Session F47: Superconducting Materials & Qubit Coherence

Mechanically induced correlated errors on superconducting qubits with relaxation times exceeding 0.4 milliseconds

Superconducting qubits are among the most advanced candidates for achieving fault-tolerant quantum computing. Despite recent significant advancements in the qubit lifetimes, the origin of the loss mechanism for state-of-the-art qubits is still subject to investigation. Furthermore, the successful implementation of quantum error correction requires negligible correlated errors between qubits. Here, we realize superconducting transmon qubits with lifetimes exceeding 0.4 ms using niobium capacitor electrodes shunted by a single Al/AlOx/Al Josephson junction on a silicon substrate and investigate their dominant error mechanism. By introducing a novel time-resolved error detection scheme synchronized with the operation of the pulse tube cooler in a dilution refrigerator, we find that mechanical shocks from the pulse tube induce nonequilibrium dynamics in highly coherent qubits, leading to strongly correlated bit-flip errors. Our observations are consistent with qubit dynamics induced by two-level systems and quasiparticles, thereby deepening our understanding of the qubit error mechanisms. Additionally, our findings provide valuable insights into potential error-mitigation strategies for achieving fault tolerance by decoupling superconducting qubits from their mechanical environments.   

Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270 μs, corresponding to Q = 5 million, and a highest observed value of 501 μs. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.   

Characterization of fabrication methods to reach high coherence superconducting quantum circuits

The fabrication of superconducting qubits and resonators with long coherence times and high quality factors is an important milestone on the way towards useful quantum applications. Although significant improvements in coherence time have been achieved over the last years, reaching qubit lifetimes well beyond 100 µs involves careful investigation of all fabrication steps. Here, we demonstrate that such high device qualities can be achieved by a combination of substrate cleaning, etching optimization and post-process sample cleaning. Thereby, we reach quality factors well above 1x10^7 for thin-film niobium CPW resonators and can observe transmon qubits with more than 700 µs lifetime. In addition, we exploit the high quality of the niobium resonators to investigate losses arising from different types of silicon substrates.   

Improving qubit performance through engineering of the substrate-Josephson junction interface.

The performance of a superconducting qubit is often limited by dissipation and two-level systems (TLS) losses. The dominant sources of these losses are believed to originate from interfaces and surfaces, likely as a result of fabrication processes, materials, or atmospheric exposure. We show an example of a chemical surface treatment that modifies the Josephson Junction-substrate interface, resulting in statistically significant improvement in T1 (>25% improvement) and a reduction in the density of strongly-coupled TLS. As this treatment primarily modifies the Josephson junction-substrate interface, we perform targeted chemical, and structural characterizations of that interface and correlate those with qubit measurements.   

Coherence Measurements of Tantalum Based Transmon Qubits

In recent years, the use of α-phase tantalum as the shunting capacitor material has resulted in improved coherence times for transmon qubits. In this talk, we discuss the design and fabrication of our Ta based transmon qubits, including the etch of the Ta thin film, the addition of the Al/AlOx/Al Josephson junction, and the cleaning methods used during the processing. We then present coherence time measurements at a temperature nominally 20 mK on a variety of devices including both planar and 3D transmon qubits.   

Coherence correlations in planar fluxonium qubits

Superconducting qubits have demonstrated promise as a technology with which to build a quantum computer. A current roadblock to creating such a processor with these circuits is their finite coherence time, which limits gate fidelity. Fluxonium qubits have exhibited long coherence times and could be an avenue towards a high-coherence quantum processor. This work studies data on the circuit parameters, coherence, and single-qubit gate fidelities of planar aluminum-on-silicon fluxoniums. We consider data gathered for the purpose of collecting statistics on device performance, understanding how measured coherence times correlate with device parameters and achievable gate fidelities, and identifying the dominant loss mechanisms limiting device coherence.  

 

This material is based upon work supported under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. government or the U.S. Air Force.    

Dependence of Transmon Coherence on Geometry and Interface Treatment

Superconducting qubits are a leading candidate for realizing a large-scale, fault-tolerant quantum computer. For continued progress, it is necessary to understand and mitigate qubit decoherence, which is largely dominated by lossy amorphous interfaces. Here, we examine the effect of qubit geometry and surface preparation on qubit energy relaxation time. Working with both single-ended and differential transmon qubits, we vary the separation between electrodes forming the qubit shunt capacitance over an order of magnitude in order to change the participation ratio of the various interfaces. Additionally, we study the effect of surface treatments both prior to and following Josephson junction formation. Finally, we examine the dependence of qubit energy relaxation and dephasing from 1/f flux noise on the geometry of the qubit junction leads.     

Characterizing the coherence of tantalum-based transmons and resonators

Scaling quantum machines requires ever-increasing numbers of qubits with ever-higher coherence. In superconducting circuits, there have been recent advances combining careful surface preparation and metals such as tantalum for non-Josephson junction circuit elements which have achieved record coherences.

