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Kohei M. Itoh

Publications and source records attributed to Kohei M. Itoh.

86 records · Page 5Linked to original sources
arXiv2010

Entanglement in a Solid State Spin Ensemble

Entanglement is the quintessential quantum phenomenon and a necessary ingredient in most emerging quantum technologies, including quantum repeaters, quantum information processing (QIP) and the strongest forms of quantum cryptography. Spin ensembles, such as those in liquid state nuclear magnetic resonance, have been powerful in the development of quantum control methods, however, these demonstrations contained no entanglement and ultimately constitute classical simulations of quantum algorithms. Here we report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in isotopically engineered phosphorus-doped silicon. We combined high field/low temperature electron spin resonance (3.4 T, 2.9 K) with hyperpolarisation of the 31P nuclear spin to obtain an initial state of sufficient purity to create a non-classical, inseparable state. The state was verified using density matrix tomography based on geometric phase gates, and had a fidelity of 98% compared with the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, to 10^10 spin pairs, and represents an essential requirement of a silicon-based quantum information processor.

quant-ph↗
arXiv2010

Storage of multiple coherent microwave excitations in an electron spin ensemble

Electron and nuclear spins have good coherence times and an ensemble of spins is a promising candidate for a quantum memory. By employing holographic techniques via field gradients a single ensemble may be used to store many bits of information. Here we present a coherent memory using a pulsed magnetic field gradient, and demonstrate the storage and retrieval of up to 100 weak 10 GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.

quant-ph↗
arXiv2010

Resonant escape over an oscillating barrier in single-electron ratchet transfer

Single-electron escape from a metastable state over an oscillating barrier is experimentally investigated in silicon-based ratchet transfer. When the barrier is oscillating on a time scale characteristic of the single-electron escape, synchronization occurs between the deterministic barrier modulation and the stochastic escape events. The average escape time as a function of its oscillation frequency exhibits a minimum providing a primary signature for resonant activation of single electrons.

cond-mat.mes-hall↗
arXiv2010

Excitonic Aharonov-Bohm Effect in Isotopically Pure 70Ge/Si Type-II Quantum Dots

We report on a magneto-photoluminescence study of isotopically pure 70Ge/Si self-assembled type-II quantum dots. Oscillatory behaviors attributed to the Aharonov-Bohm effect are simultaneously observed for the emission energy and intensity of excitons subject to an increasing magnetic field. When the magnetic flux penetrates through the ring-like trajectory of an electron moving around each quantum dot, the ground state of an exciton experiences a change in its angular momentum. Our results provide the experimental evidence for the phase coherence of a localized electron wave function in group-IV Ge/Si self-assembled quantum structures.

cond-mat.mes-hall↗
arXiv2009

Resources for Measurement-Based Quantum Carry-Lookahead Adder

We present the design of a quantum carry-lookahead adder using measurement-based quantum computation. QCLA utilizes MBQC`s ability to transfer quantum states in unit time to accelerate addition. The quantum carry-lookahead adder (QCLA) is faster than a quantum ripple-carry adder; QCLA has logarithmic depth while ripple adders have linear depth. QCLA is an order of magnitude faster than a ripple-carry adder when adding registers longer than 100 qubits but requires a cluster state that is an order of magnitude larger. Hand optimization results in a $\approx$ 26% reduction in spatial resources for the circuit.

quant-ph↗
arXiv2009

A Measurement-Based Form of the Out-of-Place Quantum Carry-Lookahead Adder

We present the design of a quantum carry-lookahead adder using measurement-based quantum computation. The quantum carry-lookahead adder (QCLA) is faster than a quantum ripple-carry adder; QCLA has logarithmic depth while ripple adders have linear depth. Our design is evaluated in terms of number of time steps and the total number of qubits used.

quant-ph↗
arXiv2007

Arithmetic on a Distributed-Memory Quantum Multicomputer

We evaluate the performance of quantum arithmetic algorithms run on a distributed quantum computer (a quantum multicomputer). We vary the node capacity and I/O capabilities, and the network topology. The tradeoff of choosing between gates executed remotely, through ``teleported gates'' on entangled pairs of qubits (telegate), versus exchanging the relevant qubits via quantum teleportation, then executing the algorithm using local gates (teledata), is examined. We show that the teledata approach performs better, and that carry-ripple adders perform well when the teleportation block is decomposed so that the key quantum operations can be parallelized. A node size of only a few logical qubits performs adequately provided that the nodes have two transceiver qubits. A linear network topology performs acceptably for a broad range of system sizes and performance parameters. We therefore recommend pursuing small, high-I/O bandwidth nodes and a simple network. Such a machine will run Shor's algorithm for factoring large numbers efficiently.

