29 research outputs found
Robust Single-Shot Spin Measurement with 99.5% Fidelity in a Quantum Dot Array
We demonstrate a new method for projective single-shot measurement of two
electron spin states (singlet versus triplet) in an array of gate-defined
lateral quantum dots in GaAs. The measurement has very high fidelity and is
robust with respect to electric and magnetic fluctuations in the environment.
It exploits a long-lived metastable charge state, which increases both the
contrast and the duration of the charge signal distinguishing the two
measurement outcomes. This method allows us to evaluate the charge measurement
error and the spin-to-charge conversion error separately. We specify conditions
under which this method can be used, and project its general applicability to
scalable quantum dot arrays in GaAs or silicon.Comment: 13 pages, 3 figure
Gaussian breeding for encoding a qubit in propagating light
Practical quantum computing requires robust encoding of logical qubits in
physical systems to protect fragile quantum information. Currently, the lack of
scalability limits the logical encoding in most physical systems, and thus the
high scalability of propagating light can be a game changer for realizing a
practical quantum computer. However, propagating light also has a drawback: the
difficulty of logical encoding due to weak nonlinearity. Here, we propose
Gaussian breeding that encodes arbitrary Gottesman-Kitaev-Preskill (GKP) qubits
in propagating light. The key idea is the efficient and iterable generation of
quantum superpositions by photon detectors, which is the most widely used
nonlinear element in quantum propagating light. This formulation makes it
possible to systematically create the desired qubits with minimal resources.
Our simulations show that GKP qubits above a fault-tolerant threshold,
including ``magic states'', can be generated with a high success probability
and with a high fidelity exceeding 0.99. This result fills an important missing
piece toward practical quantum computing.Comment: 19 pages, 2 figure
Quantum nondemolition measurement of an electron spin qubit
Measurement of quantum systems inevitably involves disturbance in various
forms. Within the limits imposed by quantum mechanics, however, one can design
an "ideal" projective measurement that does not introduce a back action on the
measured observable, known as a quantum nondemolition (QND) measurement. Here
we demonstrate an all-electrical QND measurement of a single electron spin in a
gate-defined quantum dot via an exchange-coupled ancilla qubit. The ancilla
qubit, encoded in the singlet-triplet two-electron subspace, is entangled with
the single spin and subsequently read out in a single shot projective
measurement at a rate two orders of magnitude faster than the spin relaxation.
The QND nature of the measurement protocol is evidenced by observing a
monotonic increase of the readout fidelity over one hundred repetitive
measurements against arbitrary input states. We extract information from the
measurement record using the method of optimal inference, which is tolerant to
the presence of the relaxation and dephasing. The QND measurement allows us to
observe spontaneous spin flips (quantum jumps) in an isolated system with small
disturbance. Combined with the high-fidelity control of spin qubits, these
results pave the way for various measurement-based quantum state manipulations
including quantum error correction protocols.Comment: This is a pre-print of an article published in Nature Nanotechnology.
The final authenticated version is available online at:
https://doi.org/10.1038/s41565-019-0426-
Generation of Highly Pure Single-Photon State at Telecommunication Wavelength
Telecommunication wavelength with well-developed optical communication
technologies and low losses in the waveguide are advantageous for quantum
applications. However, an experimental generation of non-classical states
called non-Gaussian states at the telecommunication wavelength is still
underdeveloped. Here, we generate highly-pure-single-photon states, one of the
most primitive non-Gaussian states, by using a heralding scheme with an optical
parametric oscillator and a superconducting nano-strip photon detector. The
Wigner negativity, the indicator of non-classicality, of the generated single
photon state is , corresponded to of single
photon and the best record of the minimum value at all wavelengths. The
quantum-optics-technology we establish can be easily applied to the generation
of various types of quantum states, opening up the possibility of
continuous-variable-quantum-information processing at telecommunication
wavelengths.Comment: 10 pages, 6 figure
Coherence of a driven electron spin qubit actively decoupled from quasi-static noise
The coherence of electron spin qubits in semiconductor quantum dots suffers
mostly from low-frequency noise. During the last decade, efforts have been
devoted to mitigate such noise by material engineering, leading to substantial
enhancement of the spin dephasing time for an idling qubit. However, the role
of the environmental noise during spin manipulation, which determines the
control fidelity, is less understood. We demonstrate an electron spin qubit
whose coherence in the driven evolution is limited by high-frequency charge
noise rather than the quasi-static noise inherent to any semiconductor device.
