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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Strategies to search for two-dimensional materials with long spin qubit coherence time

Two-dimensional (2D) materials that can host qubits with long spin coherence time (T 2 ) have the distinct advantage of integrating easily with existing microelectronic and photonic platforms, making them attractive for designing novel quantum devices with enhanced performance. However, the relative lack of 2D materials as spin qubit hosts, as well as appropriate substrates that can help maintain long T 2 , necessitates a strategy to search for candidates with robust spin coherence. Here, we develop a high-throughput computational workflow to predict the nuclear spin bath-driven qubit decoherence and T 2 in 2D materials and heterostructures. We initially screen 1172 2D materials and find 189 monolayers with T 2 > 1 ms, higher than that of naturally-abundant diamond. We then construct 1554 lattice-commensurate heterostructures between high-T 2 2D materials and select 3D substrates, and we find that T 2 is generally lower in a heterostructure than in the bare 2D host material; however, low-noise substrates (such as CeO 2 and CaO) can help maintain high T 2 . To further accelerate the material screening effort, we derive analytical models that enable rapid predictions of T 2 for 2D materials and heterostructures. The models offer a simple, yet quantitative, way to determine the relative contributions to decoherence from the nuclear spin baths of the 2D host and substrate in a heterostructural system. By developing a high-throughput workflow and analytical models, we expand the genome of 2D materials and their spin coherence times for the development of spin qubit platforms.

Toriyama, Michael Y. [Argonne National Laboratory ↗
DOE OSTI2026

Parity-doublet coherence times in optically trapped polyatomic molecules

Polyatomic molecules provide complex internal structures that are ideal for applications in quantum information science, quantum simulation and precision searches for physics beyond the standard model. A key feature of polyatomic molecules is the presence of parity-doublet states. These structures, which generically arise from the rotational and vibrational degrees of freedom afforded by polyatomic molecules, are a powerful feature to pursue diverse quantum science applications. Linear triatomic molecules contain ℓ-type parity-doublet states in the vibrational bending mode, which are predicted to exhibit robust coherence properties. Here we report optically trapped CaOH molecules prepared in ℓ-type parity-doublet states and realize a bare qubit coherence time of $T$$^{*}_{2}$, which is longer than the 0.36 s lifetime of the bending mode. We suppress differential Stark shifts by cancelling ambient electric fields using molecular spectroscopy and characterize parity-dependent trap shifts, which are found to limit the coherence time. Furthermore, the parity-doublet coherence times achieved in this work are a defining milestone for the use of polyatomic molecules in quantum science.

Quantum information↗
DOE OSTI2025

Investigating Low Loss Substrates for Improved Qubit Coherence Times

Superconducting qubits are sensitive to dielectric losses from substrates, making the identification, optimization, and preparation of low-loss materials essential for enhancing qubit coherence times. In this work, we utilize Nb superconducting radio-frequency (SRF) cavities as a high-precision measurement tool to study the dielectric loss tangent of substrates with parts-per-billion sensitivity. This technique enables temperature- and field-resolved measurements, providing deep insights into the behavior of candidate materials such as sapphire and silicon. We explore the impact of substrate preparation techniques, examining how surface treatments and cleaning protocols influence dielectric loss. These studies highlight both the microscopic origins of loss in these substrates and the critical role of preparation methods, helping to identify key materials and processes that minimize dielectric loss, crucial for improving qubit performance.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Millisecond lifetimes and coherence times in 2D transmon qubits

Materials improvement is a powerful approach to reducing loss and decoherence in superconducting qubits, because such improvements can be readily translated to large-scale processors. Recent work improved transmon coherence by using tantalum as a base layer and sapphire as a substrate. The losses in these devices are dominated by two-level systems with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve substantial improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon markedly decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q avg ) of 9.7 × 10 6 across 45 qubits. For our best qubit, we achieve a Q avg of 1.5 × 10 7 , reaching a maximum Q of 2.5 × 10 7 , corresponding to a lifetime (T 1 ) up to 1.68 ms. This low loss also allows us to observe decoherence effects related to the Josephson junction, and we use an improved, low-contamination junction deposition to achieve Hahn echo coherence times (T 2E ) exceeding T 1 . We achieve these materials improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. Here, we demonstrate single-qubit gates with 99.994% fidelity. The tantalum-on-silicon platform comprises a simple material stack that can potentially be fabricated at the wafer scale and therefore can be readily translated to large-scale quantum processors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Ultrafast x-ray imaging of coherently controlled molecular dynamics in real space and time

