Track I
Towards Scalable Quantum Computing: Theory, Materials and Technology Challenges

Session I-1 Superconducting qubits

I-1:IL01  New Material Platforms for Quantum Computing
N. DE LEON, Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ, USA

Constructing fault-tolerant quantum processors based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. However, significant improvements in the lifetime of planar transmon qubits have remained elusive for several years. We have fabricated planar transmon qubits that have both lifetimes and coherence times exceeding 0.3 milliseconds by using tantalum as the capacitor material. Following this discovery, we have parametrized the remaining sources of loss in state-of-the-art devices using systematic measurements of the dependence of loss on temperature, power, and geometry. This parametrization, complemented by direct materials characterization, allows for rational, directed improvement of superconducting qubits.


I-1:IL02  Integer Fluxonium Qubit
V. MANUCHARYAN, R. MENCIA, EPFL, Lausanne, Switzerland

Superconducting fluxonium qubit is normally regarded as a low-frequency qubit, the transition frequency of which is usually about a hundred or a few hundred MHz. The low qubit frequency helps to mitigate conventional errors, such as dielectric loss or state leakage, without limiting the ability to perform fast single and two-qubit gates. Here we demonstrate a new qubit derived from fluxonium, whose transition frequency is comparable with transmon's, that is a few GHz. This device, nicknamed Integer Fluxonium qubit, is operated at zero (integer) external flux, that is in theory it does not require any flux frustration at all. Thanks to the proper choice of circuit parameters, the qubit transition of Integer Fluxonium is as protected as the regular low-frequency fluxonium qubit, which results in similar T1 and T2 times, but much higher quality factors (up to 10^7). We expect that such a new addition to the superconducting qubits toolbox would open up new directions for constructing resilient scaled up quantum systems.


I-1:IL03  Manufacturing High-coherence Superconducting Qubits in an Advanced 300 mm Fabrication Environment
K. DE GREVE, Y. Canvel, T.Ivanov, J. Jussot, S. Kubicek, R. Leung, S. Massar, M. Mongillo, D. Perez-Lozano, A. Pacco, A. Potocnik, A.M. Vadiraj, J. Vandamme, D. Wan, imec and KU Leuven, Department of Electrical Engineering, Leuven, Belgium

In this work, we will present recent results on taking superconducting transmon qubits from lab to fab. Leveraging the advanced 300 mm fabrication facilities at imec, our team has managed to bypass the commonly-used Dolan bridge technique to create purely subtractively etched superconducting transmon qubits in a full 300 mm platform, resulting in high coherence qubits with low across-wafer variability and high yield. Using advanced morphological characterization and metrology techniques, correlated with cryogenic electrical measurements, we study the limitations on coherence and variability of these qubits, and comment on their scalability. This work is supported, in part, by the imec Industrial Affiliation Program on Quantum Computing.


I-1:IL04  Outstanding Materials Challenges & Opportunities for Developing Superconducting Quantum Information Systems
R.W. SIMMONDS, National Institute of Standards & Technology, Boulder, CO, USA

Developing a large-scale quantum information processor has become a major industrial challenge over the last few years. Of the many quantum systems available to tackle this challenge, superconducting circuits have shown impressive results thus far and appear to be posed to scale up rapidly. In this presentation, I will discuss some of the materials challenges associated with developing superconducting circuits for large scale quantum information processors. This includes providing a basic introduction to superconducting qubits, their fabrication, measurement, and coupled operations. I will then explore some of the emerging opportunities for improving, connecting, and expanding these systems for processing quantum information. In particular, I will highlight some of the efforts at the National Institute of Standards & Technology in Boulder, Colorado, USA.


