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
12
Jan
2025
Realization of tilted Dirac-like microwave cone in superconducting circuit lattices
Dirac-like band crossings are paradigms in condensed matter systems to emulate high-energy physics phenomena. They are associated with two aspects: gap and tilting. The ability to design
sign-changing gap gives rise to band topology, whereas the tilting of band crossings which is a gateway for large gravity-like effects remains uncharted. In this work, we introduce an experimental platform to realize tilted Dirac-like microwave cone in large-scale superconducting circuit lattices. The direction and magnitude of the tilt can be controlled by engineering the axially preferred second neighbor coupling. We demonstrate three lattices with 731-site LC resonator featuring tilt values of up to 59% of relative difference in the opposite-direction group velocities. This is obtained by reconstructing the density of states (DOS) of measured microwave resonance frequencies. Harnessing the tilt of Dirac-like band crossings lays the foundation for weaving the fabric of an emergent solid-state spacetime.
10
Jan
2025
Dissipating quartets of excitations in a superconducting circuit
Over the past decade, autonomous stabilization of bosonic qubits has emerged as a promising approach for hardware-efficient protection of quantum information. However, applying these
techniques to more complex encodings than the Schrödinger cat code requires exquisite control of high-order wave mixing processes. The challenge is to enable specific multiphotonic dissipation channels while avoiding unintended non-linear interactions. In this work, we leverage a genuine six-wave mixing process enabled by a near Kerr-free Josephson element to enforce dissipation of quartets of excitations in a high-impedance superconducting resonator. Owing to residual non-linearities stemming from stray inductances in our circuit, this dissipation channel is only effective when the resonator holds a specific number of photons. Applying it to the fourth excited state of the resonator, we show an order of magnitude enhancement of the state decay rate while only marginally impacting the relaxation and coherence of lower energy states. Given that stray inductances could be strongly reduced through simple modifications in circuit design and that our methods can be adapted to activate even higher-order dissipation channels, these results pave the way toward the dynamical stabilization of four-component Schrödinger cat qubits and even more complex bosonic qubits.
09
Jan
2025
Efficient Qubit Calibration by Binary-Search Hamiltonian Tracking
We present a real-time method for calibrating the frequency of a resonantly driven qubit. The real-time processing capabilities of a controller dynamically compute adaptive probing
sequences for qubit-frequency estimation. Each probing time and drive frequency are calculated to divide the prior probability distribution into two branches, following a locally optimal strategy that mimics a conventional binary search. We show the algorithm’s efficacy by stabilizing a flux-tunable transmon qubit, leading to improved coherence and gate fidelity. By feeding forward the updated qubit frequency, the FPGA-powered control electronics also mitigates non-Markovian noise in the system, which is detrimental to quantum error correction. Our protocol highlights the importance of feedback in improving the calibration and stability of qubits subject to drift and can be readily applied to other qubit platforms.
08
Jan
2025
Realizing Lattice Surgery on Two Distance-Three Repetition Codes with Superconducting Qubits
Quantum error correction is needed for quantum computers to be capable of fault-tolerantly executing algorithms using hundreds of logical qubits. Recent experiments have demonstrated
subthreshold error rates for state preservation of a single logical qubit. In addition, the realization of universal quantum computation requires the implementation of logical entangling gates. Lattice surgery offers a practical approach for implementing such gates, particularly in planar quantum processor layouts. In this work, we demonstrate lattice surgery between two distance-three repetition-code qubits by splitting a single distance-three surface-code qubit. Using a quantum circuit fault-tolerant to bit-flip errors, we achieve an improvement in the value of the decoded ZZ logical two-qubit observable compared to a similar non-encoded circuit. By preparing the surface-code qubit in initial states parametrized by a varying polar angle, we evaluate the performance of the lattice surgery operation for non-cardinal states on the logical Bloch sphere and employ logical two-qubit tomography to reconstruct the Pauli transfer matrix of the operation. In this way, we demonstrate the functional building blocks needed for lattice surgery operations on larger-distance codes based on superconducting circuits.
07
Jan
2025
Four-body coupler for superconducting qubits based on Josephson parametric oscillators
We theoretically propose a circuit of the four-body coupler for superconducting qubits based on Josephson parametric oscillators (JPOs). Our coupler for the four-body interaction has
a superconducting loop, similar to a capacitively shunted flux qubit, where an external magnetic flux set to half a flux quantum is threaded. This coupler circuit is a specific setup of the circuit called superconducting nonlinear asymmetric inductive elements (SNAIL) and also is a generalization of the previously proposed one for the four-body interaction of JPOs. We clarify roles of circuit parameters in the four-body interaction and, in particular, show that the four-body coupling constant in our circuit can be significantly increased by tuning capacitance of the coupler or the area ratio of the Josephson junctions of the coupler.
