40 research outputs found
Self-aligned fabrication process for silicon quantum computer devices
We describe a fabrication process for devices with few quantum bits (qubits),
which are suitable for proof-of-principle demonstrations of silicon-based
quantum computation. The devices follow the Kane proposal to use the nuclear
spins of 31P donors in 28Si as qubits, controlled by metal surface gates and
measured using single electron transistors (SETs). The accurate registration of
31P donors to control gates and read-out SETs is achieved through the use of a
self-aligned process which incorporates electron beam patterning, ion
implantation and triple-angle shadow-mask metal evaporation
Decoherence of quantum states and its suppression in ensemble large-scale solid state NMR quantum computers
It is discussed the decoherence problems in ensemble large-scale solid state
NMR quantum computer based on the array of P donor atoms having nuclear spin I
= 1/2. It is considered here, as main mechanisms of decoherence for low
temperature (< 0.1 K), the adiabatic processes of random modulation of qubit
resonance frequency determined by secular part of nuclear spin interaction with
electron spin of the basic atoms, with impurity paramagnetic atoms and also
with nuclear spins of impurity diamagnetic atoms. It was made estimations of
allowed concentrations of magnetic impurities and of spin temperature whereby
the required decoherence suppression is obtained. It is discussed the random
phase error suppression in the ensemble quantum register basic states.Comment: LaTeX 7 pages. Submitted to Proceedings of SPIE. International
Symposium Quantum Informatics (QI-2002), October 2002, Zvenigorod, Russi
Optical Detection of a Single Nuclear Spin
We propose a method to optically detect the spin state of a 31-P nucleus
embedded in a 28-Si matrix. The nuclear-electron hyperfine splitting of the
31-P neutral-donor ground state can be resolved via a direct frequency
discrimination measurement of the 31-P bound exciton photoluminescence using
single photon detectors. The measurement time is expected to be shorter than
the lifetime of the nuclear spin at 4 K and 10 T.Comment: 4 pages, 3 figure
The classical nature of nuclear spin noise near clock transitions of Bi donors in silicon
Whether a quantum bath can be approximated as classical noise is a
fundamental issue in central spin decoherence and also of practical importance
in designing noise-resilient quantum control. Spin qubits based on bismuth
donors in silicon have tunable interactions with nuclear spin baths and are
first-order insensitive to magnetic noise at so-called clock-transitions (CTs).
This system is therefore ideal for studying the quantum/classical nature of
nuclear spin baths since the qubit-bath interaction strength determines the
back-action on the baths and hence the adequacy of a classical noise model. We
develop a Gaussian noise model with noise correlations determined by quantum
calculations and compare the classical noise approximation to the full quantum
bath theory. We experimentally test our model through dynamical decoupling
sequence of up to 128 pulses, finding good agreement with simulations and
measuring electron spin coherence times approaching one second - notably using
natural silicon. Our theoretical and experimental study demonstrates that the
noise from a nuclear spin bath is analogous to classical Gaussian noise if the
back-action of the qubit on the bath is small compared to the internal bath
dynamics, as is the case close to CTs. However, far from the CTs, the
back-action of the central spin on the bath is such that the quantum model is
required to accurately model spin decoherence.Comment: 5 pages, 3 figure
Performing joint measurements and transformations on several qubits by operating on a single control qubit
An n-qubit quantum register can in principle be completely controlled by
operating on a single qubit that interacts with the register via an appropriate
fixed interaction. We consider a hypothetical system consisting of n spin-1/2
nuclei that interact with an electron spin via a magnetic interaction. We
describe algorithms that measure non-trivial joint observables on the register
by acting on the control spin only. For large n this is not an efficient model
for universal quantum computation but it can be modified to an efficient one if
one allows n possible positions of the control particle.
This toy model of measurements illustrates in which way specific interactions
between the register and a probe particle support specific types of joint
measurements in the sense that some joint observables can be measured by simple
sequences of operations on the probe particle.Comment: 7 pages, revtex, 3 figure
A hybrid double-dot in silicon
We report electrical measurements of a single arsenic dopant atom in the
tunnel-barrier of a silicon SET. As well as performing electrical
characterization of the individual dopant, we study series electrical transport
through the dopant and SET. We measure the triple points of this hybrid double
dot, using simulations to support our results, and show that we can tune the
electrostatic coupling between the two sub-systems.Comment: 11 pages, 6 figure
Effects of J-gate potential and interfaces on donor exchange coupling in the Kane quantum computer architecture
We calculate the electron exchange coupling for a phosphorus donor pair in
silicon perturbed by a J-gate potential and the boundary effects of the silicon
host geometry. In addition to the electron-electron exchange interaction we
also calculate the contact hyperfine interaction between the donor nucleus and
electron as a function of the varying experimental conditions. Donor
separation, depth of the P nuclei below the silicon oxide layer and J-gate
voltage become decisive factors in determining the strength of both the
exchange coupling and the hyperfine interaction - both crucial components for
qubit operations in the Kane quantum computer. These calculations were
performed using an anisotropic effective-mass Hamiltonian approach. The
behaviour of the donor exchange coupling as a function of the device parameters
varied provides relevant information for the experimental design of these
devices.Comment: 15 pages, 15 figures. Accepted for Journal of Physics: Condensed
Matte
Coherence of Spin Qubits in Silicon
Given the effectiveness of semiconductor devices for classical computation
one is naturally led to consider semiconductor systems for solid state quantum
information processing. Semiconductors are particularly suitable where local
control of electric fields and charge transport are required. Conventional
semiconductor electronics is built upon these capabilities and has demonstrated
scaling to large complicated arrays of interconnected devices. However, the
requirements for a quantum computer are very different from those for classical
computation, and it is not immediately obvious how best to build one in a
semiconductor. One possible approach is to use spins as qubits: of nuclei, of
electrons, or both in combination. Long qubit coherence times are a
prerequisite for quantum computing, and in this paper we will discuss
measurements of spin coherence in silicon. The results are encouraging - both
electrons bound to donors and the donor nuclei exhibit low decoherence under
the right circumstances. Doped silicon thus appears to pass the first test on
the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200
Two Qubit Quantum Computing in a Projected Subspace
A formulation for performing quantum computing in a projected subspace is
presented, based on the subdynamical kinetic equation (SKE) for an open quantum
system. The eigenvectors of the kinetic equation are shown to remain invariant
before and after interaction with the environment. However, the eigenvalues in
the projected subspace exhibit a type of phase shift to the evolutionary
states. This phase shift does not destroy the decoherence-free (DF) property of
the subspace because the associated fidelity is 1. This permits a universal
formalism to be presented - the eigenprojectors of the free part of the
Hamiltonian for the system and bath may be used to construct a DF projected
subspace based on the SKE. To eliminate possible phase or unitary errors
induced by the change in the eigenvalues, a cancellation technique is proposed,
using the adjustment of the coupling time, and applied to a two qubit computing
system. A general criteria for constructing a DF projected subspace from the
SKE is discussed. Finally, a proposal for using triangulation to realize a
decoherence-free subsystem based on SKE is presented. The concrete formulation
for a two-qubit model is given exactly. Our approach is novel and general, and
appears applicable to any type of decoherence. Key Words: Quantum Computing,
Decoherence, Subspace, Open System PACS number: 03.67.Lx,33.25.+k,.76.60.-kComment: 24 pages. accepted by Phys. Rev.