arXiv subjects

K. Takeda

Publications and source records attributed to K. Takeda.

At least 19 recordsLinked to original sources

arXiv2022

Noise-correlation spectrum for a pair of spin qubits in silicon

Semiconductor qubits are appealing for building quantum processors as they may be densely integrated due to small footprint. However, a high density raises the issue of noise correlated across different qubits, which is of practical concern for scalability and fault tolerance. Here, we analyse and quantify in detail the degree of noise correlation in a pair of neighbouring silicon spin qubits ~100 nm apart. We evaluate all a-priori independent auto- and cross- power spectral densities of noise as a function of frequency. We reveal strong inter-qubit noise correlation with a correlation strength as large as ~0.7 at ~1 Hz (70% of the maximum in-phase correlation), even in the regime where the spin-spin exchange interaction contributes negligibly. We furthermore find that fluctuations of single-spin precession rates are strongly correlated with exchange noise, giving away their electrical origin. Noise cross-correlations have thus enabled us to pinpoint the most influential noise in the present device among compelling mechanisms including nuclear spins. Our work presents a powerful tool set to assess and identify the noise acting on multiple qubits and highlights the importance of long-range electric noise in densely packed silicon spin qubits.

quant-ph↗

arXiv2021

Real-space observation of emergent complexity of phase evolution in micrometer-sized IrTe$_2$ crystals

We report complex behaviors in the phase evolution of transition-metal dichalcogenide IrTe$_2$ thin flakes, captured with real-space observations using scanning Raman microscopy. The phase transition progresses via growth of a small number of domains, which is unlikely in statistical models that assume a macroscopic number of nucleation events. Consequently, the degree of phase evolution in the thin flakes is quite variable for the selected specimen and for a repeated measurement sequence, representing the emergence of complexity in the phase evolution. In the $\sim$20-$μ$m$^3$-volume specimen, the complex phase evolution results in the emergent coexistence of a superconducting phase that originally requires chemical doping to become thermodynamically stable. These findings indicate that the complexity involved in phase evolution considerably affects the physical properties of a small-sized specimen.

cond-mat.str-el↗

arXiv2020

Probabilistic teleportation of a quantum dot spin qubit

Electron spin s in semiconductor quantum dot s have been intensively studied for implementing quantum computation and high fidelity single and two qubit operation s have recently been achieved . Quantum teleportation is a three qubit protocol exploiting quantum entanglement and it serv es as a n essential primitive for more sophisticated quantum algorithm s Here, we demonstrate a scheme for quantum teleportation based on direct Bell measurement for a single electron spin qubit in a triple quantum dot utilizing the Pauli exclusion principle to create and detect maximally entangled state s . T he single spin polarization is teleported from the input qubit to the output qubit with a fidelity of 0.9 1 We find this fidelity is primarily limited by singlet triplet mixing which can be improved by optimizing the device parameters Our results may be extended to quantum algorithms with a larger number of se miconductor spin qubit s

cond-mat.mes-hall↗

arXiv2020

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn$_3$P

Antiferromagnet Mn$_3$P with Neel temperature $T_N=30$ K is composed of Mn-tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at $\sim1.6$ GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn$_3$P, which is most likely effected by the underlying frustration effect.

cond-mat.str-el↗

arXiv2019

Repetitive single electron spin readout in silicon

Single electron spins confined in silicon quantum dots hold great promise as a quantum computing architecture with demonstrations of long coherence times, high-fidelity quantum logic gates, basic quantum algorithms and device scalability. While single-shot spin detection is now a laboratory routine, the need for quantum error correction in a large-scale quantum computing device demands a quantum non-demolition (QND) implementation. Unlike conventional counterparts, the QND spin readout imposes minimal disturbance to the probed spin polarization and can therefore be repeated to extinguish measurement errors. However, it has remained elusive for an electron spin in silicon as it involves exquisite exposure of the system to the external circuitry for readout while maintaining the coherence and integrity of the qubit. Here we show that an electron spin qubit in silicon can be measured in a highly non-demolition manner by probing another electron spin in a neighboring dot Ising-coupled to the qubit spin. The high non-demolition fidelity (99% on average) enables over 20 readout repetitions of a single spin state, yielding an overall average measurement fidelity of up to 95% within 1.2 ms. We further demonstrate that our repetitive QND readout protocol can realize heralded high-fidelity (> 99.6%) ground-state preparation. Our QND-based measurement and preparation, mediated by a second qubit of the same kind, will allow for a new class of quantum information protocols with electron spins in silicon without compromising the architectural homogeneity.