In this talk, we will summarize our work on improving coherence of tantalum-based superconducting transmons with aluminum Josephson junctions. We will discuss experimental data and calculations seeking to determine whether the dominant loss in our transmons and resonators is due to the tantalum-aluminum interface.  We also will show results on the effects of adopting dry-etching instead of wet-etching as the metal patterning fabrication method on the coherence of the transmons. We will present our qubits’ and resonators’ coherence data along with supporting fabrication characterization analysis using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Energy Dispersive X-ray Spectroscopy (EDX) to gain a better understanding of our samples’ surfaces.

[Place, et al., Nat. Comm (2021)].   

Materials exploration towards high coherence superconducting qubits

Superconducting quantum circuits are one of the leading platforms for quantum information processing. Tremendous progress has been made in extending the qubit lifetime over the past two decades. It has become clear that further improvement in qubit T1 is possible with reduction/elimination of lossy materials at various surfaces and interfaces by exploration of novel materials and fabrication processes. Our recent work at SQMS Center demonstrated T1 enhancement up to a factor of five in Nb qubits capped with a Ta passivation layer [1]. We attribute this improvement to the replacement of native Nb oxide layer with native Ta oxide, which is thinner and less disordered. We have expanded our work to explore annealing of Ta capped Nb films, substrate preparation, capping layers that do not form an oxide layer such as proximitized Au, and other superconducting base metal layers such as Ta and Re. The results of a systematic study, including the statistics of qubit T1 measurements, will be presented.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract number DE-AC02-07CH11359.

[1] M. Bal et al., arXiv:2304.13257 (2023)   

Rhenium as a novel superconducting material for transmon qubits

Niobium and aluminum have been widely used to realize superconducting qubits. However, the native oxide at the metal air interface limits the qubit lifetime. This layer is highly disordered and it induces losses limiting the performances of the qubits. Recently, tantalum led to great improvement in qubit relaxation time by implementation. This improvement has been attributed to a thinner and less disordered oxide layer compared to Nb and Al. Here, for the first time, we demonstrate a new superconducting material platform: rhenium. Rhenium is more resistant to oxidation; it forms a thin oxide layer that is < 1 nm. We report statistics of T1 relaxation measurements of Re qubits, and compare the results to Nb qubits as well as Re encapsulated Nb qubits. Various transmon designs with varying participation ratios of interfaces and layers are used for these studies.   

Statistical Characterization of Transmon Qubit Energy Relaxation Time

Temporal fluctuations of superconducting qubit energy relaxation time, T1, bring up additional challenges in building a fault tolerant quantum computer, and larger fluctuations have been observed in those qubits with longer T1. It is believed that the fluctuation is attributed to time varying coupling of qubits to the two-level system (TLS) defects residing in the materials, but details keep unclear. In this work we analyze the transmon qubit T1 fluctuation as a function of the temperature. By carefully modeling the interactions between qubit, TLS, and quasiparticle (QP), we disentangle the contributions of the thermally excited QP and TLS on the T1 fluctuation. We also discuss different roles of the equilibrium and non-equilibrium QPs on the qubit relaxation process.   

Effects of strongly coupled coherent two-level-systems on the coherence times of superconducting qubits

Superconducting qubits are a prominent platform for quantum computing, critical to both industrial and academic endeavors. Although these are easily scalable due to their compatibility with the recent microelectronics technology, they are prone to noise. An active area of research is the enhancement of superconducting qubit coherence times. One of the significant factors limiting the coherence times of superconducting qubits is parasitic two-level system (TLS) defects. The exact mechanism of interaction between a qubit and various types of TLS defects remains unexplored mainly due to the lack of experimental techniques to probe the form of qubit-defect couplings. Here, we present a model where the strong coupling of a TLS at specific coupling angles leads to longer coherence times. Such models can provide a pathway to transfer information through TLSs near the qubit environment as they can potentially perform the role of temporary information storage entities. We also show that this phenomenon occurs due to a correlation between the qubit and TLS. 

Reference: Shi, Jianxin. Advances in Condensed Matter Physics 2020 (2020): 1-8.   

Investigations of 1/f Magnetic Flux Noise in Superconducting Qubits with Weak Magnetic Fields

Low-frequency 1/f magnetic flux noise is known to limit the coherence of superconducting qubits, yet an understanding of its microscopic origins is still lacking. Although magnetically coupled surface defects, so-called “surface spins,” are widely accepted to be the source, their identities and interaction mechanisms remain an open question. We showed in recent experiments on aluminum C-shunt flux qubits [1] that the noise power spectrum responds anomalously to the application of weak in-plane magnetic fields. We report on the continuation of these studies, particularly focusing on advanced noise spectroscopy techniques and field-orientation dependence.

[1] D. A. Rower et al., Phys. Rev. Lett. 130, 220602 (2023).