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
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.
Quantum SWAP gate realized with CZ and iSWAP gates in a superconducting architecture
It is advantageous for any quantum processor to support different classes of two-qubit quantum logic gates when compiling quantum circuits, a property that is typically not seen with
existing platforms. In particular, access to a gate set that includes support for the CZ-type, the iSWAP-type, and the SWAP-type families of gates, renders conversions between these gate families unnecessary during compilation as any two-qubit Clifford gate can be executed using at most one two-qubit gate from this set, plus additional single-qubit gates. We experimentally demonstrate that a SWAP gate can be decomposed into one iSWAP gate followed by one CZ gate, affirming a more efficient compilation strategy over the conventional approach that relies on three iSWAP or three CZ gates to replace a SWAP gate. Our implementation makes use of a superconducting quantum processor design based on fixed-frequency transmon qubits coupled together by a parametrically modulated tunable transmon coupler, extending this platform’s native gate set so that any two-qubit Clifford unitary matrix can be realized using no more than two two-qubit gates and single-qubit gates.
18
Dez
2024
99.9%-fidelity in measuring a superconducting qubit
Despite the significant progress in superconducting quantum computation over the past years, quantum state measurement still lags nearly an order of magnitude behind quantum gate operations
in speed and fidelity. The main challenge is that the strong coupling and readout signal used to probe the quantum state may also introduce additional channels which may cause qubit state transitions. Here, we design a novel architecture to implement the long-sought longitudinal interaction scheme between qubits and resonators. This architecture not only provides genuine longitudinal interaction by eliminating residual transversal couplings, but also introduces proper nonlinearity to the resonator that can further minimize decay error and measurement-induced excitation error. Our experimental results demonstrate a measurement fidelity of 99.8% in 202 ns without the need for any first-stage amplification. After subtracting the residual preparation errors, the pure measurement fidelity is above 99.9%. Our scheme is compatible with the multiplexing readout scheme and can be used for quantum error correction.
17
Dez
2024
High dynamic-range quantum sensing of magnons and their dynamics using a superconducting qubit
Magnons can endow quantum devices with new functionalities. Assessing their potential requires precise characterization of magnon properties. Here, we use a superconducting qubit to
probe magnons in a ferrimagnet over a range of about 2000 excitations. Using qubit control and parametrically induced qubit-magnon interactions we demonstrate few-excitation sensitive detection of magnons and are able to accurately resolve their decay. These results introduce quantum circuits as high-dynamic range probes for magnons and provide an avenue toward sensitive detection of nontrivial magnon dynamics.
16
Dez
2024
Conveyor-belt superconducting quantum computer
The processing unit of a solid-state quantum computer consists in an array of coupled qubits, each locally driven with on-chip microwave lines that route carefully-engineered control
signals to the qubits in order to perform logical operations. This approach to quantum computing comes with two major problems. On the one hand, it greatly hampers scalability towards fault-tolerant quantum computers, which are estimated to need a number of qubits — and, therefore driving lines — on the order of 106. On the other hand, these lines are a source of electromagnetic noise, exacerbating frequency crowding and crosstalk, while also contributing to power dissipation inside the dilution fridge. We here tackle these two overwhelming challenges by presenting a novel quantum processing unit (QPU) for a universal quantum computer which is globally (rather than locally) driven. Our QPU relies on a string of superconducting qubits with always-on ZZ interactions, enclosed into a closed geometry, which we dub „conveyor belt“. Strikingly, this architecture requires only (N) physical qubits to run a computation on N computational qubits, in contrast to previous (N2) proposals for global quantum computation. Additionally, universality is achieved via the implementation of single-qubit gates and a one-shot Toffoli gate. The ability to perform multi-qubit operations in a single step could vastly improve the fidelity and execution time of many algorithms.
