The negatively charged nitrogen-vacancy (NV^–) center in bulk diamond is a photostable fluorophore with a radiative lifetime of 11.6 ns at room temperature. The lifetime substantially increases to ~20 ns for diamond nanoparticles (size ~ 100 nm) suspended in water due to the change in refractive index of the surrounding medium of the NV^– centers. This fluorescence decay time is much longer than that (typically 1 − 4 ns) of endogenous and exogenous fluorophores commonly used in biological imaging, making it possible to detect NV^–-containing nanodiamonds in vivo at the single particle level by fluorescence lifetime imaging microscopy (FLIM). We demonstrate the feasibility of this approach using Caenorhabditis elegans (C. elegans) as a model organism.

Fluorophores including fluorescent dyes/proteins and quantum dots (QDs) are used for fluorescence-based imaging and detection. These are based on ‘downconversion fluorescence’ and have several drawbacks: photobleaching, autofluorescence, short tissue penetration depth and tissue photo-damage. Upconversion fluorescent nanoparticles (UCNs) emit detectable photons of higher energy in the short wavelength range upon irradiation with near-infrared (NIR) light based on a process termed ‘upconversion’. UCNs show absolute photostability, negligible autofluorescence, high penetration depth and minimum photodamage to biological tissues. Lanthanide doped nanocrystals with nearinfrared NIR-to-NIR and/or NIR-to-VIS and/or NIR-to-UV upconversion fluorescence emission have been synthesized. The nanocrystals with small size and tunable multi-color emission have been developed. The emission can be tuned by doping different upconverting lanthanide ions into the nanocrystals. The nanocrystals with core-shell structure have also been prepared to tune the emission color. The surfaces of these nanocrystals have been modified to render them water dispersible and biocompatible. They can be used for ultrasensitive interference-free biodetection because most biomolecules do not have upconversion properties. UCNs are also useful for light based therapy with enhanced efficiency, for example, photoactivation.

Nanodiamond imaging is a new molecular imaging modality that takes advantage of nitrogen-vacancy (NV) defects in nanodiamonds to image the distribution of nanodiamonds within a living organism with high sensitivity and high resolution. Nanodiamond is a nontoxic material that is easily conjugated to biomolecules, such that the distribution of nanodiamond within a living organism can be used to elicit physiological information. Unlike the tracers used in other molecular imaging modalities such as positron emission tomography (PET) and single photon emission computed tomography (SPECT), nanodiamonds are stable and thus allow longitudinal imaging of the same organism over a long time span. Unlike fluorescence-based molecular imaging that has a resolution degraded by photon scattering, the resolution of nanodiamond imaging is defined by the strength of a magnetic gradient.
To form an image, a magnetic field-free region is created, such as exists halfway between two identical magnets with north poles facing each other. Optical excitation pumps the NVs into a bright fluorescence state, and microwaves transfer them to a dark state, but only for those NVs within the field-free region and resonant with the microwaves. By rastering the field-free region across the sample, the changes in fluorescence yield the nanodiamond concentration. Images of nanodiamond phantoms within chicken breast have been recorded with a prototype system. By modifying the nanodiamond particles and enhancing the imaging system, it should be possible to approach 100 μm resolution and to increase the sensitivity to a 10 nanomolar carbon concentration per root Hz in a mm³ voxel.

Electronic valleys refer to energy extrema in momentum space. In the same way as spin is utilized for spintronics, valleys can be considered as pseudo-spins for valley based electronics and optoelectronics. Although the concept of valleytronics has been proposed for many years, progress has been hampered by a lack of ideal physical systems, in particular for optically driven valleytronics. Here, we show that monolayer group VI transition metal dichalcogenides is a promising system to meet this challenge. In particular, there are circularly polarized optical selection rules associated with valley index, which allows addressing the individual valleys by optical means. We further demonstrate optical generation of valley polarization using monolayer MoS₂ as a model system. Our results open the door for optical investigation of valley physics in monolayer semiconductors.

