48 research outputs found
Quantum error correction in crossbar architectures
A central challenge for the scaling of quantum computing systems is the need
to control all qubits in the system without a large overhead. A solution for
this problem in classical computing comes in the form of so called crossbar
architectures. Recently we made a proposal for a large scale quantum
processor~[Li et al. arXiv:1711.03807 (2017)] to be implemented in silicon
quantum dots. This system features a crossbar control architecture which limits
parallel single qubit control, but allows the scheme to overcome control
scaling issues that form a major hurdle to large scale quantum computing
systems. In this work, we develop a language that makes it possible to easily
map quantum circuits to crossbar systems, taking into account their
architecture and control limitations. Using this language we show how to map
well known quantum error correction codes such as the planar surface and color
codes in this limited control setting with only a small overhead in time. We
analyze the logical error behavior of this surface code mapping for estimated
experimental parameters of the crossbar system and conclude that logical error
suppression to a level useful for real quantum computation is feasible.Comment: 29 + 9 pages, 13 figures, 9 tables, 8 algorithms and 3 big boxes.
Comments are welcom
Localized many-particle majorana modes with vanishing time-reversal symmetry breaking in double quantum dots
We introduce the concept of spinful many-particle Majorana modes with local odd operator products, thereby preserving their local statistics. We consider a superconductor-double-quantum-dot system where these modes can arise with negligible Zeeman splitting when Coulomb interactions are present. We find a reverse Mott-insulator transition, where the even- and odd-parity bands become degenerate. Above this transition, Majorana operators move the system between the odd-parity ground state, associated with elastic cotunneling, and the even-parity ground state, associated with crossed Andreev reflection. These Majorana modes are described in terms of one, three, and five operator products. Parity conservation results in a 4Ï€ periodic supercurrent in the even state and no supercurrent in the odd state
Multiple Fermi pockets revealed by Shubnikov-de Haas oscillations in WTe2
We use magneto-transport measurements to investigate the electronic structure
of WTe2 single crystals. A non-saturating and parabolic magnetoresistance is
observed in the temperature range between 2.5 to 200 K and magnetic fields up
to 8 T. Shubnikov - de Haas oscillations with beating patterns are observed.
The fast Fourier transform of the SdH oscillations reveals three oscillation
frequencies, corresponding to three pairs of Fermi pockets with comparable
effective masses , m* ~ 0.31 me. By fitting the Hall resistivity, we infer the
presence of one pair of electron pockets and two pairs of hole pockets,
together with nearly perfect compensation of the electron-hole carrier
concentration. These magnetotransport measurements reveal the complex
electronic structure in WTe2, explaining the nonsaturating magnetoresistance.Comment: Submitted to journal on 1 April, 2015, 4 Figure
Observation of topological transition of Fermi surface from a spindle-torus to a torus in large bulk Rashba spin-split BiTeCl
The recently observed large Rashba-type spin splitting in the BiTeX (X = I,
Br, Cl) bulk states due to the absence of inversion asymmetry and large charge
polarity enables observation of the transition in Fermi surface topology from
spindle-torus to torus with varying the carrier density. These BiTeX systems
with high spin-orbit energy scales offer an ideal platform for achieving
practical spintronic applications and realizing non-trivial phenomena such as
topological superconductivity and Majorana fermions. Here we use Shubnikov-de
Haas oscillations to investigate the electronic structure of the bulk
conduction band of BiTeCl single crystals with different carrier densities. We
observe the topological transition of the Fermi surface (FS) from a
spindle-torus to a torus. The Landau level fan diagram reveals the expected
non-trivial {\pi} Berry phase for both the inner and outer FSs. Angle-dependent
oscillation measurements reveal three-dimensional FS topology when the Fermi
level lies in the vicinity of the Dirac point. All the observations are
consistent with large Rashba spin-orbit splitting in the bulk conduction band.Comment: 28 pages, supplementary informatio
Many-particle Majorana bound states: derivation and signatures in superconducting double quantum dots
We consider two interacting quantum dots coupled by standard superconductors.
We derive an effective Hamiltonian, and show that over a wide parameter range a
degenerate ground state can be obtained. An exotique form of Majorana bound
states are supported at these degeneracies, and the system can be adiabatically
tuned to a limit in which it is equivalent to the one-dimensional wire model of
Kitaev. We give the form of a Majorana bound state in this system in the strong
interaction limit in the many-particle picture. We also study the Josephson
current in this system, and demonstrate that a double slit-like pattern emerges
in the presence of an extra magnetic field. This pattern is shown to disappear
with increasing interaction strength, which is able to be explained as the
current being carried by chargeless Majorana modes.Comment: 13 pages, 7 figures. Updated paper includes more details regarding
the derivation of the effective Hamiltonia
Gate modulation of the hole singlet-triplet qubit frequency in germanium
Spin qubits in germanium gate-defined quantum dots have made considerable
progress within the last few years, partially due to their strong spin-orbit
coupling and site-dependent -tensors. While this characteristic of the
-factors removes the need for micromagnets and allows for the possibility of
all-electric qubit control, relying on these -tensors necessitates the need
to understand their sensitivity to the confinement potential that defines the
quantum dots. Here, we demonstrate a qubit whose frequency is a strong
function of the voltage applied to the barrier gate shared by the quantum dots.
We find a -factor that can be approximately increased by an order of
magnitude adjusting the barrier gate voltage only by 12 mV. We attribute the
strong dependence to a variable strain profile in our device. This work not
only reinforces previous findings that site-dependent -tensors in germanium
can be utilized for qubit manipulation, but reveals the sensitivity and
tunability these -tensors have to the electrostatic confinement of the
quantum dot
Germanium wafers for strained quantum wells with low disorder
We grow strained Ge/SiGe heterostructures by reduced-pressure chemical vapor
deposition on 100 mm Ge wafers. The use of Ge wafers as substrates for epitaxy
enables high-quality Ge-rich SiGe strain-relaxed buffers with a threading
dislocation density of (61)10 cm, nearly an order of
magnitude improvement compared to control strain-relaxed buffers on Si wafers.
The associated reduction in short-range scattering allows for a drastic
improvement of the disorder properties of the two-dimensional hole gas,
measured in several Ge/SiGe heterostructure field-effect transistors. We
measure an average low percolation density of (1.220.03)10
cm, and an average maximum mobility of (3.40.1)10
cm/Vs and quantum mobility of (8.40.5)10 cm/Vs when
the hole density in the quantum well is saturated to
(1.650.02)10 cm. We anticipate immediate application
of these heterostructures for next-generation, higher-performance Ge
spin-qubits and their integration into larger quantum processors
Probing resonating valence bonds on a programmable germanium quantum simulator
Simulations using highly tunable quantum systems may enable investigations of
condensed matter systems beyond the capabilities of classical computers.
Quantum dots and donors in semiconductor technology define a natural approach
to implement quantum simulation. Several material platforms have been used to
study interacting charge states, while gallium arsenide has also been used to
investigate spin evolution. However, decoherence remains a key challenge in
simulating coherent quantum dynamics. Here, we introduce quantum simulation
using hole spins in germanium quantum dots. We demonstrate extensive and
coherent control enabling the tuning of multi-spin states in isolated, paired,
and fully coupled quantum dots. We then focus on the simulation of resonating
valence bonds and measure the evolution between singlet product states which
remains coherent over many periods. Finally, we realize four-spin states with
-wave and -wave symmetry. These results provide means to perform
non-trivial and coherent simulations of correlated electron systems.Comment: Article main text and Supplementary Information Main text: 9 pages, 5
figures Supplementary Information: 15 pages, 9 figure