QuIN LAB
2023
Carbon nanotubes (CNTs) are proving to be versatile nanomaterials that exhibit superior and attractive electrical, optical, chemical, physical, and mechanical properties. Different kinds of CNTs exist, and their associated properties have been actively explored and widely exploited from fundamental studies to practical applications. Obtaining highquality CNTs in large volumes is desirable, especially for scalable electronic, photonic, chemical, and mechanical systems. At present, abundant but random CNTs are synthesized by various growth methods including arc discharge, chemical vapor deposition, and molecular beam epitaxy. An economical way to secure pristine CNTs is to disperse the raw soot of CNTs in solutions, from which purified CNTs are collected via sorting methods. Individual CNTs are generally hydrophobic, not readily soluble, requiring an agent, known as a surfactant to facilitate effective dispersions. Furthermore, the combination of surfactants, polymers, DNA, and other additives can enhance the purity of specific types of CNTs in confidence dispersions. With highlypure CNTs, designated functional devices are built to demonstrate improved performance. This review surveys and highlights the essential roles and significant impacts of surfactants in dispersing and sorting CNTs. [DOI:10.1002/jsde.12702][arXiv:]
The phase distribution in a BoseEinstein condensate can realize various topological states which can be classified according to distinct winding numbers. While states with different winding numbers are topologically protected in the linear Schrödinger equation, when nonlinearities are introduced, violations of the topological protection can occur, leading to unwinding. Excitonpolariton condensates constitute a weakly nonlinear open dissipative system that is well suited to studying such physics. Here we show that excitonpolariton condensates display a spontaneous phase unwinding from a π– to zerostate. While such an effect was previously observed in a onedimensional polaritoncondensate array and explained as occurring due to singleparticle mode competition, we offer a new explanation in terms of collective phase unwinding of metastable states. We clarify that the collective transition is caused by the combined effect of nonlinearity and topological defects in the condensates. Reanalyzing the experimental data, we find an evidence of the collective phase unwinding. [DOI:][arXiv:2307.06550]
Timefluctuating signals are ubiquitous and diverse in many physical, chemical, and biological systems, among which random telegraph signals (RTSs) refer to a series of instantaneous switching events between two discrete levels from singleparticle movements. A reliable RTS analysis is a crucial prerequisite to identify underlying mechanisms related to device performance and sensitivity. When numerous levels are involved, complex patterns of multilevel RTSs occur and make their quantitative analysis exponentially difficult, hereby systematic approaches are often elusive. In this work, we present a threestep analysis protocol via progressive knowledgetransfer, where the outputs of the early step are passed onto a subsequent step. Especially, to quantify complex RTSs, we resort to three deep neural network architectures whose trained models can process raw temporal data directly. We furthermore demonstrate the model accuracy extensively with a large dataset of different RTS types in terms of additional background noise types and amplitude size. Our protocol offers structured schemes to extract the parameter values of complex RTSs as imperative information with which researchers can draw meaningful and relevant interpretations and inferences of given devices and systems. [DOI:10.1038/s41598023371249][arXiv:2206.00086]

NISQ is a representative keyword at present as an acronym for “noisy intermediatescale quantum”, which identifies the current era of quantum information processing (QIP) technologies. QIP science and technologies aim to accomplish unprecedented performance in computation, communications, simulations, and sensing by exploiting the infinite capacity of parallelism, coherence, and entanglement as governing quantum mechanical principles. For the last several decades, quantum computing has reached to the technology readiness level 5, where components are integrated to build midsized commercial products. While this is a celebrated and triumphant achievement, we are still a great distance away from quantumsuperior, faulttolerant architecture. To reach this goal, we need to harness technologies that recognize undesirable factors to lower fidelity and induce errors from various sources of noise with controllable correction capabilities. This review surveys noisy processes arising from materials upon which several quantum architectures have been constructed, and it summarizes leading research activities in searching for origins of noise and noise reduction methods to build advanced, largescale quantum technologies in the near future. [DOI:10.3390/ma16072561]
 2022
