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
29
Jun
2020
Improved Quantum State Tomography for Superconducting Quantum Computing Systems
Quantum device characterization via state tomography plays an important role in both validating quantum hardware and processing quantum information, unfortunately with the exponential
number of the measurements. As one of the main stream quantum platforms, superconducting quantum computing (SQC) system at least requires 3n measurement settings consisted of single-qubit readout operators in reconstructing a n-qubit state. In this work, I add the 2-qubit evolutions as the readout operators, and propose an optimal tomographic scheme with the cost-reduced measurements using the integer programming. In detail, I present the minimum number of required measurements to fully reconstruct a state for the Nearest-Neighbor, 2-Dimensional, and All-to-All connectivities on SQC qubits. It is shown that this method can reduce the number of measurements by over 60% compared with the previous state tomography on SQC systems. It is expected that the experimentalist from SQC field can directly utilize the ready-made results for reconstructing quantum states involved in their research. Besides, this method can be applied to reduce the complexity of traditional state tomography in some quantum platforms including but not limited to SQC systems.
23
Jun
2020
Topological photon pairs in a superconducting quantum metamaterial
Recent discoveries in topological physics hold a promise for disorder-robust quantum systems and technologies. Topological states provide the crucial ingredient of such systems featuring
increased robustness to disorder and imperfections. Here, we use an array of superconducting qubits to engineer a one-dimensional topologically nontrivial quantum metamaterial. By performing microwave spectroscopy of the fabricated array, we experimentally observe the spectrum of elementary excitations. We find not only the single-photon topological states but also the bands of exotic bound photon pairs arising due to the inherent anharmonicity of qubits. Furthermore, we detect the signatures of the two-photon bound edge-localized state which hints towards interaction-induced localization in our system. Our work demonstrates an experimental implementation of the topological model with attractive photon-photon interaction in a quantum metamaterial.
21
Jun
2020
Experimental Observation of Tensor Monopoles with a Superconducting Qudit
Monopoles play a center role in gauge theories and topological matter. Examples of monopoles include the Dirac monopole in 3D and Yang monopole in 5D, which have been extensively studied
and observed in condensed matter or artificial systems. However, tensor monopoles in 4D are less studied, and their observation has not been reported. Here we experimentally construct a tunable spin-1 Hamiltonian to generate a tensor monopole and then measure its unique features with superconducting quantum circuits. The energy structure of a 4D Weyl-like Hamiltonian with three-fold degenerate points acting as tensor monopoles is imaged. Through quantum-metric measurements, we report the first experiment that measures the Dixmier-Douady invariant, the topological charge of the tensor monopole. Moreover, we observe topological phase transitions characterized by the topological Dixmier-Douady invariant, rather than the Chern numbers as used for conventional monopoles in odd-dimensional spaces.
High-fidelity, high-scalability two-qubit gate scheme for superconducting qubits
High-quality two-qubit gate operations are crucial for scalable quantum information processing. Often, the gate fidelity is compromised when the system becomes more integrated. Therefore,
a low-error-rate, easy-to-scale two-qubit gate scheme is highly desirable. Here, we experimentally demonstrate a new two-qubit gate scheme that exploits fixed-frequency qubits and a tunable coupler in a superconducting quantum circuit. The scheme requires less control lines, reduces crosstalk effect, simplifies calibration procedures, yet produces a controlled-Z gate in 30ns with a high fidelity of 99.5%. Error analysis shows that gate errors are mostly coherence-limited. Our demonstration paves the way for large-scale implementation of high-fidelity quantum operations.
18
Jun
2020
Demonstration of an All-Microwave Controlled-Phase Gate between Far Detuned Qubits
A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit addressability, qubit-qubit coupling strength, circuit complexity
and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and benefit from high coherence but suffer from frequency crowding and limited addressability in multi-qubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transversely coupled transmon qubits which are far detuned compared to the qubit anharmonicity. The gate is activated by applying a single, strong microwave tone to one of the qubits, inducing a coupling between the two-qubit |f,g⟩ and |g,e⟩ states, with |g⟩, |e⟩, and |f⟩ denoting the lowest energy states of a transmon qubit. Interleaved randomized benchmarking yields a gate fidelity of 97.5±0.3% at a gate duration of 126ns, with the dominant error source being decoherence. We model the gate in presence of the strong drive field using Floquet theory and find good agreement with our data. Our gate constitutes a promising alternative to present two-qubit gates and could have hardware scaling advantages in large-scale quantum processors as it neither requires additional drive lines nor tunable couplers.
