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
26
Apr
2020
Engineering Dynamical Sweet Spots to Protect Qubits from 1/f Noise
Protecting superconducting qubits from low-frequency noise is essential for advancing superconducting quantum computation. We here introduce a protocol for engineering dynamical sweet
spots which reduce the susceptibility of a qubit to low-frequency noise. Based on the application of periodic drives, the location of the dynamical sweet spots can be obtained analytically in the framework of Floquet theory. In particular, for the example of fluxonium biased slightly away from half a flux quantum, we predict an enhancement of pure-dephasing by three orders of magnitude. Employing the Floquet eigenstates as the computational basis, we show that high-fidelity single-qubit gates can be implemented while maintaining dynamical sweet-spot operation. We further confirm that qubit readout can be performed by adiabatically mapping the Floquet states back to the static qubit states, and subsequently applying standard measurement techniques. Our work provides an intuitive tool to encode quantum information in robust, time-dependent states, and may be extended to alternative architectures for quantum information processing.
24
Apr
2020
Dynamics and multiqubit entanglement in distant resonators
We consider the dynamics of the photon states in distant resonators coupled to a common bus resonator at different positions. The frequencies of distant resonators from a common bus
resonator are equally detuned. These frequency detunings are kept larger than the coupling strengths of each resonator to the common bus resonator to satisfy the dispersive interaction regime. In the dispersive regime, we show that the time dynamics of the system evolve to an arbitrary W-type state in a single step at various interaction times. Our results show that a one-step generation of arbitrary W-type states can be achieved with high fidelity in a system of superconducting resonators.
23
Apr
2020
Robust and Fast Holonomic Quantum Gates with Encoding on Superconducting Circuits
High-fidelity and robust quantum manipulation is the key for scalable quantum computation. Therefore, due to the intrinsic operational robustness, quantum manipulation induced by geometric
phases is one of the promising candidates. However, the longer gate time for geometric operations and more physical-implementation difficulties hinder its practical and wide applications. Here, we propose a simplified implementation of universal holonomic quantum gates on superconducting circuits with experimentally demonstrated techniques, which can remove the two main challenges by introducing the time-optimal control into the construction of quantum gates. Remarkably, our scheme is also based on a decoherence-free subspace encoding, with minimal physical qubit resource, which can further immune to error caused by qubit-frequency drift, which is regarded as the main error source for large scale superconducting circuits. Meanwhile, we deliberately design the quantum evolution to eliminate gate error caused by unwanted leakage sources. Therefore, our scheme is more robust than the conventional ones, and thus provides a promising alternative strategy for scalable fault-tolerant quantum computation.
22
Apr
2020
Experimental Realization of Universal Time-optimal non-Abelian Geometric Gates
Based on the geometrical nature of quantum phases, non-adiabatic holonomic quantum control (NHQC) has become a standard technique for enhancing robustness in constructing quantum gates.
However, the conventional approach of NHQC is sensitive to control instability, as it requires the driving pulses to cover a fixed pulse area. Furthermore, even for small-angle rotations, all operations need to be completed with the same duration of time. Here we experimentally demonstrate a time-optimal and unconventional approach of NHQC (called TOUNHQC), which can optimize the operation time of any holonomic gate. Compared with the conventional approach, TOUNHQC provides an extra layer of robustness to decoherence and control errors. The experiment involves a scalable architecture of superconducting circuit, where we achieved a fidelity of 99.51% for a single qubit gate using interleaved randomized benchmarking. Moreover, a two-qubit holonomic control-phase gate has been implemented where the gate error can be reduced by as much as 18% compared with NHQC.
21
Apr
2020
Nonadiabatic geometric quantum computation with optimal control on superconducting circuits
Quantum gates, which are the essential building blocks of quantum computers, are very fragile. Thus, to realize robust quantum gates with high fidelity is the ultimate goal of quantum
manipulation. Here, we propose a nonadiabatic geometric quantum computation scheme on superconducting circuits to engineer arbitrary quantum gates, which share both the robust merit of geometric phases and the capacity to combine with the optimal control technique to further enhance the gate robustness. Specifically, in our proposal, arbitrary geometric single-qubit gates can be realized on a transmon qubit, by a resonant microwave field driving, with both the amplitude and phase of the driving being time-dependent. Meanwhile, nontrivial two-qubit geometric gates can be implemented by two capacitively coupled transmon qubits, with one of the transmon qubits‘ frequency being modulated to obtain effective resonant coupling between them. Therefore, our scheme provides a promising step towards fault-tolerant solid-state quantum computation.
