312 research outputs found
Geometric quantum gates robust against stochastic control errors
We analyze a scheme for quantum computation where quantum gates can be
continuously changed from standard dynamic gates to purely geometric ones.
These gates are enacted by controlling a set of parameters that are subject to
unwanted stochastic fluctuations. This kind of noise results in a departure
from the ideal case that can be quantified by a gate fidelity. We find that the
maximum of this fidelity corresponds to quantum gates with a vanishing
dynamical phase.Comment: 4 pager
Cooperative effects in Josephson junctions in a cavity in the strong coupling regime
We analyze the behavior of systems of two and three qubits made by Josephson
junctions, treated in the two level approximation, driven by a radiation mode
in a cavity. The regime we consider is a strong coupling one recently
experimentally reached for a single junction. Rabi oscillations are obtained
with the frequency proportional to integer order Bessel functions in the limit
of a large photon number, similarly to the case of the single qubit. A
selection rule is derived for the appearance of Rabi oscillations. A quantum
amplifier built with a large number of Josephson junctions in a cavity in the
strong coupling regime is also described.Comment: 9 pages, no figures. Version accepted for publication in Physical
Review
Protected Rabi oscillation induced by natural interactions among physical qubits
For a system composed of nine qubits, we show that natural interactions among
the qubits induce the time evolution that can be regarded, at discrete times,
as the Rabi oscillation of a logical qubit. Neither fine tuning of the
parameters nor switching of the interactions is necessary. Although
straightforward application of quantum error correction fails, we propose a
protocol by which the logical Rabi oscillation is protected against all
single-qubit errors. The present method thus opens a simple and realistic way
of protecting the unitary time evolution against noise.Comment: In this revised manuscript, new sections V, VI, VII and new
appendices A, B, C have been added to give detailed discussions. 13 pages, 4
figure
Experimental implementation of high-fidelity unconventional geometric quantum gates using NMR interferometer
Following a key idea of unconventional geometric quantum computation
developed earlier [Phys. Rev. Lett. 91, 197902 (2003)], here we propose a more
general scheme in such an intriguing way: , where and are respectively the dynamic and
geometric phases accumulated in the quantum gate operation, with as a
constant and being dependent only on the geometric feature of the
operation. More arrestingly, we demonstrate the first experiment to implement a
universal set of such kind of generalized unconventional geometric quantum
gates with high fidelity in an NMR system.Comment: 4 pages, 3 figure
Coherent shuttle of electron-spin states
We demonstrate a coherent spin shuttle through a GaAs/AlGaAs
quadruple-quantum-dot array. Starting with two electrons in a spin-singlet
state in the first dot, we shuttle one electron over to either the second,
third or fourth dot. We observe that the separated spin-singlet evolves
periodically into the spin-triplet and back before it dephases due to
nuclear spin noise. We attribute the time evolution to differences in the local
Zeeman splitting between the respective dots. With the help of numerical
simulations, we analyse and discuss the visibility of the singlet-triplet
oscillations and connect it to the requirements for coherent spin shuttling in
terms of the inter-dot tunnel coupling strength and rise time of the pulses.
The distribution of entangled spin pairs through tunnel coupled structures may
be of great utility for connecting distant qubit registers on a chip.Comment: 21 pages, 10 figure
Nanosecond-timescale spin transfer using individual electrons in a quadruple-quantum-dot device
The ability to coherently transport electron-spin states between different
sites of gate-defined semiconductor quantum dots is an essential ingredient for
a quantum-dot-based quantum computer. Previous shuttles using electrostatic
gating were too slow to move an electron within the spin dephasing time across
an array. Here we report a nanosecond-timescale spin transfer of individual
electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation
rates at a so-called `hot spot', we can upper bound the shuttle time to at most
150 ns. While actual shuttle times are likely shorter, 150 ns is already fast
enough to preserve spin coherence in e.g. silicon based quantum dots. This work
therefore realizes an important prerequisite for coherent spin transfer in
quantum dot arrays.Comment: 7 pages including 2 pages of supplementary materia
High Kinetic Inductance Superconducting Nanowire Resonators for Circuit QED in a Magnetic Field
We present superconducting microwave-frequency resonators based on NbTiN
nanowires. The small cross section of the nanowires minimizes vortex
generation, making the resonators resilient to magnetic fields. Measured
intrinsic quality factors exceed in a T in-plane magnetic
field, and in a mT perpendicular magnetic field. Due to
their high characteristic impedance, these resonators are expected to develop
zero-point voltage fluctuations one order of magnitude larger than in standard
coplanar waveguide resonators. These properties make the nanowire resonators
well suited for circuit QED experiments needing strong coupling to quantum
systems with small electric dipole moments and requiring a magnetic field, such
as electrons in single and double quantum dots
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