QuIN LAB
2024
Molybdenum disulfide (MoS2) is a representative two-dimensional layered transition-metal dichalcogenide semiconductor. Layer-number-dependent electronic properties are attractive in the development of nanomaterial-based electronics for a wide range of applications including sensors, switches, and amplifiers. MoS2 field-effect transistors (FETs) have been studied as promising future nanoelectronic devices with desirable features of atomic-level thickness and high electrical properties. When a naturally n-doped MoS2 is contacted with metals, a strong Fermi-level pinning effect adjusts a Schottky barrier and influences its electronic characteristics significantly. In this study, we investigate multilayer MoS2 Schottky barrier FETs (SBFETs), emphasizing the metal-contact impact on device performance via computational device modeling. We find that p-type MoS2 SBFETs may be built with appropriate metals and gate voltage control. Furthermore, we propose ambipolar multilayer MoS2 SBFETs with asymmetric metal electrodes, which exhibit gate-voltage dependent ambipolar transport behavior through optimizing metal contacts in MoS2 device. Introducing a dual-split gate geometry, the MoS2 SBFETs can further operate in four distinct configurations: p − p, n − n, p − n, and n − p. Electrical characteristics are calculated, and improved performance of a high rectification ratio can be feasible as an attractive feature for efficient electrical and photonic devices.[DOI:10.1088/1361-6528/ad823e][arXiv:]
The optoelectronic properties of molybdenum disulfide (MoS2) are influenced significantly by sulfur vacancy defects. While tools like electron microscopy can yield precise measurements of vacancies for small samples, they are not suitable for industrial-scale production. Estimates obtained from more scalable approaches like Raman spectroscopy are also subject to large uncertainties. This work introduces a Bayesian model that combines the information from observable Raman shifts with prior information to provide a posterior probability that comprehensively defines what is known about the sulfur vacancy in a MoS2 sample, in the context of measurement uncertainty and model error. This methodology serves as a robust tool to infer the sulfur vacancy concentration variation based on the observed Raman feature in a scalable way.[DOI:10.1557/s43580-024-00946-6][arXiv:]
Recently, 2D materials, such as graphene, have been successfully implemented as artificial conduits of molecular sizes. The extreme precision with which these structures can be fabricated provides an unprecedented framework for the development of highly specific and efficient devices. In this work, we study the electrophoretic transport of Cs+ions in a graphene membrane with effective pore heights of 3.4 Å by conducting molecular dynamics simulations. The entrance of the pore is systematically modified to investigate the effect of pore geometry on ionic conductance. Simulation results suggest a significant correlation between ionic conductance and entrance geometry, with a variation of the conductance up to 100% across the studied cases. To explain the observed correlation, two mechanisms involving an intimate relationship between ion dehydration and edge functional groups are proposed. The present study provides theoretical insights that can aid the design of graphene-based membranes with tunable ionic transport properties.[DOI:10.1039/D4CP00400K][arXiv:]
The phase distribution in a Bose-Einstein 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. Exciton-polariton condensates constitute a weakly nonlinear open dissipative system that is well suited to studying such physics. Here we show that exciton-polariton condensates display a spontaneous phase unwinding from a π– to zero-state. While such an effect was previously observed in a one-dimensional polariton-condensate array and explained as occurring due to single-particle 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:10.1038/s42005-024-01677-8][arXiv:2307.06550]
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 high-quality 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 highly-pure 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:]
Time-fluctuating 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 single-particle 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 three-step analysis protocol via progressive knowledge-transfer, 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/s41598-023-37124-9][arXiv:2206.00086]
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NISQ is a representative keyword at present as an acronym for “noisy intermediate-scale 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 mid-sized commercial products. While this is a celebrated and triumphant achievement, we are still a great distance away from quantum-superior, fault-tolerant 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, large-scale 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 solid-state platform for quantum technologies. [DOI:10.1038/s41563-022-01271-9]
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 non-uniform 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 sub-diffraction-limit resolution is highly sought-after for a wide range of critical applications. However, sophisticated tip-enhanced techniques are currently required to achieve this goal. We bypass this challenge by demonstrating an extremely broadband photodetector based on a two-dimensional (2D) van der Waals heterostructure that is sensitive to light across over a decade in energy from the mid-infrared (MIR) to deep-ultraviolet (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 atomic-layer-thickness samples placed in close proximity. This work can lead to the development of low-cost and high-throughput photosensors for hyperspectral imaging at the nanoscale.[DOI:10.1021/acs.nanolett.2c00741][arXiv:2202.00049]