I-1:IL05  Giant Atoms with Superconducting Qubits
A. FRISK KOCKUM, Chalmers University of Technology, Gothenburg, Sweden

When studying the interaction between light and matter at the quantum level, it is common to apply the dipole approximation, where atoms are assumed small compared to the wavelength of the light. However, recent experiments [1,2,3] with superconducting qubits (artificial atoms), coupled to surface acoustic waves or microwave transmission lines, have shown that it is possible to realize ”giant atoms”, i.e., atoms that couple to light (or sound) at multiple points that are wavelengths apart. In this talk, I will present an overview of theoretical and experimental work on such giant atoms [4]. In particular, I will explain how the relaxation rate of a single giant atom can be designed [5] and how multiple giant atoms can interact via a waveguide while remaining protected from relaxation into the waveguide [6]. I will also show experimental data demonstrating these phenomena [2,3]. Finally, I will give an outlook for how these properties of giant atoms may be used for quantum simulation with superconducting qubits.
[1] Gustafsson et al., Science 346, 207 (2014)
[2] Kannan et al., Nature 583, 775 (2020)
[3] Vadiraj et al., Phys. Rev. A 023710 (2021)
[4] Kockum, arXiv:1912.13012
[5] Kockum et al., Phys. Rev. A 90, 013837 (2014)
[6] Kockum et al., Phys. Rev. Lett. 120, 140404 (2018)


I-1:IL06  Quantum Error Correction Beyond Break-even 
V.V. SIVAK, A. Eickbusch, B. Royer, S. Singh, I. Tsioutsios, S. Ganjam, A. Miano, B.L. Brock, A.Z. Ding, L. Frunzio, S.M. Girvin, R.J. Schoelkopf, M.H. Devoret, Departments of Physics and Applied Physics and Yale Quantum Institute, Yale University, New Haven, CT, USA

Decoherence is a fundamental phenomenon through which classical behavior emerges from the quantum laws of nature. It is the main obstacle to harnessing these laws for information processing. Quantum error correction (QEC) is a process that allows to protect quantum information from decoherence. Previous experimental attempts to engineer such a process faced the generation of an excessive number of errors that overwhelmed the error-correcting capability of the process itself. In the recent experiment [Nature 616, 50-55 (2023)], we showed that QEC can really work in practice. We used it to extend the lifetime of quantum information by more than 2x.


I-1:L07  Two-level Defects in Superconducting Quantum Computing Chips 
A. USTINOV, Physics Institute, Karlsruhe Institute of Technology, Karlsruhe, Germany 

Progress towards reliable large-scale superconducting quantum processors requires prevention of defects by improvements in device fabrication. Material defects are currently recognized to be the main source of decoherence in superconducting quantum circuits. These defects are sparsely present in the disordered oxide barriers and on the surfaces of superconducting thin films. A tiny mechanical deformation of the film changes the energies of the atomic tunnelling systems. These changes can be extracted from the microwave spectra of superconducting qubits and resonators. Tuning the properties of individual defects by applying mechanical strain and external electric fields allow to study spectral properties, intrinsic relaxation and dephasing times, and detect mutual interactions between defects. We developed novel techniques to obtain information on locations of defects relative to the thin film edge of the qubit circuit. Resonance frequencies of defects are tuned by exposing the qubit sample to electric fields generated by electrodes surrounding the chip. By determining the defect’s coupling strength to each electrode and comparing it to a simulation of the field distribution, we obtain the probability at which location and at which interface the defect resides.



Session I-2 Defects and color centers in semiconductors

I-2:IL01  Quantum Embedding for Point Defects: Benchmarking and Applications
M. roesner, Theory of Condensed Matter Department (TCM), Institute for Molecules and Materials (IMM), Radboud University, Nijmegen, Netherlands

Model Hamiltonians are regularly derived from first-principles data to describe correlated matter. However, the standard methods for this contain a number of largely unexplored approximations. For a strongly correlated impurity model system, here we carefully compare standard downfolding techniques with the best-possible ground-truth estimates for charge-neutral excited state energies and charge densities using state-of-the-art first-principles many-body wave function approaches. To this end, we use the vanadocene molecule and analyze all downfolding aspects, including the Hamiltonian form, target basis, double counting correction, and Coulomb interaction screening models. We find that the choice of target-space basis functions emerges as a key factor for the quality of the downfolded results, while orbital-dependent double counting correction diminishes the quality. Background screening to the Coulomb interaction matrix elements primarily affects crystal-field excitations. Our benchmark uncovers the relative importance of each downfolding step and offers insights into the potential accuracy of minimal downfolded model Hamiltonians.


