I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
19
Mä
2025
2D transmons with lifetimes and coherence times exceeding 1 millisecond
Materials improvements are 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 utilizing tantalum (Ta) as a base layer and sapphire as a substrate. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q) exceeding 1.5 x 10^7, reaching a maximum Q of 2.5 x 10^7, corresponding to a lifetime (T_1) of up to 1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T_1. We achieve these material improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single qubit gates with 99.994% fidelity. The Ta-on-Si platform comprises a simple material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale quantum processors.
18
Mä
2025
Identifying Materials-Level Sources of Performance Variation in Superconducting Transmon Qubits
The Superconducting Materials and Systems (SQMS) Center, a DOE National Quantum Information Science Research Center, has conducted a comprehensive and coordinated study using superconducting
transmon qubit chips with known performance metrics to identify the underlying materials-level sources of device-to-device performance variation. Following qubit coherence measurements, these qubits of varying base superconducting metals and substrates have been examined with various nondestructive and invasive material characterization techniques at Northwestern University, Ames National Laboratory, and Fermilab as part of a blind study. We find trends in variations of the depth of the etched substrate trench, the thickness of the surface oxide, and the geometry of the sidewall, which when combined, lead to correlations with the T1 lifetime across different devices. In addition, we provide a list of features that varied from device to device, for which the impact on performance requires further studies. Finally, we identify two low-temperature characterization techniques that may potentially serve as proxy tools for qubit measurements. These insights provide materials-oriented solutions to not only reduce performance variations across neighboring devices, but also to engineer and fabricate devices with optimal geometries to achieve performance metrics beyond the state-of-the-art values.
A Cascaded Random Access Quantum Memory
Dynamic random access memory is critical to classical computing but notably absent in experimental quantum computers. Here we realize an 8-bit cascaded random access quantum memory
using superconducting circuits and cavities and showcase the ability to perform arbitrary gate operations on it. In addition to individual error channels such as photon loss, quantum memories can also experience decoherence from many-body self-interaction. We characterize the origin and contributions of many-body infidelity throughout the memory cycle. We find that individual modes can be accessed with ≲1.5% infidelity per mode and that the entire memory can be accessed in arbitrary order with an error rate below the depolarization threshold of the surface code, paving the way for fault-tolerant quantum memories.
Demonstration of High-Fidelity Entangled Logical Qubits using Transmons
Quantum error correction (QEC) codes are necessary to fault-tolerantly operate quantum computers. However, every such code is inherently limited by its inability to detect logical errors.
Here, we propose and implement a method that leverages dynamical decoupling (DD) to drastically suppress logical errors. The key to achieving this is to use the logical operators of the QEC code as DD pulses, which we refer to as logical dynamical decoupling (LDD). The resulting hybrid QEC-LDD strategy is in principle capable of handling arbitrary weight errors. We test QEC-LDD using IBM transmon devices and the [[4,2,2]] code, demonstrating performance that significantly exceeds the capabilities of using either this code or DD in isolation. We present a method that allows for the detection of logical errors affecting logically encoded Bell states, which, in this case, arise primarily from crosstalk among physical qubits. Building on this, we experimentally demonstrate high-fidelity entangled logical qubits.
Protected phase gate for the 0-π qubit using its internal modes
Protected superconducting qubits such as the 0-π qubit promise to substantially reduce physical error rates through a multi-mode encoding. This protection comes at the cost of controllability,
as standard techniques for quantum gates are ineffective. We propose a protected phase gate for the 0-π qubit that utilises an internal mode of the circuit as an ancilla. The gate is achieved by varying the qubit-ancilla coupling via a tunable Josephson element. Our scheme is a modified version of a protected gate proposed by Brooks, Kitaev and Preskill that uses an external oscillator as an ancilla. We find that our scheme is compatible with the protected regime of the 0-π qubit, and does not suffer from spurious coupling to additional modes of the 0-π circuit. Through numerical simulations, we study how the gate error scales with the circuit parameters of the 0-π qubit and the tunable Josephson element that enacts the gate.