cond-mat.mes-hall↗

arXiv2018

A fast quantum interface between different spin qubit encodings

Single-spin qubits in semiconductor quantum dots proposed by Loss and DiVincenzo (LD qubits) hold promise for universal quantum computation with demonstrations of a high single-qubit gate fidelity above 99.9 % and two-qubit gates in conjunction with a long coherence time. However, initialization and readout of a qubit is orders of magnitude slower than control, which is detrimental for implementing measurement-based protocols such as error-correcting codes. In contrast, a singlet-triplet (ST) qubit, encoded in a two-spin subspace, has the virtue of fast readout with high fidelity and tunable coupling to the electric field. Here, we present a hybrid system which benefits from the different advantages of these two distinct spin-qubit implementations. A quantum interface between the two codes is realized by electrically tunable inter-qubit exchange coupling. We demonstrate a controlled-phase (CPHASE) gate that acts within 5.5 ns, much faster than the measured dephasing time of 211 ns. The presented hybrid architecture will be useful to settle remaining key problems with building scalable spin-based quantum computers.

cond-mat.mes-hall↗

arXiv2018

Optimized electrical control of a Si/SiGe spin qubit in the presence of an induced frequency shift

Electron spins confined in quantum dots are an attractive system to realize high-fidelity qubits owing to their long coherence time. With the prolonged spin coherence time, however, the control fidelity can be limited by systematic errors rather than decoherence, making characterization and suppression of their influence crucial for further improvement. Here we report that the control fidelity of Si/SiGe spin qubits can be limited by the microwave-induced frequency shift of electric dipole spin resonance and it can be improved by optimization of control pulses. As we increase the control microwave amplitude, we observe a shift of the qubit resonance frequency, in addition to the increasing Rabi frequency. We reveal that this limits control fidelity with a conventional amplitude-modulated microwave pulse below 99.8%. In order to achieve a gate fidelity > 99.9%, we introduce a quadrature control method, and validate this approach experimentally by randomized benchmarking. Our finding facilitates realization of an ultra-high fidelity qubit with electron spins in quantum dots.

cond-mat.mes-hall↗

arXiv2017

A >99.9%-fidelity quantum-dot spin qubit with coherence limited by charge noise

Recent advances towards spin-based quantum computation have been primarily fuelled by elaborate isolation from noise sources, such as surrounding nuclear spins and spin-electric susceptibility, to extend spin coherence. In the meanwhile, addressable single-spin and spin-spin manipulations in multiple-qubit systems will necessitate sizable spin-electric coupling. Given background charge fluctuation in nanostructures, however, its compatibility with enhanced coherence should be crucially questioned. Here we realise a single-electron spin qubit with isotopically-enriched phase coherence time (20 microseconds) and fast electrical control speed (up to 30 MHz) mediated by extrinsic spin-electric coupling. Using rapid spin rotations, we reveal that the free-evolution dephasing is caused by charge (instead of conventional magnetic) noise featured by a 1/f spectrum over seven decades of frequency. The qubit nevertheless exhibits superior performance with single-qubit gate fidelities exceeding 99.9% on average. Our work strongly suggests that designing artificial spin-electric coupling with account taken of charge noise is a promising route to large-scale spin-qubit systems having fault-tolerant controllability.

