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
03
Jun
2025
Superconducting integrated random access quantum memory
Microwave quantum memory represents a critical component for the development of quantum repeaters and resource-efficient quantum processors. We report the experimental realization of
a novel architecture of superconducting random access quantum memory with cycling storage time, achieved through pulsed control of an RF-SQUID coupling element. The device demonstrates a memory cycle time of 1.51 μs and achieves 57.5\% fidelity with preservation of the input pulse shape during the first retrieval interval for near-single-photon level excitations, with subsequent exponential decay characterized by a time constant of 11.44 μs. This performance represents a several-fold improvement over previously reported implementations. Crucially, we establish that while the proposed active coupler realization introduces no measurable fidelity degradation, the primary limitation arises from impedance matching imperfections. These results highlight the potential of proposed architecture for quantum memory applications while identifying specific avenues for near-unity storage fidelity.
Floquet-Engineered Fast SNAP gates in weakly coupled cQED systems
Superconducting cavities with high quality factors, coupled to a fixed-frequency transmon, provide a state-of-the-art platform for quantum information storage and manipulation. The
commonly used selective number-dependent arbitrary phase (SNAP) gate faces significant challenges in ultra-high-coherence cavities, where the weak dispersive shifts necessary for preserving high coherence typically result in prolonged gate times. Here, we propose a protocol to achieve high-fidelity SNAP gates that are orders of magnitude faster than the standard implementation, surpassing the speed limit set by the bare dispersive shift. We achieve this enhancement by dynamically amplifying the dispersive coupling via sideband interactions, followed by quantum optimal control on the Floquet-engineered system. We also present a unified perturbation theory that explains both the gate acceleration and the associated benign drive-induced decoherence, corroborated by Floquet-Markov simulations. These results pave the way for the experimental realization of high-fidelity, selective control of weakly coupled, high-coherence cavities, and expanding the scope of optimal control techniques to a broader class of Floquet quantum systems.
Cavity-mediated cross-cross-resonance gate
We propose a cavity-mediated gate between two transmon qubits or other nonlinear superconducting elements. The gate is realized by driving both qubits at a frequency that is near-resonant
with the frequency of the cavity. Since both qubits are subject to a cross-resonant drive, we call this gate a cross-cross-resonance gate. In close analogy with gates between trapped-ion qubits, in phase space, the state of the cavity makes a circle whose area depends on the state of the two qubits, realizing a controlled-phase gate. We propose two schemes for canceling the dominant error, which is the dispersive coupling. We also show that this cross-cross-resonance gate allows one to realize simultaneous gates between multiple pairs of qubits coupled via the same metamaterial composed of an array of coupled cavities or other linear mediators.
Ultracoherent superconducting cavity-based multiqudit platform with error-resilient control
Superconducting radio-frequency (SRF) cavities offer a promising platform for quantum computing due to their long coherence times and large accessible Hilbert spaces, yet integrating
nonlinear elements like transmons for control often introduces additional loss. We report a multimode quantum system based on a 2-cell elliptical shaped SRF cavity, comprising two cavity modes weakly coupled to an ancillary transmon circuit, designed to preserve coherence while enabling efficient control of the cavity modes. We mitigate the detrimental effects of the transmon decoherence through careful design optimization that reduces transmon-cavity couplings and participation in the dielectric substrate and lossy interfaces, to achieve single-photon lifetimes of 20.6 ms and 15.6 ms for the two modes, and a pure dephasing time exceeding 40 ms. This marks an order-of-magnitude improvement over prior 3D multimode memories. Leveraging sideband interactions and novel error-resilient protocols, including measurement-based correction and post-selection, we achieve high-fidelity control over quantum states. This enables the preparation of Fock states up to N=20 with fidelities exceeding 95%, the highest reported to date to the authors‘ knowledge, as well as two-mode entanglement with coherence-limited fidelities reaching up to 99.9% after post-selection. These results establish our platform as a robust foundation for quantum information processing, allowing for future extensions to high-dimensional qudit encodings.
Exceeding the Parametric Drive Strength Threshold in Nonlinear Circuits
Superconducting quantum circuits rely on strong drives to implement fast gates, high-fidelity readout, and state stabilization. However, these drives can induce uncontrolled excitations,
so-called „ionization“, that compromise the fidelity of these operations. While now well-characterized in the context of qubit readout, it remains unclear how general this limitation is across the more general setting of parametric control. Here, we demonstrate that a nonlinear coupler, exemplified by a transmon, undergoes ionization under strong parametric driving, leading to a breakdown of coherent control and thereby limiting the accessible gate speeds. Through experiments and numerical simulations, we associate this behavior with the emergence of drive-induced chaotic dynamics, which we characterize quantitatively using the instantaneous Floquet spectrum. Our results reveal that the Floquet spectrum provides a unifying framework for understanding strong-drive limitations across a wide range of operations on superconducting quantum circuits. This insight establishes fundamental constraints on parametric control and offers design principles for mitigating drive-induced decoherence in next-generation quantum processors.