Microcavity exciton-polaritons are attractive quantum quasi-particles resulting from strong light-matter coupling in a quantum-well-cavity structure. They have become one of the most stimulating solid-state material platforms to explore beautiful collective quantum phenomena originating from macroscopic coherence in condensation and superfluidity, Berezinskii-Kosterlitz-Thouless 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 power-efficient 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 exciton-polariton spins and their anisotropic interactions, which can couple with the effective magnetic fields from mode-splitting of microcavity photons and spin-dependent relaxation processes of quantum-well excitons. Furthermore, we summarize notable theoretical and experimental research activities to reveal extraordinary quantum phenomena of spin-resolved topological states and exotic spin textures and to devise novel spin-based photonic devices based on microcavity exciton-polaritons. [DOI:10.1002/qute.202100137]
We observe rich phenomena of two-level random telegraph noise (RTN) from a commercial bulk 28-nm p-MOSFET (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 high-current RTN level dramatically switches to the low-current level. The gate-voltage 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 short-channel 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 solid-state quantum information processing by utilizing the exaggerated properties of high-lying excited states within the material. To develop practical quantum systems, high-temperature operation is desirable. Here, we study the temperature-dependence of the yellow and green Rydberg exciton resonances in a thin Cu2O crystal via broad-band phonon-assisted 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 higher-temperature 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 high-temperature Rydberg quantum information processing architecture with solid-state 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 nanometer-thick 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 edge-to-edge 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 layer-by-layer yields pinhole-free films over large areas (>cm2) with high optical transparency per deposition. The use of room temperature processing makes the approach particularly well-suited 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 electron-electron interactions—analogous to the Kohn theorem. These high-quality 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/s42254-020-0162-z]
- 2019
Josephson vortices are observed at the boundary between two exciton-polariton 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/s41566-019-0473-8]
We explore a mechanism for producing time-frequency entangled photon pairs (termed as a biphoton) from an ensemble of atom-like solid-state quantum emitters. Four distinct energy levels of the solid-state system render four spin-conserving optical transitions as observed in color centers. This feature opens up the possibility to generate a four-wave mixing biphoton based on an electromagnetic induced transparency (EIT) for long-coherence quantum communication as demonstrated in cold atomic systems. We propose a narrow EIT window below lifetime-limited linewidth of a SiV− in diamond, assuming a few hundred MHz. Consequently, the EIT-induced narrowband guarantees biphoton coherence time to be at least a few tens of nanosecond without a cavity. Assessing the criteria of solid-state parameters applicable to the existing biphoton model from cold atoms will accelerate solid-state biphoton source research. This study shows that a realization of negligible ground state dephasing of a solid-state sample will be a crucial step toward a solid-state 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 non-PZE [100] direction (θ = 45°) on GaAs. We also prepare ZnO-film sputtered GaAs substrates in order to launch SAWs eciently by IDTs even in the non-PZE 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 velocity-temperature trends at all θ-values 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:10.7567/1347-4065/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 two-dimensional honeycomb lattices for microcavity exciton- polaritons using GaAs semiconductors in the wide-range detuning values, from cavity photonlike (red-detuned) to excitonlike (blue-detuned) 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 two-kind bosons, cavity photons trapped at the lattice sites, and freely moving excitons. In particular, this two-kind band theory is absolutely essential to elucidate the exciton effect in the band structures of blue-detuned exciton-polaritons, where the flattened excitonlike dispersion appears at larger in-plane 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]
- Pre-QuIN LAB Era
- Peer-Reviewed Paper
- 2016-2015
The Berezinskii-Kosterlitz-Thouless (BKT) theorem predicts that two-dimensional bosonic condensates exhibit quasi-long-range order which is characterized by a slow decay of the spatial coherence. However previous measurements on exciton-polariton 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 exciton-polariton 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:10.1088/1367-2630/17/2/023064]