Giant exciton–polaritons come to the scene from a thin Cu2O crystal sandwiched by a microcavity. Their anticipated strong interactions may facilitate the development of a promising Rydberg solidstate platform for quantum technologies. [DOI:10.1038/s41563022012719]
We perform systematic numerical simulations of carbon nanotube (CNT) film microstrip antennas for flexible and durable applications in terms of various material parameters. The selection of appropriate materials for conductive films and a substrate of the conformable and robust microstrip antennas are crucial to increase the radiation efficiency and to reduce the losses while maintaining the mechanical properties. CNTs have been spotlighted as a promising nanomaterial, exhibiting excellent electrical and mechanical performances as desirable fea tures for microwave wearable devices. Considering the material properties of the conductor and the substrate, we examine the possible ranges of the CNT film conductivities, conductive film thickness, and a dielectric constant and thickness of a substrate. Furthermore, we model nonuniform spatial distributions of conductivity in the CNT film to assess their impact on the antenna performance. Our extensive studies of material constants and conductivity spatial patterns propose design guidelines for optimal microstrip antennas made of CNT conductive films operating in microwave frequencies.[DOI: ][arXiv: 2206.09084]
The ability to perform broadband optical spectroscopy with subdiffractionlimit resolution is highly soughtafter for a wide range of critical applications. However, sophisticated tipenhanced techniques are currently required to achieve this goal. We bypass this challenge by demonstrating an extremely broadband photodetector based on a twodimensional (2D) van der Waals heterostructure that is sensitive to light across over a decade in energy from the midinfrared (MIR) to deepultraviolet (DUV) at room temperature. The devices feature high detectivity (> 10^9 cm Hz^1/2 W^1) together with high bandwidth (2.1 MHz). The active area can be further miniaturized to submicron dimensions, far below the diffraction limit for the longest detectable wavelength of 4.1 um, enabling such devices for facile measurements of local optical properties on atomiclayerthickness samples placed in close proximity. This work can lead to the development of lowcost and highthroughput photosensors for hyperspectral imaging at the nanoscale.[DOI:10.1021/acs.nanolett.2c00741][arXiv:2202.00049]
Microcavity excitonpolaritons are attractive quantum quasiparticles resulting from strong lightmatter coupling in a quantumwellcavity structure. They have become one of the most stimulating solidstate material platforms to explore beautiful collective quantum phenomena originating from macroscopic coherence in condensation and superfluidity, BerezinskiiKosterlitzThouless transition, and various topological excitations in the form of solitons, vortices, and skyrmions. They can also provide opportunities for the development of pioneering photonic devices by exploiting bistability and parametric scatterings due to strong nonlinearity that possess remarkable performance advantages of powerefficient operation, ultrafast response time, and scalable planar geometries. This story becomes profound and fascinating when we take into account the spins of excitons, that can be directly accessed through light polarization states. The purpose of this review is to give central principles of microcavity excitonpolariton spins and their anisotropic interactions, which can couple with the effective magnetic fields from modesplitting of microcavity photons and spindependent relaxation processes of quantumwell excitons. Furthermore, we summarize notable theoretical and experimental research activities to reveal extraordinary quantum phenomena of spinresolved topological states and exotic spin textures and to devise novel spinbased photonic devices based on microcavity excitonpolaritons. [DOI:10.1002/qute.202100137]
We observe rich phenomena of twolevel random telegraph noise (RTN) from a commercial bulk 28nm pMOSFET (PMOS) near threshold at 14 K, where a Coulomb blockade (CB) hump arises from a quantum dot (QD) formed in the channel. Minimum RTN is observed at the CB hump where the highcurrent RTN level dramatically switches to the lowcurrent level. The gatevoltage dependence of the RTN amplitude and power spectral density match well with the transconductance from the DC transfer curve in the CB hump region. Our work unequivocally captures these QD transport signatures in both current and noise, revealing quantum confinement effects in commercial shortchannel PMOS even at 14 K, over 100 times higher than the typical dilution refrigerator temperatures of QD experiments (<100 mK). We envision that our reported RTN characteristics rooted from the QD and a defect trap would be more prominent for smaller technology nodes, where the quantum effect should be carefully examined in cryogenic CMOS circuit designs. [DOI:10.1109/LED.2021.3132964][arXiv:2206.09086]
 2021