Individual electron and hole quantum dot spin qubits can be coherently manipulated using picosecond modelocked laser pulses; an all-optical spin-echo was implemented that decouples slow environmental changes. While dephasing and decoherence mechanisms for electrons and holes are intrinsically different, similar qualitative results are obtained, except for dynamic nuclear polarization effects that affect the controllability of electrons. In addition, we demonstrate spin-photon entanglement in a charged InAs quantum dot, using an ultrafast downconversion technique that converts a single, spontaneously emitted photon at 900 nm into a 1560 nm photon with picosecond timing resolution. This ultrafast conversion technique allows quantum erasure of which-path frequency information in the spontaneous emission process.

The negatively-charged nitrogen-vacancy centers in diamond has motivated many groups building scalable quantum information processors based on diamond photonics. This is owning to the long-lived electronic spin coherence and the capability for spin manipulation and readout of NV centers.^1-4 The primitive operation is to create entanglement between two NV centers, based on schemes such as 'atom-photon entanglement' proposed by Cabrillo et al.⁵To scale this type of scheme beyond two qubits, one important component is an optical switch that allows light emitted from a particular device to be routed to multiple locations. With such a switch, one has choices of routing photons to specified paths and has the benefit of improving the entanglement speed by entangling multiple qubits at the same time. Yield of the existing diamond cavities coupled with NV centers are inevitably low, due to the nature of randomness for NV placement and orientation, variation of spectral stability, and variation of cavity resonance frequency and quality factor. An optical switch provides the capability to tolerate a large fraction of defective devices by routing only to the working devices. Many type of switching devices were built on conventional semiconductor materials with mechanisms from mechanical, thermal switching to carrier injection, photonics crystal, and polymer refractive index tuning .^6-8 In this paper, we build an optical-thermal switch on diamond with micro-ring waveguides, mainly for the simplicity of the diamond fabrication. The the switching function was realized by locally tuning the temperature of the diamond waveguides. Switching efficiency of 31% at 'drop' port and 73% at 'through' port were obtained.

With in-built advantages (high quantum efficiency and room temperature photostability¹) for deployment in quantum technologies as a bright on-demand source of single photons, the nitrogen vacancy (NV) center is the most widely studied optical defect in diamond. Despite significant success in controlling its spontaneous emission², the fundamental understanding of its photo-physics in various environments and host material remains incomplete. Studying NV photoemission from nanodiamonds on a glass substrate, we recently pointed out a disparity between the measured and calculated decay rates (assuming near unity quantum efficiency)³. This indicates the presence of some strong nonradiative influences from factors most likely intrinsic to nanodiamond itself. To obtain a clearer picture of the NV emission, here we remove the substrate contributions to the decay rates by embedding our nanodiamonds inside silica aerogel, a substrate-free environment of effective index n ~ 1.05. Nanodiamond doped aerogel samples were fabricated using the “two-step” process⁴. Time-resolved fluorescence measurement on ~20 centers for both coverslip and aerogel configurations, showed an increase in the mean lifetime (~37%) and narrowing of the distribution width (~40%) in the aerogel environment, which we associate with the absence of a air/cover-glass interface near the radiating dipoles³. Finite difference time domain (FDTD) calculations showed the strong influence of the irregular nanodiamond geometry on the remaining distribution width. Finally a comparison between measurements and calculations provides an estimate of the quantum efficiency of the nanodiamond NV emitters as ~0.7. This value is apparently consistent with recent reports concerning the oscillation of the NV center between negative and neutral charge states⁵.

In this work we introduce a new channel coding scheme for noisy quantum channels. The proposed scheme is called polaractivation and makes it possible to open the hidden capacity-domains of a noisy quantum channel. With the help of the proposed channel coding technique private information can be transmitted over a quantum channel that initially was not capable of private communication. We prove that the polaractivation works for arbitrary quantum channels for which a given criteria in the symmetric classical capacity is satisfied.

