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
09
Jan
2020
Multiplexed photon number measurement
The evolution of quantum systems under measurement is a central aspect of quantum mechanics. When a two level system — a qubit — is used as a probe of a larger system, it
naturally leads to answering a single yes-no question about the system state followed by its corresponding quantum collapse. Here, we report an experiment where a single superconducting qubit is counter-intuitively able to answer not a single but nine yes-no questions about the number of photons in a microwave resonator at the same time. The key ingredients are twofold. First, we exploit the fact that observing the color of a qubit carries additional information to the conventional readout of its state. The qubit-system interaction is hence designed so that the qubit color encodes the number of photons in the resonator. Secondly, we multiplex the qubit color observation by recording how the qubit reflects a frequency comb. Interestingly the amount of extracted information reaches a maximum at a finite drive amplitude of the comb. We evidence it by direct Wigner tomography of the quantum state of the resonator. Our experiment unleashes the full potential of quantum meters by bringing the measurement process in the frequency domain.
08
Jan
2020
Current detection using a Josephson parametric upconverter
We present the design, measurement and analysis of a current sensor based on a process of Josephson parametric upconversion in a superconducting microwave cavity. Terminating a coplanar
waveguide with a nanobridge constriction Josephson junction, we observe modulation sidebands from the cavity that enable highly sensitive, frequency-multiplexed output of small currents for applications such as transition-edge sensor array readout. We derive an analytical model to reproduce the measurements over a wide range of bias currents, detunings and input powers. Tuning the frequency of the cavity by more than \SI{100}{\mega\hertz} with DC current, our device achieves a minimum current sensitivity of \SI{8.9}{\pico\ampere\per\sqrt{\hertz}}. Extrapolating the results of our analytical model, we predict an improved device based on our platform, capable of achieving sensitivities down to \SI{50}{\femto\ampere\per\sqrt{\hertz}}}, or even lower if one could take advantage of parametric amplification in the Josephson cavity. Taking advantage of the Josephson architecture, our approach can provide higher sensitivity than kinetic inductance designs, and potentially enables detection of currents ultimately limited by quantum noise.
07
Jan
2020
Quantum trajectory analysis of single microwave photon detection by nanocalorimetry
We apply quantum trajectory techniques to analyze a realistic set-up of a superconducting qubit coupled to a heat bath formed by a resistor, a system that yields explicit expressions
of the relevant transition rates to be used in the analysis. We discuss the main characteristics of the jump trajectories and relate them to the expected outcomes („clicks“) of a fluorescence measurement using the resistor as a nanocalorimeter. As the main practical outcome we present a model that predicts the time-domain response of a realistic calorimeter subject to single microwave photons, incorporating the intrinsic noise due to the fundamental thermal fluctuations of the absorber and finite bandwidth of a thermometer.
06
Jan
2020
Nonlinear Effects in Superconducting Thin Film Microwave Resonators
We discuss how reactive and dissipative non-linearities affect the intrinsic response of superconducting thin-film resonators. We explain how most, if not all, of the complex phenomena
commonly seen can be described by a model in which the underlying resonance is a single-pole Lorentzian, but whose centre frequency and quality factor change as external parameters, such as readout power and frequency, are varied. What is seen during a vector-network-analyser measurement is series of samples taken from an ideal Lorentzian that is shifting and spreading as the readout frequency is changed. According to this model, it is perfectly proper to refer to, and measure, the resonant frequency and quality factor of the underlying resonance, even though the swept-frequency curves appear highly distorted and hysteretic. In those cases where the resonance curve is highly distorted, the specific shape of the trajectory in the Argand plane gives valuable insights into the second-order physical processes present. We discuss the formulation and consequences of this approach in the case of non-linear kinetic inductance, two-level-system loss, quasiparticle generation, and a generic model based on a power-law form. The generic model captures the key features of specific dissipative non-linearities, but additionally leads to insights into how general dissipative processes create characteristic forms in the Argand plane. We provide detailed formulations in each case, and indicate how they lead to the wide variety of phenomena commonly seen in experimental data. We also explain how the properties of the underlying resonance can be extracted from this data. Overall, our paper provides a self-contained compendium of behaviour that will help practitioners interpret and determine important parameters from distorted swept-frequency measurements.
