35 research outputs found
Transfer of a quantum state from a photonic qubit to a gate-defined quantum dot
Interconnecting well-functioning, scalable stationary qubits and photonic
qubits could substantially advance quantum communication applications and serve
to link future quantum processors. Here, we present two protocols for
transferring the state of a photonic qubit to a single-spin and to a two-spin
qubit hosted in gate-defined quantum dots (GDQD). Both protocols are based on
using a localized exciton as intermediary between the photonic and the spin
qubit. We use effective Hamiltonian models to describe the hybrid systems
formed by the the exciton and the GDQDs and apply simple but realistic noise
models to analyze the viability of the proposed protocols. Using realistic
parameters, we find that the protocols can be completed with a success
probability ranging between 85-97%
Towards a realistic GaAs-spin qubit device for a classical error-corrected quantum memory
Based on numerically-optimized real-device gates and parameters we study the
performance of the phase-flip (repetition) code on a linear array of Gallium
Arsenide (GaAs) quantum dots hosting singlet-triplet qubits. We first examine
the expected performance of the code using simple error models of circuit-level
and phenomenological noise, reporting, for example, a circuit-level
depolarizing noise threshold of approximately 3%. We then perform
density-matrix simulations using a maximum-likelihood and minimum-weight
matching decoder to study the effect of real-device dephasing, read-out error,
quasi-static as well as fast gate noise. Considering the trade-off between
qubit read-out error and dephasing time (T2) over measurement time, we identify
a sub-threshold region for the phase-flip code which lies within experimental
reach.Comment: 22 page
Calculation of tunnel-couplings in open gate-defined disordered quantum dot systems
Quantum computation based on semiconductor electron-spin qubits requires high
control of tunnel-couplings, both across quantum dots and between the quantum
dot and the reservoir. The tunnel-coupling to the reservoir sets the qubit
detection and initialization bandwidth for energy-resolved spin-to-charge
conversion and is essential to tune single-electron transistors commonly used
as charge detectors. Potential disorder and the increasing complexity of the
two-dimensional gate-defined quantum computing devices sets high demands on the
gate design and the voltage tuning of the tunnel barriers. We present a Green's
formalism approach for the calculation of tunnel-couplings between a quantum
dot and a reservoir. Our method takes into account in full detail the
two-dimensional electrostatic potential of the quantum dot, the tunnel barrier
and reservoir. A Markov approximation is only employed far away from the tunnel
barrier region where the density of states is sufficiently large. We calculate
the tunnel-coupling including potential disorder effects, which become
increasingly important for large-scale silicon-based spin-qubit devices.
Studying the tunnel-couplings of a single-electron transistor in Si/SiGe as a
showcase, we find that charged defects are the dominant source of disorder
leading to variations in the tunnel-coupling of four orders of magnitude.Comment: 10 pages, 4 figure
A Machine Learning Approach for Automated Fine-Tuning of Semiconductor Spin Qubits
While spin qubits based on gate-defined quantum dots have demonstrated very
favorable properties for quantum computing, one remaining hurdle is the need to
tune each of them into a good operating regime by adjusting the voltages
applied to electrostatic gates. The automation of these tuning procedures is a
necessary requirement for the operation of a quantum processor based on
gate-defined quantum dots, which is yet to be fully addressed. We present an
algorithm for the automated fine-tuning of quantum dots, and demonstrate its
performance on a semiconductor singlet-triplet qubit in GaAs. The algorithm
employs a Kalman filter based on Bayesian statistics to estimate the gradients
of the target parameters as function of gate voltages, thus learning the system
response. The algorithm's design is focused on the reduction of the number of
required measurements. We experimentally demonstrate the ability to change the
operation regime of the qubit within 3 to 5 iterations, corresponding to 10 to
15 minutes of lab-time
High-fidelity single- and two-qubit gates for two-electron spin qubits