I-2:IL03  Recombination Mechanisms in Quantum Defects
M.E. TURIANSKY, Materials Department, University of California, Santa Barbara, CA, USA; A. Alkauskas, Center for Physical Sciences and Technology (FTMC), Vilnius, Lithuania; F. Zhao, C.G. Van de Walle, Materials Department, University of California, Santa Barbara, CA, USA

Defects in semiconductors or insulators are a promising platform to realize quantum information science, including quantum computation, quantum communication, and quantum sensing. First-principles calculations based on hybrid density functional theory can be a powerful tool to identify and characterize these so-called quantum defects. In particular, there are a variety of both radiative and nonradiative transitions that may occur within a quantum defect that governs its behavior and utility. In this talk, I will discuss the development of and recent improvements to the Nonrad code, which allows us to evaluate the nonradiative transition rate from first principles. I will give two examples of the application of this code to single-photon emitters in hexagonal boron nitride, which we attribute to the boron dangling bond, and to novel defect complexes in cubic boron nitride, which we propose to be telecom-wavelength single-photon emitters. At large wavelengths, nonradiative processes become dominant, lowering the efficiency of emitters. Lastly, I will overview our newly developed methodology to address another nonradiative mechanism, trap-assisted Auger-Meitner recombination.


I-2:IL04  Quantum Networks based on Color-center Spin Qubits
T.H. TAMINIAU, QuTech and Kavli Institute of Nanoscience, Delft University of Technology, Delft, Netherlands

Color centers in diamond and related materials provide a promising qubit platform for exploring quantum simulations, computations and networks. Important challenges towards large-scale systems are to realize high-quality gate operations, robust quantum memory and fast optical entanglement interconnects. In this talk, I will discuss our latest progress in those directions. I will discuss novel gate designs that enable gate fidelities over 99.9% for all gates in a two-qubit system, our progress in ultrapure diamond growth with optimized isotopic 13C concentrations, and steps towards integrated optics to improve the optical spin-photon interface.



Session I-3 Trapped-ion, photonic and topological insulators-based qubits

I-3:IL02  State of the Art and Challenges of Scaling Ion-trap Quantum Computer
C. OSPELKAUS, for the QVLS-Q1 and ATIQ projects Leibniz Universität Hannover and PTB Braunschweig, Hannover, Germany

Trapped ions are one of the two leading experimental platforms for scalable quantum computers. All of the essential building blocks have been demonstrated experimentally. Small-scale systems are operational. The platform is characterized by its long coherence times and excellent gate fidelties as well as the all-to-all connectivity and the possibility to couple to photons for longer distance interconnects and for modularization. I will discuss our efforts to build scalable quantum cores based on fully chip-integrated microwave methods for quantum gates. Two-qubit gate infidelities approach 10-3 error rates, and single-qubit gates 10-4 error rates. I will discuss the implementation of separate registers for storage, computation, readout and ion loading. We will address challenges that need to be overcome to scale this architecture, such as the implementation of junctions, through-substrate vias, hybrid integration, integrated electronics and photonics for state preparation and readout. While there are still considerable challenges, a path is emerging for fully modularized and truly scalable implementations of quantum processors in this physical system and towards real-world applications.


I-3:IL03  Superconducting Diode Effect due to Magnetochiral Anisotropy in Topological Insulator and Rashba Nanowires
J. KLINOVAJA, H. Legg, K. Laubscher, D. Loss, University of Basel, Basel, Switzerland

The critical current of a superconductor can depend on the direction of current flow due to magnetochiral anisotropy when both inversion and time-reversal symmetry are broken, an effect known as the superconducting (SC) diode effect [1]. In our work, we consider one-dimensional (1D) systems in which superconductivity is induced via the proximity effect [2,3]. In both topological insulator and Rashba nanowires, the SC diode effect due to a magnetic field applied along the spin-polarization axis and perpendicular to the nanowire provides a measure of inversion symmetry breaking in the presence of a superconductor. Furthermore, a strong dependence of the SC diode effect on an additional component of magnetic field applied parallel to the nanowire as well as on the position of the chemical potential can be used to detect that a device is in the region of parameter space where the phase transition to topological superconductivity is expected to arise [3-6].
[1] Legg, Loss, and Klinovaja, PRB 106, 104501 (2022).
[2] Legg, Loss, and Klinovaja, PRB 104, 165405 (2021).
[3] Legg, Loss, and Klinovaja, PRB 105, 155413 (2022).
[4] Hess, Legg, Loss, and Klinovaja, PRL 130, 207001 (2023).
[5] Hess, Legg, Loss, and Klinovaja, PRB 104503 (2022).
[6] Legg, et al., Nature Nanotechnology (2022).