Coherent control aims to manipulate chemical processes on the latent length and timescales of atoms and bonds, scales that are intrinsic to molecular dynamics but not directly resolved by most experimental probes. As a result, existing coherent-control experiments, which overwhelmingly rely on spectroscopic observables, leave a crucial blind spot: the direct, real-space recovery of all atomic and molecular rearrangements in response to coherently controlled excitations. Here, we overcome this limitation by integrating a Tannor-Kosloff-Rice pump-control-probe scheme with ultrafast X-ray scattering to capture snapshots of the wavepacket motion in a benchmark molecular system. We demonstrate this by photoexciting diatomic iodine vapor with a visible pump pulse, selectively steering the wavepacket toward ground-state recombination or dissociative pathways with a time-delayed near-IR control pulse, and recording the dynamics with angstrom and femtosecond precision with an ultrashort hard X-ray probe pulse. By comparing these structural observations with numerical solutions of the time-dependent Schrödinger equation, we reveal how coherent control actively reshapes the molecular charge density distribution. Our results pave the way for leveraging structural feedback as a control handle and provide a fundamental microscopic visualization of quantum decoherence and energy redistribution at the atomic level.

Hopper, Thomas R. [SLAC National Accelerator Labor↗
DOE OSTI2025

Analysis of RF Surface Loss in a Planar 2D Qubit

The Josephson junction and shunt capacitor form a transmon qubit, which is the cornerstone of modern quantum computing platforms. For reliable quantum computing, it is important how long a qubit can remain in a superposition of quantum states, which is determined by the coherence time (T1). The coherence time of a qubit effectively sets the "lifetime" of usable quantum information, determining how long quantum computations can be performed before errors occur and information is lost. There are several sources of decoherence in transmon qubits, but the predominant one is generally considered to be dielectric losses in the natural oxide layer formed on the surface of the superconductor. In this paper, we present a numerical study of microwave surface losses in planar superconducting antennas of different transmon qubit designs. An asymptotic method for estimating the energy participation ratio in ultrathin films of nanometer scales is proposed, and estimates are given for the limits of achievable minimum RF losses depending on the electrical properties of the surface oxide and the interface of the qubit with the substrate material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Detection of atomic oxygen and its electronic coherence decays using time-resolved ultrafast coherent Raman scattering

We report the detection of atomic oxygen and quantitative measurements of its electronic Raman coherence decays in flames and low-temperature plasmas using time-resolved hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS). Atomic oxygen was detected using the Raman transitions between the spin–orbit coupled triplet ground states. Atomic oxygen was generated in an H 2 /O 2 /Ar diffusion flame and an O 2 /Ar pulsed plasma discharge. Single exponential decays were observed for the O( 3 P 2 )–O( 3 P 1 ) Raman transition at 158.3 cm −1 and the O( 3 P 2 )–O( 3 P 0 ) Raman transition at 227 cm −1 . From the decay measurements, the atomic O Raman linewidths were obtained from 25 to 150 Torr in non-equilibrium plasma and at 760 Torr in a flame. Enhanced signal-to-noise ratios (SNRs) of atomic oxygen and atomic to molecular oxygen signal contrasts were obtained by taking advantage of electronic triplet coherence beating. Enhancement of up to seven times in the atomic O SNR was observed. Furthermore, we also found that the dephasing rates of O 2 (v = 0–3, N = 37) were similar, which provides evidence for the assumption that vibrational excitation does not influence the dephasing of diatomic molecular rotational CARS transitions.

