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
21
Mä
2022
Dynamics of Transmon Ionization
Qubit measurement and control in circuit QED rely on microwave drives, with higher drive amplitudes ideally leading to faster processes. However, degradation in qubit coherence time
and readout fidelity has been observed even under moderate drive amplitudes corresponding to few photons populating the measurement resonator. Here, we numerically explore the dynamics of a driven transmon-resonator system under strong and nearly resonant measurement drives, and find clear signatures of transmon ionization where the qubit escapes out of its cosine potential. Using a semiclassical model, we interpret this ionization as resulting from resonances occurring at specific resonator photon populations. We find that the photon populations at which these spurious transitions occur are strongly parameter dependent and that they can occur at low resonator photon population, something which may explain the experimentally observed degradation in measurement fidelity.
18
Mä
2022
Conditional coherent control with superconducting artificial atoms
Controlling the flow of quantum information is a fundamental task for quantum computers, which is unpractical to realize on classical devices. Coherent devices which can process quantum
states are thus required to route the quantum states yielding the information. In this paper we demonstrate experimentally the smallest quantum transistor for superconducting processors, composed of collector and emitter qubits, and the coupler. The interaction strength between the collector and emitter is controlled by tuning the frequency and the state of the gate qubit, effectively implementing a quantum switch. From the truth-table measurement (open-gate fidelity 93.38%, closed-gate fidelity 98.77%), we verify the high performance of the quantum transistor. We also show that taking into account the third energy level of the qubits is critical to achieving a high-fidelity transistor. The presented device has a strong potential for quantum information processes in superconducting platforms.
17
Mä
2022
Compact superconducting microwave resonators based on Al-AlOx-Al capacitor
We address the scaling-up problem for superconducting quantum circuits by using lumped-element resonators based on a new fabrication method of aluminum — aluminum oxide —
aluminum (Al/AlOx/Al) parallel-plate capacitors. The size of the resonators is only 0.04 mm2, which is more than one order smaller than the typical size of coplanar resonators (1 mm2). The fabrication method we developed easily fits into the standard superconducting qubits fabrication process. We have obtained capacitance per area 14 fF/μm2 and the internal quality factor 1×103−8×103 at the single-photon level. Our results show that such devices based on Al/AlOx/Al capacitors could be further applied to the qubit readout scheme, including resonators, filters, amplifiers, as well as microwave metamaterials and novel types of qubits, such as 0−π qubit.
16
Mä
2022
Potential Nanoscale Sources of Decoherence in Niobium based Transmon Qubit Architectures
Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated remarkable improvements in recent
years, further improvements in performance through materials engineering will aid in large-scale deployment. Here, we use information retrieved from electron microscopy and analysis to conduct a detailed assessment of potential decoherence sources in transmon qubit test devices. In the niobium thin film, we observe the presence of localized strain at interfaces, which may amplify interactions between two-level systems and impose limits on T1 and T2 relaxation times. Additionally, we observe the presence of a surface oxide with varying stoichiometry and bond distances, which can generate a broad two-level system noise spectrum. Finally, a similarly disordered and rough interface is observed between Nb and the Si substrate. We propose that this interface can also degrade the overall superconducting properties.
Flux Qubit Based on Hybrid Ferromagnetic-Superconducting Device
We propose a realization of flux qubit based on the hybrid ferromagnetic-superconducting device where the flux bias is induced purely by vector potential of the vanishing magnetic field.
We support our conclusions with theoretical analysis and self-consistent three-dimensional simulations for material specific parameters.
15
Mä
2022
Combating fluctuations in relaxation times of fixed-frequency transmon qubits with microwave-dressed states
With the long coherence time, fixed-frequency transmon qubit is a promising qubit modality for quantum computing. Currently, diverse qubit architectures that utilize fixed-frequency
transmon qubits have been demonstrated with high-fidelity gate performance. Nevertheless, the relaxation times of transmon qubits can have large temporal fluctuations, causing instabilities in gate performance. The fluctuations are often believed to be caused by nearly on-resonance couplings with sparse two-level-system (TLS) defects. To mitigate their impact on qubit coherence and gate performance, one direct approach is to tune the qubits away from these TLSs. In this work, to combat the potential TLS-induced performance fluctuations in a tunable-bus architecture unitizing fixed-frequency transmon qubits, we explore the possibility of using an off-resonance microwave drive to effectively tuning the qubit frequency through the ac-Stark shift while implementing universal gate operations on the microwave-dressed qubit. We show that the qubit frequency can be tuned up to 20 MHz through the ac-stark shift while keeping minimal impacts on the qubit control. Besides passive approaches that aim to remove these TLSs through more careful treatments of device fabrications, this work may offer an active approach towards mitigating the TLS-induced performance fluctuations in fixed-frequency transmon qubit devices.
