Synchronized source of indistinguishable photons for quantum networks.

We present a source of indistinguishable photons at telecom wavelength, synchronized to an external clock, for the use in distributed quantum networks. We characterize the indistinguishability of photons generated in independent parametric down-conversion events using a Hong-Ou-Mandel interferometer, and show non-classical interference with coalescence, C = 0.83(5). We also demonstrate the synchronization to an external clock within sub-picosecond timing jitter, which is significantly shorter than the single-photon wavepacket duration of ≈ 35 ps. Our source enables scalable quantum protocols over multi-node, long-distance optical networks using network-based clock recovery systems.

Synchronization and coexistence in quantum networks.

We investigate the coexistence of clock synchronization protocols with quantum signals in a common single-mode optical fiber. By measuring optical noise between 1500 nm to 1620 nm we demonstrate a potential for up to 100 quantum, 100 GHz wide channels coexisting with the classical synchronization signals. Both "White Rabbit" and pulsed laser-based synchronization protocols were characterized and compared. We establish a theoretical limit of the fiber link length for coexisting quantum and classical channels. The maximal fiber length is below approximately 100 km for off-the-shelf optical transceivers and can be significantly improved by taking advantage of quantum receivers.

Experimental Shot-by-Shot Estimation of Quantum Measurement Confidence.

We demonstrate the single-shot confidence estimation for individual quantum measurement outcomes using the continuous measurement theory of the quantum counting process applied to the quantum state identification problem. We experimentally obtain single-shot and average confidences for quantum measurements and show that they favorably compare to that of the idealized classical measurement. Finally, we demonstrate that single-shot confidence estimations correctly represent observed experimental outcomes for a large ensemble of measurements.

In Situ Flow Cytometer Calibration and Single-Molecule Resolution via Quantum Measurement.

Fluorescent biomarkers are used to detect target molecules within inhomogeneous populations of cells. When these biomarkers are found in trace amounts it becomes extremely challenging to detect their presence in a flow cytometer. Here, we present a framework to draw a detection baseline for single emitters and enable absolute calibration of a flow cytometer based on quantum measurements. We used single-photon detection and found the second-order autocorrelation function of fluorescent light. We computed the success of rare-event detection for different signal-to-noise ratios (SNR). We showed high-accuracy identification of the events with occurrence rates below 10-5 even at modest SNR levels, enabling early disease diagnostics and post-disease monitoring.

Portable bioluminescent platform for in vivo monitoring of biological processes in non-transgenic animals.

Bioluminescent imaging (BLI) is one of the most powerful and widely used preclinical imaging modalities. However, the current technology relies on the use of transgenic luciferase-expressing cells and animals and therefore can only be applied to a limited number of existing animal models of human disease. Here, we report the development of a "portable bioluminescent" (PBL) technology that overcomes most of the major limitations of traditional BLI. We demonstrate that the PBL method is capable of noninvasive measuring the activity of both extracellular (e.g., dipeptidyl peptidase 4) and intracellular (e.g., cytochrome P450) enzymes in vivo in non-luciferase-expressing mice. Moreover, we successfully utilize PBL technology in dogs and human cadaver, paving the way for the translation of functional BLI to the noninvasive quantification of biological processes in large animals. The PBL methodology can be easily adapted for the noninvasive monitoring of a plethora of diseases across multiple species.

Coherent optical processes with an all-optical atomic simulator.

We show how novel photonic devices such as broadband quantum memory and efficient quantum frequency transduction can be implemented using three-wave mixing processes in a 1D array of nonlinear waveguides evanescently coupled to nearest neighbors. We do this using an analogy of an atom interacting with an external optical field using both classical and quantum models of the optical fields and adapting well-known coherent processes from atomic optics, such as electromagnetically induced transparency and stimulated Raman adiabatic passage to design. This approach allows the implementation of devices that are very difficult or impossible to implement by conventional techniques.

Simple autocorrelation method for thoroughly characterizing single-photon detectors.

We introduce and demonstrate a simple and highly sensitive method for characterizing single-photon detectors. This method is based on analyzing multi-order correlations among time-tagged detection events from a device under calibrated continuous-wave illumination. First- and second-order properties such as detection efficiency, dark count rate, afterpulse probability, dead time, and reset behavior are measured with high accuracy from a single data set, as well as higher-order properties such as higher-order afterpulse effects. While the technique is applicable to any type of click/no-click detector, we apply it to two different single-photon avalanche diodes, and we find that it reveals a heretofore unreported afterpulse effect due to detection events that occur during the device reset.

Quantum frequency bridge: high-accuracy characterization of a nearly-noiseless parametric frequency converter.

We demonstrate an efficient and inherently ultra-low noise frequency conversion via a parametric sum frequency generation. Due to the wide separation between the input and pump frequencies and the low pump frequency relative to the input photons, the upconversion results in only ≈100 background photons per hour. To measure such a low rate, we introduced a dark count reduction algorithm for an optical transition edge sensor.

Statistically background-free, phase-preserving parametric up-conversion with faint light.

We demonstrate up-conversion with no statistically significant background photons and a dynamic range of 15 decades. Near-infrared 920 nm photons were converted into the visible at 577 nm using periodically poled lithium niobate waveguides pumped by a 1550 nm laser. In addition to achieving statistically noiseless frequency up-conversion, we report a high degree of phase preservation (with fringe visibilities ≥ 0.97) at the single-photon level using an up-converting Mach-Zehnder interferometer. This background-free process opens a path to single-photon detection with no intrinsic dark count. Combined with a demonstrated photon-number preserving property of an up-converter, this work demonstrates the feasibility of noiseless frequency up-conversion of entangled photon pairs.