I-3:IL06  III-V Nanowire Heterostructures for Quantum Photonics
N. LOVERGINE, Università del Salento, Lecce, Italy

Solid state single-photon emitters and detectors are at the very core of modern quantum technologies. Among different types, integrated quantum photonic (IQP) devices promise reliable, scalable and cost-effective alternatives to bulk optics. IQP devices based on different on-chip waveguide platforms (including Si, GaAs and inP) are being intensively investigated. In particular, single-photon sources based on III-V semiconductor quantum dots (QDs) allow for deterministic generation of identical single photons. III-V QDs are usually fabricated in the form of planar arrays, but light extraction efficiency is very limited in such samples; embedding the QDs in a photonic nanowire allows superior light extraction, and such QD-in-nanowire structures have shown ultra-bright single-photon emission. This talk reviews the status of III-V compound QD-in-nanowire self-assembly via metalorganic vapor phase epitaxy (MOVPE) technologies, such as selective-area epitaxy (SAE) and vapor-liquid-solid (VLS) growth along with pros and cons of each technology. Examples of GaAs based QD-in-nanowire structures will be presented.


I-3:IL07  Visualizing Topological Phases of Matter – towards future anyonic braiding operations
ZHI-XUN SHEN, Dept of Physics and Applied Physics, Stanford University, Stanford, CA, USA

In this talk, we present visualization of topological states of matter, with focus on the bulk-edge correspondence in quantum Hall state (QHS), quantum spin Hall state (QSHS), quantum anomalous Hall state (QAHS) and fractional quantum anomalous Hall state ((FQAHS) or fractional Chern insulator state (FCI). The material systems include GaAs quantum well and graphene (QHS), 1T’ WTe2 (QSHS), delta doped Cr/(Bi,Sb)2Te3 (QAHS) and twisted bi-layer MoTe2 (FQAHS or FCI). The emphasis will be on recent results on FQAHS where we observed the co-existence of neighboring -3/5 and -2/3 FCI states. This breakthrough enables the investigation of topological protected 1D interface formed between different anyonic orders at zero magnetic field. These possibilities include the edge state scattering/coupling between different FCI states, creation of new anionic states, and future anyonic braiding operations for topological quantum information processing.



Session I-4 Superconducting quantum dot and dopant-based qubits

I-4:IL01  Detecting Electric, Magnetic and Strain Fields with a Single High-spin Nucleus in Silicon
A. MORELLO, UNSW Sydney, Sydney, Australia

Material-related properties such as lattice strain, electric and magnetic fields, can subtly or dramatically influence the behavior of charges and spins confined in semiconductors. Group-V donors in silicon are a simple, elegant, and highly coherent platform for quantum information processing. In this talk I will present novel results on using the 123Sb donor in silicon as platform for encoding quantum information. The 123Sb atom comprises a nuclear spin I = 7/2, which possesses an electric quadrupole moment, which makes it sensitive to both electric and magnetic fields, and to lattice strain. The nuclear spin retains an exceptionally long coherence time, corresponding to a 2 Hz magnetic resonance linewidth, and can thus be used as a sensitive detector of minute fluctuations in its environment. Crucially, we can rigorously distinguish electric from magnetic noise, and sense lattice strain. This capability represents an unprecedented opportunity to understand material and noise properties in semiconductor devices at the atomic scale.