Rydberg excitons in Cu2O can be an emergent platform for solidstate quantum information processing by utilizing the exaggerated properties of highlying excited states within the material. To develop practical quantum systems, hightemperature operation is desirable. Here, we study the temperaturedependence of the yellow and green Rydberg exciton resonances in a thin Cu2O crystal via broadband phononassisted absorption spectra between 4 K and 100 K. At 4 K, we can identify the principal quantum number n = 11 yellow and n = 4 green Rydberg exciton states, beyond which we are limited by the spectral resolution of standard absorption techniques. Above liquid nitrogen boiling temperature (~80 K), the n = 6 yellow and n = 4 green Rydberg exciton states are readily captured and highertemperature yellow Rydberg exciton optical properties still exhibit the standard scaling laws seen at low temperatures. This promising result lays the groundwork for a new route to build a hightemperature Rydberg quantum information processing architecture with solidstate Cu2O. [DOI:10.1103/PhysRevB.103.205203][arXiv:2105.00326]
 2020
 Hexagonal boron nitride (hBN) is being increasingly used in optoelectronic devices to electronically and/or chemically isolate materials like graphene and carbon nanotubes from the environment or other device components. Solution exfoliation is a scalable method to produce large quantities of nanometerthick hBN but it remains challenging to integrate these discrete flakes into uniform thin films by conventional solution processing approaches. In this work, we demonstrate how a modified Langmuir–Blodgett coating approach can be used to assemble densely tiled layers of the exfoliated hBN flakes arranged edgetoedge at the air–water interface. These floating films can be transferred to effectively any solid substrate yielding pure hBN films without utilizing any surfactant or polymer additive. Building films layerbylayer yields pinholefree films over large areas (>cm^{2}) with high optical transparency per deposition. The use of room temperature processing makes the approach particularly wellsuited for use in transparent and flexible devices. [DOI:10.1039/D0TC02933E]
The harmonic oscillator is a foundational concept in both theoretical and experimental quantum mechanics. Here, we demonstrate harmonic oscillators in a semiconductor platform by faithfully implementing continuously graded alloy semiconductor quantum wells. Unlike current technology, this technique avoids interfaces that can hamper the system and allows for the production of multiwell stacks several micrometers thick. The experimentally measured system oscillations are at 3 THz for two structures containing 18 and 54 parabolic quantum wells. Absorption at room temperature is achieved: this is as expected from a parabolic potential and is unlike square quantum wells that require cryogenic operation. Linewidths below 11% of the central frequency are obtained up to 150 K, with a 5.6% linewidth obtained at 10 K. Furthermore, we show that the system correctly displays an absence of nonlinearity despite electronelectron interactions—analogous to the Kohn theorem. These highquality structures already open up several new experimental vistas. [DOI:10.1103/PhysRevLett.125.097403]
Nine researchers, editors and science communicators share their views about the barriers that Asian scientists encounter in publishing their work and becoming more visible on the international level. [DOI:10.1038/s422540200162z]
 2019
Josephson vortices are observed at the boundary between two excitonpolariton condensates, with lasers used to create the required local phase twist. The finding opens new opportunities for exploring fundamental physics and engineering novel quantum devices. [DOI:10.1038/s4156601904738]
We explore a mechanism for producing timefrequency entangled photon pairs (termed as a biphoton) from an ensemble of atomlike solidstate quantum emitters. Four distinct energy levels of the solidstate system render four spinconserving optical transitions as observed in color centers. This feature opens up the possibility to generate a fourwave mixing biphoton based on an electromagnetic induced transparency (EIT) for longcoherence quantum communication as demonstrated in cold atomic systems. We propose a narrow EIT window below lifetimelimited linewidth of a SiV− in diamond, assuming a few hundred MHz. Consequently, the EITinduced narrowband guarantees biphoton coherence time to be at least a few tens of nanosecond without a cavity. Assessing the criteria of solidstate parameters applicable to the existing biphoton model from cold atoms will accelerate solidstate biphoton source research. This study shows that a realization of negligible ground state dephasing of a solidstate sample will be a crucial step toward a solidstate biphoton generation for more than a hundred nanosecond time scale with a subnatural atomic linewidth of a few MHz.[DOI:10.1364/JOSAB.36.000646]