Atomic and molecular spectroscopy↗
DOE OSTI2026

Electron charge coherence on a solid neon surface

Recent experiments show ∼0.1 ms coherence time for a single electron charge qubit on a solid neon surface. This remarkably long coherence time is believed to result from the intrinsic purity of solid neon as a qubit host. In this paper, we present theoretical studies on the decoherence mechanisms of an electron’s charge (lateral motional) states on solid neon. At the typical experimental temperature of ∼10 mK, the two main decoherence mechanisms are the phonon-induced displacement of neon surface and phonon-induced modulation of neon permittivity (dielectric constant). With a qubit frequency increasing from 1 GHz to 10 GHz, the charge coherence time decreases from about 366 s to 7 ms and from about 864 s to 24 ms, respectively, limited by the two mechanisms above. Here, the calculated coherence times are at least one order longer than the observed ones at ∼6.4 GHz qubit frequency, suggesting plenty of room for experimental improvement.

Zou, Shan [University of Notre Dame, IN (United St↗
DOE OSTI2026

Sapphire Substrate Trenching to Advance Superconducting High-Coherence Quantum Devices

Achieving longer coherence times in superconducting qubits is essential for advancing quantum information processing and enabling scalable quantum computing. This work presents an innovative fabrication strategy focused on sapphire trenching of the substrate of superconducting high-coherence quantum devices, developed by the SQMS Nanofabrication Taskforce in collaboration with the materials characterization team. In this talk, we present a method to efficiently and consistently trench into the sapphire substrate, without compromising the surface roughness or profile of the etched sapphire. In the literature, trenching in the substrate has been shown to decrease losses, unwanted coupling and thermal stress. Sapphire’s strong covalent bonds make it harder to etch compared to silicon, yet it is more favorable for high-coherence quantum devices due to its lower dielectric losses. This innovative technique opens new possibilities for superconducting qubit fabrication: we fabricated high coherence qubit chips on sapphire substrates with trenching, to demonstrate that it will help pushing towards higher coherence times. By integrating this work with ongoing junction process optimization, design innovation, materials characterization and exploration, we outline a clear pathway toward transmon coherence times reaching millisecond scales and beyond.

Garattoni, S. [Fermilab]↗
DOE OSTI2026

Sapphire Substrate Trenching to Advance Superconducting High-Coherence Quantum Devices

Achieving longer coherence times in superconducting qubits is essential for advancing quantum information processing and enabling scalable quantum computing. This work presents an innovative fabrication strategy focused on sapphire trenching of the substrate of superconducting high-coherence quantum devices, developed by the SQMS Nanofabrication Taskforce in collaboration with the materials characterization team. In this talk, we present a method to efficiently and consistently trench into the sapphire substrate, without compromising the surface roughness or profile of the etched sapphire. In the literature, trenching in the substrate has been shown to decrease losses, unwanted coupling and thermal stress. Sapphire’s strong covalent bonds make it harder to etch compared to silicon, yet it is more favorable for high-coherence quantum devices due to its lower dielectric losses. This innovative technique opens new possibilities for superconducting qubit fabrication: we fabricated high coherence qubit chips on sapphire substrates with trenching, to demonstrate that it will help pushing towards higher coherence times. By integrating this work with ongoing junction process optimization, design innovation, materials characterization and exploration, we outline a clear pathway toward transmon coherence times reaching millisecond scales and beyond.

Garattoni, S. [Fermilab]↗
DOE OSTI2024

The role of the dopant in the electronic structure of erbium-doped TiO 2 for quantum emitters

Erbium-doped TiO 2 materials are promising candidates for advancing quantum technologies, necessitating a thorough understanding of their electronic and crystal structures to tailor their properties and enhance coherence times. This study explored epitaxial erbium-doped rutile TiO 2 films deposited on r-sapphire substrates using molecular beam epitaxy. Photoluminescence excitation spectroscopy demonstrated decreasing photoluminescence lifetimes with erbium doping, indicating limited optical coherence times. Lattice distortions associated with Er 3+ were probed by x-ray absorption spectroscopy, indicating that erbium primarily occupies Ti 4+ sites and influences oxygen vacancies. Significant lattice distortions in the higher-order shells and apparent full coordination around erbium suggest that additional defects are likely prevalent in these regions. These findings indicate that defects contribute to limited coherence times by introducing alternative decay pathways, leading to shorter photoluminescence lifetimes.

