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Efficient optical readout of a single, solid-state electronic spin at room temperature is a key challenge for nanoscale quantum sensing. Here we apply the technique of spin-to-charge conversion to enhance the optical spin-state readout of a single Nitrogen-Vacancy (NV) color center in room temperature diamond, with no degradation in the NV spin coherence time. We demonstrate an order-of-magnitude improvement in spin readout noise per shot and about a factor of five improvement in AC magnetometry sensitivity, compared to the conventional NV spin-state optical readout method. This improvement is realized in a widely-applicable bulk diamond system. We show that selecting for successful charge state initialization leads to possible further improvement in sensitivity. This technique is well suited to sensing applications involving low duty cycle pulsed signals, e.g., in biomagnetometry, where long deadtimes demand optimized sensitivity per shot., 11 pages more...

Nitrogen vacancy (NV) color centers in diamond are a leading modality for both superresolution optical imaging and nanoscale magnetic field sensing. In this work, we solve the remaining key challenge of performing optical magnetic imaging and spectroscopy selectively on multiple NV centers that are located within a diffraction-limited field-of-view. We use spin-RESOLFT microscopy to enable precision nanoscale mapping of magnetic field patterns with resolution down to ~20 nm, while employing a low power optical depletion beam. Moreover, we use a shallow NV to demonstrate the detection of proton nuclear magnetic resonance (NMR) signals exterior to the diamond, with 50 nm lateral imaging resolution and without degrading the proton NMR linewidth., Comment: main text: 5 pages, 4 figures. Supplementary Material: 5 pages, 8 figures more...

Results of active phase tracking system deployment for quantum communication are presented. The system allows high-visibility single photon interferometry over 100 km of standard optical fibre. Single photon detector optimization is performed and the estimates for long-distance quantum key distribution with heralded single photon source are presented. more...

A brief overview is given of single photon detector performance requirements for quantum cryptography applications. The analysis is made with respect to restrictions necessary to secure the quantum key distribution channel. InGaAs/InP avalanche photodiode performance is analysed for single photon counting at 1550 nm. Quantum efficiency, dark current and afterpulsing probability (for times up to 100μs after an initial avalanche) are studied in a wider temperature range than previously reported (0deg; C to –80deg;C). We show that photon counting is a bottle-neck in current quantum key distribution systems and provides the source for future performance improvement. more...

In an interferometer, path information and interference visibility are incompatible quantities. Complete determination of the path will exclude any possibility of interference, rendering the visibility zero. However, if the composite object and probe state is pure, it is, under certain conditions, possible to trade the path information for improved (conditioned) visibility. Such a procedure is called quantum erasure. We have performed such experiments with polarization entangled photon pairs. Using a partial polarizer we could vary the degree of entanglement between object and probe. We could also vary the interferometer splitting ratio and thereby vary the a priori path predictability. We have tested quantum erasure under a number of different experimental conditions and found good agreement between experiments and theory., Comment: 8 pages, 8 figures, REVTeX more...

We have carried out polarization measurements on a two-photon quantum state generated by spontaneous parametric down conversion. Our measurements show that the state is unpolarized in the classical theory, but they also show that the state is not invariant under geometric rotation. Therefore, it is not unpolarized in the quantum theory. This is another example of a rather simple experiment that clearly shows the effects of quantum interference. It also confirms the theoretical investigation by Klyshko [Phys. Lett. A 163 (1992) 349]. The experiment is explained theoretically and a systematic treatment of polarization in quantum theory is outlined. more...

We have experimentally demonstrated a realization of the two-mode relative phase operator introduced by Luis and Sanchez-Soto (Luis A and Sanchez-Soto L L 1993 Phys. Rev. A 48 4702). The relative phase distribution function was measured for a weakly excited relative phase eigenstate and weakly excited two-mode coherent states. The experiment is also (using the eigenstates) a demonstration of Heisenberg-limited interferometry. more...