A key ingredient for fault-tolerant quantum computers are sufficiently accurate logic gates on single and multiple qubits in the presence of decohering noise. In this thesis, we theoretically develop and experimentally demonstrate such high-fidelity quantum gates for semiconductor-based quantum computing. Specifically, we consider a qubit encoded in the singlet and one triplet state of two electron spins in GaAs. While its potential for optical coupling makes GaAs an attractive material, highly auto-correlated magnetic field noise from fluctuating nuclear spins leads to considerably lower coherence times than for Si-based devices, where nuclear spins can be removed by isotopic purification. In addition to noise, the control methods used in earlier experiments are based on unrealistic approximations. For these reasons, fidelities well above 99 %, as required by current quantum error correction (QEC) schemes, have not been obtained before this thesis. To tackle these issues, we extend the filter function formalism to describe quantum gates and processes in the presence of experimentally relevant non-Markovian noise. Since this formalism can consider all relevant noise sources in a computationally efficient manner, it can be used to find optimal control pulses by numerical optimization, leading to predicted single-qubit gate fidelities of 99.9 %. Furthermore, we deal with the considerable control challenges associated with this qubit type by experimental calibration of the optimized control pulses. To this end, we extend our previous experimental gate set calibration (GSC) routine to remove systematic gate errors on an arbitrary number of qubits. We apply the numerically optimized single-qubit control pulses to our GaAs sample and experimentally calibrate them with GSC. This procedure yields an average gate fidelity of 99.50 ± 0.04 % and a low leakage rate of 0.13 ± 0.03 % out of the computational subspace, characterized by randomized benchmarking. We also optimize realistic two-qubit control pulses, considering current control hardware as well as interqubit Coulomb and exchange coupling that cannot be fully turned off. Using measured noise spectra, we show that two-qubit gate fidelities of 99.90 % can be reached in GaAs, while 99.99 % can be achieved in Si. These results demonstrate that high-fidelity gates can be realized even in the presence of nuclear spins as in GaAs
Filter Functions for Quantum Processes under Correlated Noise
Many qubit implementations are afflicted by correlated noise not captured by
standard theoretical tools that are based on Markov approximations. While
independent gate operations are a key concept for quantum computing, it is
actually not possible to fully describe noisy gates locally in time if noise is
correlated on times longer than their duration. To address this issue, we
develop a method based on the filter function formalism to perturbatively
compute quantum processes in the presence of correlated classical noise. We
derive a composition rule for the filter function of a sequence of gates in
terms of those of the individual gates. The joint filter function allows to
efficiently compute the quantum process of the whole sequence. Moreover, we
show that correlation terms arise which capture the effects of the
concatenation and thus yield insight into the effect of noise correlations on
gate sequences. Our generalization of the filter function formalism enables
both qualitative and quantitative studies of algorithms and state-of-the-art
tools widely used for the experimental verification of gate fidelities like
randomized benchmarking, even in the presence of noise correlations.Comment: 6 pages, 1 figure. Letter accompanying arXiv:2103.02403. Open-source
software available at https://github.com/qutech/filter_functions. v2:
published versio
Filter-function formalism and software package to compute quantum processes of gate sequences for classical non-Markovian noise
Correlated, non-Markovian noise is present in many solid-state systems
employed as hosts for quantum information technologies, significantly
complicating the realistic theoretical description of these systems. In this
regime, the effects of noise on sequences of quantum gates cannot be described
by concatenating isolated quantum operations if the environmental correlation
times are on the scale of the typical gate durations. The filter function
formalism has been successful in characterizing the decay of coherence under
the influence of such classical, non-Markovian environments and here we show it
can be applied to describe unital evolution within the quantum operations
formalism. We find exact results for the quantum process and a simple
composition rule for a sequence of operations. This enables the detailed study
of effects of noise correlations on algorithms and periodically driven systems.
Moreover, we point out the method's suitability for numerical applications and
present the open-source Python software package filter_functions. Amongst other
things, it facilitates computing the noise-averaged transfer matrix
representation of a unital quantum operation in the presence of universal
classical noise for arbitrary control sequences. We apply the presented methods
to selected examples.Comment: 35 pages, 8 figures. In-depth article accompanying arXiv:2103.02385.
Open-source software available at https://github.com/qutech/filter_functions.
v2: published versio