We measure the phase velocities of surface acoustic waves (SAWs) propagating at dierent crystal orientations on (001)cut GaAs substrates and their temperature dependance. We design and fabricate sets of interdigital transducers (IDTs) to induce 4 μm SAWs via the inverse piezoelectric (PZE) eect between the PZE [110] direction (set as θ = 0°) and the nonPZE [100] direction (θ = 45°) on GaAs. We also prepare ZnOfilm sputtered GaAs substrates in order to launch SAWs eciently by IDTs even in the nonPZE direction. We quantify acoustic velocities between 1.4 K and 300 K from the resonant frequencies in the S11 parameter using a network analyzer. We observe parabolic velocitytemperature trends at all θvalues both on GaAs and ZnO/GaAs substrates. Below 200 K, in ZnO/GaAs substrates slower SAW modes appear around the [110] direction, which are unseen at room temperature.[DOI:10.7567/13474065/ab0008][arXiv:2105.00319]
The electronic band structure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the band structure with band parameters explains intrinsic physical, chemical, and mechanical properties of the solid. Here we report the artificial polaritonic band structures of twodimensional honeycomb lattices for microcavity exciton polaritons using GaAs semiconductors in the widerange detuning values, from cavity photonlike (reddetuned) to excitonlike (bluedetuned) regimes. In order to understand the experimental band structures and their band parameters, such as gap energies, bandwidths, hopping integrals, and density of states, we originally establish a polariton band theory within an augmented plane wave method with twokind bosons, cavity photons trapped at the lattice sites, and freely moving excitons. In particular, this twokind band theory is absolutely essential to elucidate the exciton effect in the band structures of bluedetuned excitonpolaritons, where the flattened excitonlike dispersion appears at larger inplane momentum values captured in our experimental access window. We reach an excellent agreement between theory and experiments in all detuning values.[DOI:10.1103/PhysRevB.99.045302]
 PreQuIN LAB Era
 PeerReviewed Paper
 20162015
The BerezinskiiKosterlitzThouless (BKT) theorem predicts that twodimensional bosonic condensates exhibit quasilongrange order which is characterized by a slow decay of the spatial coherence. However previous measurements on excitonpolariton condensates revealed that their spatial coherence can decay faster than allowed under the BKT theory, and different theoretical explanations have already been proposed. Through theoretical and experimental study of excitonpolariton condensates, we show that the fast decay of the coherence can be explained through the simultaneous presence of multiple modes in the condensate. [DOI:10.1103/PhysRevA.93.053622]
We study the very strong coupling effect in a semiconductor optical microcavity in which the exciton radius is dramatically modified due to the presence of strong photon–exciton coupling by means of nonlinear numerical optimization. To experimentally verify the features of very strong coupling and distinguish them from those of strong coupling, we propose two schemes and show that our proposals can provide unequivocal experimental proof of the existence of very strong coupling. [DOI:10.1088/13672630/17/2/023064]
2014
Recently a new type of system exhibiting spontaneous coherence has emerged—the exciton–polariton condensate. Exciton–polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The polaritons have a lifetime that is typically comparable to or shorter than thermalization times, giving them an inherently nonequilibrium nature. Nevertheless, they exhibit many of the features that would be expected of equilibrium Bose–Einstein condensates (BECs). The nonequilibrium nature of the system raises fundamental questions as to what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus on several physical phenomena exhibited by exciton–polariton condensates. In particular, we examine topics such as the difference between a polariton BEC, a polariton laser and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, and Berezinskii–Kosterlitz–Thouless and Bardeen–Cooper–Schrieffer physics. We also discuss the physics and applications of engineered polariton structures.[DOI:10.1038/nphys3143][arXiv:1411.6822]
We report the condensation of microcavity excitonpolaritons at Γ points located on the boundary between the third and higher Brillouin zones in hexagonal lattices: triangular and honeycomb geometries. We collect experimental evidence that supports the finite momentum condensation: (1) the coherent Bragg peaks formed at nonzero Γ points; (2) the nonlinear intensity increase in the excitonpolariton emission at quantum degeneracy threshold; (3) the spectral linewidth behavior: narrowing near threshold and broadening above threshold; and (4) the equivalent 4fy3−3x2ylike orbital symmetry in real space. The forbital state at Γ points appears as a metastable momentum valley to trap excitonpolaritons, which is explained by singleparticle bandstructure calculations.[DOI:10.1103/PhysRevB.89.085306]
2013
