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Yoshihisa Yamamoto

Publications and source records attributed to Yoshihisa Yamamoto.

At least 109 records · Page 6Linked to original sources
arXiv2007

Quantum simulation of Fermi-Hubbard models in semiconductor quantum dot arrays

We propose a device for studying the Fermi-Hubbard model with long-range Coulomb interactions using an array of quantum dots defined in a semiconductor two-dimensional electron gas system. Bands with energies above the lowest energy band are used to form the Hubbard model, which allows for an experimentally simpler realization of the device. We find that depending on 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 fabrication technologies.

quant-ph↗
arXiv2007

Strongly Correlated Polaritons in a Two-Dimensional Array of Photonic Crystal Microcavities

We propose a practical scheme to observe the polaritonic quantum phase transition (QPT) from the superfluid (SF) to Bose-glass (BG) to Mott-insulator (MI) states. The system consists of a two-dimensional array of photonic crystal microcavities doped with substitutional donor/acceptor impurities. Using realistic parameters, we show that such strongly correlated polaritonic systems can be constructed using the state-of-art semiconductor technology.

quant-ph↗
arXiv2007

Spatial Coherence of a Polariton Condensate

We perform Young's double-slit experiment to study the spatial coherence properties of a two-dimensional dynamic condensate of semiconductor microcavity polaritons. The coherence length of the system is measured as a function of the pump rate, which confirms a spontaneous buildup of macroscopic coherence in the condensed phase. An independent measurement reveals that the position and momentum uncertainty product of the condensate is close to the Heisenberg limit. An experimental realization of such a minimum uncertainty wave packet of the polariton condensate opens a door to coherent matter-wave phenomena such as Josephson oscillation, superfluidity, and solitons in solid state condensate systems.

cond-mat.mes-hall↗
arXiv2007

Generation of 10-GHz clock sequential time-bin entanglement

This letter reports telecom-band sequential time-bin entangled photon-pair generation at a repetition rate of 10 GHz in periodically-poled reverse-proton-exchange lithium niobate waveguides based on mode demultiplexing. With up-conversion single-photon detectors, we observed a two-photon-interference-fringe visibility of 85.32% without subtraction of accidental noise contributions, which can find broad application in quantum information and quantum entanglement research.

quant-ph↗
arXiv2007

Coherent Population Trapping of Electron Spins in a Semiconductor

In high-purity n-type GaAs under strong magnetic field, we are able to isolate a lambda system composed of two Zeeman states of neutral-donor bound electrons and the lowest Zeeman state of bound excitons. When the two-photon detuning of this system is zero, we observe a pronounced dip in the excited-state photoluminescence indicating the creation of the coherent population-trapped state. Our data are consistent with a steady-state three-level density-matrix model. The observation of coherent population trapping in GaAs indicates that this and similar semiconductor systems could be used for various EIT-type experiments.

cond-mat.mes-hall↗
arXiv2007

Quantum computers based on electron spins controlled by ultra-fast, off-resonant, single optical pulses

We describe a fast quantum computer based on optically controlled electron spins in charged quantum dots that are coupled to microcavities. This scheme uses broad-band optical pulses to rotate electron spins and provide the clock signal to the system. Non-local two-qubit gates are performed by phase shifts induced by electron spins on laser pulses propagating along a shared waveguide. Numerical simulations of this scheme demonstrate high-fidelity single-qubit and two-qubit gates with operation times comparable to the inverse Zeeman frequency.

quant-ph↗
arXiv2007

Quantum key distribution over 40 dB channel loss using superconducting single photon detectors

Quantum key distribution (QKD) offers an unconditionally secure means of communication based on the laws of quantum mechanics. Currently, a major challenge is to achieve a QKD system with a 40 dB channel loss, which is required if we are to realize global scale QKD networks using communication satellites. Here we report the first QKD experiment in which secure keys were distributed over 42 dB channel loss and 200 km of optical fibre. We employed the differential phase shift quantum key distribution (DPS-QKD) protocol implemented with a 10-GHz clock frequency, and superconducting single photon detectors (SSPD) based on NbN nanowire. The SSPD offers a very low dark count rate (a few Hz) and small timing jitter (60 ps full width at half maximum). These characteristics allowed us to construct a 10-GHz clock QKD system and thus distribute secure keys over channel loss of 42 dB. In addition, we achieved a 17 kbit/s secure key rate over 105 km of optical fibre, which is two orders of magnitude higher than the previous record, and a 12.1 bit/s secure key rate over 200 km of optical fibre, which is the longest terrestrial QKD yet demonstrated. The keys generated in our experiment are secure against both general collective attacks on individual photons and a specific collective attack on multi-photons, known as a sequential unambiguous state discrimination (USD) attack.

quant-ph↗
arXiv2007

Correlated photon-pair generation in reverse-proton-exchange PPLN waveguides with integrated mode demultiplexer at 10 GHz clock

We report 10-ps correlated photon pair generation in periodically-poled reverse-proton-exchange lithium niobate waveguides with integrated mode demultiplexer at a wavelength of 1.5-um and a clock of 10 GHz. Using superconducting single photon detectors, we observed a coincidence to accidental count ratio (CAR) as high as 4000. The developed photon-pair source may find broad application in quantum information systems as well as quantum entanglement experiments.

