PCSI2027 Session PCSI-MoM1: Superconducting & Hybrid Qubits

In this seminar, I will present our team's recent efforts in developing high-performance superconducting qubits. Topics will include improving performance and processing windows of base layer films made from niobium and tantalum on silicon substrates, with special emphasis on the correlations between microwave quantum device performance and surface and interface materials properties [1,2]. Particularly, we found that we could reduce substrate temperatures for BCC Ta deposition well into the back-end-of-the-line process window via use of krypton as a sputtering gas. Transmon qubits with a twenty-micron capacitor gap made from these films exhibit quality factors exceeding fifteen million (coherence times up to 1 millisecond), demonstrating their performance with processing that is compatible with a greater number of tools. Additionally, I will highlight high-performance qubits made with Josephson junctions formed by a resist-free fabrication method [3] and our efforts to make high-performance qubit fabrication more broadly available through developing standard process flows at the Cornell Nanoscale Facility.

[1] Olszewski, Maciej W., Jadrien T. Paustian, Tathagata Banerjee, et al. “Low-Loss Nb on Si Superconducting Resonators from a Dual-Use Spintronics Deposition Chamber and with Acid-Free Post-Processing.” Applied Physics Letters 128, no. 21 (2026): 214001. https://doi.org/10.1063/5.0325435.

[2] Olszewski, Maciej W., Lingda Kong, Simon Reinhardt, et al. “Krypton-Sputtered Tantalum Films for Scalable High-Performance Quantum Devices.” arXiv:2601.20091. Preprint, arXiv, January 27, 2026. https://doi.org/10.48550/arXiv.2601.20091.

This prototype was primarily supported by the Microelectronics Commons Program, a DoW initiative, under award number N00164-23-9-G061.Funding (or Partial funding) for shared facilities used in this prototype was provided by the Microelectronics Commons Program, a DoW initiative, under award number N00164-23-9-G061.

[3] Banerjee, Tathagata, Stephen Daniel Funni, Saswata Roy, Judy J. Cha, and Valla Fatemi. “Resist-Free Shadow Deposition Using Silicon Trenches for Josephson Junctions in Superconducting Qubits.” arXiv:2604.09796. Preprint, arXiv, April 10, 2026. https://doi.org/10.48550/arXiv.2604.09796.

Josephson Junctions (JJs) are a central building block of superconducting qubits and related technologies, but are known to host defects (so-called Two Level Systems, or TLS) which negatively impact the performance of quantum devices. Traditionally, JJs have been built around amorphous oxide barriers, which have long been thought to inherently host TLS. Here, we present a Josephson Junction based on a crystalline silicon fin, as proposed in the “FinMET” concept [1,2].

Our fin-based Josephson Junctions are fabricated using an anisotropic KOH etch on 110 Si using a SiN hard mask, creating fins approximately 1-2 μm tall and less than 20 nm thick. [3] Angle deposited aluminum forms electrodes on either side of the silicon fin, with the SiN forming a shadow mask that prevents shorting of the two electrodes over the top of the fin.

Conventional Josephson junctions (JJs), superconductor/tunnel barrier/superconductor structures, are created using polycrystalline and amorphous materials. Typically, the superconducting metal layers are polycrystalline, and the tunnel barrier is amorphous, which leads to dangling bonds at the superconductor/tunnel barrier interfaces and in the tunnel barrier itself. These dangling bonds are believed to house two-level energy loss systems (TLS) that limit qubit and other quantum device performance.

Thus, one way to make progress in this field is to carefully select materials that minimize this loss. Cobalt disilicide (CoSi₂) can be grown epitaxially on silicon (Si), and its close lattice match with Si also enables the growth of single-crystalline Si tunnel barriers directly on the superconducting CoSi₂ layer [1, 2]. This prospect makes CoSi2 particularly interesting for superconducting circuits because it eliminates the usage of amorphous or polycrystalline materials, which could potentially reduce loss. The vertical Josephson junction can then be completed by depositing another superconducting layer. In this talk, we demonstrate the growth and characterization of aluminum (Al)/Si/CoSi₂/Si(111) structures using molecular beam epitaxy. The reflection high-energy electron diffraction (RHEED) shows the signature 1x1 surface reconstruction for CoSi2, 7x7 surface reconstruction post Si growth, and a 1x1 surface reconstruction following Al deposition. Following synthesis, atomic force microscopy of the CoSi₂ shows a smooth surface with terraces, which is indicative of a high-quality, epitaxial surface and suggests that the film grows in a well-ordered manner (Fig. 1 (a)). Additionally, the films exhibited sharp peaks in X-ray diffraction (Fig. 1 (b)) and oscillations in X-ray reflectivity, indicating well-defined crystalline structure and interfaces. Correlation of the growth parameters to the superconducting critical temperature of CoSi₂ were also investigated, with the highest critical temperature of 1.25 K.

