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We experimentally demonstrate multi-order quasi-phase-matching conditions for spectral shaping of photon pairs produced by spontaneous parametric downconversion. Incorporated with a group-velocity matching condition, our multi-order quasi-phase-matching scheme generates single photons high spectral purity.

We demonstrate size-selective optical trapping and transport for nanoparticles near an optical nanofiber taper. Using a two-wavelength counterpropagating mode configuration, we show that 100-nm-diameter and 150-nm-diameter gold nanospheres (GNSs) are trapped by the evanescent field in the taper region at different optical powers. Conversely, when one nanoparticle species is trapped the other may be transported, leading to a sievelike effect. Our results show that sophisticated optical manipulation can be achieved in a passive configuration by taking advantage of mode behavior in nanophotonic devices.

We demonstrate trapping and transport of liposomes in the evanescent field of an optical nanofiber with ~10 mW of optical power. Interesting regimes with non-negligible interaction of liposomes with the nanofiber surface give unique dynamics.

Enhanced manipulation and analysis of bio-particles using light confined in nano-scale dielectric structures has proceeded apace in the last several years. Small mode volumes, along with the lack of a need for bulky optical elements give advantages in sensitivity and scalability relative to conventional optical manipulation. However, manipulation of lipid vesicles (liposomes) remains difficult, particularly in the sub-micron diameter regime. Here we demonstrate the optical trapping and transport of sub-micron diameter liposomes along an optical nanofiber using the nanofiber mode’s evanescent field. We find that nanofiber diameters below a nominal diffraction limit give optimal results. Our results pave the way for integrated optical transport and analysis of liposome-like bio-particles, as well as their coupling to nano-optical resonators.

Recently, much research concerning the combination of nano-scale waveguides with nano-crystals and other nano-particles has been reported because of possible applications in the field of quantum information and communication. The most useful and convenient method to verify the nature of such systems is optical detection. However, due to the diffraction limit, optical identification of characteristics such as particle type, particle position, etc., is difficult or impossible. However, if such particles are placed on a waveguide, the coupling of scattered light to the waveguide-guided modes can reveal the information about the particles. Here we consider how illumination with light of arbitrary polarization can reveal the difference between isotropic and non-isotropic nano-particles placed on the surface of an optical nanofiber. Specifically, we measure the polarization response function of gold nano-rods (GNRs) on an optical nanofiber surface and show that it is qualitatively different to that for gold nano-spheres (GNSs). This experimental technique provides a simple new tool for the optical characterization of hybrid nano-optical devices.

Photon-pair generation via spontaneous parametric downconversion (SPDC) has been widely used for optical quantum information experiments. We demonstrate high-precision spectral measurements of SPDC sources, utilizing frequency-resolved sum-frequency generation, a reverse process of SPDC.

Tailoring spectral properties of photon pairs is of great importance for optical quantum information and measurement applications. High-resolution spectral measurement is a key technique for engineering spectral properties of photons, making them ideal for various quantum applications. Here we demonstrate spectral measurements and optimization of frequency-entangled photon pairs produced via spontaneous parametric downconversion (SPDC), utilizing frequency-resolved sum-frequency generation (SFG), the reverse process of SPDC. A joint phase-matching spectrum of a nonlinear crystal around 1580 nm is captured with a 40 pm resolution and a > 40 dB signal-to-noise ratio, which is significantly improved compared to traditional frequency-resolved coincidence measurements. Moreover, our scheme is applicable to collinear degenerate sources whose characterization is difficult with previously demonstrated stimulated difference frequency generation (DFG). We also illustrate that the observed phase-matching function is useful for finding an optimal pump spectrum to maximize the spectral indistinguishability of SPDC photons. We expect that our precise spectral characterization technique will be useful tool for characterizing and tailoring SPDC sources for a wide range of optical quantum applications.

We develop a transient photoinduced Kerr rotation spectroscopy technique using a heterodyne detection scheme to study spin dynamics of microscopic quantum states in solids, such as single quantum dots and spin helixes. The use of the heterodyne beat note signal generated by the interference of the frequency-shifted probe and reference pulses realizes the Kerr rotation measurements in combination with micro-spectroscopy, even when the probe pulse propagates collinearly with the strong pump pulse, which resonantly excites the probing state. In addition, the interference gives an optical amplification of the Kerr signal, which provides a clear observation of the photoinduced spin dynamics by the weak probe intensity. Here, we present results of Kerr rotation measurements for a single quantum dot exciton, which shows a maximum rotation angle of few µrad.

