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Y. Nakamura

Publications and source records attributed to Y. Nakamura.

At least 253 records · Page 14Linked to original sources
arXiv2004

Coherent dynamics of a flux qubit coupled to a harmonic oscillator

In the emerging field of quantum computation and quantum information, superconducting devices are promising candidates for the implementation of solid-state quantum bits or qubits. Single-qubit operations, direct coupling between two qubits, and the realization of a quantum gate have been reported. However, complex manipulation of entangled states - such as the coupling of a two-level system to a quantum harmonic oscillator, as demonstrated in ion/atom-trap experiments or cavity quantum electrodynamics - has yet to be achieved for superconducting devices. Here we demonstrate entanglement between a superconducting flux qubit (a two-level system) and a superconducting quantum interference device (SQUID). The latter provides the measurement system for detecting the quantum states; it is also an effective inductance that, in parallel with an external shunt capacitance, acts as a harmonic oscillator. We achieve generation and control of the entangled state by performing microwave spectroscopy and detecting the resultant Rabi oscillations of the coupled system.

cond-mat.mes-hall↗
arXiv2004

Single-shot measurement of the Josephson charge qubit

We demonstrate single-shot readout of quantum states of the Josephson charge qubit. The quantum bits are transformed into and stored as classical bits (charge quanta) in a dynamic memory cell - a superconducting island. The transformation of state |1> (differing form state |0> by an extra Cooper pair) is a result of a controllable quasiparticle tunneling to the island. The charge is then detected by a conventional single-electron transistor, electrostatically decoupled from the qubit. We study relaxation dynamics in the system and obtain the readout efficiency of 87% and 93% for |1> and |0> states, respectively.

cond-mat.mes-hall↗
arXiv2004

Finite Temperature QCD with Two Flavors of Non-perturbatively Improved Wilson Fermions

We study QCD with two flavors of non-perturbatively improved Wilson fermions at finite temperature on the $16^3 8$ lattice. We determine the transition temperature at lattice spacings as small as $a \sim 0.12$ fm, and study string breaking below the finite temperature transition. We find that the static potential can be fitted by a two-state ansatz, including a string state and a two-meson state. We investigate the role of Abelian monopoles at finite temperature.

hep-lat↗
arXiv2003

Demonstration of conditional gate operation using superconducting charge qubits

Since the first demonstration of coherent control of a quantum state of a superconducting charge qubit a variety of Josephson-junction-based qubits have been implemented with remarkable progress in coherence time and read-out schemes. Although the current level of this solid-state device is still not as advanced as that of the most advanced microscopic-system-based qubits, these developments, together with the potential scalability, have renewed its position as a strong candidate as a building block for the quantum computer. Recently, coherent oscillation and microwave spectroscopy in capacitively-coupled superconducting qubits have been reported. The next challenging step toward quantum computation is a realization of logic gates. Here we demonstrate a conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that they can be transformed by controlled-NOT (C-NOT) gate operation in the amplitude of the states. Although the phase evolution during the gate operation is still to be clarified, the present results are a major step toward the realization of a universal solid-state quantum gate.

cond-mat.mes-hall↗
arXiv2003

Dynamics of Monopoles and Flux Tubes in Two-Flavor Dynamical QCD

We investigate the confining properties of the QCD vacuum with $N_f=2$ flavors of dynamical quarks, and compare the results with the properties of the quenched theory. We use non-perturbatively $\mathcal{O}(a)$ improved Wilson fermions to keep cut-off effects small. We focus on color magnetic monopoles. Among the quantities we study are the monopole density and the monopole screening length, the static potential and the profile of the color electric flux tube. We furthermore derive the low-energy effective monopole action. Marked differences between the quenched and dynamical vacuum are found.

hep-lat↗
arXiv2003

Finite temperature phase transition in full QCD with $N_f=2$ flavors of clover fermions at $N_t=8$ and 10

We present results for QCD with $N_f=2$ flavors of dynamical quarks using nonperturbatively improved Wilson fermions at finite temperature on $16^3 \times 8$ and $24^3 \times 10$ lattices. We determine the transition temperature in the range of quark masses $0.6 \lesssim m_π/m_ρ\leq 0.8$. After fixing the Maximal Abelian gauge we investigate the contribution of Abelian monopoles to the Polyakov loop, Polyakov loop susceptibility and confirm Abelian and monopole dominance in full QCD.

hep-lat↗
arXiv2003

Coherent Quantum Dynamics of a Superconducting Flux Qubit

We have observed coherent time evolution between two quantum states of a superconducting flux qubit comprising three Josephson junctions in a loop. The superposition of the two states carrying opposite macroscopic persistent currents is manipulated by resonant microwave pulses. Readout by means of switching-event measurement with an attached superconducting quantum interference device revealed quantum-state oscillations with high fidelity. Under strong microwave driving it was possible to induce hundreds of coherent oscillations. Pulsed operations on this first sample yielded a relaxation time of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These results are promising for future solid-state quantum computing.

cond-mat.mes-hall↗
arXiv2003

Heavy quark potential in lattice QCD at finite temperature

Results of the study of lattice QCD with two flavors of nonperturbatively improved Wilson fermions at finite temperature are presented. The transition temperature for m_π/m_ρ \sim 0.8 and lattice spacing a \sim 0.12 fm is determined. A two-exponent ansatz is successfully applied to describe the heavy quark potential in the confinement phase.

