J. Sleight’s research while affiliated with IBM Research - Thomas J. Watson Research Center and other places

Superconducting qubits have been used in the most advanced demonstrations of quantum information processing, and they can be manufactured at-scale using proven semiconductor techniques. This makes them one of the leading technologies in the race to demonstrate useful quantum computers. Since their initial demonstration, advances in design, fabrication, and materials have extended the timescales over which fragile quantum information can be stored and manipulated on superconducting qubits. Ubiquitous atomic-scale material defects have been identified as a primary cause of qubit energy-loss and decoherence. Here we study transmon qubits that exhibit energy relaxation times exceeding 2.5 ms. Even at these long timescales, our qubit energy loss is dominated by two level systems (TLS). We observe large variations in these energy-loss times that would make it extremely difficult to accurately evaluate and compare qubit fabrication processes and to perform studies that require precise measurements of energy loss. To address this issue, we present a technique for characterizing qubit quality factor. In this method, we apply a slowly varying electric field to TLS near the qubit to stabilize the measured energy relaxation time, enabling us to replace hundreds of hours of measurements with ones that span several minutes.

Non-equilibrium quasiparticles are possible sources for decoherence in superconducting qubits because they can lead to energy decay or dephasing upon tunneling across Josephson junctions (JJs). Here, we investigate the impact of the intrinsic properties of two-dimensional transmon qubits on quasiparticle tunneling (QPT) and discuss how we can use quasiparticle dynamics to gain critical information about the quality of JJ barrier. We find the tunneling rate of the non-equilibrium quasiparticles to be sensitive to the choice of the shunting capacitor material and their geometry in qubits. In some devices, we observe an anomalous temperature dependence of the QPT rate below 100 mK that deviates from a constant background associated with non-equilibrium quasiparticles. We speculate that this behavior is caused by high transmission sites/defects within the oxide barriers of the JJs, leading to spatially localized subgap states. We model this by assuming that such defects generate regions with a smaller effective gap. Our results present a unique in situ characterization tool to assess the uniformity of tunnel barriers in qubit junctions and shed light on how quasiparticles can interact with various elements of the qubit circuit.

Quantum computing relies on the operation of qubits in an environment as free of noise as possible. Assessing the quality of this environment is a key aspect of ensuring high-fidelity implementations based on superconducting qubits. Relaxation, decoherence, dephasing, and quasiparticle tunneling rates have been measured for various shielding configurations used in the measurement environment for state-of-the-art transmon qubits. An ensemble of approximately 120 control devices was used for this study, with five different capacitor pad designs. The shielding elements varied in the configuration included an indium gasket at the qubit can's lid, Cryoperm magnetic shielding, the mixing chamber shield of the dilution refrigerator, the inclusion of a vacuum pump-out port, and capping unused subminiature version A connectors at the top of the measurement can's lid. It was found that the qubit lifetimes T1, T2, and Tϕ are robust to the all of configuration changes tried until the mixing chamber shield was removed, significantly increasing blackbody radiation levels in the qubit measurement space, where in that limit it was found that tapering the qubit pads reduced the amount of loss. In contrast, the quasiparticle tunneling rates were found to be extremely sensitive to all configuration changes tested. Consistent with earlier reports [McEwen et al., arXiv:2104.05219 (2021); Cardani et al., Nat. Commun. 12, 2733 (2021); Wilen et al., Nature 594, 369–373 (2021); Ristè et al. Nat. Commun. 4, 1913 (2013)], the findings from this study indicate that non-equilibrium quasiparticles do not currently limit the lifetimes of well-shielded transmon qubits.