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 non-equilibrium nature. Nevertheless, they exhibit many of the features that would be expected of equilibrium Bose–Einstein condensates (BECs). The non-equilibrium 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 exciton-polaritons 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 exciton-polariton emission at quantum degeneracy threshold; (3) the spectral linewidth behavior: narrowing near threshold and broadening above threshold; and (4) the equivalent 4fy3−3x2y-like orbital symmetry in real space. The f-orbital state at Γ points appears as a metastable momentum valley to trap exciton-polaritons, which is explained by single-particle band-structure calculations.[DOI:10.1103/PhysRevB.89.085306]
2013
Entanglement between stationary quantum memories and photonic qubits is crucial for future quantum communication networks. Although high-fidelity spin–photon entanglement was demonstrated in well-isolated atomic and ionic systems, in the solid-state, where massively parallel, scalable networks are most realistically conceivable, entanglement fidelities are typically limited due to intrinsic environmental interactions. Distilling high-fidelity entangled pairs from lower-fidelity 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 high-fidelity entangled pairs becomes imperative for practical realization of such networks. Here we report the first results of complete state tomography of a solid-state spin–photon-polarization-entangled qubit pair, using a single electron-charged indium arsenide quantum dot. We demonstrate record-high fidelity in the solid-state of well over 90%, and the first (99.9%-confidence) achievement of a fidelity that will unambiguously allow for entanglement distribution in solid-state 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 energy-efficient 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 weak-coupling 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 room-temperature operation using wide-bandgap 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 solid-state 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 high-orbital condensation in non-equilibrium 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 solid-state platform, where the system order parameter can be accessed in both real and momentum spaces.[DOI:10.1088/1367-2630/15/3/035032][arXiv:1210.2153]
We explore the exciton-polariton condensation in two degenerate orbital states. In the honeycomb lattice potential, at the third band we have two degenerate vortex-antivortex lattice states at inequivalent K– and K′-points. We have observed energetically degenerate condensates within the linewidth ∼0.3 meV, and directly measured the vortex-antivortex 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 exciton-microcavity photon coupling constant can be made substantially larger than the exciton binding energy in GaAs-based 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 exciton-polariton laser without resorting to wide band-gap semiconductors.[DOI:10.1103/PhysRevB.87.115303][arXiv:1210.0294]
2012
Hightlights: [Nature News&View][Stanford News][Physics World][IEEE Spectrum]
Long-distance 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 quantum-dot spin qubit and a flying (propagating) photonic qubit has yet to be demonstrated. Moreover, many quantum dots emit single photons at visible to near-infrared wavelengths, where silica fibre losses are so high that long-distance quantum communication protocols become difficult to implement. Here we demonstrate entanglement between an InAs quantum-dot 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 sub-10-picosecond pulses at a wavelength of 2.2 micrometres in the frequency downconversion process provides the necessary quantum erasure to eliminate which-path information in the photon energy. Together with previously demonstrated indistinguishable single-photon emission at high repetition rates, the present technique advances the III–V semiconductor quantum-dot spin system as a promising platform for long-distance quantum communication.[DOI:10.1038/nature11577]
We describe a design to implement a two-qubit 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 electron-electron Ruderman-Kittel-Kasuya-Yosida interactions, offers high speed, high fidelity two-qubit 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 low-energy quantum state dynamics. Recently, exciton condensates were trapped in various artificial periodic potential geometries: one-dimensional (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 meta-stable condensation in high-energy bands; the third band exhibits a signature of weaker dispersive band structures, a flat band. We perform a single-particle band structure calculation to compare measured band structures.[DOI:10.1088/1367-2630/14/6/065002]
2011-2006