We study the photon echo spectrum of self-assembled semiconductor quantum dots using femtosecond light pulses. The spectrum shape changes from a single-peaked to a double-peaked structure as the time delay between the two excitation pulses is increased. The spectrum change is reproduced by numerical calculations, which include the depolarization field induced by the biexciton-exciton transition as well as the conventional local-field effect for the exciton--ground-state transition in a quantum dot. Our findings suggest that various optical transitions in tightly localized systems generate a depolarization field, which renormalizes the resonant frequency with a change in the polarization itself, leading to unique optical properties.

Two port optical devices couple light to either port dependent on the input photon state. An important class of two-port devices is that of evanescently-coupled interfaces where chirality of photon coupling can lead to important technological applications. Here, we perform a fundamental characterization of such an interface, reconstructing the two-port polarization response over the surface of the Poincaré sphere for an optical nanofibre. From this result, we derive a chirality measure which is universal, obeying a one parameter scaling law independent of the exact parameters of the nanofibre and wavelength of light. Additionally, we note that the polarization response differs qualitatively for single and multiple coupled emitters, with possible implications for sensing and the characterization of waveguide coupled spins.

Single-photon sources are essential tools for quantum information technology and testing the foundations of quantum theory. To date, single-photon sources which emit purely polarized single photons (e.g. horizontal polarization and vertical polarization) have been realized in various ways. On the other hand, single-photon sources which emit completely unpolarized single photons are also important for applications such as realizing hardware random number generators and testing the foundations of quantum theory such as the error-disturbance relation for mixed states [1, 2] which are quantum states of unpolarized polarization states. Here, we present a dynamically and statically unpolarized single-photon source using a single nitrogen-vacancy (NV) center in diamond. In particular, we demonstrate a new method for evaluation of dynamical unpolarization using the second order correlation function [3].

Nanophotonic devices are becoming increasingly important for enabling the controlled and efficient production of photons in well defined modes of optical resonators and waveguides. A particular class of such devices which has seen extensive application in recent years is that of evanescently coupled waveguides which can be used to create fiber-coupled single photon sources or to couple light to other nanophotonic systems. However, evanescent coupling typically results in equal intensities coupled to either port of the waveguide, leading to a 50% if it is desired to send photons to a particular receiver.

We studied the population and coherence dynamics of a biexciton in a planar CuCl microcavity. We developed a time-resolved two-photon polarization spectroscopy technique, by which we extracted the population dynamics of the biexciton even when the biexciton was spectrally overlapped with the lower cavity polariton. The observed lifetime of the cavity biexciton in the weak-coupling regime was much shorter than that of a bare CuCl thin film without a cavity, which directly demonstrates cavity enhancement of the radiative decay rate of the biexciton. The cavity enhancement originates from the hybridization of the unbound two-cavity-polariton state to the biexciton wave function by the biexciton-cavity coupling. By comparing the population and coherence decay curves of the biexciton, we found that the coherence of the cavity biexciton was limited by the radiative population decay.

We studied the CuCl microcavity designed for the enhancement of the biexciton-photon coupling, in terms of spectrally resolved four-wave mixing. We observed the strong dependence of the spectrum on the polarization of the pump pulses and the polarization correlation between the pump pulses and the four-wave mixing signal, suggesting that the biexciton state is successfully formed in the CuCl microcavity. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the excitonic Rabi oscillations in self assembled quantum dots observed by photon echo spectroscopy. The amplitude of the observed Rabi oscillations increases with the pulse area, which is different from the results of previous reports. A numerical calculation of the Rabi oscillations including the local field effect in a quantum dot well reproduces the experimental oscillations, indicating that the local field effect is one of the important origins of the modification of the Rabi oscillations in quantum dots (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coherent electron spin dynamics is observed by tomographic Kerr rotation measurement in a GaAs/AlGaAs quantum well. Coherent measurement of the electron spin state prepared via the light-hole exciton was reported previously. In this study, we confirm that the electron spin state prepared via the heavy-hole exciton is also observable coherently and directly. Additionally, the magneto-optical spectrum is measured around the energy of the light-hole exciton under an in-plane magnetic field. These results lead us to better understanding of the tomographic Kerr rotation measurement.

Optical fibres have proved to be an important medium for manipulating and generating light in applications including soliton transmission1, light amplification2, all-optical switching3 and supercontinuum generation4. In the quantum regime, fibres may prove useful for ultralow-power all-optical signal processing5 and quantum information processing6. Here, we demonstrate the first experimental observation of optical nonlinearity at the single-photon level in an optical fibre. Taking advantage of the large nonlinearity and managed dispersion of photonic crystal fibres7,8, we report very small (1 × 10−7 to ∼1 × 10−8 rad) conditional phase shifts induced by weak coherent pulses that contain one or less than one photon per pulse on average. We discuss the feasibility of quantum information processing using optical fibres, taking into account the observed Kerr nonlinearity, accompanied by ultrafast response time and low induced loss. The tiny phase changes introduced by nonlinear optics performed at the single-photon level is reported in a photonic crystal fibre with carefully designed nonlinear and dispersion properties. The approach may prove useful in future quantum information processing schemes.

