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
30
Jan
2024
Qplacer: Frequency-Aware Component Placement for Superconducting Quantum Computers
Noisy Intermediate-Scale Quantum (NISQ) computers face a critical limitation in qubit numbers, hindering their progression towards large-scale and fault-tolerant quantum computing.
A significant challenge impeding scaling is crosstalk, characterized by unwanted interactions among neighboring components on quantum chips, including qubits, resonators, and substrate. We motivate a general approach to systematically resolving multifaceted crosstalks in a limited substrate area. We propose Qplacer, a frequency-aware electrostatic-based placement framework tailored for superconducting quantum computers, to alleviate crosstalk by isolating these components in spatial and frequency domains alongside compact substrate design. Qplacer commences with a frequency assigner that ensures frequency domain isolation for qubits and resonators. It then incorporates a padding strategy and resonator partitioning for layout flexibility. Central to our approach is the conceptualization of quantum components as charged particles, enabling strategic spatial isolation through a ‚frequency repulsive force‘ concept. Our results demonstrate that Qplacer carefully crafts the physical component layout in mitigating various crosstalk impacts while maintaining a compact substrate size. On device topology benchmarks, Qplacer can reduce the required area for theoretical crosstalk-free layout by 2.61x and 2.25x on average, compared to the results of manual design and classical placement engines, respectively.
29
Jan
2024
Closed and open superconducting microwave waveguide networks as a model for quantum graphs
We report on high-precision measurements that were performed with superconducting waveguide networks with the geometry of a tetrahedral and a honeycomb graph. They consist of junctions
of valency three that connect straight rectangular waveguides of incommensurable lengths. The experiments were performed in the frequency range of a single transversal mode, where the associated Helmholtz equation is effectively one dimensional and waveguide networks may serve as models of quantum graphs with the joints and waveguides corresponding to the vertices and bonds. The tetrahedral network comprises T junctions, while the honeycomb network exclusively consists of Y junctions, that join waveguides with relative angles 90 degree and 120 degree, respectively. We demonstrate that the vertex scattering matrix, which describes the propagation of the modes through the junctions strongly depends on frequency and is non-symmetric at a T junction and thus differs from that of a quantum graph with Neumann boundary conditions at the vertices. On the contrary, at a Y junction, similarity can be achieved in a certain frequeny range. We investigate the spectral properties of closed waveguide networks and fluctuation properties of the scattering matrix of open ones and find good agreement with random matrix theory predictions for the honeycomb waveguide graph.
Experimental Demonstration of Thermodynamics of Three-level Quantum Heat Engine using Superconducting Quantum Circuits
The three-level system represents the smallest quantum system capable of autonomous cycling in quantum heat engines. This study proposes a method to demonstrate the actual thermodynamics
of a three-level quantum heat engine by designing and implementing superconducting quantum circuits. Following error mitigation, the outcomes from the quantum circuit model designed in this study, when executed on a real quantum device, closely align with theoretical predictions, thereby validating the effectiveness of the circuit model. This study offers a novel approach for investigating three-level quantum heat engines, enabling the verification of theoretical research findings while also reducing the complexity and cost of experimental procedures.
26
Jan
2024
Low loss hybrid Nb/Au superconducting resonators for quantum circuit applications
Superconducting resonators play a crucial role in developing forthcoming quantum computing schemes. The complete integration of molecular spin-based quantum bits with superconducting
resonators still requires further developments, notably in maintaining low-loss resonances and high quality factors. In this work, we have developed a superconducting device combining a niobium (Nb) circuit with a 10 nm gold (Au) capping layer, which supports low microwave losses and enables new functionalities such as the integration of magnetic molecules into solid-state devices. Our investigation across a wide temperature and driving power range reveals that adding the Au layer reduces the density of two-level system (TLS) defects present in the device. Moreover, the presence of the thin Au layer induces a higher kinetic inductance at low temperatures, leading to enhanced responsivity. Cryogenic characterization confirms the good performance of the device, allowing these resonators to serve as platforms for hybrid devices involving molecular spin qubits/gates where the gold can anchor alkyl thiol groups to form self-assembled monolayers. Our findings suggest the potential of Nb/Au lumped element resonators (LERs) as versatile and promising tools for advancing superconducting quantum technologies and the integration of quantum functionalities into solid-state devices.