quant-ph↗
arXiv2007

Quantum simulation of spin ordering with nuclear spins in a solid state lattice

An experiment demonstrating the quantum simulation of a spin-lattice Hamiltonian is proposed. Dipolar interactions between nuclear spins in a solid state lattice can be modulated by rapid radio-frequency pulses. In this way, the effective Hamiltonian of the system can be brought to the form of an antiferromagnetic Heisenberg model with long range interactions. Using a semiconducting material with strong optical properties such as InP, cooling of nuclear spins could be achieved by means of optical pumping. An additional cooling stage is provided by adiabatic demagnetization in the rotating frame (ADRF) down to a nuclear spin temperature at which we expect a phase transition from a paramagnetic to antiferromagnetic phase. This phase transition could be observed by probing the magnetic susceptibility of the spin-lattice. Our calculations suggest that employing current optical pumping technology, observation of this phase transition is within experimental reach.

quant-ph↗
arXiv2007

Dynamics of Quantum Dot Photonic Crystal Lasers

Quantum dot photonic crystal membrane lasers were fabricated and the large signal modulation characteristics were studied. We find that the modulation characteristics of quantum dot lasers can be significantly improved using cavities with large spontaneous emission coupling factor. Our experiments show, and simulations confirm, that the modulation rate is limited by the rate of carrier capture into the dots to around 30GHz in our present system.

quant-ph↗
arXiv2007

Photon Antibunching from a Single Quantum Dot-Microcavity System in the Strong Coupling Regime

We observe antibunching in the photons emitted from a strongly-coupled single quantum dot and pillar microcavity in resonance. When the quantum dot was spectrally detuned from the cavity mode, the cavity emission remained antibunched, and also anticorrelated from the quantum dot emission. Resonant pumping of the selected quantum dot via an excited state enabled these observations by eliminating the background emitters that are usually coupled to the cavity. This device demonstrates an on-demand single photon source operating in the strong coupling regime, with a Purcell factor of 61 and quantum effciency of 97%

quant-ph↗
arXiv2006

Hybrid quantum repeater based on dispersive CQED interactions between matter qubits and bright coherent light

We describe a system for long-distance distribution of quantum entanglement, in which coherent light with large average photon number interacts dispersively with single, far-detuned atoms or semiconductor impurities in optical cavities. Entanglement is heralded by homodyne detection using a second bright light pulse for phase reference. The use of bright pulses leads to a high success probability for the generation of entanglement, at the cost of a lower initial fidelity. This fidelity may be boosted by entanglement purification techniques, implemented with the same physical resources. The need for more purification steps is well compensated for by the increased probability of success when compared to heralded entanglement schemes using single photons or weak coherent pulses with realistic detectors. The principle cause of the lower initial fidelity is fiber loss; however, spontaneous decay and cavity losses during the dispersive atom/cavity interactions can also impair performance. We show that these effects may be minimized for emitter-cavity systems in the weak-coupling regime as long as the resonant Purcell factor is larger than one, the cavity is over-coupled, and the optical pulses are sufficiently long. We support this claim with numerical, semiclassical calculations using parameters for three realistic systems: optically bright donor-bound impurities such as 19-F:ZnSe with a moderate-Q microcavity, the optically dim 31-P:Si system with a high-Q microcavity, and trapped ions in large but very high-Q cavities.

quant-ph↗
arXiv2006

Millisecond spin-flip times of donor-bound electrons in GaAs

We observe millisecond spin-flip relaxation times of donor-bound electrons in high-purity n-GaAs . This is three orders of magnitude larger than previously reported lifetimes in n-GaAs . Spin-flip times are measured as a function of magnetic field and exhibit a strong power-law dependence for fields greater than 4 T . This result is in qualitative agreement with previously reported theory and measurements of electrons in quantum dots.

cond-mat.mes-hall↗
arXiv2006

Tomonaga-Luttinger Liquid Features in Ballistic Single-Walled Carbon Nanotubes: Conductance and Shot Noise

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 non-equilibrium Keldysh formalism and find qualitative and quantitative agreement between experiment and theory.

cond-mat.mes-hall↗
arXiv2006

Generation and transfer of single photons on a photonic crystal chip

We present a basic building block of a quantum network consisting of a quantum dot coupled to a source cavity, which in turn is coupled to a target cavity via a waveguide. The single photon emission from the high-Q/V source cavity is characterized by a twelve-fold spontaneous emission (SE) rate enhancement that results in a SE coupling efficiency near 0.98 into the source cavity mode. Single photons are efficiently transferred into the target cavity through the waveguide, with a source/target field intensity ratio of 0.12 (up to 0.49 observed in other structures without coupled quantum dots). This system shows great promise as a building block of future on-chip quantum information processing systems.

quant-ph↗
arXiv2006

100 km secure differential phase shift quantum key distribution with low jitter up-conversion detectors

We present a quantum key distribution experiment in which keys that were secure against all individual eavesdropping attacks allowed by quantum mechanics were distributed over 100 km of optical fiber. We implemented the differential phase shift quantum key distribution protocol and used low timing jitter 1.55 um single-photon detectors based on frequency up-conversion in periodically poled lithium niobate waveguides and silicon avalanche photodiodes. Based on the security analysis of the protocol against general individual attacks, we generated secure keys at a practical rate of 166 bit/s over 100 km of fiber. The use of the low jitter detectors also increased the sifted key generation rate to 2 Mbit/s over 10 km of fiber.

quant-ph↗
arXiv2006

Quantum simulator for the Hubbard model with long-range Coulomb interactions using surface acoustic waves

A practical 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/AlGaAs heterojunction. Two surface acoustic waves are then induced in the GaAs substrate, which create a two dimensional ``egg-carton'' potential. The dynamics of the electrons in this potential is described by a Hubbard model with long-range Coulomb interactions. The state of the electrons in this system can be probed via its conductance and noise properties. This allows the identification of a metallic or insulating state. Numerical estimates for the parameters appearing in the effective Hubbard model are calculated using the proposed experimental system. These calculations suggest that observations of quantum phase transition phenomena of the electrons in the potential array are within experimental reach.

cond-mat.str-el↗