hep-lat↗
arXiv2002

The profile of the broken string in the confined and deconfined phase in full QCD

We study the profile of the broken string (flux tube) in the maximally abelian gauge below and above the finite temperature phase transition in full QCD. In the deconfinement phase, the flux tube disappears and the electric field apperas to be Coulomb-like. In the confinement phase, but near $T_c$, at shorter distances a flux tube is formed like at zero temperature, while at larger distances the tube disappears similar to the deconfinement phase.

hep-lat↗
arXiv2002

Quantum oscillations in two coupled charge qubits

Despite an apparent progress in implementing individual solid-state qubits, there have been no experimental reports so far on multi-bit gates required for building a real quantum computer. Here we report a new circuit comprising two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations whose spectrum reflects interaction between the qubits. Our results demonstrate the feasibility of coupling of multiple solid-state qubits and indicate the existence of entangled two-qubit states.

cond-mat.mes-hall↗
arXiv2002

The Crab Nebula: 3-dimensional Modeling

We construct an axisymmetric model for the Crab Nebula. The flow dyamics is based on the model by Kennel and Coroniti (1984), but we assume that the kinetic-energy-dominant wind is confined in the equatorial region. We reproduce spacially resolved spectra which agree well with the Chandra results. However, the reproduced image is not a ring but lip-shaped. In addition, brightness contrast between fore and back sides of the ring cannot be reproduced if we assumes that the sigma-parameter is as small as 10**-3. We suggest that the nebula field is highly disordered due to for instance magnetic reconnection. The estimate of sigma can be larger than previously expected.

astro-ph↗
arXiv2002

Thermodynamics and heavy quark potential in N_f=2 dynamical QCD

We study N_f=2 lattice QCD with nonperturbatively improved Wilson fermions at finite temperature on 16^3 \cdot 8 lattices. We determine the transition temperature at m_π/m_ρ \sim 0.8 and lattice spacing as small as 0.12fm. The string breaking at T < T_c is also studied. We find that the static potential can be fitted by a simple expression involving string model potential at finite temperature.

hep-lat↗
arXiv2002

Pulsar Wind Nebulae and Particle Acceleration in the Pulsar Magnetosphere

This paper deals with the Crab Nebula problem to suggest that particle acceleration takes place not only at the inner shock but also over a larger region in the nebula with disordered magnetic field. Kennel and Cornoniti (1984) constructed a spherically symmetric model of the Crab Nebula and concluded that the pulsar wind which excites the nebular is kinetic-energy dominant (KED) because the nebula flow induced by KED wind is favorable to explain the nebula spectrum and expansion speed. This is true even with new Chandra observation, which provides newly the spatially resolved spectra. We have shown below with 3D modelling and the Chandra image that pure toroidal magnetic field and KED wind are incompatible with the Chandra observation.

astro-ph↗
arXiv2001

Dynamics of Josephson junction systems in the computational subspace

The quantum dynamics of the Josephson junction system in the computational subspace is investigated. A scheme for the controlled not operation is given for two capasitively coupled SQUIDs. In this system, there is no systematic error for the two qubit operation. For the inductively coupled SQUIDs, the effective Hamiltonian causes systematic errors in the computational subspace for the two qubit operation. Using the purterbation theory, we construct a more precise effective Hamiltonian. This new effective Hamiltonian reduces the systematic error to the level much lower than the threshold of the fault resilent quantum computation.

quant-ph↗
arXiv2001

On the dynamics of color magnetic monopoles in full QCD

We present first results on the dynamics of monopoles in full QCD with N_f=2 flavors of dynamical quarks. Among the quantities being studied are the monopole density and the monopole screening length, the static potential as well as the profile of the color electric flux tube. Furthermore, we derive the low-energy effective monopole action.

hep-lat↗
arXiv2001

Charge echo in a Cooper-pair box

A spin-echo-type technique is applied to an artificial two-level system that utilizes charge degree of freedom in a small superconducting electrode. Gate-voltage pulses are used to produce the necessary pulse sequence in order to eliminate the inhomogeneity effect in the time-ensemble measurement and to obtain refocused echo signals. Comparison of the decay time of the observed echo signal with estimated decoherence time suggests that low-frequency energy-level fluctuations due to the 1/f charge noise dominate the dephasing in the system.

cond-mat.mes-hall↗
arXiv2000

Lorentz Angle Measurement for CO2/Isobutane Gas Mixtures

We have developed a Lorentz angle measurement system for cool gas mixtures in the course of our R&D for a proposed JLC central drift chamber (JLC-CDC). The measurement system is characterized by the use of two laser beams to produce primary electrons and flash ADCs to read their signals simultaneously. With this new system, we have measured Lorentz angles for CO2/isobutane gas mixtures with different proportions (95:5, 90:10, and 85:15), varying drift field from 0.6 to 2.0 kV/cm and magnetic field up to 1.5 T. The results of the measurement are in good agreement with GARFIELD/MAGBOLTZ simulations.

hep-ex↗