17
Mä
2025
Reversing Hydrogen-Related Loss in α-Ta Thin Films for Quantum Device Fabrication
α-Tantalum (α-Ta) is an emerging material for superconducting qubit fabrication due to the low microwave loss of its stable native oxide. However, hydrogen absorption during fabrication,
particularly when removing the native oxide, can degrade performance by increasing microwave loss. In this work, we demonstrate that hydrogen can enter α-Ta thin films when exposed to 10 vol% hydrofluoric acid for 3 minutes or longer, leading to an increase in power-independent ohmic loss in high-Q resonators at millikelvin temperatures. Reduced resonator performance is likely caused by the formation of non-superconducting tantalum hydride (TaHx) precipitates. We further show that annealing at 500°C in ultra-high vacuum (10−8 Torr) for one hour fully removes hydrogen and restores the resonators‘ intrinsic quality factors to ~4 million at the single-photon level. These findings identify a previously unreported loss mechanism in α-Ta and offer a pathway to reverse hydrogen-induced degradation in quantum devices based on Ta and, by extension also Nb, enabling more robust fabrication processes for superconducting qubits.
Optimizing the frequency positioning of tunable couplers in a circuit QED processor to mitigate spectator effects on quantum operations
We experimentally optimize the frequency of flux-tunable couplers in a superconducting quantum processor to minimize the impact of spectator transmons during quantum operations (single-qubit
gates, two-qubit gates and readout) on other transmons. We adapt a popular transmon-like tunable-coupling element, achieving high-fidelity, low-leakage controlled-Z gates with unipolar, fast-adiabatic pulsing only on the coupler. We demonstrate the ability of the tunable coupler to null residual ZZ coupling as well as exchange couplings in the one- and two-excitation manifolds. However, the nulling of these coherent interactions is not simultaneous, prompting the exploration of tradeoffs. We present experiments pinpointing spectator effects on specific quantum operations. We also study the combined effect on the three types of operations using repeated quantum parity measurements.
Low-loss Nb on Si superconducting resonators from a dual-use spintronics deposition chamber and with acid-free post-processing
Magnetic impurities are known to degrade superconductivity. For this reason, physical vapor deposition chambers that have previously been used for magnetic materials have generally
been avoided for making high-quality superconducting resonator devices. In this article, we show by example that such chambers can be used: with Nb films sputtered in a chamber that continues to be used for magnetic materials, we demonstrate compact (3 {\mu}m gap) coplanar waveguide resonators with low-power internal quality factors near one million. We achieve this using a resist strip bath with no post-fabrication acid treatment, which results in performance comparable to previous strip baths with acid treatments. We also find evidence that this improved resist strip bath provides a better surface chemical template for post-fabrication hydrogen fluoride processing. These results are consistent across three Si substrate preparation methods, including a \SI{700}{\celsius} anneal.
Realizing a Symmetry Protected Topological Phase in a Superconducting Circuit
We propose a superconducting quantum circuit whose low-energy degrees of freedom are described by the sine-Gordon (SG) quantum field theory. For suitably chosen parameters,
the circuit
hosts a symmetry protected topological (SPT) phase protected by a discrete ℤ2 symmetry. The ground state of the system is twofold degenerate and exhibits local spontaneous symmetry breaking of the ℤ2 symmetry close to the edges of the circuit, leading to spontaneous localized edge supercurrents. The ground states host Majorana zero modes (MZM) at the edges of the circuit. On top of each of the two ground states, the system exhibits localized bound states at both edges, which are topologically protected against small disorder in the bulk. The spectrum of these boundary excitations should be observable in a circuit-QED experiment with feasible parameter choices.
Mixed spin-boson coupling for qubit readout with suppressed residual shot-noise dephasing
Direct dipole coupling between a two-level system and a bosonic mode describes the interactions present in a wide range of physical platforms. In this work, we study a coupling that
is mixed between two pairs of quadratures of a bosonic mode and a spin. In this setting, we can suppress the dispersive shift while retaining a nonzero Kerr shift, which remarkably results in a cubic relationship between shot noise dephasing and thermal photons in the oscillator. We demonstrate this configuration with a simple toy model, quantify the expected improvements to photon shot-noise dephasing of the spin, and describe an approach to fast qubit readout via the Kerr shift. Further, we show how such a regime is achievable in superconducting circuits because magnetic and electric couplings can be of comparable strength, using two examples: the Cooper pair transistor and the fluxonium molecule.