cond-mat.mes-hall↗

arXiv2017

A triangular triple quantum dot with tunable tunnel couplings

A two-dimensional arrangement of quantum dots with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple quantum dot with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov-Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple quantum dot from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages.

cond-mat.mes-hall↗

arXiv2016

A fault-tolerant addressable spin qubit in a natural silicon quantum dot

Fault-tolerant quantum operation is a key requirement for the development of quantum computing. This has been realized in various solid-state systems including isotopically purified silicon which provides a nuclear spin free environment for the qubits, but not in industry standard natural (unpurified) silicon. Here we demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet optimally designed for fast spin control. This optimized design allows us to achieve the optimum Rabi oscillation quality factor Q = 140 at a Rabi frequency of 10 MHz in the frequency range two orders of magnitude higher than that achieved in previous studies. This leads to a qubit fidelity of 99.6 %, which is the highest reported for natural silicon qubits and comparable to that obtained in isotopically purified silicon quantum-dot-based qubits. This result can inspire contributions from the industrial and quantum computing communities.

cond-mat.mes-hall↗

arXiv2016

Quantum dephasing in a gated GaAs triple quantum dot due to nonergodic noise

We extract the phase coherence of a qubit defined by singlet and triplet electronic states in a gated GaAs triple quantum dot, measuring on timescales much shorter than the decorrelation time of the environmental noise. In this non-ergodic regime, we observe that the coherence is boosted and several dephasing times emerge, depending on how the phase stability is extracted. We elucidate their mutual relations, and demonstrate that they reflect the noise short-time dynamics.

cond-mat.mes-hall↗

arXiv2013

Charge-noise-free Lateral Quantum Dot Devices with Undoped Si/SiGe Wafer

We develop quantum dots in a single layered MOS structure using an undoped Si/SiGe wafer. By applying a positive bias on the surface gates, electrons are accumulated in the Si channel. Clear Coulomb diamond and double dot charge stability diagrams are measured. The temporal fluctuation of the current is traced, to which we apply the Fourier transform analysis. The power spectrum of the noise signal is inversely proportional to the frequency, and is different from the inversely quadratic behavior known for quantum dots made in doped wafers. Our results indicate that the source of charge noise for the doped wafers is related to the 2DEG dopant.

cond-mat.mes-hall↗

arXiv2013

Characterization and Suppression of Low-frequency Noise in Si/SiGe Quantum Point Contacts and Quantum Dots

We report on the effects of a global top gate on low-frequency noise in Schottky gate-defined quantum point contacts (QPCs) and quantum dots (QDs) in a modulation-doped Si/SiGe heterostructure. For a relatively large top gate voltage, the QPC current shows frequent switching with 1/f2 Lorentzian type charge noise. As the top gate voltage is decreased, the QPC pinch-off voltage becomes less negative, and the 1/f2 noise becomes rapidly suppressed in a homogeneous background 1/f noise. We apply this top-gating technique to double QDs to stabilize the charge state for the electron number down to zero.

cond-mat.mes-hall↗

arXiv2013

Nucleon strange quark content from N_f=2+1 lattice QCD with exact chiral symmetry

We calculate the strange quark content of the nucleon in 2+1-flavor lattice QCD. Chirally symmetric overlap fermion formulation is used to avoid the contamination from up and down quark contents due to an operator mixing between strange and light scalar operators, \bar{s}s and \bar{u}u+\bar{d}d. At a lattice spacing a=0.112(1) fm, we perform calculations at four values of degenerate up and down quark masses, which cover a range of the pion mass M_pi \simeq 300-540 MeV. We employ two different methods: one is a direct method where we calculate the strange quark content by directly inserting the strange scalar operator. The other is an indirect method where the quark content is extracted from a derivative of the nucleon mass in terms of the strange quark mass. With these two methods we obtain consistent results with each other. Our best estimate f_{T_s}=0.009(15)(16) is in good agreement with our previous studies in two-flavor QCD.

hep-lat↗

arXiv2013

Histogram Monte Carlo Simulation of the Geometrically Frustrated XY Antiferromagnet with Biquadratic Exchange