I-4:IL02  Hybrid Circuit Quantum Electrodynamics with Semiconductor QDs
P. SCARLINO, Institute of Physics, Hybrid Quantum Circuits Laboratory, École Polytechnique Fédérale de Lausanne ‐ EPFL, Lousanne, Switzerland
 
I-4:IL03  Circuit Quantum Electrodynamics Experiments in Planar Germanium
G. KATSAROS, Institute of Science and Technology Austria, Klosterneuburg, Austria

Spin qubits in Germanium are promising candidates for the realization of quantum processors. [1]. From 2018 and within a few years a Loss-DiVincenzo [1], a singlet-triplet hole spin qubit [2], a two-qubit [3], a four-qubit Ge quantum processor [4] and a 16 quantum dot crossbar array [5] have been realized demonstrating the potential of Ge for quantum information. One of the still open challenges is their long distance coupling. In this talk I will report on circuit quantum electrodynamics experiments in planar Ge. With the use of granular Aluminum as material for the realization of high impedance resonators, charge-photon coupling with a strength of about 600MHz has been realized. Finally, I will report on the development of hybrid Al/Ge devices [7], paving the way towards Andreev spin qubits.
[1] G. Scappucci et al., Nature Reviews Materials 6, 926 (2021).
[2] H. Watzinger et al., Nature Communications 9, 3902 (2018).
[3] D. Jirovec et al., Nature Materials 20, 1106 (2021).
[4] N. W. Hendrickx et al., Nature 577, 487-491 (2020).
[5] N. W. Hendrickx et al., Nature 591, 580-585 (2021).
[6] F. Borsoi et al., Nature Nanotechnology (2023).
[7] M. Valentini et al., arXiv:2306.07109v1 (2023).


I-4:IL05  Quantum Computation with Spins in Silicon - Coherence, Integration, and Scaling
XIAO XUE, L.M.K. Vandersypen, QuTech and Kavli Institute of Nanoscience, Delft University of Technology, Delft, Netherlands

In this talk, I will give an overview of our vision on quantum computing with electron spin qubits, and discuss the progress and challenges in realizing fault-tolerant, fully integrated silicon quantum circuits [1]. I will first present our work on characterizing gate fidelities, including the realization of a high-fidelity two-qubit gate that meets the requirement for implementing quantum error correction codes [2, 3, 4]. Then I will steer the focus to quantum control of spin qubits using a cryo-CMOS chip, named "Horse Ridge", which is the first step towards solving the wiring issues at the quantum-classical interface [5]. Finally, I will introduce our recent spin qubit experiments, in particular the realization of two-qubit iSWAP-class logic between distant spins in silicon, which is achieved by using an on-chip superconducting resonator [6].
[1] L. M. K. Vandersypen, et al., npj Quantum Information 3, 34 (2017).
[2] X. Xue, et al., PRX 9, 021011 (2019).
[3] J. Helsen, X. Xue, et al., npj Quantum Information 5, 71 (2019).
[4] X. Xue, et al., Nature 601, 343-347 (2022).
[5] X. Xue, et al., Nature 593, 205–210 (2021).
[6] J. Dijkema, X. Xue, et al., in preparation.


I-4:L07  Investigating Frequency Shifts in Silicon Spin Qubits influenced by Environmental Coupling 
I. HEINZ, G. Burkard, Department of Physics, University of Konstanz, Konstanz, Germany

Silicon quantum dots hosting spin qubits demonstrate great potential for quantum computation. Therefore, an electron is confined in a silicon quantum well and a potential induced by gate electrodes. A magnetic gradient field gives rise to a splitting of the electron spin states, effectively creating the qubit states. Single qubit operations rely on electric-dipole spin resonance (EDSR), aligning the driving frequency with the qubit frequency. Recent experiments [1,2] indicate frequency shifts linked to environmental properties such as temperature. We investigate the origin of such frequency shifts by employing an effective phonon-qubit coupling in our description arising from spin-orbit coupling to gain physical intuition to the temperature dependence of the qubit frequency. Additionally, we explore the influence of valley splitting on the EDSR response of a spin qubit.
[1] B. Undseth et al., Phys. Rev. Appl. 19 (2023)

I-4:IL08  Hole Spin Qubits for Quantum Computing in Si and Ge Quantum Dots  
D. LOSS, University of Basel, Basel, Switzerland

Semiconductor spin qubits offer a unique opportunity for scalable quantum computation by leveraging classical transistor technology. This has triggered a worldwide effort to develop spin qubits, in particular, in Si and Ge based quantum dots, both for electrons and for holes. Due to strong spin orbit interaction, hole spin qubits benefit from ultrafast all-electrical qubit control and sweet spots to counteract charge and nuclear spin noise . In this talk I will present an overview of the state-of-the art in the field and focus, in particular, on recent developments on hole spin physics in Ge and Si nanowires, Si FinFETs, and Ge heterostructures.  