02
Jun
2025
Demonstrating magnetic field robustness and reducing temporal T1 noise in transmon qubits through magnetic field engineering
The coherence of superconducting transmon qubits is often disrupted by fluctuations in the energy relaxation time (T1), limiting their performance for quantum computing. While background
magnetic fields can be harmful to superconducting devices, we demonstrate that both trapped magnetic flux and externally applied static magnetic fields can suppress temporal fluctuations in T1 without significantly degrading its average value or qubit frequency. Using a three-axis Helmholtz coil system, we applied calibrated magnetic fields perpendicular to the qubit plane during cooldown and operation. Remarkably, transmon qubits based on tantalum-capped niobium (Nb/Ta) capacitive pads and aluminum-based Josephson junctions (JJs) maintained T1 lifetimes near 300 {\mu}s even when cooled in fields as high as 600 mG. Both trapped flux up to 600 mG and applied fields up to 400 mG reduced T1 fluctuations by more than a factor of two, while higher field strengths caused rapid coherence degradation. We attribute this stabilization to the polarization of paramagnetic impurities, the role of trapped flux as a sink for non-equilibrium quasiparticles (QPs), and partial saturation of fluctuating two-level systems (TLSs). These findings challenge the conventional view that magnetic fields are inherently detrimental and introduce a strategy for mitigating noise in superconducting qubits, offering a practical path toward more stable and scalable quantum systems.
30
Mai
2025
Strongly driven transmon as an incoherent noise source
Under strong drives, which are becoming necessary for fast high-fidelity operations, transmons can be structurally unstable. Due to chaotic effects, the computational manifold is no
longer well separated from the remainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. We show here that these detrimental effects can further propagate to other degrees of freedom, for example to neighboring qubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise to another circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noise spectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certain limit, the interaction with the driven transmon can be modeled as a stochastic diffusive process driving the TLS.
29
Mai
2025
Fluxonium as a control qubit for bosonic quantum information
Bosonic codes in superconducting resonators are a hardware-efficient avenue for quantum error correction and can harness the favorable error hierarchies provided by long-lived cavities
compared to typical superconducting qubits. These benefits can be negated, however, by the necessary coupling to an ancillary control qubit, which often induces highly detrimental effects such as excess decoherence and undesired nonlinearities. It is thus an important question whether a qubit-cavity coupling can be realized that avoids such effects. Here, we investigate the fluxonium as control qubit, motivated by its long lifetime and controllability of Hamiltonian parameters that suggest an avenue toward controlled elimination of undesired nonlinearities. In a proof-of-concept experiment we use the fluxonium to measure the coherence properties of a storage resonator and demonstrate the predictability of the cavity’s inherited nonlinearities from the fluxonium. We demonstrate universal control by preparing and characterizing resonator Fock states and their superpositions using selective number-dependent arbitrary phase gates. The fidelities of state preparation and tomography are accounted for by incoherent resonator decay errors in our planar prototype device. Finally, we predict that the fluxonium can achieve beneficial cavity-coupling regimes compared to the transmon, with the potential to eliminate undesirable cavity nonlinearities. These results demonstrate the potential of the fluxonium as a high-performance bosonic control qubit for superconducting cavities.
27
Mai
2025
Probing the quantum motion of a macroscopic mechanical oscillator with a radio-frequency superconducting qubit
Long-lived mechanical resonators like drums oscillating at MHz frequencies and operating in the quantum regime offer a powerful platform for quantum technologies and tests of fundamental
physics. Yet, quantum control of such systems remains challenging, particularly owing to their low energy scale and the difficulty of achieving efficient coupling to other well-controlled quantum devices. Here, we demonstrate repeated, and high-fidelity interactions between a 4 MHz suspended silicon nitride membrane and a resonant superconducting heavy-fluxonium qubit. The qubit is initialized at an effective temperature of 27~μK and read out in a single-shot with 77% fidelity. During the membrane’s 6~ms lifetime, the two systems swap excitations more than 300 times. After each interaction, a state-selective detection is performed, implementing a stroboscopic series of weak measurements that provide information about the mechanical state. The accumulated records reconstruct the membrane’s position noise-spectrum, revealing both its thermal occupation nth≈47 at 10~mK and the qubit-induced back-action. By preparing the qubit either in its ground or excited state before each interaction, we observe an imbalance between the emission and absorption spectra, proportional to nth and nth+1, respectively-a hallmark of the non-commutation of phonon creation and annihilation operators. Since the predicted Diósi-Penrose gravitational collapse time is comparable to the measured mechanical decoherence time, our architecture enters a regime where gravity-induced decoherence could be tested directly.
26
Mai
2025
Parasitic RF-SQUIDs in superconducting qubits due to wirebonds
Superconducting qubits show great promise to realize practical quantum computers from micro-fabricated integrated circuits. However, their solid-state architecture bears the burden
of parasitic modes in qubit materials and the control circuitry which cause decoherence and interfere with qubits. Here, we present evidence that wirebonds, which are used to contact the micro-circuits and to realize chip-to-chip airbridges, may contain parasitic Josephson junctions. In our experiment, such a junction was enclosed in a superconducting loop and so gave rise to the formation an RF-SQUID which interfered with a nearby flux-tunable transmon qubit. Periodic signatures observed in magnetic field sweeps revealed a strong AC-dispersive coupling of the parasitic RF-SQUID to both the qubit and its readout resonator, in addition to the DC-inductive coupling between RF-SQUID and qubit. Our finding sheds light on a previously unknown origin of decoherence due to parasitic Josephson junctions in superconducing circuits.