Entanglement between stationary quantum memories and photonic qubits is crucial for future quantum communication networks. Although highfidelity spin–photon entanglement was demonstrated in wellisolated atomic and ionic systems, in the solidstate, where massively parallel, scalable networks are most realistically conceivable, entanglement fidelities are typically limited due to intrinsic environmental interactions. Distilling highfidelity entangled pairs from lowerfidelity precursors can act as a remedy, but the required overhead scales unfavourably with the initial entanglement fidelity. With spin–photon entanglement as a crucial building block for entangling quantum network nodes, obtaining highfidelity entangled pairs becomes imperative for practical realization of such networks. Here we report the first results of complete state tomography of a solidstate spin–photonpolarizationentangled qubit pair, using a single electroncharged indium arsenide quantum dot. We demonstrate recordhigh fidelity in the solidstate of well over 90%, and the first (99.9%confidence) achievement of a fidelity that will unambiguously allow for entanglement distribution in solidstate quantum repeater networks.[DOI:10.1038/ncomms3228]
Hightlights: [Stanford News][LaserFocusWorld][Photonics][Optics.org][Asme.org][Design products & applications]
Conventional semiconductor laser emission relies on stimulated emission of photons, which sets stringent requirements on the minimum amount of energy necessary for its operation. In comparison, exciton–polaritons in strongly coupled quantum well micro cavities can undergo stimulated scattering that promises more energyefficient generation of coherent light by ‘polariton lasers’. Polariton laser operation has been demonstrated in optically pumped semiconductor microcavities at temperatures up to room temperature, and such lasers can outperform their weakcoupling counterparts in that they have a lower threshold density. Even though polariton diodes have been realized, electrically pumped polariton laser operation, which is essential for practical applications, has not been achieved until now. Here we present an electrically pumped polariton laser based on a microcavity containing multiple quantum wells. To prove polariton laser emission unambiguously, we apply a magnetic field and probe the hybrid light–matter nature of the polaritons. Our results represent an important step towards the practical implementation of polaritonic light sources and electrically injected condensates, and can be extended to roomtemperature operation using widebandgap materials. [DOI:10.1038/nature12036]
Dirac particles, massless relativistic entities, obey linear energy dispersions and hold important implications in particle physics. The recent discovery of Dirac fermions in condensed matter systems including graphene and topological insulators has generated a great deal of interest in exploring the relativistic properties associated with Dirac physics in solidstate materials. In addition, there are stimulating research activities to engineer Dirac particles, elucidating their exotic physical properties in a controllable setting. One of the successful platforms is the ultracold atom–optical lattice system, whose dynamics can be manipulated and probed in a clean environment. A microcavity exciton–polariton–lattice system offers the advantage of forming highorbital condensation in nonequilibrium conditions, which enables one to explore novel quantum orbital order in two dimensions. In this paper, we experimentally construct the band structures near Dirac points, the vertices of the first hexagonal Brillouin zone with exciton–polariton condensates trapped in a triangular lattice. Due to the finite spectral linewidth, the direct map of band structures at Dirac points is elusive; however, we identify the linear part above Dirac points and its associated velocity value is ~0.9–2 × 10^8 cm/s, consistent with the theoretical estimate 1 × 10^8 cm/s with a 2 μm lattice constant. We envision that the exciton–polariton condensates in lattices would be a promising solidstate platform, where the system order parameter can be accessed in both real and momentum spaces.[DOI:10.1088/13672630/15/3/035032][arXiv:1210.2153]
We explore the excitonpolariton condensation in two degenerate orbital states. In the honeycomb lattice potential, at the third band we have two degenerate vortexantivortex lattice states at inequivalent K– and K′points. We have observed energetically degenerate condensates within the linewidth ∼0.3 meV, and directly measured the vortexantivortex lattice phase order of the order parameter. We have also observed the intensity anticorrelation between polariton condensates at the K– and K′points. We relate this intensity anticorrelation to the dynamical feature of polariton condensates induced by the stochastic relaxation from the common particle reservoir.[DOI:10.1103/PhysRevB.87.214503][arXiv:1211.3833]