[1]Ditchek, B. et al. APL.47, 1200(1985), [2] Hensel, J. et al. APL. 47, 151 (1985)

Refractory metals are important materials for superconducting electronics and quantum circuits. Many of these applications require thin films with controlled morphology and crystal structure, but achieving this control becomes particularly challenging at thicknesses of only a few nanometers. In conventional physical-vapor deposition, morphology and crystal structure depend strongly on adatom mobility, which is influenced by the substrate temperature relative to the melting temperature. Therefore, the high melting temperatures of refractory metals limit adatom mobility at typical substrate temperatures. However, increasing substrate temperatures can promote unwanted interfacial reactions, particularly with underlying III-V semiconductor heterostructures.

Here, we use cryogenic deposition to intentionally suppress atomic mobility and separate deposition from crystallization in ultrathin Nb, Ta, Re, and V films on III-V heterostructures. In situ reflection high-energy electron diffraction (RHEED) patterns show that Ta (Fig. 1(a)), Nb, and V films are initially highly disordered or amorphous and become polycrystalline upon heating to 250 °C (Fig. 1(b)). Ex situ Grazing-incidence X-ray diffraction (GIXRD) reveals that thinner Ta and Re films remain amorphous, whereas thicker films crystallize into α-Ta and hexagonal close-packed (hcp) Re (Fig. 1(c) and 1(d)). This thickness dependent behavior is confirmed for Re using scanning transmission electron microscopy (STEM) (Fig. 1(e) and 1(f)). In Ta/Nb bilayers, Ta crystallizes at a thickness of only 2 nm, indicating that adjacent metal layers can modify crystallization. Transport measurements reveal relationships between crystal structure and superconductivity. The α-Ta film shows a higher superconducting transition temperature (T_c) compared to amorphous Ta. In contrast, a 4 nm amorphous Re film has an enhanced T_c of 6.1 K, whereas 6 nm hcp Re has a T_c of 2.6 K. These results show how material choice, film thickness, and adjacent layers influence crystallization following cryogenic deposition, providing a pathway to integrate ultrathin refractory superconductors on III-V semiconductor heterostructures.

Epitaxial inorganic interfaces lie at the heart of semiconductor, spintronic, and quantum devices. At an interface between two dissimilar materials physical properties and functionalities may arise, which do not exist in any of the isolated constituents in the bulk. The resulting device performance hinges on the electronic and magnetic properties of the interface. The configuration space of possible inorganic interfaces is vast and largely underexplored, owing to the almost infinite number of ways different materials can be combined to form interfaces. Computer simulations may significantly accelerate the discovery and design of new inorganic interfaces with desirable properties.

To predict the structure of domain-matched epitaxial interfaces, we have developed the Ogre code [1]. Ogre starts by performing “lattice matching” to identify all possible domain-matched commensurate interfaces within the range of user-defined Miller indices, maximum strain, and maximum area. For each orientation, Ogre constructs interface structures by identifying and combining all possible surface terminations of the two materials. Subsequently, for each interface, Ogre performs “surface matching” to identify the best in-plane registry and interfacial distance between the two materials. Finally, all candidate interfaces are ranked by stability.

To study the electronic properties of interfaces we use density functional theory (DFT). Within DFT, the many-body interactions between electrons are described by approximate exchange-correlation functionals. The accuracy of the results hinges on an appropriate choice of functional. We have developed a method of machine learning the Hubbard U correction added to a DFT functional by Bayesian optimization (BO) [2]. The DFT+U(BO) method balances accuracy with computational cost, enabling unprecedented simulations of large surface and interface models of interest for applications in quantum computing. For example, we have used DFT+U(BO) to assess candidate materials for tunnel barriers at semiconductor/ superconductor interfaces [3,4].

[1] S. Toso, D. Dardzinski, L. Manna, N. Marom, ACS Nano 19, 5326 (2025)

[2] M. Yu, S. Yang, C. Wu, and N. Marom, npj Computational Materials 6, 180 (2020)

[3] M. J. A. Jardine, D. Dardzinski, M. Yu, A. Purkayastha, A.-H. Chen, Y.-H. Chang, A. Engel, V. N. Strocov, M. Hocevar, C. J. Palmstrøm, S. M. Frolov, and N. Marom, ACS Applied Materials & Interfaces 15, 16288 (2023)

[4] M. J. A. Jardine, D. Dardzinski, Z. Cai, V. N. Strocov, M. Hocevar, C. J. Palmstrøm, S. M. Frolov, and N. Marom, ACS Applied Materials & Interfaces 17, 5462 (2025)