1,740 research outputs found
Quantum computing and single-qubit measurements using the spin filter effect
Many things will have to go right for quantum computation to become a reality
in the lab. For any of the presently-proposed approaches involving spin states
in solids, an essential requirement is that these spins should be measured at
the single-Bohr-magneton level. Fortunately, quantum computing provides a
suggestion for a new approach to this seemingly almost impossible task: convert
the magnetization into a charge, and measure the charge. I show how this might
be done by exploiting the spin filter effect provided by ferromagnetic tunnel
barriers, used in conjunction with one-electron quantum dots.Comment: 11 pages, LaTeX, 1 figure. To be published in J. Appl. Phys., paper
given at the 43rd Annual MMM Conferenc
Simple operation sequences to couple and interchange quantum information between spin qubits of different kinds
Efficient operation sequences to couple and interchange quantum information
between quantum dot spin qubits of different kinds are derived using exchange
interactions. In the qubit encoding of a single-spin qubit, a singlet-triplet
qubit, and an exchange-only (triple-dot) qubit, some of the single-qubit
interactions remain on during the entangling operation; this greatly simplifies
the operation sequences that construct entangling operations. In the ideal
setup, the gate operations use the intra-qubit exchange interactions only once.
The limitations of the entangling sequences are discussed, and it is shown how
quantum information can be converted between different kinds of quantum dot
spin qubits.Comment: 9 pages, 4 figure
Hall Effect Gyrators and Circulators
The electronic circulator, and its close relative the gyrator, are invaluable
tools for noise management and signal routing in the current generation of
low-temperature microwave systems for the implementation of new quantum
technologies. The current implementation of these devices using the Faraday
effect is satisfactory, but requires a bulky structure whose physical dimension
is close to the microwave wavelength employed. The Hall effect is an
alternative non-reciprocal effect that can also be used to produce desired
device functionality. We review earlier efforts to use an ohmically-contacted
four-terminal Hall bar, explaining why this approach leads to unacceptably high
device loss. We find that capacitive coupling to such a Hall conductor has much
greater promise for achieving good circulator and gyrator functionality. We
formulate a classical Ohm-Hall analysis for calculating the properties of such
a device, and show how this classical theory simplifies remarkably in the
limiting case of the Hall angle approaching 90 degrees. In this limit we find
that either a four-terminal or a three-terminal capacitive device can give
excellent circulator behavior, with device dimensions far smaller than the a.c.
wavelength. An experiment is proposed to achieve GHz-band gyration in
millimetre (and smaller) scale structures employing either semiconductor
heterostructure or graphene Hall conductors. An inductively coupled scheme for
realising a Hall gyrator is also analysed.Comment: 18 pages, 15 figures, ~5 MB. V3: sections V-VIII revisited plus other
minor changes, Fig 2 added. Submitted to PR
Effective fault-tolerant quantum computation with slow measurements
How important is fast measurement for fault-tolerant quantum computation?
Using a combination of existing and new ideas, we argue that measurement times
as long as even 1,000 gate times or more have a very minimal effect on the
quantum accuracy threshold. This shows that slow measurement, which appears to
be unavoidable in many implementations of quantum computing, poses no essential
obstacle to scalability.Comment: 9 pages, 11 figures. v2: small changes and reference addition
Noise-Protected Gate for Six-Electron Double-Dot Qubits
Singlet-triplet spin qubits in six-electron double quantum dots, in moderate
magnetic fields, can show superior immunity to charge noise. This immunity
results from the symmetry of orbitals in the second energy shell of circular
quantum dots: singlet and triplet states in this shell have identical charge
distributions. Our phase-gate simulations, which include charge noise
from fluctuating traps, show that this symmetry is most effectively exploited
if the gate operation switches rapidly between sweet spots deep in the (3,3)
and (4,2) charge stability regions; fidelities very close to one are predicted
if subnanosecond switching can be performed.Comment: 7 pages, 3 figure
Multiport Impedance Quantization
With the increase of complexity and coherence of superconducting systems made
using the principles of circuit quantum electrodynamics, more accurate methods
are needed for the characterization, analysis and optimization of these quantum
processors. Here we introduce a new method of modelling that can be applied to
