34 research outputs found
Long-Time Correlations in Single-Neutron Interferometry Data
We present a detailed analysis of the time series of time-stamped neutron
counts obtained by single-neutron interferometry. The neutron counting
statistics display the usual Poissonian behavior, but the variance of the
neutron counts does not. Instead, the variance is found to exhibit a dependence
on the phase-shifter setting which can be explained by a probabilistic model
that accounts for fluctuations of the phase shift. The time series of the
detection events exhibit long-time correlations with amplitudes that also
depend on the phase-shifter setting. These correlations appear as damped
oscillations with a period of about 2.8 s. By simulation, we show that the
correlations of the time differences observed in the experiment can be
reproduced by assuming that, for a fixed setting of the phase shifter, the
phase shift experienced by the neutrons varies periodically in time with a
period of 2.8 s. The same simulations also reproduce the behavior of the
variance. Our analysis of the experimental data suggests that time-stamped data
of singleparticle interference experiments may exhibit transient features that
require a description in terms of non-stationary processes, going beyond the
standard quantum model of independent random events
Fragility of gate-error metrics in simulation models of flux-tunable transmon quantum computers
Constructing a quantum computer requires immensely precise control over a quantum system. A lack of precision is often quantified by gate-error metrics, such as the average infidelity or the diamond distance. However, usually such gate-error metrics are only considered for individual gates and not the errors that accumulate over consecutive gates. Furthermore, it is not well known how susceptible the metrics are to the assumptions which make up the model. Here we investigate these issues using realistic simulation models of quantum computers with flux-tunable transmons and coupling resonators. Our main findings reveal that (i) gate-error metrics are indeed affected by the many assumptions of the model, (ii) consecutive gate errors do not accumulate linearly, and (iii) gate-error metrics are poor predictors for the performance of consecutive gates. Additionally, we discuss a potential limitation in the scalability of the studied device architecture.</p
Gate-error analysis in simulations of quantum computers with transmon qubits
In the model of gate-based quantum computation, the qubits are controlled by
a sequence of quantum gates. In superconducting qubit systems, these gates can
be implemented by voltage pulses. The success of implementing a particular gate
can be expressed by various metrics such as the average gate fidelity, the
diamond distance, and the unitarity. We analyze these metrics of gate pulses
for a system of two superconducting transmon qubits coupled by a resonator, a
system inspired by the architecture of the IBM Quantum Experience. The metrics
are obtained by numerical solution of the time-dependent Schr\"odinger equation
of the transmon system. We find that the metrics reflect systematic errors that
are most pronounced for echoed cross-resonance gates, but that none of the
studied metrics can reliably predict the performance of a gate when used
repeatedly in a quantum algorithm
Optical switch based on a fluid-filled photonic crystal fiber Bragg grating
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Fragility of gate-error metrics in simulation models of flux-tunable transmon quantum computers
Constructing a quantum computer requires immensely precise control over a quantum system. A lack of precision is often quantified by gate-error metrics, such as the average infidelity or the diamond distance. However, usually such gate-error metrics are only considered for individual gates and not the errors that accumulate over consecutive gates. Furthermore, it is not well known how susceptible the metrics are to the assumptions which make up the model. Here we investigate these issues using realistic simulation models of quantum computers with flux-tunable transmons and coupling resonators. Our main findings reveal that (i) gate-error metrics are indeed affected by the many assumptions of the model, (ii) consecutive gate errors do not accumulate linearly, and (iii) gate-error metrics are poor predictors for the performance of consecutive gates. Additionally, we discuss a potential limitation in the scalability of the studied device architecture.</p
Testing quantum fault tolerance on small systems
We extensively test a recent protocol to demonstrate quantum fault tolerance on three systems: (1) a real-time simulation of five spin qubits coupled to an environment with two-level defects, (2) a real-time simulation of transmon quantum computers, and (3) the 16-qubit processor of the IBM Q Experience. In the simulations, the dynamics of the full system is obtained by numerically solving the time-dependent Schrödinger equation. We find that the fault-tolerant scheme provides a systematic way to improve the results when the errors are dominated by the inherent control and measurement errors present in transmon systems. However, the scheme fails to satisfy the criterion for fault tolerance when decoherence effects are dominant
Numerical analysis of effective models for flux-tunable transmon systems
Simulations and analytical calculations that aim to describe flux-tunable transmons are usually based on effective models of the corresponding lumped-element model. However, when a control pulse is applied, in most cases it is not known how much the predictions made with the effective models deviate from the predictions made with the original lumped-element model. In this work we compare the numerical solutions of the time-dependent Schrödinger equation for both the effective and the lumped-element models, for microwave and unimodal control pulses (external fluxes). These control pulses are used to model single-qubit (X) and two-qubit gate (iswap and cz) transitions. First, we derive a nonadiabatic effective Hamiltonian for a single flux-tunable transmon and compare the pulse response of this model to the one of the corresponding circuit Hamiltonian. Here we find that both models predict similar outcomes for similar control pulses. Then, we study how different approximations affect single-qubit (X) and two-qubit gate (iswap and cz) transitions in two different two-qubit systems. For this purpose we consider three different systems in total: a single flux-tunable transmon and two two-qubit systems. In summary, we find that a series of commonly applied approximations (individually and/or in combination) can change the response of a system substantially, when a control pulse is applied
Characterization of PAHs in geological samples by charge-transfer liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry
Separation of conditions as a prerequisite for quantum theory
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