The viability of neutral atom based quantum computers is dependent upon scalability to large numbers of qubits. Diffractive optical elements (DOEs) offer the possibility to scale up to many qubit systems by enabling the manipulation of light to collect signal or deliver a tailored spatial trapping pattern. DOEs have an advantage over refractive microoptics since they do not have measurable surface sag, making significantly larger numerical apertures (NA) accessible with a smaller optical component. The smaller physical size of a DOE allows the micro-lenses to be placed in vacuum with the atoms, reducing aberration effects that would otherwise be introduced by the cell walls of the vacuum chamber. The larger collection angle accessible with DOEs enable faster quantum computation speeds. We have designed a set of DOEs for collecting the 852 nm fluorescence from the D2 transition in trapped cesium atoms, and compare these DOEs to several commercially available refractive micro-lenses. The largest DOE is able to collect over 20% of the atom’s radiating sphere whereas the refractive micro-optic is able to collect just 8% of the atom’s radiating sphere.

In this paper we report the first demonstration of “cavity enhanced rephased amplified spontaneous emission”. The rephased amplified spontaneous emission (RASE) protocol provides an all-in-one photon-pair source and quantum memory that has applications as a quantum repeater node. Cavity enhancement of the interaction of the optical mode with the ensemble has the potential to improve the fidelity of the entanglement of the photon pairs. Using heterodyne detection, amplified emission and photon echo induced rephased amplified emission were observed from a Pr³⁺ doped Y₂SiO₅ crystal placed in a Fabry Perot cavity with a finesse of 70. Modifications to the experiment to allow non-classical correlations to be observed are discussed.

Atomic vapor cells with buffer gas have a number of advantages when employed as quantum memory blocks based on the DLCZ (Duan-Lukin-Cirac-Zoller) protocol: operation slightly above room temperature, ease of handling, as well as commercial availability. Nevertheless, the signal-to-noise ratio in the current implementations is severely limited by the simultaneous presence of collisional fluorescence and the four-wave mixing noise. In our previous work, we have shown how to minimize the influence of the former on the writing process and provided an unambiguous demonstration of quantum memory lasting for 4 μs. An elegant approach to suppress the four-wave-mixing noise by pre-pumping to the state with the hyperfine sublevel with the maximum value was proposed by Walther et al., Int. J. Quantum Inform. 5, 51 (2007). Here we show that this approach is fundamentally limited by the cancellation of the Raman matrix elements involving the Fˊ= 1 and Fˊ= 2 levels, which occurs for all experimental conditions in the S → P transitions of all alkali atoms. A detuning that maximizes the signal-to-noise ratio is shown to exist for a given detector dark-count rate.

Compressing the temporal correlation of two photons to the monocycle regime (3.56 fs, center wavelength: 1064 nm) is expected to open up new perspectives in quantum metrology, allowing applications such as submicron quantum optical coherence tomography and novel nonlinear optical experiments. To achieve this, the two-photon state must essentially be ultra-broadband in the frequency domain and ultra-short in the time domain. Here, we report the successful generation of such ultra-broadband, frequency-correlated two-photon states via type-0, cw-pumped (532 nm) spontaneous parametric down conversion using four PPMgSLT crystals with different chirp rates of their poling periods. For the collinear condition, single-photon spectra are detected using a Si-CCD and an InGaAs photodiode array with a monochromator, while for a noncollinear condition, an NbN meander-type superconducting single photon detector (SNSPD) and an InP/GaAs photomultiplier tube (PMT) with a laser line Bragg tunable bandpass filter are used. The broadband sensitivity of the SNSPD and PMT in the near-infrared wavelength range enable singleshot observations with a maximum bandwidth of 820 nm among the four samples. Such spectra can in principle achieve a temporal correlation as short as 1.2 cycles (4.4 fs) with the use of appropriate phase compensation, which can be measured using the sum-frequency signal. We also discuss several detection strategies for measuring coincidence counts in the presence of wavelength-dependent optical elements as a step towards frequency correlation measurements.

We study the dynamics of the interaction between two weak light beams mediated by a strongly coupled quantum dot-photonic crystal cavity system. We demonstrate switching between two weak pulsed beams (40 ps pulses), observing an increase of the systems transmission when the signal and the control pulses overlap inside the cavity. Our results show that the quantum dot-nanocavity system enables fast, controllable optical switching at the single-photon level.

