Session D41: New Techniques and Design in Superconducting Qubits II

Improving Realization of New Geometries in Coplanar-Waveguide Lattices

The field of circuit QED has emerged as a rich platform for both quantum computation and quantum simulation. Lattices of coplanar waveguide (CPW) resonators realize artificial photonic materials in the tight-binding limit [1] capable of realizing non-Eucliudean geometries [2] and unconventional unit cells [3]. Combined with strong qubit-photon interactions, these systems can be used to study dynamical phase transitions, many-body phenomena, and spin models in driven-dissipative systems. In practice, fabrication of new lattice requires precise control over resonator frequencies and geometries. Here we will present progress towards rapid development of new CPW lattice by characterizing systematic and parasitic effects between different geometric configurations of CPW resonators.

[1] D. Underwood et al., Phys. Rev. A 86, 023837 (2012)

[2] A. J. Koll ́ar et al., Nature 571, 45 (2019)

[3] A. J. Koll ́ar et al., Comm. Math. Phys. 376,1909 (2019)   

Analysis and mitigation of interface losses in transmon qubit

Reducing losses in superconducting qubit circuits is critical for enabling the development of large-scale quantum computing architectures.  Qualitative and quantitative models of qubit performance are a powerful tool for understanding and reducing these losses.  To generate such models, we tailor device geometies and circuit parameters to maximize sensitivity to specific loss mechanisms. These tailored devices function as 'test structures' that can be co-fabricated with standard designs to develop accurate qubit loss models.   We present the results of a series of studies that investigate losses in transmon qubits resulting from capacitor and Josephson junction dielectric layers, fabrication residues, microwave packaging, and background quasiparticles.  As part of this approach, we develop the fabrication processes and EM modeling techniques necessary for accurately modeling dielectric losses.  We furthermore apply these results to improve our qubits and demonstrate mean T₁ and T₂ times in excess of 200 microseconds (Q ~ 4.5 million).   

In-situ Transmission Electron Microscopy Study on Niobium Oxide Annealing in Nb Resonator for Quantum Computing

Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. The coherence time of the superconducting qubits is mainly limited by microwave dissipation attributed to two-level system defects present at interfaces, such as the Nb/air interface. One way to improve the Nb/air interface quality is by thermal annealing, as shown by extensive studies in 3D cavities. However, it is unclear how the microstructure and chemistry of the oxide layer change during heat treatment. To better understand and optimize device processing at the atomic scale we have developed an in-situ method using an aberration-corrected transmission electron microscope. Combining atomic level imaging and spectroscopy we were able to monitor the niobium oxide layer evolution on Nb thin film during heating from room temperature to ~400 ˚C. Complex structure evolution, including structure, chemistry, and oxidation state of the niobium oxide will be discussed.   

The Merged Element Transmon

The superconducting transmon qubit consists of a Josephson junction and a shunt capacitor, which together form a nonlinear resonant circuit. This basic concept allows for a variety of implementations. One such variation is the merged element transmon (MET), in which the Josephson junction is engineered to act as its own parallel shunt capacitor.[1,2] The MET offers a potentially smaller footprint and a very different electric field profile than conventional transmons. We have produced devices based on micrometer scale Al/AlOx/Al junctions which show surprisingly good T1 times given the relatively large junction size, with T1> 100 microseconds in some cases. Because the MET concentrates the energy in the junction, it may be possible to achieve even better coherence times and reduce the effect of two level systems by making more perfect dielectric barriers in the junction, for example by epitaxial means. For conventional small junction transmons, our results, combined with scaling arguments, suggest that the junction is not normally the dominant source of energy loss.

 

[1]. R. Zhao, S. Park, T. Zhao, M. Bal, C.R.H. McRae, J. Long, and D.P. Pappas, Phys. Rev. Applied 14, 064006 (2020).

