Session M73: Superconducting Qubits: Qubit materials

Evaluating Niobium-Germanium Heterostructures for Voltage-Tunable Superconducting Quantum Devices

Voltage-tunable hybrid superconductor-semiconductor Josephson junctions are promising building blocks for low-loss frequency-tunable quantum devices such as qubits and resonator couplers. The realization of hybrid devices in group IV semiconductors such as Ge is of particular interest due to higher scalability and low dielectric loss at microwave frequencies. However, inducing superconductivity in Ge via proximity effect has been proven to be challenging because of large interfacial energy barriers and defect densities. Here, we utilize ultrahigh vacuum evaporation to deposit high-quality Nb layers on Ge (001) substrates. Through various thermal cycling schemes, we tune the interface structures. Through X-ray photoelectron spectroscopy and femtosecond ultraviolet photoelectron spectroscopy we determine the impact the thermal cycling has on the band alignment and chemical composition of the interface. This is complemented by cryogenic measurements of interface losses for the Nb/Ge heterostructures as thin films and as coplanar waveguide resonators (at microwave frequencies).   

Single crystal nm-thick aluminum superconducting microwave resonators with internal quality factors above one million

Single-crystal thin films with a highly ordered atomic structure can host controlled interfaces to their adjacent layers, as the key to reducing dielectric loss in superconducting quantum circuits. In this work, we have demonstrated single-crystal aluminum (Al) thin films on sapphire substrates with very high crystallinity and high internal quality factors (Qi). Using in-situ oxide deposition instead of a conventional oxidation process, we have achieved a well-controlled interface between oxide and Al film and protected these nm-thick Al films from oxidation in the ambient.

High crystallinity of single-crystal Al films was characterized using synchrotron radiation X-ray diffraction. Clear Pendellosung fringes were observed, indicative of high crystallinity of the Al films and small interface roughness (< 0.5 nm) between the Al films and the adjacent layers. The full-width at half-maximum (FWHM) of θ-rocking curves of the Al films are record-low values of 0.015 degrees (54 arcsec) from samples with thicknesses from 3 nm to 20 nm-thick. The samples exhibited smooth surfaces and interfaces confirmed by X-ray reflectivity (XRR) and atomic force microscope. Microstrip resonators made from such single-crystal Al films reveal internal quality factors over 1 million around 7 GHz in the low power limit at 10 mK. Such single-crystal materials offer excellent potential of low dielectric loss, thus improving the coherence of superconducting quantum circuits.   

Nitridized aluminum thin films for superconducting quantum technologies

Building quantum technologies with superconducting qubits puts stringent requirements on the choice of materials, as their quality and properties ultimately limit the circuit performance. In this work, we propose a novel superconducting material attractive for superconducting quantum circuits: thin-film nitridized aluminum fabricated by magnetron sputtering in nitrogen-argon flows. At room temperature, the samples behave as a normal conductor (low N₂ partial pressures), and as an insulator (high N₂ partial pressures), finding the limit between both regimes in the sample processed with 15% N₂. Measurements at low temperature show that the thin films display superconductivity for 0% - 15% N₂, with critical temperatures that vary non-monotonically from hundreds of mK for the most resistive samples up to more than 3K for samples fabricated with 5-10% N₂. We also report an important decrease in the critical current density for the most resistive samples and qualitative differences of the resistance behavior as a function temperature. These results indicate that nitridized aluminum may be used in a variety of applications, ranging from a superinductor in the highly resistive regime to a larger-gap material in the low resistance regime.   

Characterizations of Superconducting Ta Films for Quantum Computing

Recently, a breakthrough was reported in the long coherence time (> 0.5 ms) transmon qubit using tantalum films as the base superconductor in the devices. Here we report a study of superconducting properties in Ta films showing a similar normal state resistive behavior and superconducting transition temperature T_C. Using noncontact ac magnetic susceptibility measurements, we found strikingly different response in their imaginary part of magnetic susceptibility χ''. A sharp peak and near zero loss in χ′′ below T_C were observed in some films, indicating a strongly coupled superconductor. A broad peak and massive loss in χ′′ below T_C were observed in another films, that could be a leading source of decoherence. This study demonstrates that the ac susceptibility is an excellent method for characterizing bulk superconducting properties of the constituent materials used in qubits.   

High-performance transmon qubits with an epitaxially grown TiN film on Si (100) substrate

The transmon qubit is a fundamental superconducting quantum element, and its performance directly affects the performance of integrated quantum circuits. The energy decay time of a transmon is known to be sensitive to the surface condition of the superconducting electrodes, and there have been many attempts to improve the performance of transmons by changing the materials and surface treatments. Here we present the properties of our transmons consisting of a shadow-evaporated Al Josephson junction and TiN electrodes grown on Si (100) substrate with domain-matching epitaxy at high temperature. The energy decay time of 250 μs is obtained for a transmon with the transition frequency of 4.5 GHz. We evaluate the loss mechanism of our transmons by changing their designs.   

