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arXiv:2106.08406

Low frequency correlated charge noise measurements across multiple energy transitions in a tantalum transmon

Daniel Tennant, Luis A. Martinez, Kristin M. Beck, Sean R. O’Kelley, Christopher D. Wilen, Robert McDermott, Jonathan L. DuBois, Yaniv Jacob Rosen

Published Jun 15, 2021• 1 citation
arXivHTMLPDF

Abstract

Transmon qubits fabricated with tantalum metal have been shown to possess energy relaxation times greater than 300 $μ$s and, as such, present an attractive platform for high precision, correlated noise studies across multiple higher energy transitions. Tracking the multi-level fluctuating qudit frequencies with a precision enabled by the high coherence of the device allows us to extract the charge offset and quasi-particle dynamics. We observe qualitatively different charge offset behavior in the tantalum device than those measured in previous low frequency charge noise studies. In particular, we find the charge offset dynamics are dominated by rare, discrete jumps between a finite number of quasi-stationary charge configurations, a previously unobserved charge noise process in superconducting qubits.

DOI
10.48550/arxiv.2106.08406

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Transmon qubits fabricated with tantalum metal have been shown to possess energy relaxation times greater than 300 μμμs and, as such, present an attractive platform for high precision, correlated noise studies across multiple higher energy transitions. Tracking the multi-level fluctuating qudit frequencies with a precision enabled by the high coherence of the device allows us to extract the charge offset and quasi-particle dynamics. We observe qualitatively different charge offset behavior in the tantalum device than those measured in previous low frequency charge noise studies. In particular, we find the charge offset dynamics are dominated by rare, discrete jumps between a finite number of quasi-stationary charge configurations, a previously unobserved charge noise process in superconducting qubits.
We report observations of discrete charge states of a coherent two-level system (TLS) that is strongly coupled to an offset-charge-sensitive superconducting transmon qubit. We measure an offset charge of 0.072e associated with the two TLS eigenstates, which have a transition frequency of 2.9 GHz and a relaxation time exceeding 3 ms. Combining measurements in the strong dispersive and resonant regime, we quantify both transverse and longitudinal coupling of the TLS-qubit interaction. We further perform joint tracking of TLS transitions and quasiparticle tunneling dynamics but find no intrinsic correlations. This Letter demonstrates microwave-frequency TLS as a source of low-frequency charge noise.
Understanding and suppressing sources of decoherence is a leading challenge in building practical quantum computers. In superconducting qubits, low-frequency charge noise is a well-known decoherence mechanism that is effectively suppressed in the transmon qubit. Devices with multiple charge-sensitive modes can exhibit more complex behaviors, which can be exploited to study charge fluctuations in superconducting qubits. Here we characterize charge sensitivity in a superconducting qubit with two transmonlike modes, each of which is sensitive to multiple charge-parity configurations and charge-offset biases. Using Ramsey interferometry, we observe sensitivity to four charge-parity configurations and track two independent charge-offset drifts over hour time-scales. We provide a predictive theory for charge sensitivity in such multimode qubits which agrees with our results. Finally, we demonstrate the utility of a multimode qubit as a charge detector by spatially tracking local-charge drift.
Superconducting quantum circuits are sensitive to their electrostatic environment: uncontrolled charges accumulating on the electrodes of a Josephson junction shift the energy levels of a qubit, perturbing its operation and restricting their design. This effect is captured by a single parameter - the charge offset - whose slow, unpredictable drift has proven difficult to eliminate in practice. Here, we report a tantalum-based transmon qubit in which the charge offset remains pinned at zero over nearly three months of measurements, including two thermal cycles, with no observable compromise to the qubit lifetime. This exceptional stability disappears in later cooldowns, indicating a fragile mechanism at play. We attribute it to the inductance of a thin superconducting layer inadvertently formed in parallel with the Josephson junction during fabrication. X-ray surface spectroscopy suggests this layer arises from an incomplete wet-etch of tantalum on sapphire. Deliberately engineering such a layer offers a route to eliminating charge-offset drift in superconducting circuits more broadly.
Transmons are widely adopted in quantum computing architectures for their engineered insensitivity to charge noise and correspondingly long relaxation times. Despite this advantage, transmons often exhibit large fluctuations in dephasing times across different devices and also within qubits on the same device. Existing transmon qubits are assumed to be insensitive to charge noise. However, very little recent attention has been paid to the dependence of dephasing on the local charge environment. In this study, we see fluctuations in the dephasing time, TϕT_ϕTϕ​, which correlate to charge offset. While charge offset fluctuations are slow, parity switches are fast processes tied to the charge offset and can affect TϕT_ϕTϕ​ in Ramsey experiments. We implement a protocol to detect parity switching events using single-shot methods, which are interleaved within a Ramsey measurement. We find that events that remain in the same parity state have a higher T2T_2T2​ than measurements averaged over both parities. Our results show that transmons can be limited by charge-noise, even with EJ/EC≈50E_\text{J}/E_\text{C} \approx 50EJ​/EC​≈50. Consequently, parity flip rates must be considered as a device characterization metric.
Transmon qubits are a cornerstone of modern superconducting quantum computing platforms. Temporal fluctuations of energy relaxation in these qubits are widely attributed to microscopic two-level systems (TLSs) in device dielectrics and interfaces, yet isolating individual defects typically relies on tuning the qubit or the TLS into resonance. We demonstrate a novel spectroscopy method for fixed-frequency transmons based on multilevel relaxation: repeated preparation of the second excited state and simultaneous T1T_1T1​ extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by ≳100 MHz\gtrsim 100\,\mathrm{MHz}≳100MHz from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.