Spin Environment of a Superconducting Qubit in High Magnetic Fields
Superconducting qubits equipped with quantum non-demolishing readout and active feedback can be used as information engines to probe and manipulate microscopic degrees of freedom, whether
intentionally designed or naturally occurring in their environment. In the case of spin systems, the required magnetic field bias presents a challenge for superconductors and Josephson junctions. Here we demonstrate a granular aluminum nanojunction fluxonium qubit (gralmonium) with spectrum and coherence resilient to fields beyond one Tesla. Sweeping the field reveals a paramagnetic spin-1/2 ensemble, which is the dominant gralmonium loss mechanism when the electron spin resonance matches the qubit. We also observe a suppression of fast flux noise in magnetic field, suggesting the freezing of surface spins. Using an active state stabilization sequence, the qubit hyperpolarizes long-lived two-level systems (TLSs) in its environment, previously speculated to be spins. Surprisingly, the coupling to these TLSs is unaffected by magnetic fields, leaving the question of their origin open. The robust operation of gralmoniums in Tesla fields offers new opportunities to explore unresolved questions in spin environment dynamics and facilitates hybrid architectures linking superconducting qubits with spin systems.
06
Jan
2025
Cavity-assisted quantum transduction between superconducting qubits and trapped atomic particles mediated by Rydberg levels
In this work, we present an approach for transferring quantum states from superconducting qubits to the internal states of trapped atoms or ions. To achieve state transfer, we utilize
the Rydberg levels of those trapped atomic particles, which interact indirectly with the superconducting qubits via their mutual coupling to a microwave cavity. We investigate two protocols for this transfer: resonant interaction between the cavity, the atom/ion and the superconducting qubit, and dispersive interaction. We analyze the robustness of the state transfer fidelity against various noise and damping mechanisms, including cavity decay, Rydberg state decay, and the relaxation and dephasing of the superconducting qubit. For experimentally demonstrated parameters of interaction strengths, dissipation, and dephasing, our scheme achieves fidelities above 95\%.
Compact superconducting vacuum-gap capacitors with low microwave loss and high mechanical coherence for scalable quantum circuits
Vacuum gap capacitors have recently gained considerable attention in superconducting circuit platforms due to their compact design and low dielectric losses in the microwave regime.
Their ability to support mechanical vibrational modes makes them ideal candidates for circuit optomechanics. However, precise control of gap size and achieving high coherence in mechanical modes remain long-standing challenges. Here, we present a detailed fabrication process for scalable vacuum gap capacitors that support ultra-high-coherence mechanical motion, exhibit low microwave loss, and maintain a small footprint compared to planar geometries. We fabricate arrays of up to 24 LC resonators, with capacitors featuring nanometer-scale gap size variations. We demonstrate that the mechanical quality factors can reach up to 40×106, a 100-fold improvement over other platforms, with microwave quality factors (105) at low photon number levels. This platform also achieves a sizable single-photon optomechanical coupling rate of approximately 20 Hz. Using this, we cooled the mechanical oscillator to its ground state (0.07 quanta) and squeezed its motion below the vacuum level by 2.7 dB. We further demonstrate the scalability of this platform by implementing large-scale optomechanical arrays, a strained graphene model, and observing quantum collective phenomena in a mechanical hexamer. These vacuum gap capacitors are promising candidates for coupling superconducting qubits with mechanical systems, serving as storage elements in quantum computing, and exploring gravitational effects on quantum mechanics.
Propagation velocity measurements of substrate phonon bursts using MKIDs for superconducting circuits
High-energy bursts in superconducting quantum circuits from various radiation sources have recently become a practical concern due to induced errors and their propagation in the chip.
The speed and distance of these disturbances have practical implications. We used a linear array of multiplexed MKIDs on a single silicon chip to measure the propagation velocity of a localized high-energy burst, introduced by driving a Normal metal- Insulator-Superconductor (NIS) junction. We observed a reduction in the apparent propagation velocity with NIS power, which is due to the combined effect of reduced phonon flux with distance and the existence of a minimum detectable QP density in the MKIDs. A simple theoretical model is fitted to extract the longitudinal phonon velocity in the substrate and the conversion efficiency of phonons to QPs in the superconductor.
19
Dez
2024
Multiplexed Readout of Superconducting Qubits Using a 3D Re-entrant Cavity Filter
Hardware efficient methods for high fidelity quantum state measurements are crucial for superconducting qubit experiments, as qubit numbers grow and feedback and state reset begin to
be employed for quantum error correction. We present a 3D re-entrant cavity filter designed for frequency-multiplexed readout of superconducting qubits. The cavity filter is situated out of the plane of the qubit circuit and capacitively couples to an array of on-chip readout resonators in a manner that can scale to large qubit arrays. The re-entrant cavity functions as a large-linewidth bandpass filter with intrinsic Purcell filtering. We demonstrate the concept with a four-qubit multiplexed device.