68 research outputs found
Two-Qubit Pulse Gate for the Three-Electron Double Quantum Dot Qubit
The three-electron configuration of gate-defined double quantum dots encodes
a promising qubit for quantum information processing. I propose a two-qubit
entangling gate using a pulse-gated manipulation procedure. The requirements
for high-fidelity entangling operations are equivalent to the requirements for
the pulse-gated single-qubit manipulations that have been successfully realized
for Si QDs. This two-qubit gate completes the universal set of all-pulse-gated
operations for the three-electron double-dot qubit and paves the way for a
scalable setup to achieve quantum computation.Comment: 8 pages, 4 figure
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
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
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
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
Two-Qubit Couplings of Singlet-Triplet Qubits Mediated by One Quantum State
We describe high-fidelity entangling gates between singlet-triplet qubits
(STQs) which are coupled via one quantum state (QS). The QS can be provided by
a quantum dot itself or by another confined system. The orbital energies of the
QS are tunable using an electric gate close to the QS, which changes the
interactions between the STQs independent of their single-qubit parameters.
Short gating sequences exist for the controlled NOT (CNOT) operations. We show
that realistic quantum dot setups permit excellent entangling operations with
gate infidelities below , which is lower than the quantum error
correction threshold of the surface code. We consider limitations from
fabrication errors, hyperfine interactions, spin-orbit interactions, and charge
noise in GaAs and Si heterostructures.Comment: 12 pages, 6 figure
Charge-noise tolerant exchange gates of singlet-triplet qubits in asymmetric double quantum dots
In the semi-conductor double quantum dot singlet-triplet qubit architecture,
the decoherence caused by the qubit's charge environment poses a serious
obstacle in the way towards large scale quantum computing. The effects of the
charge decoherence can be mitigated by operating the qubit in the so called
sweet spot regions where it is insensitive to electrical noise. In this paper,
we propose singlet-triplet qubits based on two quantum dots of different sizes.
Such asymmetric double dot systems allow the implementation of exchange gates
with controllable exchange splitting operated in the doubly occupied charge
region of the larger dot, where the qubit has high resilience to charge noise.
In the larger dot, can be quenched to a value smaller than the intra-dot
tunneling using magnetic fields, while the smaller dot and its larger splitting
can be used in the projective readout of the qubit
Adiabatic two-qubit gates in capacitively coupled quantum dot hybrid qubits
The ability to tune qubits to flat points in their energy dispersions ("sweet
spots") is an important tool for mitigating the effects of charge noise and
dephasing in solid-state devices. However, the number of derivatives that must
be simultaneously set to zero grows exponentially with the number of coupled
qubits, making the task untenable for as few as two qubits. This is a
particular problem for adiabatic gates, due to their slower speeds. Here, we
propose an adiabatic two-qubit gate for quantum dot hybrid qubits, based on the
tunable, electrostatic coupling between distinct charge configurations. We
confirm the absence of a conventional sweet spot, but show that controlled-Z
(CZ) gates can nonetheless be optimized to have fidelities of 99% for a
typical level of quasistatic charge noise (1
eV). We then develop the concept of a dynamical sweet spot (DSS), for
which the time-averaged energy derivatives are set to zero, and identify a
simple pulse sequence that achieves an approximate DSS for a CZ gate, with a
5 improvement in the fidelity. We observe that the results depend on
the number of tunable parameters in the pulse sequence, and speculate that a
more elaborate sequence could potentially attain a true DSS.Comment: 14 pages, 9 figure
Validity of the single-particle description and charge noise resilience for multielectron quantum dots
We construct an optimal set of single-particle states for few-electron
quantum dots (QDs) using the method of natural orbitals (NOs). The NOs include
also the effects of the Coulomb repulsion between electrons. We find that they
agree well with the noniteracting orbitals for GaAs QDs of realistic
parameters, while the Coulomb interactions only rescale the radius of the NOs
compared to the noninteracting case. We use NOs to show that four-electron QDs
are less susceptible to charge noise than their two-electron counterparts.Comment: 11+ pages, 5 figure
Fault-Tolerant Quantum Computation for Singlet-Triplet Qubits with Leakage Errors
We describe and analyze leakage errors of singlet-triplet qubits. Even though
leakage errors are a natural problem for spin qubits encoded using quantum dot
arrays, they have obtained little attention in previous studies. We describe
the realization of leakage correction protocols that can be implemented
together with the quantum error correction protocol of the surface code.
Furthermore we construct explicit leakage reduction units that need, in the
ideal setup, as few as three manipulation steps. Our study shows that leakage
errors can be corrected without the need of measurements and at the cost of
only a few additional ancilla qubits and gate operations compared to standard
quantum error correction codes.Comment: 7+ pages, 5 figure
- …