We model frozen light stored as a spin wave via electromagnetically induced transparency quantum-memory techniques in a Bose-Einstein condensate. The joint evolution of the condensate and the frozen light is typically modeled using coupled Gross-Pitaevskii equations for the two atomic fields, but these equations are only valid in the mean-field limit. Even when the mean-field limit holds for the host condensate, coupling between the host and the spin wave component could lead to a breakdown of the mean-field approximation if the host fluctuations are large compared the mean-field value of the spin wave. We develop a theoretical framework for modeling the corrections to the mean-field theory of a two-component condensate. Our analysis commences with a full second-quantized Hamiltonian for a two-component condensate. The field operators are broken up into a mean-field and a quantum fluctuation component. The quantum fluctuations are truncated to lowest non-vanishing order. We find the transformation diagonalizing the second-quantized approximate Hamiltonian and show that it can be described using the solutions to a system of coupled differential equations.

We demonstrate efficient photon pair generation for quantum communication using an all-semiconductor approach. In an AlGaAs Bragg-reflection waveguide we employ spontaneous parametric down-conversion to produce photon pairs at telecommunication wavelengths. The various phase-matching solutions present in our device can be used to create timebin or polarization entanglement. This approach can to lead to a fully integrated photon pair source with the pump laser, active and passive optical devices all on a single semiconductor chip.

In recent years, great efforts have been devoted to the miniaturization of quantum information technology on semiconductor chips. In the context of photon pair sources, the bi-exciton cascade of a quantum dot and the four wave mixing in a Silicon waveguide have been used to demonstrate the generation of entangled states. Spontaneous parametric down-conversion in III-V semiconductor waveguides combines the advantages of room temperature and telecom wavelength operation, while keeping open the possibility of electrically pumping of the device. Here we present a source consisting of a multilayer AlGaAs waveguide grown on a GaAs substrate and then chemically etched to achieve lateral confinement in a ridge. The structure design is such that a pump beam (around 759 nm), impinging on the waveguide surface with an incidence angle θ generates two counterpropagating orthogonally polarized beams (around 1518 nm). The waveguide core is surrounded by distributed Bragg reectors to enhance the pump field within the device. We demonstrate the direct emission of polarization entangled photons by pumping the device at two symmetric angles of incidence corresponding to frequency degeneracy and performing a quantum tomography measurement. Most common entanglement witnesses are satisfied and a raw fidelity of F = 0:86 to a Bell state is obtained. These results open the route to the demonstration of other interesting features of our device such as the generation of hyper-entangled states via the control of the frequency correlation degree through the spatial and spectral pump beam profile, leading to a new generation of completely integrated devices for quantum information.

Further improvement of infrared single photon sources is a major challenge for future implementations of quantum information and quantum communication applications. In this paper, we give further insight into a recently presented, conceptually novel method for the generation of single photons.¹ The method is of particular interest for spectral domains where stable room temperature single photon sources are not available. For example, this is the presently the case for the near-infrared. This wavelength regime is important for data transfer over long distances where optical losses in fibers are minimal. The presented method is based on the following idea. The fundamental key requirement for single photon generation is the generation of a single excitation in an optically active system. It is not the presence of a single quantum system. The presented method is applied to realize a stable, non-blinking, room temperature infrared single photon source by converting visible single photons from a defect center in diamond to the near infrared. For the presented implementation, the theoretical conversion efficiency was estimated to be 26 %. In a first prove of principle experiment, a conversion efficiency of 0.1 % was achieved.

We report a novel component for integrated quantum photonic applications, a waveguide single-photon autocorrelator. It is based on two superconducting nanowire detectors patterned onto the same GaAs ridge waveguide. Combining the electrical output of the two detectors in a correlation card enables the measurement of the second-order correlation function g⁽²⁾ (τ), which realizes the functionality of a Hanbury-Brown and Twiss experiment in a very compact integrated device. Each detector shows a polarization-independent quantum efficiency of ~0.5-1% at 1300 nm. This autocorrelator represents a key building block for quantum photonic integrated circuits including single-photon sources and linear optics.