Crystallographic defects↗
DOE OSTI2025

Multitaper Magnitude‐Squared Coherence for Time Series With Missing Data: Understanding Oscillatory Processes Traced by Multiple Observables

To explore the hypothesis of a common source of variability in two time series, observers may estimate the magnitude-squared coherence (MSC), which is a frequency-domain view of the cross correlation. For time series that do not have uniform observing cadence, MSC can be estimated using Welch's overlapping segment averaging. However, multitaper has superior statistical properties to Welch's method in terms of the tradeoff between bias, variance, and bandwidth. The classical multitaper technique has recently been extended to accommodate time series with underlying uniform observing cadence from which some observations are missing. This situation is common for solar and geomagnetic data sets, which may have gaps due to breaks in satellite coverage, instrument downtime, or poor observing conditions. We demonstrate the scientific use of missing-data multitaper magnitude-squared coherence by detecting known solar mid-term oscillations in simultaneous, missing-data time series of solar Lyman α flux and geomagnetic Disturbance Storm Time index. Due to their superior statistical properties, we recommend that multitaper methods be used for all heliospheric time series with underlying uniform observing cadence.

Astro-statistics techniques (1886)↗
DOE OSTI2024

Correlating aluminum layer deposition rates, Josephson junction microstructure, and superconducting qubits’ performance

An ultra-thin aluminum oxide layer is a key component for Josephson junctions (JJ) in superconducting quantum bits (qubits). This layer serves as a barrier layer for Cooper pairs tunneling between the superconducting electrodes and significantly influences the overall performance of the junction. In this study, we investigate the impact of aluminum deposition rates on the microstructure and chemical variation of the aluminum oxide layer, as well as the device's yields and qubits’ lifetimes. Surprisingly, although the oxide layer roughness and thickness variation improve noticeably with an increasing Al deposition rate from 0.5 Å/s to 5 Å/s, the qubit's coherence time is almost unchanged. We attribute this to the fact that Cooper pairs tunnel only through the thinnest region of the barrier. Additionally, we revealed a stress-induced grain boundary sliding short-circuit failure mechanism in the JJ. Our discovery provides a vital understanding of oxide microstructure and JJ functionality, which are critical for improving the coherence time of superconducting qubits.

Coherence time↗
DOE OSTI2025

Increasing Radon Daughter Plate-out onto Superconducting Qubits

Radon daughter plate-out may pose a fundamental limitation to realizing large-scale superconducting quantum computers. Radon daughters plated-out onto a superconducting qubit chip can decay, releasing energy into the substrate, producing phonons. Phonon induced quasiparticles generate bit flip errors and reduce coherence times. We seek to increase plate-out on chips to enable future study of the impact of Radon on coherence times. In this project, we utilize COMSOL Multiphysics simulations to finalize design for a device to increase radon plate-out.

Kay, Dax [Northwestern U.]↗
DOE OSTI2026

Increasing Radon Daughter Plate-out onto Superconducting Qubits

Radon daughter plate-out may pose a fundamental limitation to realizing large-scale superconducting quantum computers. Radon daughters plated-out on a superconducting qubit can decay, releasing energy into the substrate, producing phonons. Phonon-induced quasiparticles then generate bit-flip errors and reduce coherence times. We seek to increase plate-out on chips to enable future study of the impact of radon on coherence times. In this project, we utilize COMSOL Multiphysics simulations to visualize electric field configuration and model ion transport to finalize design of a device to increase radon plate-out on qubit chips.