We discuss generation and measurement strategies for maximum-likelihood estimation of small phase shifts. We demonstrate that optimal encoding of small phase shifts and optimal measurements of small phase shifts require different states. We also show that states with identical phase resolution may have rather different Pegg-Barnett phase distributions. Hence, the connection between phase distribution and quantum phase resolution is rather weak. more...

We report a systematic theoretical and experimental study of the limits to spatial resolution for stimulated emission depletion (STED) superresolution fluorescence microscopy using continuous wave (CW) laser beams. We develop a theoretical framework for CW STED imaging from point fluorescent emitters and calculate the dependence of 2D spatial resolution on the power of the CW excitation (pump) beam, as well as the power, contrast, and polarization of the CW STED “doughnut” beam. We perform CW STED experiments on (non-bleaching) nitrogen vacancy (NV) color centers in diamond and find good agreement with the theoretical expressions for CW STED spatial resolution. Our results will aid the optimization and application of CW STED microscopy in both the physical and life sciences. more...

Correlative light and electron microscopy promises to combine molecular specificity with nanoscale imaging resolution. However, there are substantial technical challenges including reliable co-registration of optical and electron images, and rapid optical signal degradation under electron beam irradiation. Here, we introduce a new approach to solve these problems: imaging of stable optical cathodoluminescence emitted in a scanning electron microscope by nanoparticles with controllable surface chemistry. We demonstrate well-correlated cathodoluminescence and secondary electron images using three species of semiconductor nanoparticles that contain defects providing stable, spectrally-distinguishable cathodoluminescence. We also demonstrate reliable surface functionalization of the particles. The results pave the way for the use of such nanoparticles for targeted labeling of surfaces to provide nanoscale mapping of molecular composition, indicated by cathodoluminescence colour, simultaneously acquired with structural electron images in a single instrument. more...

Part of Focus on Diamond-Based Photonics and Spintronics We demonstrate a method of imaging spatially varying magnetic fields using a thin layer of nitrogen-vacancy (NV) centers at the surface of a diamond chip. Fluorescence emitted by the two-dimensional NV ensemble is detected by a CCD array, from which a vector magnetic field pattern is reconstructed. As a demonstration, ac current is passed through wires placed on the diamond chip surface, and the resulting ac magnetic field patterns are imaged using an echo-based technique with sub-micron resolution over a 140 μm×140 μm field of view, giving single-pixel sensitivity \sim100\,{\rm nT}/\sqrt{{\rm Hz}} . We discuss ongoing efforts to further improve the sensitivity, as well as potential bioimaging applications such as real-time imaging of activity in functional, cultured networks of neurons. PACS 61.72.J- Point defects and defect clusters 78.55.Hx Other solid inorganic materials 87.50.C- Static and low-frequency electric and magnetic fields effects 85.30.Tv Field effect devices Subjects Electronics and devices Condensed matter: electrical, magnetic and optical Semiconductors Medical physics Biological physics Condensed matter: structural, mechanical & thermal, United States. Defense Advanced Research Projects Agency, National Institute of Standards and Technology (U.S.), National Science Foundation (U.S.) more...

We use a group theoretical approach to model the nitrogen-vacancy defect in diamond. In our analysis we clarify several properties of this defect that have been source of controversy such as the ordering of the singlets and the mechanism that leads to spin mixing in the excited state of this defect. In particular, we demonstrate that the ordering of the ground state configuration (e2) is {3A2, 1E, 1A1} and that the spin-spin interaction causes the mixing in the excited state. In addition, we analyze the angular momentum and spin properties of the excited state structure that enables a spin photon entanglement scheme that has been recently demonstrate experimentally. Our description is general and it can be easily applied to other defects in solid-state systems. more...

Quantum entanglement is among the most fascinating aspects of quantum theory. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state spin qubit associated with thenitrogen vacancy centre in diamond. more...

