem approach to designing photonic crystal cavities that we have developed enables their rapid optimization in a single step, thereby reducing the cavity optimization time from weeks to hours [5]. We are also pursuing theoretical and experimental work on integration of a number of photonic crystal components (cavities and waveguides) into functional circuits for classical and quantum information processing, including nontrivial two-qubit quantum gates [6-7].

We demonstrate coupling between two photonic crystal cavities via a row defect waveguide. A coupled mode theory is then introduced to better describe cavity waveguide coupling near the waveguide band edge.

We design efficient photonic crystal (PC) cavity-waveguide couplers using the Fourier-space optimization. This system is a fundamental building block for integrated photonic circuits with direct applications in quantum information processing.

We experimentally realize a photonic crystal microcavity-quantum dot system where a quantum dot is simultaneously coupled to evanescently coupled optical cavities with modes spectrally separated by 2.4 nm.

We describe our work on engineering nonlinear optical devices using quantum dots coupled to optical resonators. These systems enable large optical Stark shifts and all optical switching with only a few photons.

We will discuss our experimental and theoretical work on a number of solid-state quantum information processing devices enabled by combining nanophotonic structures - photonic crystals with artificial atoms (such as quantum dots).