superconducting structures involving multiple Josephson junctions, high-Q
superconducting cavities, external ports, and voltage sources. Our technique,
an extension of our previous work on single-port structures [1], permits the
derivation of system Hamiltonians that are capable of representing every
feature of the physical system over a wide frequency band and the computation
of T1 times for qubits. We begin with a black box model of the linear and
passive part of the system. Its response is given by its multiport impedance
function Zsim(w), which can be obtained using a finite-element electormagnetics
simulator. The ports of this black box are defined by the terminal pairs of
Josephson junctions, voltage sources, and 50 Ohm connectors to high-frequency
lines. We fit Zsim(w) to a positive-real (PR) multiport impedance matrix Z(s),
a function of the complex Laplace variable s. We then use state-space
techniques to synthesize a finite electric circuit admitting exactly the same
impedance Z(s) across its ports; the PR property ensures the existence of this
finite physical circuit. We compare the performance of state-space algorithms
to classical frequency domain methods, justifying their superiority in
numerical stability. The Hamiltonian of the multiport model circuit is obtained
by using existing lumped element circuit quantization formalisms [2, 3]. Due to
the presence of ideal transformers in the model circuit, these quantization
methods must be extended, requiring the introduction of an extension of the
Kirchhoff voltage and current laws
Inverted Singlet-Triplet Qubit Coded on a Two-Electron Double Quantum Dot
The spin configuration of two electrons confined at a double quantum
dot (DQD) encodes the singlet-triplet qubit (STQ). We introduce the inverted
STQ (ISTQ) that emerges from the setup of two quantum dots (QDs) differing
significantly in size and out-of-plane magnetic fields. The strongly confined
QD has a two-electron singlet ground state, but the weakly confined QD has a
two-electron triplet ground state in the subspace. Spin-orbit
interactions act nontrivially on the subspace and provide universal
control of the ISTQ together with electrostatic manipulations of the charge
configuration. GaAs and InAs DQDs can be operated as ISTQs under realistic
noise conditions.Comment: 10 pages, 4 figure
Quantum Computation and Spin Physics
A brief review is given of the physical implementation of quantum computation
within spin systems or other two-state quantum systems. The importance of the
controlled-NOT or quantum XOR gate as the fundamental primitive operation of
quantum logic is emphasized. Recent developments in the use of quantum
entanglement to built error-robust quantum states, and the simplest protocol
for quantum error correction, are discussed.Comment: 21 pages, Latex, 3 eps figures, prepared for the Proceedings of the
Annual MMM Meeting, November, 1996, to be published in J. Appl. Phy
Topological Quantum Computing
This set of lecture notes forms the basis of a series of lectures delivered
at the 48th IFF Spring School 2017 on Topological Matter: Topological
Insulators, Skyrmions and Majoranas at Forschungszentrum Juelich, Germany. The
first part of the lecture notes covers the basics of abelian and non-abelian
anyons and their realization in the Kitaev's honeycomb model. The second part
discusses how to perform universal quantum computation using Majorana fermions.Comment: In Topological Matter: Topological Insulators, Skyrmions and
Majoranas, Lecture notes of the 48th IFF Spring School 2017, eds. S. Bluegel,
Y. Mokrusov, T. Schaepers, and Y. Ando (Forschungszentrum Juelich, Key
Technologies, Vol. 139, 2017), Sec. D
Noise Analysis of Qubits Implemented in Triple Quantum Dot Systems in a Davies Master Equation Approach
We analyze the influence of noise for qubits implemented using a triple
quantum dot spin system. We give a detailed description of the physical
realization and develop error models for the dominant external noise sources.
We use a Davies master equation approach to describe their influence on the
qubit. The triple dot system contains two meaningful realizations of a qubit:
We consider a subspace and a subsystem of the full Hilbert space to implement
the qubit. We test the robustness of these two implementations with respect to
the qubit stability. When performing the noise analysis, we extract the initial
time evolution of the qubit using a Nakajima-Zwanzig approach. We find that the
initial time evolution, which is essential for qubit applications, decouples
from the long time dynamics of the system. We extract probabilities for the
qubit errors of dephasing, relaxation and leakage. Using the Davies model to
describe the environment simplifies the noise analysis. It allows us to
construct simple toy models, which closely describe the error probabilities.Comment: 30 pages, 18 figure
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