[2] H.J. Mamin, E. Huang, S. Carnevale, C.T. Rettner, N. Arellano, M.H. Sherwood, C. Kurter, B. Trimm, M. Sandberg, R.M. Shelby, M.A. Mueed, B.A. Madon, A. Pushp, M. Steffen, and D. Rugar, Phys. Rev. Applied 16, 024023 (2021)   

Transmon performance with different electrode energy gaps

We have repeatedly measured the behavior of gap-engineered Al/AlOx/Al transmons in a 3-D cavity and observed significant fluctuations. In our devices, one electrode was formed from nominally pure aluminum while the other electrode was formed from oxygen-doped aluminum. The superconducting energy gap of the aluminum films depends on the grain size, which depends on the oxygen doping as well as the layer thickness. In a device with a thin first layer of pure Al and thick doped second Al layer, T₁ varied from about 100 to 300 μs at 20 mK. A device with a thin doped-Al first layer and thick pure Al second layer showed T₁ fluctuations of a similar size. This device also showed large fluctuations in T₂ , with a maximum value over 100 μs. Observation of the transition spectrum in this low E_j/E_c device showed significant  charge dispersion, with quasiparticle switching and periods of slow drifting (hour) in the offset charge.    

Suppression of quasiparticle poisoning in superconducting circuits by local ion bombardment of Aluminum electrodes

Tunneling of non-equilibrium quasiparticles across Josephson junctions is a significant source of dephasing in superconducting qubits. The rate of quasiparticle tunneling can be reduced by local enhancement of the superconducting energy gap Δ in the junction electrodes. Aluminum offers a unique opportunity for gap engineering, because Δ in Al films significantly increases with disorder. We observed an increase of Δ by 0.3 – 0.5K in thin Al films treated with a focused beam of 25 keV Ne ions using a Zeiss Orion Plus microscope and applied this technique for local modification of Al films in prefabricated transmon qubits. We will discuss the results of measurements of the coherence time and thermal occupation in the qubits before and after ion bombardment.   

High Impedance Niobium Geometric Inductors

Planar spiral resonators made from superconducting Aluminum have recently been shown to achieve impedances that surpass the resistance quantum owing to their high geometric inductance. These geometric superinductors are an interesting new element in the quantum circuit toolbox with applications in noise protected qubits, quantum metrology and coupling to single spins. However, to integrate these devices with typical spin qubit architectures, they need to be magnetic-field compatible. Here we present fabrication and characterization of Niobium spiral resonators with high critical fields. We use a subtractive process where thin film Niobium is sputtered on Si wafers, patterns are written with ebeam lithography and etched with a reactive ion etch. We characterize the devices in a 1K setup and show quality factors in excess of Q>10⁴ for resonators with impedances exceeding 1 kΩ. We validate our model for the resonators by extracting the ratio of geometric/kinetic inductance from temperature sweep data and verify that indeed over 95% of the inductance is geometric. Finally, we will discuss the prospects of integrating these devices with quantum-dot devices for coupling between microwave photons and electron spins.   

Spurious mode suppression in inductively shunted enclosures for large superconducting quantum devices

Embedding superconducting quantum circuits in enclosures with a clean electromagnetic environment is a key factor in maintaining high qubit performance. As devices are scaled up in size, the electromagnetic modes of sufficiently large enclosures will appear at low enough frequencies to interfere with the qubit and resonator modes of interest, reducing coherence and causing long-range crosstalk. Introducing an array of inductive shunts to an enclosure results in a cut-off frequency which is independent of its lateral size, exponentially suppressing enclosure-mediated qubit crosstalk at arbitrary chip size [1]. Here, we present simulations and measurements of enclosures large enough to hold a 16-qubit device, demonstrating suppression of the enclosure modes when inductively shunting pillars are included. These results are a promising indicator that this architecture will maintain the previously achieved high qubit performance [2] as these devices are scaled up.

 

[1] P. A. Spring et al., Phys. Rev. Appl. 14, 24061 (2020)

[2] P. A. Spring et al., arXiv:2107.11140 [quant-ph] (2021)   

Aluminum properties beyond the thin film regime for superconducting qubit circuits

Aluminum is the most commonly used material in superconducting qubit circuits, yet its properties are not fully characterized in the thin film regime. In particular, the penetration depth of thin film aluminum is not well documented, even though its value is of utmost importance in the design of, e. g., flux qubits and couplers. Very relevant device parameters are affected by penetration depth, particularly the kinetic inductance and the flux exclusion properties of the thin film, leading to device design uncertainties and loss, respectively.