Kay, Dax [Fermilab; Northwestern U.]↗
DOE OSTI2026

Enhancing Coherence Limits in Superconducting Quantum Systems for Computing and Sensing

This talk will highlight recent efforts at the SQMS Center to develop qudit-based quantum computing architectures using superconducting three-dimensional (3D) cavities, as well as the use of these ultra-coherent cavities for quantum sensing. I will present systematic studies of materials and devices aimed at identifying and mitigating the dominant sources of decoherence—including two-level systems (TLS), quasiparticles, and other noise mechanisms—in both transmons and 3D cavities. These investigations include microwave loss characterization of niobium, tantalum, aluminum, their native oxides, and substrate materials such as silicon and sapphire. By combining measurements on qubits and cavities, we disentangle subsystem-specific loss mechanisms and establish a hierarchy of mitigation strategies, leading to transmon coherence times exceeding one millisecond. I will also discuss studies of quasiparticle dynamics, including quasiparticle bursts observed in qubits operated both above ground and at the Gran Sasso underground laboratory, and the observation that applied magnetic fields can suppress temporal T₁ fluctuations. Building on these advances, we demonstrate a record-coherence two-cell cavity-qudit system with coherence times exceeding 20 milliseconds. Leveraging tunable sideband interactions together with error-resilient protocols, including measurement-based error correction and post-selection, we achieve high-fidelity quantum state control, including the preparation of Fock states up to N=20 with fidelities above 95% and the generation of high-fidelity two-mode entangled states. Finally, I will discuss how these ultra-coherent quantum systems are enabling emerging quantum sensing applications, including searches for dark matter and gravitational waves.

Roy, Tanay [Fermilab] (ORCID:000000019442862X)↗
DOE OSTI2025

Pushing the Boundaries of Coherence in Superconducting Quantum Systems at the SQMS Center for Computing, Sensing, and Metrology

This talk will highlight recent advances at the SQMS Center in understanding and mitigating decoherence in superconducting quantum systems, focusing on both transmon qubits and 3D superconducting cavities. I will present results from systematic studies of materials and devices, aimed at identifying key sources of loss: including two-level systems (TLS), quasiparticles, and other noise mechanisms. These studies span microwave loss characterization in materials such as niobium, tantalum, aluminum, and their native oxides, as well as substrate losses in silicon and sapphire. Using both qubits and cavities, we disentangle subsystem contributions to loss and develop a hierarchy of mitigation strategies. Through these efforts, we have achieved transmon coherence times exceeding one millisecond. I will also present investigations into quasiparticle dynamics, including bursts observed in qubits located above ground and in the Gran Sasso underground laboratory. Additional studies explore temperature dependence to distinguish between TLS and quasiparticle losses, and reveal that applying a magnetic field can reduce temporal T1 fluctuations in transmons. Building on these results, we demonstrate a record-coherence two-cell cavity-qudit system with coherence times exceeding 20 milliseconds. By leveraging sideband interactions and error-resilient protocols including measurement-based correction and post-selection, we achieve high-fidelity quantum state control. This includes preparation of Fock states up to N = 20 with fidelities over 95%, and two-mode entanglement with coherence-limited fidelities reaching 99.9% after post-selection. These achievements position the SQMS platform as a powerful foundation for scalable quantum information processing and high-dimensional qudit encodings. Finally, I will discuss how these ultra-coherent systems are being deployed in emerging quantum sensing applications, including dark matter searches and gravitational wave detection.

Roy, Tanay [Fermilab]↗
DOE OSTI2026

Spin-transparent storage rings for quantum computing

Spin-transparent storage rings, where any spin direction repeats after one full turn, can be used in conjunction with ion traps as a new quantum computing platform [1]. Advantages of spin-transparent rings for quantum computing include: large numbers of stored qubits; long quantum coherence times of up to several hours; long storage lifetimes; and room temperature operation. These exceptional qualities mean rings could provide a scalable way to implement algorithms with deep complexity requiring many quantum operations while simultaneously providing a large number of qubits. This new platform where the qubit has long quantum coherence time can also be used as a quantum sensor or a part of a quantum memory.

Accelerator Physics↗