We employed a feedback control technique to actively suppress the latter,
demonstrating a -flip gate fidelity as high as in a
gallium arsenide quantum dot. We show that the driven-evolution coherence is
limited by the longitudinal noise at the Rabi frequency, whose spectrum
resembles the noise observed in isotopically purified silicon qubits.Comment: 10 pages, 7 figure
Quantum arbitrary waveform generator
Controlling the waveform of light is the key for a versatile light source in
classical and quantum electronics. Although pulse shaping of classical light is
a mature technique and has been used in various fields, more advanced
applications would be realized by a light source that generates arbitrary
quantum light with arbitrary temporal waveform. We call such a device a quantum
arbitrary waveform generator (Q-AWG). The Q-AWG must be able to handle
versatile quantum states of light, which are fragile. Thus, the Q-AWG requires
a radically different methodology from classical pulse shaping. In this paper,
we invent an architecture of Q-AWGs that can operate semi-deterministically at
a repetition rate over GHz in principal. We demonstrate its core technology via
generating highly non-classical states with waveforms that have never been
realized before. This result would lead to powerful quantum technologies based
on Q-AWGs such as practical optical quantum computing.Comment: 24 pages, 5 figure
Role of STAT4 polymorphisms in systemic lupus erythematosus in a Japanese population: a case-control association study of the STAT1-STAT4 region
IntroductionRecent studies identified STAT4 (signal transducers and activators of transcription-4) as a susceptibility gene for systemic lupus erythematosus (SLE). STAT1 is encoded adjacently to STAT4 on 2q32.2-q32.3, upregulated in peripheral blood mononuclear cells from SLE patients, and functionally relevant to SLE. This study was conducted to test whether STAT4 is associated with SLE in a Japanese population also, to identify the risk haplotype, and to examine the potential genetic contribution of STAT1. To accomplish these aims, we carried out a comprehensive association analysis of 52 tag single nucleotide polymorphisms (SNPs) encompassing the STAT1-STAT4 region.MethodsIn the first screening, 52 tag SNPs were selected based on HapMap Phase II JPT (Japanese in Tokyo, Japan) data, and case-control association analysis was carried out on 105 Japanese female patients with SLE and 102 female controls. For associated SNPs, additional cases and controls were genotyped and association was analyzed using 308 SLE patients and 306 controls. Estimation of haplotype frequencies and an association study using the permutation test were performed with Haploview version 4.0 software. Population attributable risk percentage was estimated to compare the epidemiological significance of the risk genotype among populations.ResultsIn the first screening, rs7574865, rs11889341, and rs10168266 in STAT4 were most significantly associated (P < 0.01). Significant association was not observed for STAT1. Subsequent association studies of the three SNPs using 308 SLE patients and 306 controls confirmed a strong association of the rs7574865T allele (SLE patients: 46.3%, controls: 33.5%, P = 4.9 × 10-6, odds ratio 1.71) as well as TTT haplotype (rs10168266/rs11889341/rs7574865) (P = 1.5 × 10-6). The association was stronger in subgroups of SLE with nephritis and anti-double-stranded DNA antibodies. Population attributable risk percentage was estimated to be higher in the Japanese population (40.2%) than in Americans of European descent (19.5%).ConclusionsThe same STAT4 risk allele is associated with SLE in Caucasian and Japanese populations. Evidence for a role of STAT1 in genetic susceptibility to SLE was not detected. The contribution of STAT4 for the genetic background of SLE may be greater in the Japanese population than in Americans of European descent