Fast single-qubit gates for continuous dynamically decoupled systems
Environmental noise that couples longitudinally to a quantum system dephases that system and can limit its coherence lifetime. Performance using quantum superposition in clocks, information
processors, communication networks, and sensors depends on careful state and external field selection to lower sensitivity to longitudinal noise. In many cases time varying external control fields–such as the Hahn echo sequence originally developed for nuclear magnetic resonance applications–can passively correct for longitudinal errors. There also exist continuous versions of passive correction called continuous dynamical decoupling (CDD), or spin-locking depending on context. However, treating quantum systems under CDD as qubits has not been well explored. Here, we develop universal single-qubit gates that are „fast“ relative to perturbative Rabi gates and applicable to any CDD qubit architecture. We demonstrate single-qubit gates with fidelity =0.9947(1) on a frequency tunable CDD transmon superconducting circuit operated where it is strongly sensitive to longitudinal noise, thus establishing this technique as a potentially useful tool for operating qubits in applications requiring high fidelity under non-ideal conditions.
Gatemon Qubit Revisited for Improved Reliability and Stability
The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building novel superconducting
devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit. However, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times. Here we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1 MHz precision over a range of several GHz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design.
15
Dez
2024
Fabrication of low-loss Josephson parametric devices
Superconducting circuits incorporating Josephson elements represent a promising hardware platform for quantum technologies. Potential applications include scalable quantum computing,
microwave quantum networks, and quantum-limited amplifiers. However, progress in Josephson junction-based quantum technologies is facing the ongoing challenge of minimizing loss channels. This is also true for parametric superconducting devices based on nonlinear Josephson resonators. In this work, we report on the fabrication and characterization of low-loss Josephson parametric devices operated in the GHz frequency range, showing record internal quality factors. Specifically, we achieve internal quality factors significantly above 105 for both Josephson parametric converters and the Josephson parametric amplifiers at low microwave power ranging in the single-photon regime. These low-loss devices mark a significant step forward in realizing high-performance quantum circuits, enabling further advancements in superconducting quantum technologies.
A hybrid classical-quantum approach to highly constrained Unit Commitment problems
The unit commitment (UC) problem stands as a critical optimization challenge in the electrical power industry. It is classified as NP-hard, placing it among the most intractable problems
to solve. This paper introduces a novel hybrid quantum-classical algorithm designed to efficiently (approximately) solve the UC problem in polynomial time. In this approach, the UC problem is decomposed into two subproblems: a QUBO (Quadratic Unconstrained Binary Optimization) problem and a quadratic optimization problem. The algorithm employs the Quantum Approximate Optimization Algorithm (QAOA) to identify the optimal unit combination and classical methods to determine individual unit powers. The proposed hybrid algorithm is the first to include both the spinning reserve constraint (thus improving its applicability to real-world scenarios) and to explore QAOA warm-start optimization in this context. The effectiveness of this optimization was illustrated for specific instances of the UC problem, not only in terms of solution accuracy but also by reducing the number of iterations required for QAOA convergence. Hybrid solutions achieved using a single-layer warm-start QAOA (p=1) are within a 5.1 % margin of the reference (approximate) classical solution, while guaranteeing polynomial time complexity on the number of power generation units and time intervals.
11
Dez
2024
Interplay of coupling, residual, and quasiparticle losses for the frequency- and temperature-dependent quality factor of superconducting resonators
The overall, loaded quality factor QL quantifies the loss of energy stored in a resonator. Here we discuss on general grounds how QL of a planar microwave resonator made of a conventional
superconductor should depend on temperature and frequency. We consider contributions to QL due to dissipation by thermal quasiparticles (QQP), due to residual dissipation (QRes), and due to coupling (QC). We present experimental data obtained with superconducting stripline resonators fabricated from lead (Pb), with different center conductor widths and different coupling gaps. We probe the resonators at various harmonics between 0.7 GHz and 6 GHz and at temperatures between 1.5 K and 7 K. We find a strongly frequency- and temperature-dependent QL, which we can describe by a lumped-element model. For certain resonators at lowest temperatures we observe a maximum in the frequency-dependent QL when QRes and QC match, and here the measured QL can exceed 2×105.