14
Mä
2022
Effective non-local parity-dependent couplings in qubit chains
For the efficient implementation of quantum algorithms, practical ways to generate many-body entanglement are a basic requirement. Specifically, coupling multiple qubit pairs at once
can be advantageous and can lead to multi-qubit operations useful in the construction of hardware-tailored algorithms. Here we harness the simultaneous coupling of qubits on a chain and engineer a set of non-local parity-dependent quantum operations suitable for a wide range of applications. The resulting effective long-range couplings directly implement a parametrizable Trotter-step for Jordan-Wigner fermions and can be used for simulations of quantum dynamics, efficient state generation in variational quantum eigensolvers, parity measurements for error-correction schemes, and the generation of efficient multi-qubit gates. Moreover, we present numerical simulations of the gate operation in a superconducting quantum circuit architecture, which show a high gate fidelity of >99.9% for realistic experimental parameters.
Fluctuation Spectroscopy of Two-Level Systems in Superconducting Resonators
Superconducting quantum computing is experiencing a tremendous growth. Although major milestones have already been achieved, useful quantum-computing applications are hindered by a
variety of decoherence phenomena. Decoherence due to two-level systems (TLSs) hosted by amorphous dielectric materials is ubiquitous in planar superconducting devices. We use high-quality quasilumped element resonators as quantum sensors to investigate TLS-induced loss and noise. We perform two-tone experiments with a probe and pump electric field; the pump is applied at different power levels and detunings. We measure and analyze time series of the quality factor and resonance frequency for very long time periods, up to 1000 h. We additionally carry out simulations based on the TLS interacting model in presence of a pump field. We find that loss and noise are reduced at medium and high power, matching the simulations, but not at low power.
13
Mä
2022
Phonon downconversion to suppress correlated errors in superconducting qubits
Quantum error correction can preserve quantum information in the presence of local errors; however, errors that are correlated across a qubit array are fatal. For superconducting qubits,
high-energy particle impacts due to background radioactivity or cosmic ray muons produce bursts of energetic phonons that travel throughout the substrate and create excitations out of the superconducting ground state, known as quasiparticles, which poison all qubits on the chip. Here we use thick normal metal reservoirs on the back side of the chip to promote rapid downconversion of phonons to sufficiently low energies where they can no longer poison qubits. We introduce a pump-probe scheme involving controlled injection of pair-breaking phonons into the qubit chips. We examine quasiparticle poisoning on chips with and without backside metallization and demonstrate a reduction in the flux of pair-breaking phonons by more than a factor of 20. In addition, we use a Ramsey interferometer scheme to simultaneously monitor quasiparticle parity on three qubits for each chip and observe a two-order of magnitude reduction in correlated poisoning due to ambient radiation. Our approach reduces correlated errors due to background radiation below the level necessary for fault-tolerant operation of a multiqubit array.
Quasiparticle Poisoning of Superconducting Qubits from Resonant Absorption of Pair-breaking Photons
The ideal superconductor provides a pristine environment for the delicate states of a quantum computer: because there is an energy gap to excitations, there are no spurious modes with
which the qubits can interact, causing irreversible decay of the quantum state. As a practical matter, however, there exists a high density of excitations out of the superconducting ground state even at ultralow temperature; these are known as quasiparticles. Observed quasiparticle densities are of order 1~μm−3, tens of orders of magnitude larger than the equilibrium density expected from theory. Nonequilibrium quasiparticles extract energy from the qubit mode and induce discrete changes in qubit offset charge, a potential source of dephasing. Here we show that a dominant mechanism for quasiparticle poisoning in superconducting qubits is direct absorption of high-energy photons at the qubit junction. We use a Josephson junction-based photon source to controllably dose qubit circuits with millimeter-wave radiation, and we use an interferometric quantum gate sequence to reconstruct the charge parity on the qubit island. We find that the structure of the qubit itself acts as a resonant antenna for millimeter-wave radiation, providing an efficient path for photons to generate quasiparticle excitations. A deep understanding of this physics will pave the way to realization of next-generation superconducting qubits that are robust against quasiparticle poisoning and could enable a new class of quantum sensors for dark matter detection.