Macroscopic order appears as the collective behaviour of many interacting particles. Prime examples are superfluidity in helium, atomic Bose–Einstein condensation, s-wave 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, high-orbital states of bosons exhibit exotic orders distinct from the orders with real-valued bosonic ground states. Recently, a wide range of related phenomena have been studied using atom condensates in optical lattices, but the experimental observation of high-orbital orders has been limited to momentum space. Here we establish microcavity exciton–polariton condensates as a promising alternative for exploring high-orbital 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 focus-tunable 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:10.1088/1367-2630/16/9/093028] [arXiv:1005.1064]We propose a device for studying the Fermi-Hubbard model with long-range Coulomb interactions using an array of coupled quantum dots defined in a semiconductor two-dimensional electron-gas 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 two-band 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 metal-Mott insulator quantum phase transition and a d-wave 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 in-plane lateral potential in a semiconductor microcavity using a metal thin-film. Two types of potential are produced: a circular aperture and a one-dimensional (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 blue-shifted. 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 in-plane spatial trap potential for lower exciton-polaritons (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 many-particle 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 single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The long-range spatial coherence leads to the existence of phase-locked 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 meta-stable -state was discovered in a weak link of superfluid 3He, which is characterized by a ‘p-wave’ 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 build-up of in-phase (‘zero-state’) and antiphase (‘-state’) ‘superfluid’ states in a solid-state system; an array of exciton–polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton–polariton condensates.[DOI:10.1038/nature06334]
We study the electrical transport properties of well-contacted ballistic single-walled carbon nanotubes in a three-terminal configuration at low temperatures. We observe signatures of strong electron-electron interactions: the conductance exhibits bias-voltage-dependent amplitudes of quantum interference oscillation, and both the current noise and Fano factor manifest bias-voltage-dependent power-law scalings. We analyze our data within the Tomonaga-Luttinger liquid model using the nonequilibrium Keldysh formalism and find qualitative and quantitative agreement between experiment and theory.[DOI:10.1103/PhysRevLett.99.036802][arXiv:cond-mat/0610196]
An experimental scheme for a quantum simulator of strongly correlated electrons is proposed. Our scheme employs electrons confined in a two-dimensional electron gas in a GaAs/AlGaAsheterojunction. Two surface acoustic waves are then induced in the substrate, creating a two-dimensional “egg-carton” potential. The dynamics of the electrons in this potential are described by a Hubbard model with long-range 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:cond-mat/0608142]
We present a detailed theoretical investigation of transport through a single-walled 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 Fabry-Perot (FP) oscillations in conductance and shot noise. We include the electron-electron interaction and the finite length of the SWNT within the inhomogeneous Tomonaga-Luttinger liquid (TLL) model and treat the nonequilibrium effects due to an applied bias voltage within the Keldysh approach. In low-frequency transport properties, the TLL effect is apparent mainly via power-law 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 spin-mode 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 shot-noise 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:cond-mat/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 ‘polariton-laser’ 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 light-matter interaction for ultrafast optical manipulation and read-out. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about μs-coherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a Λ-type energy level system of a single quantum-dot electron spin in a magnetic field and ultrafast non-linear frequency conversion, quantum-dot spin-photon entanglement is observed.[DOI:10.1117/12.2025332]
We report an ultrafast downconversion quantum interface, where 910-nm single photons from a quantum dot are downconverted to the 1.5-μm telecom band with sub-10 picosecond pulses at 2.2-μm, enabling the demonstration of quantum-dot spin-photon 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
exciton-polariton mediated two-qubit operation and single shot quantum non-demolition
(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 exciton-polaritons. This paper describes a single qubit rotation, which together with two-qubit controlled-z 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 non-equilibrium differential conductance and current fluctuations in a single quantum point contact. The two-terminal electrical transport properties — differential conductance and shot noise — are measured at 1.5 K as a function of the drain-source voltage and the Schottky split-gate voltage. In differential conductance measurements, conductance plateaus appear at integer multiples of 2e^2/h when the drain-source voltage is small, and the plateaus evolve to a fractional of 2e^2/h as the drain-source voltage increases. Our shot noise measurements correspondingly show that the shot noise signal is highly suppressed at both the integer and the non-integer 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 single-electron model cannot explain the 0.7 structure and the noise suppression, suggesting that this characteristic relates to the electron-electron interactions.[arXiv:cond-mat/0311435]