We studied photoinduced Kerr effect in cavity polaritons in a semiconductor microcavity by carefully measuring the time-resolved Kerr rotation and ellipticity spectra. The Kerr rotation angle of the microcavity polaritons is a hundred times larger than a conventional single quantum well. A spectral shape analysis of the observed spectra suggests that the photoindeced Kerr effect mainly results from a line broadening of the polarions due to spin-polarized polariton-polariton scattering. Enhancement in oscillator strength also contributes to the large rotation angle. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum information can be stored and manipulated by an electron spin if its state was faithfully transferred from messenger photon qubit. Such a quantum state transfer needs to have a function of conversion from photon polarization to electron spin state and flagging the safe conversion without destroying the spin state. We have achieved single photon response by using a quantum dot as an electron trap and a quantum point contact as a charge sensor on GaAs/AlGaAs-based modulation doped structure. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We measured time-resolved photon echo signals by using the heterodyne detection technique on GaAs multiple quantum wells. Oscillation structure with a period of about 0.8 ps is observed when the time interval (delay time) of two pulses which generate the photon echo signal is shorter than 0.4 ps. The oscillating structure disappears when the delay time is longer than 0.6 ps. Temperature and excitation density dependence indicate that the origin of this oscillation is the interaction between a normal exciton and an exciton coupled with a specific acoustic phonon mode which is identified by calculating the Rytov model.

We report on the polarization dependence for hyper-Rayleigh scattering (HRS) in two-photon resonance with exciton states in ZnSe. The HRS signal is enhanced drastically when twice the incident photon energy is resonant with the excitation energy of the 2P excitons in ZnSe/GaAs thin films. The selection rules for the HRS in two-photon resonance with the 2P excitons are different from those for nonlinear susceptibility determined by crystal symmetry. Further, we report a study of the symmetries of the 2P excitonic wave function with linearly- and circularly-polarized incident beams. The heavy and light hole 2P excitons belong to different symmetries of the base functions.

We develop a theory of active dephasing control of qubit systems in array structures by using the multi-wave mixing technique. Assuming an acoustic phonon with a cutoff frequency ω c as the reservoir boson, we evaluate the state evolution of the qubit–reservoir system under irradiation of an optical pulse sequence. The effect of active dephasing control by the optical π pulses would be observed as diffracted echo signals in the phase-matching direction. The analysis for the case of weak pulse control is also presented.

The phonon bottleneck effect is studied in InAs/GaInP quantum dots (QDs). The time-resolved photoluminescence spectra from the lowest ( n =1) and second lowest ( n =2) exciton states in QDs are measured in the temperature range of 5– 200 K . We find that the rise time of the n =1 level is about 500 ps when the excitation energy is in resonance with the n =2 level. This is a long time compared to about 200 ps in non-resonant excitation, and therefore the bottleneck effect exists between the levels n =2 and 1. In order to understand the source of the bottleneck, the temperature dependence of carrier relaxation time between the exciton levels is investigated in the assigned temperature range. With increasing temperature, the relaxation time decreases to 200 ps at 200 K . The data are in good agreement with the theoretical fit of the relaxation probability for multi-phonon processes.

The signals due to six-wave mixing (SWM) processes are observed for two-dimensional excitons and biexcitons. The signal intensity of the SWM process strongly depends on the pulse area of the excitation light. The pulse area is estimated by comparing the intensities of four-wave mixing (FWM) signals in two different directions. The photon echo and the transient-grating signals due to the biexcitonic state of GaSe were observed by SWM. When we change the order of the excitation pulses in our SWM experiments, the excitonic phase relaxation process can be observed in the negative delay-time region.

We present experimental data obtained from ferromagnetic p-(Ga,Mn)As layers that indicate the collective rotation of ferromagnetically coupled Mn spins by optically generated spin-polarized holes through the p–d exchange interaction. The rotation occurs reversibly between the in-plane and perpendicular directions, causing a large change in perpendicular magnetization without the application of a magnetic field. Pump-and-probe experiments suggest that the rotation of Mn spins take place in the picosecond time domain.