20
Apr
2020
Measuring and controlling radio-frequency quanta with superconducting circuits
In this PhD thesis, we will present the theoretical and experimental work that led to the realization of a radio-frequency circuit quantum electrodynamics system (RFcQED). In chapter
2, we provide a detailed derivation of the Hamiltonian of circuit QED formulated in the context of the Rabi model, and extract expressions for the cross-Kerr interaction. The resulting requirements for the coupling rate in RFcQED are discussed, one of them being the need to dramatically increase the coupling rate compared to typical circuit QED device. In chapter 3 we cover two experimental approaches to increasing the coupling in a circuit QED system, one making use of a high impedance resonator, the second utilizing a large coupling capacitor. In chapter 4, we combine these two approaches to implement RFcQED. Through strong dispersive coupling, we could measure individual photons in a megahertz resonator, demonstrate quantum control by cooling the resonator to the ground state or preparing Fock states, and finally observe with nanosecond resolution the re-thermalization of these states. In chapter 5 we present QuCAT or Quantum Circuit Analyzer Tool in Python, a software package that can be used for the design of circuit QED systems such as the one presented in this thesis. In chapter 6 we discuss how certain interplays between general relativity and quantum mechanics cannot be described using our current laws of physics. In particular, we show how radio-frequency mechanical oscillators are perfect candidates to perform experiments in this regime. In chapter 7 we present the prospects for coupling such mechanical oscillator to weakly anharmonic superconducting circuits such as the transmon qubit or RFcQED systems.
17
Apr
2020
Ramsey-biased spectroscopy of superconducting qubits under dispersion
We proposed a spectroscopic method that extends Ramsey’s atomic spectroscopy to detect the transition frequency of a qubit fabricated on a superconducting circuit. The method
uses a multi-interval train of qubit biases to implement an alternate resonant and dispersive couplings to an incident probe field. The consequent absorption spectrum of the qubit has a narrower linewidth at its transition frequency than that obtained from constantly biasing the qubit to resonance while the middle dispersive evolution incurs only a negligible shift in detected frequency. Modeling on transmon qubits, we find that the linewidth reduction reaches 23% and Ramsey fringes are simultaneously suppressed at extreme duration ratio of dispersion over resonance for a double-resonance scheme. If the scheme is augmented by an extra resonance segment, a further 37% reduction can be achieved.
16
Apr
2020
Emergent PT symmetry in a double-quantum-dot circuit QED set-up
Open classical and quantum systems with effective parity-time (PT) symmetry, over the past five years, have shown tremendous promise for advances in lasers, sensing, and non-reciprocal
devices. And yet, the microscopic origin of such effective, non-Hermitian models is not well understood. Here, we show that a non-Hermitian Hamiltonian emerges naturally in a double-quantum-dot-circuit-QED (DQD-circuit QED) set-up, which can be controllably tuned to the PT-symmetric point. This effective Hamiltonian, derived from a microscopic model for the set-up, governs the dynamics of two coupled circuit-QED cavities with a voltage-biased DQD in one of them. Our analysis also reveals the effect of quantum fluctuations on the PT symmetric system. The PT-transition is, then, observed both in the dynamics of cavity observables as well as via an input-output experiment. As a simple application of the PT-transition in this set-up, we show that loss-induced enhancement of amplification and lasing can be observed in the coupled cavities. Our results pave the way for an on-chip realization of a potentially scalable non-Hermitian system with a gain medium in quantum regime, as well as its potential applications for quantum technology.
Simultaneous excitation of two noninteracting atoms with time-frequency correlated photon pairs in a superconducting circuit
Here we report the first observation of simultaneous excitation of two noninteracting atoms by a pair of time-frequency correlated photons in a superconducting circuit. The strong coupling
regime of this process enables the synthesis of a three-body interaction Hamiltonian, which allows the generation of the tripartite Greenberger-Horne-Zeilinger state in a single step with a fidelity as high as 0.95. We further demonstrate the quantum Zeno effect of inhibiting the simultaneous two-atom excitation by continuously measuring whether the first photon is emitted. This work provides a new route in synthesizing many-body interaction Hamiltonian and coherent control of entanglement.
Accelerating complex control schemes on a heterogeneous MPSoC platform for quantum computing
Control and readout of superconducting quantum bits (qubits) require microwave pulses with gigahertz frequencies and nanosecond precision. To generate and analyze these microwave pulses,
we developed a versatile FPGA-based electronics platform. While basic functionality is directly handled within the FPGA, guaranteeing highest accuracy on the nanosecond timescale, more complex control schemes render impractical to implement in hardware.
To provide deterministic timing and low latency with high flexibility, we developed the Taskrunner framework. It enables the execution of complex control schemes, so-called user tasks, on the real-time processing unit (RPU) of a heterogeneous Multiprocessor System-on-Chip (MPSoC). These user tasks are specified conveniently using standard C language and are compiled automatically by the MPSoC platform when loaded onto the RPU. We present the architecture of the Taskrunner framework as well as timing benchmarks and discuss applications in the field of quantum computing.