Non-equilibrium quasiparticles are possible sources for decoherence in superconducting qubits because they can lead to energy decay or dephasing upon tunneling across Josephson junctions. Here, we investigate the impact of the intrinsic properties of two-dimensional transmon qubits on quasiparticle tunneling (QPT) and discuss how we can use QPT to gain critical information about the Josephson junction quality and device performance. We find the tunneling rate of the non-equilibrium quasiparticles to be sensitive to the choice of the shunting capacitor material and their geometry in qubits. In some devices, we observe an anomalous temperature dependence of the QPT rate below 100 mK that deviates from a constant background associated with non-equilibrium quasiparticles. We speculate that high transmission sites within the Josephson junction's tunnel barrier can lead to this behavior, which we can model by assuming that the defect sites have a smaller effective superconducting gap than the leads of the junction. Our results present a unique characterization tool for tunnel barrier quality in Josephson junctions and shed light on how quasiparticles can interact with various elements of the qubit circuit.

Quantum computing relies on the operation of qubits in an environment as free of noise as possible. This work reports on measuring the impact of environmental radiation on lifetimes of fixed frequency transmon qubits with various capacitor pad geometries by varying the amount of shielding used in the measurement space. It was found that the qubit lifetimes are robust against these shielding changes until the most extreme limit was tested without a mixing chamber shield in the refrigerator. In contrast, the quasiparticle tunneling rates were found to be extremely sensitive to all configurations tested, indicating these devices are not yet limited by losses related to superconducting quasiparticles.

Superconducting qubits are sensitive to a variety of loss mechanisms including dielectric loss from interfaces. By changing the physical footprint of the qubit it is possible to modulate sensitivity to surface loss. Here we show a systematic study of planar superconducting transmons of differing physical footprints to optimize the qubit design for maximum coherence. We find that qubits with small footprints are limited by surface loss and that qubits with large footprints are limited by other loss mechanisms which are currently not understood.

We present results from gate-all-around (GAA) silicon nanowire (SiNW) MOSFETs fabricated using a process flow capable of achieving a nanowire pitch of 30 nm and a scaled gate pitch of 60 nm. We demonstrate for the first time that GAA SiNW devices can be integrated to density targets commensurate with CMOS scaling needs of the 10 nm node and beyond. In addition, this work achieves the highest performance for GAA SiNW NFETs at a gate pitch below 100 nm.

We demonstrate for the first time, Si1-xGex channel trigate PFETs on insulator with aggressively scaled fin width WFIN, gate length LG, and high-K/metal-gate stack (inversion oxide thickness TINV = 1.5 nm) using an implant-free raised source/drain (RSD) process. We report excellent electrostatic control down to LG = 18 nm for WFIN ≤ 18 nm. Using an optimized RSD process, we achieved high-performance SiGe-channel PFETs with oncurrent ION = 1.1 mA/μm at off-current IOFF = 100 nA/μm and supply voltage VDD = 1 V, which is attributed to high hole source injection velocity vx0 exceeding 1 × 107 cm/s.

Anneal in reduced pressure hydrogen ambient is known to induce morphological changes in silicon microstructures via markedly increased surface self-diffusivity on exposed silicon surfaces. Here, we investigate the capillary instability of silicon nanostructures under hydrogen anneal. We demonstrate that a surface diffusion mask can significantly improve stability by isolating vulnerable segments from large mass reservoirs. In addition, we find that Plateau-Rayleigh instability shows strong crystallographic dependence, which is explained by the surface energy anisotropy of silicon. We observe that nanowires are the least stable when their axial orientation corresponds to 〈100〉 and are increasingly stable for 〈111〉, 〈112〉, and 〈110〉.

We demonstrate undoped-body, gate-all-around (GAA) Si nanowire (NW) MOSFETs with excellent electrostatic scaling. These NW devices, with a TaN/Hf-based gate stack, have high drive-current performance with NFET/PFET I_DSAT = 825/950 μA/μm (circumference-normalized) or 2592/2985 μA/μm (diameter-normalized) at supply voltage V_DD = 1 V and off-current I_OFF = 15 nA/μm. Superior NW uniformity is obtained through the use of a combined hydrogen annealing and oxidation process. Clear scaling of short-channel effects versus NW size is observed. Additionally, we observe a divergence of the nanowire capacitance from the planar limit, as expected, as well as enhanced device self-heating for smaller diameter nanowires. We have also applied this method to making functional 25-stage ring oscillator circuits.