Recently, we have reported that the spin-polarized holes generated by the irradiation with circularly polarized light can change the magnetization orientation of III–V ferromagnetic semiconductor (Ga,Mn)As via p–d exchange interaction. In this paper, we report that a small portion of change does not return to the initial state after the light is turned off. This residual component, named as the memorization effect, exhibits ferromagnetic characteristics. This fact strongly suggests that small magnetic domains having the perpendicular magnetic axis are rotated by photogenerated carrier spins.

We performed process tomography of a coherent state transfer from light polarization into electron spin polarization in a semiconductor quantum well. The density matrix of the generated electron spin state was reconstructed by a three-dimensional tomographic Kerr rotation measurement so that we could quantitatively evaluate the transfer process. We found that the process fidelity of the state transfer was 81% and clarified that the phase-flip error caused by mixing with another Zeeman sublevel at the hole state was the major contributor to fidelity degradation.

The phase characteristics of transmitted optical pulses were investigated in the energy- and the temporal domain using the spectrally resolved cross-correlation technique. The temporal evolution of fs pulses passing through GaAs/AlGaAs quantum wells indicates a long tail due to the coherent emission from the exciton. The phase discontinuity was observed in the time-domain data. The phase of the coherent emission signal is found to differ by 180° from that of the incident light field. Furthermore, we report the pulse propagation effect when the pre-excited coherent exciton exists in the sample. The resultant data shows that the ultrashort optical pulses are strongly affected by the phase difference between the coherent emission and the coherent exciton polarization.

Time-resolved three-pulse photon echoes of the exciton in layered semiconductor GaSe are investigated by using up-conversion technique. The time-resolved echo signal depends on the separation time between the first and second pulses. The shape of the echo signal changes as the separation time increases. The time-resolved signal also depends on the waiting time between the second and third pulses. For a short waiting time, the temporal evolution of the echo signal shows an ideal photon-echo-like (Gaussian-like) shape. On the other hand, for longer waiting times, the temporal shape of the echo signal changes dramatically and has an extra peak at the arrival time of the third pulse. This behavior can be explained by spectral diffusion in the three-pulse photon echo process.

The coherent emission in the direction of the reflection has been investigated under the Brewster angle of incidence by using phase-locked pulse pairs to excite resonantly the excitons in the anisotropic crystal GaSe. The interferogram of the reflected pulse has a long tail corresponding to the free induction decay signal. We also observed the phase difference between the coherent emission from the Γ4 and Γ6 excitonic polarizations in the direction of the reflection.

We report the demonstration of decoherence control of excitons on a layered compound semiconductor GaSe by using successive three femtosecond pulses, i.e., the six-wave mixing configuration. The second pulse acts as a π pulse which reverses the time evolution of non-Markovian dynamics. By changing the pulse interval conditions, we confirmed for the first time the suppression of exciton decoherence by π pulse irradiation.

We observed the antibound state, as well as the bound state, for cavity polariton pairs in a planar CuCl microcavity by spectrally resolved four-wave mixing. We obtained dispersion curves of the bound and antibound states by changing the incident angle of the pump pulses corresponding to the cavity detuning. The dispersion curve for the bound state suggests that the bound state is mainly composed of a bare biexciton and is weakly coupled to the cavity photons. The dephasing time of the bound state was faster than that of a bare biexciton in a thin sample, supporting the hypothesis that the bound state is coupled to the cavity photons. On the other hand, the antibound state consists of two lower polaritons having the same spin. The clear observation of the antibound state can be qualitatively explained by the phase-space filling, which reduces the Rabi splitting.

We demonstrate that a coherent superposition state of two temporally separated optical pulses, called a time-bin state, can be transferred to that of up/down electron spins in a semiconductor by synchronizing the time separation to the precession period of either electrons or holes. The time-bin transfer scheme does not require polarization mode degeneracy and can map the time-bin state to the electron spin state that is not accessible directly using only polarization. The scheme offers a new approach for quantum interfaces between photons and electron spins.

We have shown that the normally forbidden second harmonic generation signal is enhanced drastically when twice the incident photon energy is resonant with the excitation energy of a 2P exciton in ZnSe/GaAs thin films. Using this resonant effect and a novel spectroscopic technique which utilizes a phase-locked laser pulse pair, we have investigated the dynamical behavior of 2P excitons. The interferogram of exciton polarization has a tail of a few picoseconds. Beating structure was observed in the decay profile. This structure can be explained by polarization interference between the heavy hole and light hole excitons split by strain. Further, we have studied the fine structure of the 2P exciton in ZnSe films grown on GaAs substrates using this resonance phenomenon. The 2P exciton line is found to show distinct fine structure caused by envelope–hole coupling, in addition to splitting of the exciton state due to strain effects.

