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
27
Feb
2025
Experimental realization of a quantum heat engine based on dissipation-engineered superconducting circuits
Quantum heat engines (QHEs) have attracted long-standing scientific interest, especially inspired by considerations of the interplay between heat and work with the quantization of energy
levels, quantum superposition, and entanglement. Operating QHEs calls for effective control of the thermal reservoirs and the eigenenergies of the quantum working medium of the engine. Although superconducting circuits enable accurate engineering of controlled quantum systems, beneficial in quantum computing, this framework has not yet been employed to experimentally realize a cyclic QHE. Here, we experimentally demonstrate a quantum heat engine based on superconducting circuits, using a single-junction quantum-circuit refrigerator (QCR) as a two-way tunable heat reservoir coupled to a flux-tunable transmon qubit acting as the working medium of the engine. We implement a quantum Otto cycle by a tailored drive on the QCR to sequentially induce cooling and heating, interleaved with flux ramps that control the qubit frequency. Utilizing single-shot qubit readout, we monitor the evolution of the qubit state during several cycles of the heat engine and measure positive output powers and efficiencies that agree with our simulations of the quantum evolution. Our results verify theoretical models on the thermodynamics of quantum heat engines and advance the control of dissipation-engineered thermal environments. These proof-of-concept results pave the way for explorations on possible advantages of QHEs with respect to classical heat engines.
Digital Simulation of Non-Abelian Parafermions in Superconducting Circuits
Parafermions, which can be viewed as a fractionalized version of Majorana modes, exhibit non-Abelian statistics and emerge in topologically ordered systems, while their realization
in experiment has been challenging. Here we propose an experimental scheme for the digital simulation of parafermions and their non-Abelian braiding in superconducting circuits by realizing the ℤd plaquette model on a two-dimensional lattice. Two protocols using quantum circuits and non-destructive measurements are introduced to prepare the ground state, on which the parafermion pairs are created by engineering dislocations. We further develop a generalized code deformation approach to realize the fusion and non-Abelian braiding statistics of parafermion modes, in which the concrete example for d=3 parafermions is studied in detail. We also examine the real parameter regime to confirm the feasibility in superconducting devices. This work extends previous methods for twist defects in superconducting qubits to qudit systems, and may open up a way for parafermion-based high-dimensional topological quantum computing with experimental feasibility.
26
Feb
2025
Scalable Low-overhead Superconducting Non-local Coupler with Exponentially Enhanced Connectivity
Quantum error correction codes with non-local connections such as quantum low-density parity-check (qLDPC) incur lower overhead and outperform surface codes on large-scale devices.
These codes are not applicable on current superconducting devices with nearest-neighbor connections. To rectify the deficiency in connectivity of superconducting circuit system, we experimentally demonstrate a convenient on-chip coupler of centimeters long and propose an extra coupler layer to map the qubit array to a binary-tree connecting graph. This mapping layout reduces the average qubit entangling distance from O(N) to O(logN), demonstrating an exponentially enhanced connectivity with eliminated crosstalk. The entangling gate with the coupler is performed between two fluxonium qubits, reaching a fidelity of 99.37 % while the system static ZZ rate remains as low as 144 Hz without active cancellation or circuit parameter targeting. With the scalable binary tree structure and high-fidelity non-local entanglement, novel quantum algorithms can be implemented on the superconducting qubit system, positioning it as a strong competitor to other physics systems regarding circuit connectivity.
25
Feb
2025
Enhancing Intrinsic Quality Factors Approaching 10 Million in Superconducting Planar Resonators via Spiral Geometry
This study investigates the use of spiral geometry in superconducting resonators to achieve high intrinsic quality factors, crucial for applications in quantum computation and quantum
sensing. We fabricated Archimedean Spiral Resonators (ASRs) using domain-matched epitaxially grown titanium nitride (TiN) on silicon wafers, achieving intrinsic quality factors of Qi=(9.6±1.5)×106 at the single-photon level and Qi=(9.91±0.39)×107 at high power, significantly outperforming traditional coplanar waveguide (CPW) resonators.
We conducted a comprehensive numerical analysis using COMSOL to calculate surface participation ratios (PRs) at critical interfaces: metal-air, metal-substrate, and substrate-air. Our findings reveal that ASRs have lower PRs than CPWs, explaining their superior quality factors and reduced coupling to two-level systems (TLSs).
Demolition measurement protocol for transmon qubits
The process of measuring a qubit and re-initializing it to the ground state practically lead to long qubit idle times between re-runs of experiments on a superconducting quantum computer.
Here, we propose a protocol for a demolition measurement of a transmon qubit that integrates qubit readout with the reset process to minimize qubit idle time. We present a three-staged implementation of this protocol, involving a combined qubit readout and resonator reset scheme that unconditionally resets the resonator at the end of the readout; a leakage removal scheme that can be integrated with the measurement stage; and an unconditional qubit reset. We demonstrate that this protocol could be implemented in 1μs with greater than 95% reset fidelity and a 99% readout fidelity without any hardware overhead beyond those commonly used. This provides at least a tenfold speed-up compared to the passive decay of the qubit, hence significantly increasing the data-acquisition rate.
24
Feb
2025
High quality superconducting tantalum resonators with beta phase defects
For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials.
Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from α-tantalum, avoiding the meta-stable β-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing β-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors (Qi) up to (5.0±2.5)×106 in the single photon regime. They outperform resonators with no sign of the β-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of β-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.
21
Feb
2025
Non-degenerate noise-resilient superconducting qubit
We propose a superconducting qubit based on engineering the first and second harmonics of the Josephson energy and phase relation EJ1cosφ and EJ2cos2φ. By constructing a circuit such
that EJ2 is negative and |EJ1|≪|EJ2|, we create a periodic potential with two non-degenerate minima. The qubit, which we dub „harmonium“, is formed from the lowest-energy states of each minimum. Bit-flip protection of the qubit arises due to the localization of each qubit state to their respective minima, while phase-flip protection can be understood by considering the circuit within the Born-Oppenheimer approximation. We demonstrate with time-domain simulations that single- and two-qubit gates can be performed in approximately one hundred nanoseconds. Finally, we compute the qubit coherence times using numerical diagonalization of the complete circuit in conjunction with state-of-the-art noise models. We estimate out-of-manifold heating times on the order of milliseconds, which can be treated as erasure errors using conventional dispersive readout. We estimate pure-dephasing times on the order of many tens of milliseconds, and bit-flip times on the order of seconds.
EDA-Q: Electronic Design Automation for Superconducting Quantum Chip
Electronic Design Automation (EDA) plays a crucial role in classical chip design and significantly influences the development of quantum chip design. However, traditional EDA tools
cannot be directly applied to quantum chip design due to vast differences compared to the classical realm. Several EDA products tailored for quantum chip design currently exist, yet they only cover partial stages of the quantum chip design process instead of offering a fully comprehensive solution. Additionally, they often encounter issues such as limited automation, steep learning curves, challenges in integrating with actual fabrication processes, and difficulties in expanding functionality. To address these issues, we developed a full-stack EDA tool specifically for quantum chip design, called EDA-Q. The design workflow incorporates functionalities present in existing quantum EDA tools while supplementing critical design stages such as device mapping and fabrication process mapping, which users expect. EDA-Q utilizes a unique architecture to achieve exceptional scalability and flexibility. The integrated design mode guarantees algorithm compatibility with different chip components, while employing a specialized interactive processing mode to offer users a straightforward and adaptable command interface. Application examples demonstrate that EDA-Q significantly reduces chip design cycles, enhances automation levels, and decreases the time required for manual intervention. Multiple rounds of testing on the designed chip have validated the effectiveness of EDA-Q in practical applications.
20
Feb
2025
Randomized benchmarking of a high-fidelity remote CNOT gate over a meter-scale microwave interconnect
In the modular superconducting quantum processor architecture, high-fidelity, meter-scale microwave interconnect between processor modules is a key technology for extending system size
beyond constraints imposed by device manufacturing equipment, yield, and signal delivery. While there have been many demonstrations of remote state transfer between modules, these relied on tomographic experiments for benchmarking, but this technique does not reliably separate State Preparation And Measurement (SPAM) error from error per state transfer. Recent developments based on randomized benchmarking provide a compatible theory for separating these two errors. In this work, we present a module-to-module interconnect based on Tunable-Coupling Qubits (TCQs) and benchmark, in a SPAM error tolerant manner, a remote state transfer fidelity of 0.988 across a 60cm long coplanar waveguide (CPW). The state transfer is implemented via superadiabatic transitionless driving method, which suppresses intermediate excitation in internal modes of CPW. We also introduce the frame tracking technique to correct unintended qubit phase rotations before and after the state transfers, which enables the SPAM-error-tolerant benchmarking of the state transfers. We further propose and construct a remote CNOT gate between modules, composed of local CZ gates in each module and remote state transfers, and report a high gate fidelity of 0.933 using randomized benchmarking method. The remote CNOT construction and benchmarking we present is a more complete metric that fully characterizes the module to module link operation going forward as it more closely represents interconnect operation in a circuit.
Experimental demonstrations of Josephson threshold detectors for broadband microwave photons detection
Current-biased Josephson junctions (CBJJs) have been demonstrated as sensitive Josephson threshold detectors (JTDs) in the infrared range. In this letter, we show this kind of detector
could also be used to detect broadband microwave photons. Based on the numerical simulations of the noise-driving phase dynamics of an underdamped Josephson junction, driven by the low-frequency triangular wave current, we argue that the microwave photons flowing across the JJ can be detected by probing the voltage switched signals of the JJ. Experimentally, we designed and fabricated the relevant Al/AlOx/Al Josephson device and measured its response to microwave photons at 50~mK temperature. Experimental results indicate that the weak microwave signals could be threatened as the additional noises modify the phase dynamics of the CBJJ, which could thus be detected by the generated JTD. The detection sensitivity is characterized by using the Kumar-Caroll index to differentiate the junction switched duration distributions, with and without microwave signal input. Although the demonstrated detection sensitivity is just −92~dBm (corresponding to approximately 30~photon/ns) for the microwave photons at ∼5GHz (which is manifestly deviated from the plasma frequency of the fabricated JJ), we argued, based on the relevant numerical simulations, that the generated JTD could be used to achieve the sensitive detection of the microwave photons at the plasma frequency of the JJ.