Bulk loss in silicon substrates for superconducting quantum circuits

Superconducting qubits are a leading candidate for large-scale, fault-tolerant quantum computing. It is critical to understand and minimize losses originating in the materials used to realize the qubit. While loss is generally dominated by two-level state defects in amorphous interfaces, advances in qubit design and materials have led to steady reductions in interface losses over the years. Ultimately, qubit coherence will be limited by bulk losses in the silicon or sapphire substrate, about which surprisingly little is known. Here we describe a novel testbed based on a 3D stub cavity resonator designed to probe bulk loss in candidate qubit substrates. We present the results of numerical simulations that have guided optimization of the 3D cavity resonator. We present a thorough study of bulk loss in silicon grown according to different methods, subjected to different post-growth anneal protocols, and with different crystal orientation.   

Probing insulating state of a Josephson junction in dc and microwave domains

A single Josephson junction in an Ohmic environment undergoes the Schmid-Bulgadaev quantum phase transition between superconducting and insulating states. Here we present a characterization of the junction's insulating state, combining conventional dc measurements and a novel technique, which probes the junction's response to microwave photons. With both techniques, we observe the junction's behavior, which is consistent with the behavior of an element dual to a superconducting Josephson junction. In the junction's response to a dc excitation, we find Coulomb blockade and Bloch oscillation effects. Accordingly, in the microwave domain, the junction responds to incoming photons as a non-linear Bloch capacitance, which we reveal through the frequency dependance of the phase shift induced by the junction to scattered photons. Our experimental technique allows for the simple characterization of insulating Josephson junctions with potential applications in quantum information science and metrology.   

Room temperature growth of niobium titanium alloy thin films on sapphire substrates for low loss superconducting quantum devices

Superconducting microwave resonators with high quality factors (high-Q) have become a key technology in a wide range of applications. The quest for materials that have low loss property at low temperatures is an area of great interest for quantum computation and photon detection. Niobium-titanium (NbTi) is a disordered superconducting alloy with a relatively high transition temperature (T_c) and has been widely used in industrial applications as practical superconducting wires. Here, we present the growth of high quality NbTi thin films on c-plane sapphire (Al₂O₃) substrates using room temperature magnetron sputtering techniques. We investigate the strain, crystallinity, surface morphology, and superconducting properties of NbTi films with various thicknesses. A nanoscale surface morphology exhibits a sixfold symmetric grain structure following the hexagonal Al₂O₃ substrate. Detailed XRD analyses revealed that the bcc NbTi with higher T_c are grown epitaxially on Al₂O₃ substrates with bcc (110) and (100) aligned along Al₂O₃ (0001) and (10-10), respectively. We found that the film quality and the concomitant superconducting property can be widely controlled by changing the sputtering conditions and relevant parameters.   

Comparison of Nb and Ta Oxides for Potential Magnetic Pairbreaking and Quasiparticle Generation in Superconducting Qubits

Recent studies of transmon qubits where the Nb capacitor is replaced with Ta have shown increases in T₁ decay time by a factor of 3 to 10. We suggest that this dramatic improvement may be due to differences in the native oxide.  Nb sub-oxides are shown to be magnetic based on scanning point contact tunneling spectroscopy and magnetic susceptibility. There is evidence of magnetic pairbreaking that leads to sub-gap quasiparticle states.  The question of whether Ta exhibits magnetic sub oxides is addressed.  High purity recrystallized foils of Nb and Ta have been prepared by the identical procedure of UHV annealing above 2000C and subsequently exposed to air for periods of time from 30 minutes up to two weeks or longer.  XRD measurements reveal similar grain orientations in each foil.  TEM measurements show that the Ta oxide is roughly 40% thinner than the Nb oxide after long term air exposure. XPS measurements reveal several notable differences in the Ta oxide including the apparent absence of a TaO₂ sub-oxide whereas the Nb foil, as well as other Nb samples, consistently exhibit a corresponding NbO₂ sub-oxide.  Density functional theory calculations of formation energies are presented to aid in the analysis.