We report on the experimental generation of an entangled state with a spectrally pure heralded single-photon state and a weak coherent state. By choosing group-velocity matching in the nonlinear crystal, our system for producing entangled photons was 60 times brighter than that in the earlier experiment [Phys. Rev. Lett. 90, 240401 (2003)], with no need of bandpass filters. This entanglement system is useful for quantum information protocols that require indistinguishable photons from independent sources., Comment: 7 pages, 5 figures

We experimentally studied the excitonic Rabi oscillations in self-assembled semiconductor quantum dots using two-pulse photon echo spectroscopy. We observed the change in the shape of the Rabi oscillations by the second pulse intensity. Our experimental results were systematically reproduced by numerical calculations taking into account the local field effect arising from the Coulomb interaction between photoinduced charge densities in a quantum dot. Our experimental findings indicated that the local field effect contributes considerably to the coherent optical responses of the excitons in quantum dots.

We demonstrate magneto-optical double resonance of a single NV center in diamond for quantum media conversion between a photon and an electron or a nuclear spin via resonant photo-absorption.

The polarization dependence of 3-pulse four wave mixing (FWM) at the exciton resonance has been studied in GaSe. A quantum beat modulation can be observed in 3-pulse FWM signals. The polarization dependence of the quantum beat has also been studied. The quantum beat and the polarization interference can be experimentally distinguished by the time-integrated 3-pulse FWM.

The polarization dependence of the stimulated photon echo due to excitons has been studied in GaSe. A quantum beat modulation can be observed in the stimulated photon echo signals. The polarization dependence of the quantum beat is also studied. The quantum beat and the polarization interference are experimentally distinguished by the time-integrated stimulated photon echo.

Entanglement is one of the essential resources in quantum information and communication technology (QICT). The entanglement thus far explored and applied to QICT has been pure and distillable entanglement. Yet there is another type of entanglement, called 'bound entanglement', which is not distillable by local operations and classical communication (LOCC). We demonstrate the experimental 'activation' of the bound entanglement held in the four-qubit Smolin state, unleashing its immanent entanglement in distillable form, with the help of auxiliary two-qubit entanglement and LOCC. We anticipate that it opens the way to a new class of QICT applications that utilize more general classes of entanglement than ever, including bound entanglement., 6pages, 6figures

We report the efficient generation of a four-qubit bound entangled Smolin state. We also demonstrate the activation of the bound entanglement; we make the bound entanglement in the Smolin state distillable with the help of an additional Bell pair.

We report the experimental realization of the four-partite bound-entangled state, i.e., Smolin state [1] and demonstrate the ‘superactivation’ of the bound entanglement. Bound entanglement is a class of multipartite entanglement that cannot be distilled into pure entanglement under local operations and classical communication (LOCC). Despite its undistillable property, this class of entanglement has interesting properties that are expected to be useful in quantum information processing (QIP). For instance, in the Smolin state, the bound entanglement can be activated into distillable entanglement when two of the parties get together. The process is called “unlocking” [1], and has been demonstrated experimentally [2]. More importantly, the bound entanglement can be activated with the help of entanglement in other systems [3]. The activation of bound entanglement, i.e., superactivation, plays an essential role in the QIP protocols using bound entangled states (e.g., remote information concentration [4]).

The reduction of de Broglie wavelength is one of the fascinating characteristics of multiphoton quantum interference. It will be applicable to, for instance, super-resolution imaging beyond the classical diffraction limit and ultrasensitive phase measurement beyond the standard quantum limit. Thus far, the reduced de Broglie wavelength has been experimentally demonstrated using visible or near infrared photons [1–3]. However, it is still a challenging task to demonstrate multiphoton quantum interference in a telecom-wavelength region, because of insufficient quantum efficiencies of single-photon detectors working at telecom wavelength. Also, efficient photon sources of multiple photons that are mutually indistinguishable are essential to carry out various experiments including multiphoton quantum interference and quantum information protocols. In this context, we have developed an efficient photon-pair source designed for such multiphoton experiments at telecom wavelength. Making use of this source, we demonstrate the first measurement of reduced de Broglie wavelength of the four-photon state at the telecom wavelength.

We present an experiment of nonclassical interference between an intrinsically pure heralded single-photon state and a weak coherent state. Our experiment demonstrates that, without the use of bandpass filters, spectrally pure single photons can have high-visibility ($89.4\ifmmode\pm\else\textpm\fi{}0.5%$) interference with photons from a weak coherent field. Our scheme lays the groundwork for future experiments requiring quantum interference between photons in nonclassical states and those in coherent states.