We present a procedure that makes use of group theory to analyze and predict the main properties of the negatively charged nitrogen-vacancy (NV) center in diamond. We focus on the relatively low temperatures limit where both the spin-spin and spin-orbit effects are important to consider. We demonstrate that group theory may be used to clarify several aspects of the NV structure, such as ordering of the singlets in the ($e^2$) electronic configuration, the spin-spin and the spin-orbit interactions in the ($ae$) electronic configuration. We also discuss how the optical selection rules and the response of the center to electric field can be used for spin-photon entanglement schemes. Our general formalism is applicable to a broad class of local defects in solids. The present results have important implications for applications in quantum information science and nanomagnetometry., Comment: 30 pages, 6 figures more...

We present a new derivation of the unpolarized quantum states of light, whose general form was first derived by Prakash and Chandra [Phys. Rev. A 4, 796 (1971)]. Our derivation makes use of some basic group theory, is straightforward, and offers some new insights., 3 pages, REVTeX, presented at ICQO'2000 more...

Quantum entanglement is among the most fascinating aspects of quantum theory. Entangled optical photons are now widely used for fundamental tests of quantum mechanics and applications such as quantum cryptography. Several recent experiments demonstrated entanglement of optical photons with trapped ions, atoms and atomic ensembles, which are then used to connect remote long-term memory nodes in distributed quantum networks. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique, and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks. more...

Considering a general closed system with a discrete energy spectrum and a given mean energy, we derive the states that minimize the time it takes to evolve into an orthogonal state under free evolution. The implications on optical interferometry are discussed. more...

We investigate the use of photon number states to identify eavesdropping attacks on quantum key distribution (QKD) schemes. The technique is based on the fact that different photon numbers traverse a channel with different transmittivity. We then describe two QKD schemes that utilize this method, one of which overcomes the upper limit on the key generation rate imposed by the dead time of detectors when using a heralded source of photons., Comment: 15 pages, 6 figures; Accepted to J. Mod. Opt more...

The efficiency of spontaneous parametric down conversion is calculated and measured for several nonlinear crystals and waveguides in single spatial mode regime. Efficiency of waveguide sources is found to be far superior compared to the bulk crystal sources. more...

We report on two quantum polarization experiments. The first one shows that three geometrical rotations can in principle be distinguished with certainty using two photons. The second experiment shows quantum polarization properties of classically unpolarized light. We outline a new treatment of polarization in quantum optics. Our formalism differs from the usual quantum description, which uses the Stokes operators. more...

From the experimentalist’ spoint of view, it is natural to think of aphase measurement as a measurement that involves a reference time. As a part of the system, the reference time generating “clock” must be taken into account in the quantum treatment of the measuring process. This could be argued to be the reason why there does not exist any observable corresponding to the absolute phase in the unrestricted Hilbert space.Luis and Sanchez-Soto [Phys. Rev. A 48, 4702 (1993)] have investigated the case where the “clock” is another harmonic oscillator and defined a Hermitian phase-difference operator, which is valid in the infinite Hilbert space. more...

We describe a robust heralded single photon source based on parametric down conversion of CW 532-nm light in a periodically polled KTP waveguide. Low required pump power (sub-mW), reasonable operational temperature (43oC), high heralding efficiency (60%), and narrow spectral width of the heralded photons (sub-nm) make it an ideal light source for long-distance quantum communications. more...

We report our recent results in development of the secure fiber-optics communication system based upon quantum key distribution (QKD). Emphasize is made on the limitation imposed by the state-of-the-art components crucial for the system performance. We discuss the problem of the interferometer design and highlight the possible security loopholes known. Together with single photon counting performance it places the main restriction on the distance range and the secure key rate of the QKD system based upon the weak coherent pulses. Finally we describe the result of the first test of the system using single photons produced by non-degenerate parametric down-conversion as a source. more...

We report our results for quantum key distribution over 76 km of commercial optical fiber using one‐way interferometer and heralded photon source based on parametric down conversion.