    

Kinetic Inductance Measurement of Niobium Diselenide (NbSe2) using MicrowaveTechniques

Superconductors with high kinetic inductance at microwave frequencies can suppress charge fluctuations in quantum circuits. This property provides a platform for creating the so-called protected qubits such as fluxonium and zero-pi qubits as well as other quantum devices. In this experiment, we measure the kinetic inductance of thin (thickness <10 nm) NbSe₂, a van der Waals superconductor, by using circuit quantum electrodynamics (cQED) architecture. Thin NbSe₂ flakes, entirely encapsulated by hexagonal boron nitride (hBN), have been incorporated into a superconducting coplanar resonator. By measuring the resonance characteristics of this resonator in the low-temperature, low-photon number limit, we extract the kinetic inductance and the quality factor of this 2D crystalline superconductor. Our approach can be applied to study a wide variety of 2D superconductors relevant to constructing high coherence superconducting quantum circuits.

    

Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits

Niobium films, 160 nm thick, used as leads to aluminum Josephson junctions in superconducting qubits (transmons) have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and magneto-optical imaging. In the Meissner state, the films show a very sharp superconducting transition at T_c = 9.35 K, a fairly clean superconducting gap, and signs of stronger coupling consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb. The behavior in the applied magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat channeling to a silicone substrate leading to thermo-magnetic dendritic avalanches. The latter may signify an issue for transmons driven at several GHz. Possible mitigation strategies are discussed.

    

SQMS nanofabrication taskforce: Towards fabrication of high coherence quantum devices

Since its inception, SQMS Center has carried out extensive materials characterization and modeling of superconducting qubits yielding new knowledge and high impact results. The Center has recently launched SQMS Nanofabrication Taskforce to implement novel materials and fabrication processes for high coherence superconducting quantum devices. The Taskforce brings together experts in nanofabrication and materials science from Fermilab, NIST, Rigetti, Northwestern University, and Ames Lab and has access to state-of-the-art foundries at Boulder Microfabrication Facility at NIST, Pritzker Nanofabrication Facility at the University of Chicago, Riggeti nanofab facilities, and NuFab facility at Northwestern University. The first coordinated study of the taskforce has been to inhibit the surface oxides of Nb, believed to be a significant source of decoherence in Nb based superconducting quantum devices. A wide range of materials, such as Al, Ta, and TiN, has been explored as a passivation layer. The preliminary results of microwave loss measurements of superconducting transmission line resonators and coherence measurements of transmon qubit devices 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.

    

Superconducting Ge thin films by molecular beam epitaxy for quantum information

Extreme doping concentrations in group IV semiconductors has been shown to induce not only the expected metal-insulator transition [1] but also at high enough compositions (~5% or more), superconductivity is achieved [2]. However, all reports of such superconductors are grown through highly non-equilibrium growth methods such as laser or ion implantation with subsequent annealing. These methods suffer from difficulties in controlling resultant film thickness [2] or scalability [3]. Here, we will discuss our work on doped Ga:Ge films grown by molecular beam epitaxy (MBE) that show superconductivity. We report growth parameter phase space and the resulting film electronic properties. Our MBE-grown films display T_c ~ 0.8K-2K, B_c^oop ~ 0.05T-0.25T, and B_c^IP ~ 0.3T-1.3T, displaying significant reliance on film thickness. The key links between film growth parameters and resulting critical temperature and field will be discussed.

[1] C. Persson, et. al., Phys. Rev. B. 63, 205119 (2001).

[2] K. Sardashti, et. al., Appl. Phys. Lett. 118, 073102 (2021).

[3] D. Cammilleri, et. al., Thin Solid Films 517, 75-79 (2008).   

Hybrid Bulk/Thin-Film 3D Resonators for Superconducting Quantum Circuits

One of the leading candidate platforms for quantum computing is superconducting circuits. The resonant modes of superconducting microwave resonators offer a long-lived and hardware-efficient way to store quantum information, and offer significant advantages for quantum error correction. To optimize coherence, it is desirable to find ways of combining the long lifetimes and low surface participations of 3D cavities with the control of materials properties and mass producibility of conventional circuits. In this talk, I will present a way to improve the scalability of bosonic modes without sacrificing their coherence times. Our design combines a high-quality on-chip film with a simpler but high coherence package, and effectively eliminates the effects of the substrate bulk and surface losses. We combine this design with a transmon ancilla and show that it enables millisecond coherence times, on par with 3D cavities.   

Fano Interference in Microwave Resonator Measurements

Resonator measurements are a simple but powerful tool to characterize a material’s microwave response. The losses of a resonant mode are quantified by its internal quality factor Q_i, which can be extracted from the scattering coefficient in a microwave reflection or transmission measurement. Here we show that a systematic error on Q_i arises from Fano interference of the signal with a background path. Limited knowledge of the interfering paths in a given setup translates into a range of uncertainty for Q_i, which increases with the coupling coefficient. We experimentally illustrate the relevance of Fano interference in typical microwave resonator measurements and the associated pitfalls encountered in extracting Q_i. On the other hand, we also show how to characterize and utilize the Fano interference to eliminate the systematic error.