Dynamics of nonclassical light from a single solid-state quantum emitter.

We measure the dynamics of a nonclassical optical field using two-time second-order correlations in conjunction with pulsed excitation. The technique quantifies single-photon purity and coherence during the excitation-decay cycle of an emitter, illustrated here using a quantum dot. We observe that for certain pump wavelengths, photons detected early in the cycle have reduced single-photon purity and coherence compared to those detected later. A model indicates that the single-photon purity dynamics are due to exciton recapture after initial emission and within the same pulse cycle.

Coalescence of single photons emitted by disparate single-photon sources: the example of InAs quantum dots and parametric down-conversion sources.

Single photons produced by fundamentally dissimilar physical processes will in general not be indistinguishable. We show how photons produced from a quantum dot and by parametric down-conversion in a nonlinear crystal can be manipulated to be indistinguishable. The measured two-photon coalescence probability is 16%, and is limited by quantum-dot decoherence. Temporal filtering to the quantum-dot coherence time and accounting for detector time response increases this to 61% while retaining 25% of the events. This technique can connect different elements in a scalable quantum network.

Analytical method for parameterizing the random profile components of nanosurfaces imaged by atomic force microscopy.

The functional properties of many technological surfaces in biotechnology, electronics, and mechanical engineering depend to a large degree on the individual features of their nanoscale surface texture, which in turn is a function of the surface manufacturing process. Among these features, the surface irregularities and self-similarity structures at different spatial scales, especially in the range of 1 to 100 nm, are of high importance because they greatly affect the surface interaction forces acting at a nanoscale distance. An analytical method for parameterizing the surface irregularities and their correlations in nanosurfaces imaged by atomic force microscopy (AFM) is proposed. In this method, flicker noise spectroscopy--a statistical physics approach--is used to develop six nanometrological parameters characterizing the high-frequency contributions of jump- and spike-like irregularities into the surface texture. These contributions reflect the stochastic processes of anomalous diffusion and inertial effects, respectively, in the process of surface manufacturing. The AFM images of the texture of corrosion-resistant magnetite coatings formed on low-carbon steel in hot nitrate solutions with coating growth promoters at different temperatures are analyzed. It is shown that the parameters characterizing surface spikiness are able to quantify the effect of process temperature on the corrosion resistance of the coatings. It is suggested that these parameters can be used for predicting and characterizing the corrosion-resistant properties of magnetite coatings.

Interference of single photons from two separate semiconductor quantum dots.

We demonstrate and characterize interference between discrete photons emitted by two separate semiconductor quantum dot states in different samples excited by a pulsed laser. Their energies are tuned into resonance using strain. The photons have a total coalescence probability of 18.1% and the coincidence rate is below the classical limit. Postselection of coincidences within a narrow time window increases the coalescence probability to 47%. The probabilities are reduced from unity because of dephasing and the postselection value is also reduced by the detector time response.

Single-photon propagation through dielectric bandgaps.

Theoretical models of photon traversal through quarter-wave dielectric stack barriers that arise due to Bragg reflection predict the saturation of the propagation time with the barrier length, known as the Hartman effect. This saturation is sensitive to the addition of single dielectric layers, varying significantly from sub-luminal to apparently super-luminal and vice versa. Our research tests the suitability of photonic bandgaps as an optical model for the tunneling process. Of particular importance is our observation of subtle structural changes in dielectric stacks drastically affecting photon traversal times, allowing for apparent sub- and super-luminal effects. We also introduce a simple model to link HOM visibility to wavepacket distortion that allows us to exclude this as a possible cause of the loss of contrast in the barrier penetration process.

Experimental test of nonclassicality for a single particle.

In a recent paper [R. Alicki and N. Van Ryn, J. Phys. A: Math. Theor., 41, 062001 (2008)] a test of nonclassicality for a single qubit was proposed. Here, we discuss the class of hidden variables theories to which this test applies and present an experimental realization.

High accuracy verification of a correlated-photon- based method for determining photoncounting detection efficiency.

We have characterized an independent primary standard method to calibrate detection efficiency of photon-counting detectors based on twophoton correlations. We have verified this method and its uncertainty by comparing it to a substitution method using a conventionally calibrated transfer detector tied to a national primary standard detector scale. We obtained a relative standard uncertainty for the correlated-photon method of 0.18 % (k=1) and for the substitution method of 0.17 % (k=1). From a series of measurements we found that the two independent calibration techniques differ by 0.14 (14) %, which is within the established uncertainty of comparison. We believe this is the highest accuracy characterization and independent verification of the correlated-photon method yet achieved.

Existence and properties of quadratic solitons in anisotropic media: variational approach.

Stationary quadratic solitons associated with second harmonic generation in optically anisotropic media have been investigated both numerically and analytically using the variational approach. The solitons were found to have elliptical shapes, both for the fundamental and second harmonic, and their approximate beam waists and amplitudes as a function of the anisotropy and the soliton parameter were found. The important limits of anisotropic diffraction were compared to the well-known model of isotropic diffraction. The stability of anisotropic solitons was addressed via the Vakhitov-Kolokolov criterion and the regions of parameter space for which the solitons are stable were identified. Direct numerical simulations of the coupled field equations were performed to illustrate the existence, stability, and ellipticity of anisotropic quadratic solitons. In general, good agreement was found between approximate analytical approaches and numerical experiments.