Fast, Lifetime-Preserving Readout for High-Coherence Quantum Annealers
We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit
(CSFQ). By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that T1 of the CSFQ is not impacted by Purcell loss from its low-Q readout resonator (Qe=760) despite being detuned by only 40 MHz. When annealed, the QFP amplifies the qubit’s persistent current signal such that it generates a flux qubit-state-dependent frequency shift of 85 MHz in the readout resonator, which is over 9 times its linewidth. The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing 98.6% with only 80 ns integration, and reaches fidelities of 99.6% when integrated for 1 μs. This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.
Process tomography of Robust Dynamical Decoupling in Superconducting Qubits
Dynamical decoupling is a technique that protects qubits against noise, the ability to preserve quantum coherence in the presence of noise is essential for the development of quantum
devices. Here the Rigetti quantum computing platform was used to test different dynamical decoupling sequences. The performance of the sequences was characterized by quantum process tomography and analyzed using the quantum channels formalism. It is shown that dynamical decoupling does not merely change the coherence times of the qubit, instead, it creates an effective environment. As in early dynamical decoupling experiments, it is also shown here that the performance of the sequences can be limited by pulse imperfections. However, the performance can be improved using robust sequences, i.e. dynamical decoupling sequences that are robust against experimental imperfections.
17
Jun
2020
Simulating and mitigating crosstalk
We describe an efficient and scalable framework for modeling crosstalk effects on quantum information processors. By applying optimal control techniques, we show how to tuneup arbitrary
high-fidelity parallel operations on systems with substantial local and nonlocal crosstalk. Simulations show drastically lower error rates for a 2D square array of 100 superconducting transmon qubits. These results suggest that rather than striving to engineer away undesirable interactions during fabrication, we can largely mitigate their effects through careful characterization and control optimization.
A superconducting circuit realization of combinatorial gauge symmetry
We propose a superconducting wire array that realizes a family of quantum Hamiltonians that possess combinatorial gauge symmetry — a local symmetry where monomial transformations
play a central role. This physical system exhibits a rich structure. In the classical limit its ground state consists of two superimposed spin liquids; one is a crystal of small loops containing disordered U(1) degrees of freedom, and the other is a soup of loops of all sizes associated to Z2 topological order. We show that the classical results carry over to the quantum case when fluctuations are gradually tuned via the wire capacitances, yielding Z2 quantum topological order. In an extreme quantum limit where the capacitances are all small, we arrive at an effective quantum spin Hamiltonian that we conjecture would sustain Z2 quantum topological order with a gap of the order of the Josephson coupling in the array. The principles behind the construction for superconducting arrays extends to other bosonic and fermionic systems, and offers a promising path towards topological qubits and the study of other many-body systems.
15
Jun
2020
Voltage-tunable superconducting resonators: a platform for random access quantum memory
In quantum computing architectures, one important factor is the trade-off between the need to couple qubits to each other and to an external drive and the need to isolate them well
enough in order to protect the information for an extended period of time. In the case of superconducting circuits, one approach is to utilize fixed frequency qubits coupled to coplanar waveguide resonators such that the system can be kept in a configuration that is relatively insensitive to noise. Here, we propose a scalable voltage-tunable quantum memory (QuMem) design concept compatible with superconducting qubit platforms. Our design builds on the recent progress in fabrication of Josephson field effect transistors (JJ-FETs) which use InAs quantum wells. The JJ-FET is incorporated into a tunable coupler between a transmission line and a high-quality resonator in order to control the overall inductance of the coupler. A full isolation of the high-quality resonator can be achieved by turning off the JJ-FET. This could allow for long coherence times and protection of the quantum information inside the storage cavity. The proposed design would facilitate the implementation of random access memory for storage of quantum information in between computational gate operations.