Histogram Monte Carlo simulation is used to investigate effects of biquadratic exchange $J_{2}$ on phase transitions of a 3D classical XY antiferromagnet with frustration induced by the antiferromagnetic exchange $J_{1}$ and the stacked triangular lattice geometry. The biquadratic exchange is considered negative (antiferroquadrupolar) within the triangular planes and positive (ferroquadrupolar) between the planes. The phase diagram obtained features a variety of interesting phenomena arising from the presence of both the biquadratic exchange and frustration. In a strong biquadratic exchange limit ($|J_{1}|/|J_{2}| \leq 0.25$), the antiferroquadrupolar phase transition which is of second order is followed by the antiferromagnetic one which can be either first or second order. The separate antiferroquadrupolar and antiferromagnetic second-order transitions are found to belong to the chiral XY and Ising universality classes, respectively. If the biquadratic exchange is reduced both transitions are found to be first order and occur simultaneously in a wide region of $|J_{1}|/|J_{2}|$. However, if $|J_{2}| \rightarrow 0$ the transition changes to the second-order one with the chiral universality class critical behavior.

cond-mat.stat-mech↗

arXiv2013

Effect of biquadratic exchange on phase transitions of a planar classical Heisenberg ferromagnet

Effect of biquadratic exchange on phase transitions of a planar classical Heisenberg (or XY) ferromagnet on a stacked triangular lattice is investigated by Standard Monte Carlo and Histogram Monte Carlo simulations in the region of a bilinear to biquadratic exchange interaction ratio $J_{1}/J_{2} \leq 1$. The biquadratic exchange is found to cause separate second-order phase transitions in a strong biquadratic exchange limit, followed by simultaneous dipole and quadrupole ordering, which is of first order for an intermediate range of the exchange ratio and changes to a second-order one again as $J_{1}/J_{2}$ is further increased. Thus, a phase diagram featuring both triple and tricritical points is obtained. Furthermore, a finite-size scaling analysis is used to calculate the critical indices for both dipole and quadrupole kinds of ordering.

cond-mat.stat-mech↗

arXiv2013

Chiral universality class behavior of a non-chiral antiferroquadrupole system

The planar Heisenberg system with antiferroquadrupolar exchange on 3D stacked triangular lattice is shown to belong to the new "chiral" universality class, predicted for chiral antiferromagnets. The present system, however, displays no such chirality, which, according to the currently widely accepted concept, is supposed to be a key ingredient for the new critical behavior. Our claim that the new universality class should not be limited only to the chiral antiferromagnets is based on a simple mapping between the chiral antiferromagnetic and antiferroquadrupolar systems and supported by actual calculations of the transition temperatures and critical indices from finite-size scaling analysis of data issued from Histogram Monte Carlo simulations. In order to demonstrate the existence of two different universality classes in the behavior of quadrupolar systems, the finite-size scaling is also performed for the system with ferroquadrupolar exchange, which, in contrast to the antiferroquadrupolar exchange system case, produced standard critical behavior, as could also be anticipated from the mapping.

cond-mat.stat-mech↗

arXiv2011

Nucleon strange quark content from two-flavor lattice QCD with exact chiral symmetry

Strange quark content of the nucleon is calculated in dynamical lattice QCD employing the overlap fermion formulation. For this quantity, exact chiral symmetry guaranteed by the Ginsparg-Wilson relation is crucial to avoid large contamination due to a possible operator mixing with $\bar{u}u+\bar{d}d$. Gauge configurations are generated with two dynamical flavors on a 16^3 x 32 lattice at a lattice spacing a \simeq 0.12fm. We directly calculate the relevant three-point function on the lattice including a disconnected strange quark loop utilizing the techniques of all-to-all quark propagator and low-mode averaging. Our result f_{T_s} = 0.032(8)(22), is in good agreement with our previous indirect estimate using the Feynman-Hellmann theorem.

hep-lat↗