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

New multiplexing scheme for monitoring fiber optic Bragg grating sensors in the coherence domain

A new multiplexing scheme for monitoring fiber optic Bragg gratings in the coherence domain has been developed. Grating pairs with different grating distances are distributed along a fiber line, and interference between their reflections is monitored with a scanning Michelson interferometer. The Bragg wavelength of the individual sensor elements is determined from the interference signal frequency

Quantum Nondemolition Dispersive Readout of a Superconducting Artificial Atom Using Large Photon Numbers

Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number $\overline{n}$ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing $\overline{n}$ is generally observed to also monotonously increase these transition rates. Here we present a fluxonium artificial atom in which, despite the fact that the measured transition rates show nonmonotonous fluctuations within a factor of 6, for photon numbers up to $\overline{n}$≈200, the signal-to-noise ratio continuously improves with increasing $\overline{n}$. Even without the use of a parametric amplifier, at $\overline{n}$=74, we achieve fidelities of 99% and 93% for feedback-assisted ground and excited state preparations, respectively. At higher $\overline{n}$, leakage outside the qubit computational space can no longer be neglected and it limits the fidelity of quantum state preparation

Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers

Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number $\bar{n}$ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing $\bar{n}$ is generally observed to also increase these transition rates. Here we present a fluxonium artificial atom in which we measure an overall flat dependence of the transition rates between its first two states as a function of $\bar{n}$, up to $\bar{n}\approx200$. Despite the fact that we observe the expected decrease of the dispersive shift with increasing readout power, the signal-to-noise ratio continuously improves with increasing $\bar{n}$. Even without the use of a parametric amplifier, at $\bar{n}=74$, we measure fidelities of 99% and 93% for feedback-assisted ground and excited state preparation, respectively.Comment: typos corrected, added figure at p.10 (section IV of the Supplemental Material), added reference

Observation of Josephson Harmonics in Tunnel Junctions

Superconducting quantum processors have a long road ahead to reach fault-tolerant quantum computing. One of the most daunting challenges is taming the numerous microscopic degrees of freedom ubiquitous in solid-state devices. State-of-the-art technologies, including the world's largest quantum processors, employ aluminum oxide (AlO$_x$) tunnel Josephson junctions (JJs) as sources of nonlinearity, assuming an idealized pure $\sin\varphi$ current-phase relation (C$\varphi$R). However, this celebrated $\sin\varphi$ C$\varphi$R is only expected to occur in the limit of vanishingly low-transparency channels in the AlO$_x$ barrier. Here we show that the standard C$\varphi$R fails to accurately describe the energy spectra of transmon artificial atoms across various samples and laboratories. Instead, a mesoscopic model of tunneling through an inhomogeneous AlO$_x$ barrier predicts %-level contributions from higher Josephson harmonics. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The reality of Josephson harmonics transforms qubit design and prompts a reevaluation of models for quantum gates and readout, parametric amplification and mixing, Floquet qubits, protected Josephson qubits, etc. As an example, we show that engineered Josephson harmonics can reduce the charge dispersion and the associated errors in transmon qubits by an order of magnitude, while preserving anharmonicity

Observation of Josephson harmonics in tunnel junctions

Approaches to developing large-scale superconducting quantum processors must cope with the numerous microscopic degrees of freedom that are ubiquitous in solid-state devices. State-of-the-art superconducting qubits employ aluminium oxide (AlO$_x$) tunnel Josephson junctions as the sources of nonlinearity necessary to perform quantum operations. Analyses of these junctions typically assume an idealized, purely sinusoidal current–phase relation. However, this relation is expected to hold only in the limit of vanishingly low-transparency channels in the AlO$_x$ barrier. Here we show that the standard current–phase relation fails to accurately describe the energy spectra of transmon artificial atoms across various samples and laboratories. Instead, a mesoscopic model of tunnelling through an inhomogeneous AlO$_x$ barrier predicts percent-level contributions from higher Josephson harmonics. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The presence and impact of Josephson harmonics has important implications for developing AlOx-based quantum technologies including quantum computers and parametric amplifiers. As an example, we show that engineered Josephson harmonics can reduce the charge dispersion and associated errors in transmon qubits by an order of magnitude while preserving their anharmonicity

Optical switch based on a fluid-filled photonic crystal fiber Bragg grating

Version of RecordPublishe

Estimation and application of the thermodynamic properties of aqueous phenanthrene and isomers of methylphenanthrene at high temperature

Estimates of standard molal Gibbs energy (ΔGf°) and enthalpy (ΔHf°) of formation, entropy (S°), heat capacity (Cp°) and volume (V°) at 25 °C and 1 bar of aqueous phenanthrene (P) and 1-, 2-, 3-, 4- and 9-methylphenanthrene (1-MP, 2-MP, 3-MP, 4-MP, 9-MP) were made by combining reported standard-state properties of the crystalline compounds, solubilities and enthalpies of phenanthrene and 1-MP, and relative Gibbs energies, enthalpies and entropies of aqueous MP isomers from published quantum chemical simulations. The calculated properties are consistent with greater stabilities of the β isomers (2-MP and 3-MP) relative to the α isomers (1-MP and 9-MP) at 25 °C. However, the metastable equilibrium values of the abundance ratios 2-MP/1-MP (MPR) and (2-MP + 3-MP)/(1-MP + 9-MP) (MPI-3) decrease with temperature, becoming <1 at ~375–455 °C. The thermodynamic model is consistent with observations of reversals of these organic maturity parameters at high temperature in hydrothermal and metamorphic settings. Application of the model to data reported for the Paleoproterozoic Here’s Your Chance (HYC) Pb–Zn–Ag ore deposit (McArthur River, Northern Territory, Australia) indicates a likely effect of high-temperature equilibration on reported values of MPR and MPI-3, but this finding is contingent on the location within the deposit. If metastable equilibrium holds, a third aromatic maturity ratio, 1.5 × (2-MP + 3-MP)/(P + 1-MP + 9-MP) (MPI-1), can be used as a proxy for oxidation potential. Values of log aH2(aq) determined from data reported for HYC and for a sequence of deeply buried source rocks are indicative of more reducing conditions at a given temperature than those inferred from data reported for two sets of samples exposed to contact or regional metamorphism. These results are limiting-case scenarios for the modeled systems that do not account for effects of non-ideal mixing or kinetics, or external sources or transport of the organic matter.Nevertheless, quantifying the temperature dependence of equilibrium constants of organic reactions enables the utilization of organic maturity parameters as relative geothermometers at temperatures higher than the nominal limits of the oil window