We show that when following a simple cavity design metric, a quantum well excitonmicrocavity photon coupling constant can be made substantially larger than the exciton binding energy in GaAsbased optical microcavities. Consequently the very strong coupling regime becomes accessible in which a strong asymmetry between upper and lower polariton branches may be observed experimentally. We further show that the corresponding polariton dissociation and saturation boundaries on the phase diagram are much extended, which suggests the possibility of constructing a room temperature, high power excitonpolariton laser without resorting to wide bandgap semiconductors.[DOI:10.1103/PhysRevB.87.115303][arXiv:1210.0294]
2012
Hightlights: [Nature News&View][Stanford News][Physics World][IEEE Spectrum]
Longdistance quantum teleportation and quantum repeater technologies require entanglement between a single matter quantum bit (qubit) and a telecommunications (telecom)wavelength photonic qubit. Electron spins in III–V semiconductor quantum dots are among the matter qubits that allow for the fastest spin manipulation and photon emission, but entanglement between a single quantumdot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to nearinfrared wavelengths, where silica fibre losses are so high that longdistance quantum communication protocols become difficult to implement. Here we demonstrate entanglement between an InAs quantumdot electron spin qubit and a photonic qubit, by frequency downconversion of a spontaneously emitted photon from a singly charged quantum dot to a wavelength of 1,560 nanometres. The use of sub10picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate whichpath information in the photon energy. Together with previously demonstrated indistinguishable singlephoton emission at high repetition rates, the present technique advances the III–V semiconductor quantumdot spin system as a promising platform for longdistance quantum communication.[DOI:10.1038/nature11577]
We describe a design to implement a twoqubit geometric phase gate, by which a pair of electrons confined in adjacent quantum dots are entangled. The entanglement is a result of the Coulomb exchange interaction between the optically excited exciton polaritons and the localized spins. This optical coupling, resembling the electronelectron RudermanKittelKasuyaYosida interactions, offers high speed, high fidelity twoqubit gate operation with moderate cavity quality factor Q. The errors due to the finite lifetime of the polaritons can be minimized by optimizing the optical pulse parameters (duration and energy). The proposed design, using electrostatic quantum dots, maximizes entanglement and ensures scalability. [DOI:10.1103/PhysRevB.85.241403][arXiv:1201.3725]
Microcavity exciton–polariton condensates, as coherent matter waves, have provided a great opportunity to investigate hydrodynamic vortex properties, superfluidity and lowenergy quantum state dynamics. Recently, exciton condensates were trapped in various artificial periodic potential geometries: onedimensional (1D), 2D square, triangular and hexagonal lattices. The 2D kagome lattice, which has been of interest for many decades, exhibits spin frustration, giving rise to magnetic phase order in real materials. In particular, flat bands in the 2D kagome lattice are physically interesting in that localized states in the real space are formed. Here, we realize exciton–polariton condensates in a 2D kagome lattice potential and examine their photoluminescence properties. Above quantum degeneracy threshold values, we observe metastable condensation in highenergy bands; the third band exhibits a signature of weaker dispersive band structures, a flat band. We perform a singleparticle band structure calculation to compare measured band structures.[DOI:10.1088/13672630/14/6/065002]
20112006
Macroscopic order appears as the collective behaviour of many interacting particles. Prime examples are superfluidity in helium, atomic Bose–Einstein condensation, swave and d wave superconductivity and metal–insulator transitions. Such physical properties are tightly linked to spin and charge degrees of freedom and are greatly enriched by orbital structures. Moreover, highorbital states of bosons exhibit exotic orders distinct from the orders with realvalued bosonic ground states. Recently, a wide range of related phenomena have been studied using atom condensates in optical lattices, but the experimental observation of highorbital orders has been limited to momentum space. Here we establish microcavity exciton–polariton condensates as a promising alternative for exploring highorbital orders. We observe the formation of d orbital condensates on a square lattice and characterize their coherence properties in terms of population distributions both in real and momentum space.[DOI:10.1038/nphys2012]
We present an optical setup with focustunable lenses to dynamically control the waist and focus position of a laser beam, in which we transport a trapped ultracold cloud of 87Rb over a distance of . The scheme allows us to shift the focus position at constant waist, providing uniform trapping conditions over the full transport length. The fraction of atoms that are transported over the entire distance comes near to unity, while the heating of the cloud is in the range of a few microkelvin. We characterize the position stability of the focus and show that residual drift rates in focus position can be compensated for by counteracting with the tunable lenses. Beyond being a compact and robust scheme to transport ultracold atoms, the reported control of laser beams makes dynamic tailoring of trapping potentials possible. As an example, we steer the size of the atomic cloud by changing the waist size of the dipole beam.[DOI:10.1088/13672630/16/9/093028] [arXiv:1005.1064]
