An accurate understanding of the Josephson effect is the keystone of quantum information processing with superconducting hardware. Here we show that the celebrated sinφ current-phaserelation (CφR) of Josephson junctions (JJs) fails to fully describe the energy spectra of transmon artificial atoms across various samples and laboratories. While the microscopic theory of JJs contains higher harmonics in the CφR, these have generally been assumed to give insignificant corrections for tunnel JJs, due to the low transparency of the conduction channels. However, this assumption might not be justified given the disordered nature of the commonly used AlOx tunnel barriers. Indeed, a mesoscopic model of tunneling through an inhomogeneous AlOx barrier predicts contributions from higher Josephson harmonics of several %. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The measurement of Josephson harmonics in the CφR of standard tunnel junctions prompts a reevaluation of current models for superconducting hardware and it offers a highly sensitive probe towards optimizing tunnel barrier uniformity.
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 infidelityor 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. We show that the gate-error metrics are susceptible to many of the assumptions which make up the model. Additionally, we find that consecutive gate errors do not accumulate linearly. Previous work showed that the gate-error metrics are poor predictors for the performance of consecutive gates. Here, we provide further evidence and a concise theoretical explanation for this finding. Furthermore, we discuss a problem that potentially limits the overall scaling capabilities of the device architecture we study in this work.
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 circulatingphoton number n¯ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing 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 n¯, up to n¯≈200. 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 n¯. Even without the use of a parametric amplifier, at n¯=74, we measure fidelities of 99% and 93% for feedback-assisted ground and excited state preparation, respectively.
The real-time dynamics of systems with up to three SQUIDs is studied by numerically solving the time-dependent Schrödinger equation. The numerical results are used to scrutinize themapping of the flux degrees of freedom onto two-level systems (the qubits) as well as the performance of the intermediate SQUID as a tunable coupling element. It is shown that the two-level representation yields a good description of the flux dynamics during quantum annealing, and the presence of the tunable coupling element does not have negative effects on the overall performance. Additionally, data obtained from a two-level spin dynamics simulation of quantum annealing is compared to experimental data produced by the D-Wave 2000Q quantum annealer. The effects of finite temperature are incorporated in the simulation by coupling the qubit-system to a bath of spin-1/2 particles. It is shown that including an environment modeled as non-interacting two-level systems that couple only to the qubits can produce data which matches the experimental data much better than the simulation data of the isolated qubits, and better than data obtained from a simulation including an environment modeled as interacting two-level systems coupling to the qubits.