In this work, we study the dependence of penetration depth with film thickness. Our experiment consists of measuring the resonance of LC resonators with different aluminum thickness and extracting the kinetic inductance contribution to the resonance. Separately, four-probe measurements of the normal state resistance of aluminum at 4K provide an alternative estimate of the kinetic inductance.

A proper choice of film thickness leads to penetration depths low enough to favor the removal of vortexes, which are a source of qubit decoherence and resonator noise, while keeping the rest of device properties minimally altered.   

Ultrahigh-inductance materials from spinodal decomposition

Disordered superconducting nitrides with kinetic inductance have long been considered a leading material candidate for high-inductance quantum-circuit applications. Despite continuing efforts in reducing material dimensions to increase the kinetic inductance and the corresponding circuit impedance, it becomes a fundamental challenge to improve further without compromising material qualities. To this end, we propose a method to drastically increase the kinetic inductance of superconducting materials via spinodal decomposition while keeping a low microwave loss. We use epitaxial Ti_0.48Al_0.52N as a model system, and for the first time demonstrate the utilization of spinodal decomposition to trigger the insulator-to-superconductor transition with a drastically enhanced material disorder. The measured kinetic inductance has increased by 2-3 orders of magnitude compared with all the best reported disordered superconducting nitrides. Our work paves the way for substantially enhancing and deterministically controlling the inductance of superconducting circuit elements for advanced quantum-circuit fabrication.   

Robustness of Dolan-bridge and Manhattan-style Josephson junctions in through-silicon via integrated substrates

Vertical input-output routing of signals using metallized through-silicon vias (TSVs) is a promising path to scaling monolithic superconducting quantum processors. Bridgeless (i.e., Manhattan) junctions have been believed to be less sensitive to resist height variations compared to Niemeyer-Dolan bridge junctions, therefore benefiting frequency targeting of qubits in the presence of TSVs.  We compared the performance of Manhattan and Dolan-style variants of Josephson junctions fabricated on TSV-integrated substrates. The residual standard deviation of junction resistances is 30% lower on average for Manhattan-style junctions than Dolan-bridge junctions at the intra-die level in the TSV layout. The number of defective junctions (opens and shorts) is one order of magnitude lower in Manhattan-style junctions. Comparison of resistance of identically patterned junctions between planar and TSV dies shows a near doubling of resistance only for the Dolan-bridge variant in the TSV layout. Ongoing optimization of the resist stack and junction pattern post-processing steps tailored for TSV-integrated substrates may further narrow the resistance spread of Manhattan-style junctions.   

Fast and high-fidelity readout of transmon qubits in scalable QPU architecture

The speed and fidelity of single- and two-qubit gates of superconducting quantum processing units (QPU) have been the subject of optimization but most multi qubit devices only feature modest readout performance in comparison. The high-fidelity dispersive readout with low readout crosstalk requires individual Purcell filters for readout of each qubit. However, the Purcell filters interfere with each other and are sensitive to the QPU package impedance due to the strong coupling to the feedline. These effects make it hard to achieve targeted detuning between the readout resonators and Purcell filters. We present a microwave circuit with reduced sensitivity to loading, and a computationally efficient numerical optimization method for the readout structure parameters. Our approach is validated by a collection of measured readout circuit spectrums with targeted properties and a state-of-the-art qubit readout fidelity.   

Constraining the absolute 3D dopant density in aluminum delta layers

Using four unique measurement techniques, we report the saturation 2D dopant density of 2.8(5) x 10¹⁴  atoms/cm² for aluminum on the silicon (Si(100)) surface, the effective carrier activation, and the emergence of a second, high mobility conduction band above ~30 K — outcomes of an effort to achieve an Al doped superconducting phase in Si. Scanning tunneling microscopy (STM) is used to directly measure dopants on the Si(100) surface before solid phase epitaxial growth of the Si capping layer. Subsequent secondary ion mass spectrometry (SIMS) and atom probe tomography (APT) are used to separately measure the number and distribution of dopants, while Hall transport measurements are used to measure carrier density at various temperatures. At this time, we believe a density of ~6 x 10²⁰ atoms/cm³ (~1.2 at %) was reached, but no evidence of superconductivity has been observed.