We propose a device for studying the FermiHubbard model with longrange Coulomb interactions using an array of coupled quantum dots defined in a semiconductor twodimensional electrongas system. Bands above the lowest energy band are used to form the Hubbard model, so that a high average electron density may be used to implement the device. We find that depending on the average electron density, the system is well described by a one or twoband Hubbard model. Our device design enables the control of the ratio of the Coulomb interaction to the kinetic energy of the electrons independently to the filling of the quantum dots, such that a large portion of the Hubbard phase diagram may be probed. Estimates of the Hubbard parameters suggest that a metalMott insulator quantum phase transition and a dwave superconducting phase should be observable using current technologies. [DOI:0.1103/PhysRevB.78.075320] [arXiv:0711.2841]
We present a simple method to create an inplane lateral potential in a semiconductor microcavity using a metal thinfilm. Two types of potential are produced: a circular aperture and a onedimensional (1D) periodic grating pattern. The amplitude of the potential induced by a 24 nm – 6 nm Au/Ti film is on the order of a few hundreds of μeV measured at 6–8 K. Since the metal layer makes the electromagnetic fields to be close to zero at the metal–semiconductor interface, the photon mode is confined more inside of the cavity. As a consequence, the effective cavity length is reduced under the metal film, and the corresponding cavity resonance is blueshifted. Our experimental results are in a good agreement with theoretical estimates. In addition, by applying a DC electric voltage to the metal film, we are able to modify the quantum well exciton mode due to the quantum confined Stark effect, inducing a ∼1 meV potential at ∼20 kV/cm. Our method produces a controllable inplane spatial trap potential for lower excitonpolaritons (LPs), which can be a building block towards 1D arrays and 2D lattices of LP condensates.[DOI:10.1002:pssb.200777610] [arXiv:0805.4673]
The effect of quantum statistics in quantum gases and liquids results in observable collective properties among manyparticle systems. One prime example is Bose–Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose–Einstein condensate is generally defined as a macroscopic occupation of a singleparticle quantum state, a phenomenon technically referred to as offdiagonal longrange order due to nonvanishing offdiagonal components of the singleparticle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The longrange spatial coherence leads to the existence of phaselocked multiple condensates in an array of superfluid helium, superconducting Josephson junctions or atomic Bose–Einstein condensates. Under certain circumstances, a quantum phase difference of is predicted to develop among weakly coupled Josephson junctions. Such a metastable state was discovered in a weak link of superfluid ^{3}He, which is characterized by a ‘pwave’ order parameter. The possible existence of such a state in weakly coupled atomic Bose–Einstein condensates has also been proposed, but remains undiscovered. Here we report the observation of spontaneous buildup of inphase (‘zerostate’) and antiphase (‘state’) ‘superfluid’ states in a solidstate system; an array of exciton–polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These inphase and antiphase states reflect the band structure of the onedimensional polariton array and the dynamic characteristics of metastable exciton–polariton condensates.[DOI:10.1038/nature06334]
We study the electrical transport properties of wellcontacted ballistic singlewalled carbon nanotubes in a threeterminal configuration at low temperatures. We observe signatures of strong electronelectron interactions: the conductance exhibits biasvoltagedependent amplitudes of quantum interference oscillation, and both the current noise and Fano factor manifest biasvoltagedependent powerlaw scalings. We analyze our data within the TomonagaLuttinger liquid model using the nonequilibrium Keldysh formalism and find qualitative and quantitative agreement between experiment and theory.[DOI:10.1103/PhysRevLett.99.036802][arXiv:condmat/0610196]
An experimental scheme for a quantum simulator of strongly correlated electrons is proposed. Our scheme employs electrons confined in a twodimensional electron gas in a GaAs/AlGaAsheterojunction. Two surface acoustic waves are then induced in the substrate, creating a twodimensional “eggcarton” potential. The dynamics of the electrons in this potential are described by a Hubbard model with longrange Coulomb interactions. Estimates of the Hubbard parameters suggest that observations of quantum phase transition phenomena are within experimental reach. [DOI:10.1103/PhysRevLett.99.016405][arXiv:condmat/0608142]
