9 research outputs found
Enhancing qubit readout through dissipative sub-Poissonian dynamics
Single-shot qubit readout typically combines high readout contrast with
long-lived readout signals, leading to large signal-to-noise ratios and high
readout fidelities. In recent years, it has been demonstrated that both readout
contrast and readout signal lifetime, and thus the signal-to-noise ratio, can
be enhanced by forcing the qubit state to transition through intermediate
states. In this work, we demonstrate that the sub-Poissonian relaxation
statistics introduced by intermediate states can reduce the single-shot readout
error rate by orders of magnitude even when there is no increase in
signal-to-noise ratio. These results hold for moderate values of the
signal-to-noise ratio () and a small number of
intermediate states (). The ideas presented here could have
important implications for readout schemes relying on the detection of
transient charge states, such as spin-to-charge conversion schemes for
semiconductor spin qubits and parity-to-charge conversion schemes for
topologically protected Majorana qubits.Comment: 10 pages, 6 figures. Two appendices have been added. This version is
close to the final published versio
Four-Majorana qubit with charge readout: dynamics and decoherence
We present a theoretical analysis of a Majorana-based qubit consisting of two
topological superconducting islands connected via a Josephson junction. The
qubit is operated by electrostatic gates which control the coupling of two of
the four Majorana zero modes. At the end of the operation, readout is performed
in the charge basis. Even though the operations are not topologically
protected, the proposed experiment can potentially shed light on the coherence
of the parity degree of freedom in Majorana devices and serve as a first step
towards topological Majorana qubits. We discuss in detail the charge-stability
diagram and its use for characterizing the parameters of the devices, including
the overlap of the Majorana edge states. We describe the multi-level spectral
properties of the system and present a detailed study of its controlled
coherent oscillations, as well as decoherence resulting from coupling to a
non-Markovian environment. In particular, we study a gate-controlled protocol
where conversion between Coulomb-blockade and transmon regimes generates
coherent oscillations of the qubit state due to the overlap of Majorana modes.
We show that, in addition to fluctuations of the Majorana coupling,
considerable measurement errors may be accumulated during the conversion
intervals when electrostatic fluctuations in the superconducting islands are
present. These results are also relevant for several proposed implementations
of topological qubits which rely on readout based on charge detection
High-fidelity single-shot readout for a spin qubit via an enhanced latching mechanism
The readout of semiconductor spin qubits based on spin blockade is fast but
suffers from a small charge signal. Previous work suggested large benefits from
additional charge mapping processes, however uncertainties remain about the
underlying mechanisms and achievable fidelity. In this work, we study the
single-shot fidelity and limiting mechanisms for two variations of an enhanced
latching readout. We achieve average single-shot readout fidelities > 99.3% and
> 99.86% for the conventional and enhanced readout respectively, the latter
being the highest to date for spin blockade. The signal amplitude is enhanced
to a full one-electron signal while preserving the readout speed. Furthermore,
layout constraints are relaxed because the charge sensor signal is no longer
dependent on being aligned with the conventional (2, 0) - (1, 1) charge dipole.
Silicon donor-quantum-dot qubits are used for this study, for which the dipole
insensitivity substantially relaxes donor placement requirements. One of the
readout variations also benefits from a parametric lifetime enhancement by
replacing the spin-relaxation process with a charge-metastable one. This
provides opportunities to further increase the fidelity. The relaxation
mechanisms in the different regimes are investigated. This work demonstrates a
readout that is fast, has one-electron signal and results in higher fidelity.
It further predicts that going beyond 99.9% fidelity in a few microseconds of
measurement time is within reach.Comment: Supplementary information is included with the pape
Repetitive Quantum Nondemolition Measurement and Soft Decoding of a Silicon Spin Qubit
Quantum error correction is of crucial importance for fault-tolerant quantum computers. As an essential step toward the implementation of quantum error-correcting codes, quantum nondemolition measurements are needed to efficiently detect the state of a logical qubit without destroying it. Here we implement quantum nondemolition measurements in a Si/SiGe two-qubit system, with one qubit serving as the logical qubit and the other serving as the ancilla. Making use of a two-qubit controlled-rotation gate, the state of the logical qubit is mapped onto the ancilla, followed by a destructive readout of the ancilla. Repeating this procedure enhances the logical readout fidelity from 75.5±0.3% to 94.5±0.2% after 15 ancilla readouts. In addition, we compare the conventional thresholding method with an improved signal processing method called soft decoding that makes use of analog information in the readout signal to better estimate the state of the logical qubit. We demonstrate that soft decoding leads to a significant reduction in the required number of repetitions when the readout errors become limited by Gaussian noise, for instance, in the case of readouts with a low signal-to-noise ratio. These results pave the way for the implementation of quantum error correction with spin qubits in silicon.</p