I-4:IL09  Two-qubit Operations in Silicon Quantum Dots made on a 300mm Process measured using a Radiofrequency Electron Cascade  
J.F. Chittock-Wood^{1, 2}, R.C.C. Leon², M.A. Fogarty², S. Patomäki^{1, 2}, F. Ekkehard von Horstig^{2, 3}, N. Johnson¹, A. Seigel^{2, 4}, H. Jnane^{2, 4}, J. Jussot⁵, S. Kubicek⁵, B. Govoreanu⁵, S.C. Benjamin^{2, 4}, M.F. Gonzalez-Zalba², J.J.L. Morton^{1, 2}, ¹University College London, UK; ²Quantum Motion, London, UK; ³University of Cambridge, UK; ⁴University of Oxford, UK; ⁵IMEC, Belgium

We present two novel advancements: (i) coherent control over the exchange interaction in a 300mm wafer processed natural silicon metal-oxide-semiconductor (MOS) quantum dot (QD) array; (ii) the realization of a radio-frequency (rf) driven electron cascade readout technique. Recent studies of MOS QD arrays fabricated by industrial grade processes have demonstrated control and readout of individual electron spin qubits – here we present control over the exchange interaction between two electron spins, over a range of 7.5 – 144 MHz. We characterize a spin-orbit interaction and tune it to optimize coherence for exchange control, forming the basis for √SWAP operations with dephasing times up to T_2^SWAP≈400 ns, and gate quality factors ≈15. With spin-echo sequences, we extend this dephasing time by up to a factor of 5. These results were obtained using dispersive spin-projective measurements through a new rf electron cascade technique. Interdot charge transitions driven by the rf excitation cyclically force electron tunneling in a nearby QD, creating an amplification of the induced charge on the resonator. Our results demonstrate the feasibility of two qubit operations in foundry fabricated QDs, alongside integration with scalable dispersive sensing techniques for array-wide readout.


I-4:IL10  Tuning Quantum Dot Arrays with Rays  
J.P. ZWOLAK, National Institute of Standards and Technology, Gaithersburg, MD, USA

Over the past decade, there has been several advances aimed at automation of the various aspect of tuning QD devices, from testing device functionality and bootstrapping to setting the device topology to charge tuning. While the initial attempts relied on the appealingly intuitive and relatively easy-to-implement conventional algorithms involving a combination of techniques from regression analysis, pattern matching, and quantum control theory, more recently researchers began to take advantage of the tools provided by the field of artificial intelligence. In this talk, I will discuss how the recently proposed ray-based classification framework can be combined with an “action-based” algorithm to tune a bootstrapped QD device into a specific few-electron charge configuration. I will also discuss our approach to resolving certain tuning failure modes by targeted identification of specific nonidealities. Among others, we have developed systems to flag devices with unintended dots near the operating regime autonomously and to identify high levels of telegraph noise. These autonomous systems will enable both high throughput screening of quantum dot devices as well as more reliable tuning to a regime suitable for qubit operations.


I-4:IL11  High-throughput Spectroscopic Characterization of Nanowire-based Quantum Structures for Quantum Information Technologies  
P. PARKINSON, N. PATEL, S. CHURCH, University of Manchester, Manchester, UK; A. Sanchez, University of Warwick, UK; H. Liu, University College London, UK

Defect-based or quantum-confinement based structures have long been explored as potential physical implementations of stationary qubits, for conversion of stationary to flying qubits, or for generation of single photons. Prototypical examples include NV centres in diamond, defects in hexagonal-boron nitride, implanted systems in ultrapure silicon and many varieties of colloidal and epitaxial quantum dot. Each platform has different advantages and disadvantages, and to date no technology provides an obvious route to deliver repeatable, room temperature, easily manufacturable and integrable quantum structures. Over the past decades many of the initial scientific questions have now been answered; however, scale-up now presented unique challenges. In this presentation, I will discuss the role of high-throughput photonic measurements to understand, optimize and exploit variation in potential quantum platforms. I will speak about our work on mechanically structured quantum systems, about recent work expanding the role of colloidal quantum dots and more fundamental challenges around metrology and standardization for high-throughput study.