quant-ph↗
arXiv2006

Architecture-Dependent Execution Time of Shor's Algorithm

We show how the execution time of algorithms on quantum computers depends on the architecture of the quantum computer, the choice of algorithms (including subroutines such as arithmetic), and the ``clock speed'' of the quantum computer. The primary architectural features of interest are the ability to execute multiple gates concurrently, the number of application-level qubits available, and the interconnection network of qubits. We analyze Shor's algorithm for factoring large numbers in this context. Our results show that, if arbitrary interconnection of qubits is possible, a machine with an application-level clock speed of as low as one-third of a (possibly encoded) gate per second could factor a 576-bit number in under one month, potentially outperforming a large network of classical computers. For nearest-neighbor-only architectures, a clock speed of around twenty-seven gates per second is required.

quant-ph↗
arXiv2005

Line Broadening and Decoherence of Electron Spins in Phosphorus-Doped Silicon Due to Environmental 29^Si Nuclear Spins

Phosphorus-doped silicon single crystals with 0.19 % <= f <= 99.2 %, where f is the concentration of 29^Si isotopes, are measured at 8 K using a pulsed electron spin resonance technique, thereby the effect of environmental 29^Si nuclear spins on the donor electron spin is systematically studied. The linewidth as a function of f shows a good agreement with theoretical analysis. We also report the phase memory time T_M of the donor electron spin dependent on both f and the crystal axis relative to the external magnetic field.

cond-mat.mtrl-sci↗
arXiv2000

Localization length and impurity dielectric susceptibility in the critical regime of the metal-insulator transition in homogeneously doped p-type Ge

We have determined the localization length ξand the impurity dielectric susceptibility χ_{\rm imp} as a function of Ga acceptor concentrations (N) in nominally uncompensated ^{70}Ge:Ga just below the critical concentration (N_c) for the metal-insulator transition. Both ξand χ_{\rm imp} diverge at N_c according to the functions ξ\propto(1-N/N_c)^{-ν} and χ_{\rm imp}\propto(N_c/N-1)^{-ζ}, respectively, with ν=1.2\pm0.3 and ζ=2.3\pm0.6 for 0.99N_c< N< N_c. Outside of this region (N<0.99N_c), the values of the exponents drop to ν=0.33\pm0.03 and ζ=0.62\pm0.05. The effect of the small amount of compensating dopants that are present in our nominally uncompensated samples, may be responsible for the change of the critical exponents at N\approx0.99N_c.

cond-mat.dis-nn↗
arXiv1999

Metal-insulator transition of isotopically enriched neutron-transmutation-doped ^{70}Ge:Ga in magnetic fields

We have investigated the temperature dependence of the electrical conductivity sigma(N,B,T) of nominally uncompensated, neutron-transmutation-doped ^{70}Ge:Ga samples in magnetic fields up to B=8 T at low temperatures (T=0.05-0.5 K). In our earlier studies at B=0, the critical exponent mu=0.5 defined by sigma(N,0,0) \propto (N-N_c)^{mu} has been determined for the same series of ^{70}Ge:Ga samples with the doping concentration N ranging from 1.861 \times 10^{17} cm^{-3} to 2.434 \times 10^{17} cm^{-3}. In magnetic fields, the motion of carriers loses time-reversal symmetry, the universality class may change and with it the value of mu. In this work, we show that magnetic fields indeed affect the value of mu (mu changes from 0.5 at B=0 to 1.1 at B \geq 4 T). The same exponent mu'=1.1 is also found in the magnetic-field-induced MIT for three different ^{70}Ge:Ga samples, i.e., sigma(N,B,0) \propto [B_c(N)-B]^{mu'} where B_c(N) is the concentration-dependent critical magnetic induction. We show that sigma(N,B,0) obeys a simple scaling rule on the (N,B) plane. Based on this finding, we derive from a simple mathematical argument that mu=mu' as has been observed in our experiment.

cond-mat.dis-nn↗
arXiv1998

Electrical properties of isotopically enriched neutron-transmutation-doped ^{70} Ge:Ga near the metal-insulator transition

We report the low temperature carrier transport properties of a series of nominally uncompensated neutron-transmutation doped (NTD) ^{70} Ge:Ga samples very close to the critical concentration N_c for the metal-insulator transition. The concentration of the sample closest to N_c is 1.0004N_c and it is unambiguously shown that the critical conductivity exponent is 0.5. Properties of insulating samples are discussed in the context of Efros and Shklovskii's variable range hopping conduction.

cond-mat.dis-nn↗