We present a comprehensive and self-consistent theory of relative-phase measurements and the associated Hermitian relative-phase operator of two harmonic oscillators. We find that since Nature does ... more...

Interference phenomena lead to a wealth of applications in many areas of physics. Entangled quantum states allow one to surpass the classical measurement sensitivity or resolution in polarimetry, interferometry, and imaging. In this paper we shall review, in some depth, polarization properties of quantized two-mode electromagnetic fields and show how interference and quantum entanglement lead to new phenomena. We shall also briefly discuss subwavelength quantum lithography. more...

Different quantum information schemes, such as eavesdropping in quantum cryptography, dictate the necessity of extracting information about pair conjugate observables from a single copy of a quantum system. Mathematically, quantum measurements are usually described by an uncertainty relation. The difference between the simultaneous measurement uncertainty relation form those known from the textbooks on quantum mechanics is the additional uncertainty associated with the measurement procedure itself in contrast to the state preparation uncertainties described by the Schroedinger-Robertson type uncertainty relations. We present here an overview of our approach based on the state estimation theory and maximum likelihood strategy. We make a theoretical analysis and an experimental verification of minimum-uncertainty product of the two-states quantum system simultaneous measurement based on partially entangled photon pairs. more...

Linearly polarized classical light can be expressed in a vertical and a horizontal component. Geometrically rotating vertically polarized light by 90 degrees will convert it to the orthogonal horizontal polarization. We have experimentally generated a two-photon state of light which evolves into an orthogonal state upon geometrical rotation by 60 degrees. Rotating this state by an additional 60 degrees will yield a state which is mutually orthogonal to the first two states. Generalizing this procedure, one can generate N+1 mutually orthogonal N-photon states that cyclicly evolve from one to another upon a geometric rotation by 180/(N+1) degrees. more...

We have made the first experimental demonstration of the simultaneous minimum uncertainty product between two complementary observables for a two-state system (a qubit). A partially entangled two-photon state was used to perform such measurements. Each of the photons carries (partial) information of the initial state thus leaving a room for measurements of two complementary observables on every member in an ensemble., Comment: 4 pages, 4 figures, REVTeX, submitted to PRL more...

The combination of superconducting single-photon detectors and phase-shift keying technology has enabled a leap in the performance of quantum cryptography.

The results of photon flux noise measurements in a quantum well semiconductor laser with 1.3 micrometer wavelength and antireflection coating on the front surface are presented. The results are compared with the theory based on quantum mechanical Langevin-Heisenberg equations. more...

The investigations of correlation between intensity noise of semiconductor laser and junction voltage noise were made. The laser was high-power quantum well separate confinement heterostructure (SCH) InGaAsP/GaAs laser with (lambda) equals 0.8 micrometers . The full negative correlation near the threshold between photon flux noise and junction voltage noise in constant-current regime was obtained. It was shown that at room temperature the laser exhibited constant-current regime due to base resistance. The theoretical treatment of correlation coefficient versus pump current was proposed and the role of optical losses has been revealed. The comparison of theoretical and experimental behavior near the threshold were made. more...

We characterized backscattering effects in optical fiber using a photon counting technique and considered its implications for quantum key distribution (QKD). We found that Rayleigh (elastic) backscattering can put strong limitations on a two-way QKD system’s performance. Raman (inelastic) scattering can restrict the ability of wavelength multiplexing of a quantum channel with strong classical data channel(s). more...

We formulate a general complementarity relation starting from any Hermitian operator with discrete non-degenerate eigenvalues. We then elucidate the relationship between quantum complementarity and the Heisenberg-Robertson's uncertainty relation. We show that they are intimately connected. Finally we exemplify the general theory with some specific suggested experiments., 9 pages, 4 figures, REVTeX, uses epsf.sty and multicol.sty more...

Summary form only given. In quantum optics the usual way of treating polarization has been to use the Stokes parameters. Here we present a different approach starting from the quantum description of geometric rotation of polarization. more...