Many-excitation removal of a transmon qubit using a single-junction quantum-circuit refrigerator and a two-tone microwave drive
Achieving fast and precise initialization of qubits is a critical requirement for the successful operation of quantum computers. The combination of engineered environments with all-microwave
techniques has recently emerged as a promising approach for the reset of superconducting quantum devices. In this work, we experimentally demonstrate the utilization of a single-junction quantum-circuit refrigerator (QCR) for an expeditious removal of several excitations from a transmon qubit. The QCR is indirectly coupled to the transmon through a resonator in the dispersive regime, constituting a carefully engineered environmental spectrum for the transmon. Using single-shot readout, we observe excitation stabilization times down to roughly 500 ns, a 20-fold speedup with QCR and a simultaneous two-tone drive addressing the e-f and f0-g1 transitions of the system. Our results are obtained at a 48-mK fridge temperature and without postselection, fully capturing the advantage of the protocol for the short-time dynamics and the drive-induced detrimental asymptotic behavior in the presence of relatively hot other baths of the transmon. We validate our results with a detailed Liouvillian model truncated up to the three-excitation subspace, from which we estimate the performance of the protocol in optimized scenarios, such as cold transmon baths and fine-tuned driving frequencies. These results pave the way for optimized reset of quantum-electric devices using engineered environments and for dissipation-engineered state preparation.
Superconducting flux qubit operating at zero magnetic field
The operation of a conventional superconducting flux qubit requires the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the
qubit loop, which makes the scaling of quantum circuits based on this type of qubits difficult. It has been proposed that, by inducing a pi phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as pi-junction, it is possible to realize a flux qubit operating at zero magnetic flux. We report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic pi-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. With further improvements in the materials of the ferromagnetic junction, the zero-flux-biased flux qubits can become a promising platform for quantum computing.
24
Jan
2024
Entanglement assisted probe of the non-Markovian to Markovian transition in open quantum system dynamics
We utilize a superconducting qubit processor to experimentally probe the transition from non-Markovian to Markovian dynamics of an entangled qubit pair. We prepare an entangled state
between two qubits and monitor the evolution of entanglement over time as one of the qubits interacts with a small quantum environment consisting of an auxiliary transmon qubit coupled to its readout cavity. We observe the collapse and revival of the entanglement as a signature of quantum memory effects in the environment. We then engineer the non-Markovianity of the environment by populating its readout cavity with thermal photons to show a transition from non-Markovian to Markovian dynamics, reaching a regime where the quantum Zeno effect creates a decoherence-free subspace that effectively stabilizes the entanglement between the qubits.
22
Jan
2024
Observation of discrete charge states of a coherent two-level system in a superconducting qubit
We report observations of discrete charge states of a coherent dielectric two-level system (TLS) that is strongly coupled to an offset-charge-sensitive superconducting transmon qubit.
We measure an offset charge of 0.072e associated with the two TLS eigenstates, which have a transition frequency of 2.9 GHz and a relaxation time exceeding 3 ms. Combining measurements in the strong dispersive and resonant regime, we quantify both transverse and longitudinal couplings of the TLS-qubit interaction. We further perform joint tracking of TLS transitions and quasiparticle tunneling dynamics but find no intrinsic correlations. This study demonstrates microwave-frequency TLS as a source of low-frequency charge noise.
Geometric Phase of a Transmon in a Dissipative Quantum Circuit
Superconducting circuits reveal themselves as promising physical devices with multiple uses. Within those uses, the fundamental concept of the geometric phase accumulated by the state
of a system shows up recurrently, as, for example, in the construction of geometric gates. Given this framework, we study the geometric phases acquired by a paradigmatic setup: a transmon coupled to a superconductor resonating cavity. We do so both for the case in which the evolution is unitary and when it is subjected to dissipative effects. These models offer a comprehensive quantum description of an anharmonic system interacting with a single mode of the electromagnetic field within a perfect or dissipative cavity, respectively. In the dissipative model, the non-unitary effects arise from dephasing, relaxation, and decay of the transmon coupled to its environment. Our approach enables a comparison of the geometric phases obtained in these models, leading to a thorough understanding of the corrections introduced by the presence of the environment.
19
Jan
2024
Spectral signatures of non-trivial topology in a superconducting circuit
Topology, like symmetry, is a fundamental concept in understanding general properties of physical systems. In condensed matter systems, non-trivial topology may manifest itself as singular
features in the energy spectrum or the quantization of observable quantities such as electrical conductance and magnetic flux. Using microwave spectroscopy, we show that a superconducting circuit with three Josephson tunnel junctions in parallel can possess energy degeneracies indicative of \emph{intrinsic} non-trivial topology. We identify three topological invariants, one of which is related to a hidden quantum mechanical supersymmetry. Depending on fabrication parameters, devices are gapless or not, and fall on a simple phase diagram which is shown to be robust to perturbations including junction imperfections, asymmetry, and inductance. Josephson tunnel junction circuits, which are readily fabricated with conventional microlithography techniques, allow access to a wide range of topological systems which have no condensed matter analog. Notable spectral features of these circuits, such as degeneracies and flat bands, may be leveraged for quantum information applications, whereas quantized transport properties could be useful for metrology applications.