We present a detailed theoretical investigation of transport through a singlewalled carbon nanotube (SWNT) in good contact to metal leads where weak backscattering at the interfaces between SWNT and source and drain reservoirs gives rise to electronic FabryPerot (FP) oscillations in conductance and shot noise. We include the electronelectron interaction and the finite length of the SWNT within the inhomogeneous TomonagaLuttinger liquid (TLL) model and treat the nonequilibrium effects due to an applied bias voltage within the Keldysh approach. In lowfrequency transport properties, the TLL effect is apparent mainly via powerlaw characteristics as a function of bias voltage or temperature at energy scales above the finite level spacing of the SWNT. The FP frequency is dominated by the noninteracting spinmode velocity due to two degenerate subbands rather than the interacting charge velocity. At higher frequencies, the excess noise is shown to be capable of resolving the splintering of the transported electrons arising from the mismatch of the TLL parameter at the interface between metal reservoirs and SWNT’s. This dynamics leads to a periodic shotnoise suppression as a function of frequency and with a period that is determined solely by the charge velocity. At large bias voltages, these oscillations are dominant over the ordinary FP oscillations caused by two weak backscatterers. This makes shot noise an invaluable tool to distinguish the two mode velocities in the SWNT.[DOI:10.1103/PhysRevB.74.235438][arXiv:condmat/0604613]
Book Chapters
Conference Proceedings & ArXiv
A new type of electrically pumped semiconductor laser has been demonstrated which promises an energy efficient laser operation: Recent achievements in the field of ‘polaritonlaser’ development are presented and an outlook is given. [DOI:10.1364/ACPC.2013.AW3B.6]
A Polariton Laser Diode Operated Under Electrical Pumping [article]
Semiconductor quantum dots can be utilized to capture single electron or hole spins and they have therewith promise for various applications in fields like spintronics, spin based quantum information processing and chiral photonics. We integrate quantum dots into semiconductor microcavities to enhance lightmatter interaction for ultrafast optical manipulation and readout. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about μscoherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a Λtype energy level system of a single quantumdot electron spin in a magnetic field and ultrafast nonlinear frequency conversion, quantumdot spinphoton entanglement is observed.[DOI:10.1117/12.2025332]
We report an ultrafast downconversion quantum interface, where 910nm single photons from a quantum dot are downconverted to the 1.5μm telecom band with sub10 picosecond pulses at 2.2μm, enabling the demonstration of quantumdot spinphoton entanglement. [10.1364/CLEO_QELS.2013.QM3B.7]
We propose a scheme for universal quantum computation with electron spin qubits.
The scheme requires electrical control and manipulation of single spin qubit, along with
excitonpolariton mediated twoqubit operation and single shot quantum nondemolition
(QND) readout.[DOI:10.1364/CLEO_QELS.2013.QM3C.5]
We propose an all optical quantum computation scheme, with trapped electron spin qubits, using their Coulomb exchange interaction with optically excited microcavity excitonpolaritons. This paper describes a single qubit rotation, which together with twoqubit controlledz gate presented in PRB 85, 241403(R) (2012), form a set of universal logic gates. The errors due to finite cavity lifetime and incorrect orientation of the rotation axis are minimized by optimizing pump pulse parameters. With projective homodyne phase measurement and initialization, our scheme is a promising candidate for the physical realization of a universal quantum computer.[arXiv: 1208.2252]
We study nonequilibrium differential conductance and current fluctuations in a single quantum point contact. The twoterminal electrical transport properties — differential conductance and shot noise — are measured at 1.5 K as a function of the drainsource voltage and the Schottky splitgate voltage. In differential conductance measurements, conductance plateaus appear at integer multiples of 2e^2/h when the drainsource voltage is small, and the plateaus evolve to a fractional of 2e^2/h as the drainsource voltage increases. Our shot noise measurements correspondingly show that the shot noise signal is highly suppressed at both the integer and the noninteger conductance plateaus. This main feature can be understood by the induced electrostatic potential model within a single electron picture. In addition, we observe the 0.7 structure in the differential conductance and the suppressed shot noise around 0.7 (2e^2/h); however, the previous singleelectron model cannot explain the 0.7 structure and the noise suppression, suggesting that this characteristic relates to the electronelectron interactions.[arXiv:condmat/0311435]