23
Dez
2019
Engineering Cross Resonance Interaction in Multi-modal Quantum Circuits
Existing scalable superconducting quantum processors have only nearest-neighbor coupling. This leads to reduced circuit depth, requiring large series of gates to perform an arbitrary
unitary operation in such systems. Recently, multi-modal devices have been demonstrated as a promising candidate for small quantum processor units. Always on longitudinal coupling in such circuits leads to implementation of native high fidelity multi-qubit gates. We propose an architecture using such devices as building blocks for a highly connected larger quantum circuit. To demonstrate a quantum operation between such blocks, a standard transmon is coupled to the multi-modal circuit using a 3D bus cavity giving rise to small exchange interaction between the transmon and one of the modes. We study the cross resonance interaction in such systems and characterize the entangling operation as well as the unitary imperfections and cross-talk as a function of device parameters. Finally, we tune up the cross resonance drive to implement multi-qubit gates in this architecture.
A tunable coupler for suppressing adjacent superconducting qubit coupling
Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev.
Appl. 10, 054061 (2018)] we experimentally demonstrate a simply-designed and flux-controlled tunable coupler with continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a controlled-phase (CZ) gate by tuning one qubit frequency into and out of the usual operating point while dynamically keeping the qubit-qubit coupling off. This scheme not only efficiently suppresses the leakage out of the computational subspace but also allows for the acquired two-qubit phase being geometric at the operating point only where the coupling is on. We achieve an average CZ gate fidelity of 98.3%, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.
Solid-state qubits integrated with superconducting through-silicon vias
As superconducting qubit circuits become more complex, addressing a large array of qubits becomes a challenging engineering problem. Dense arrays of qubits benefit from, and may require,
access via the third dimension to alleviate interconnect crowding. Through-silicon vias (TSVs) represent a promising approach to three-dimensional (3D) integration in superconducting qubit arrays — provided they are compact enough to support densely-packed qubit systems without compromising qubit performance or low-loss signal and control routing. In this work, we demonstrate the integration of superconducting, high-aspect ratio TSVs — 10 μm wide by 20 μm long by 200 μm deep — with superconducting qubits. We utilize TSVs for baseband control and high-fidelity microwave readout of qubits using a two-chip, bump-bonded architecture. We also validate the fabrication of qubits directly upon the surface of a TSV-integrated chip. These key 3D integration milestones pave the way for the control and readout of high-density superconducting qubit arrays using superconducting TSVs.
22
Dez
2019
Quantum approximate optimization of the exact-cover problem on a superconducting quantum processor
Present-day, noisy, small or intermediate-scale quantum processors—although far from fault-tolerant—support the execution of heuristic quantum algorithms, which might enable
a quantum advantage, for example, when applied to combinatorial optimization problems. On small-scale quantum processors, validations of such algorithms serve as important technology demonstrators. We implement the quantum approximate optimization algorithm (QAOA) on our hardware platform, consisting of two transmon qubits and one parametrically modulated coupler. We solve small instances of the NP-complete exact-cover problem, with 96.6\% success probability, by iterating the algorithm up to level two.
20
Dez
2019
Quantum Fourier Transform in Oscillating Modes
Quantum Fourier transform (QFT) is a key ingredient of many quantum algorithms. In typical applications such as phase estimation, a considerable number of ancilla qubits and gates are
used to form a Hilbert space large enough for high-precision results. Qubit recycling reduces the number of ancilla qubits to one, but it is only applicable to semi-classical QFT and requires repeated measurements and feedforward within the coherence time of the qubits. In this work, we explore a novel approach based on resonators that forms a high-dimensional Hilbert space for the realization of QFT. By employing the perfect state-transfer method, we map an unknown multi-qubit state to a single resonator, and obtain the QFT state in the second oscillator through cross-Kerr interaction and projective measurement. A quantitive analysis shows that our method allows for high-dimensional and fully-quantum QFT employing the state-of-the-art superconducting quantum circuits. This paves the way for implementing various QFT related quantum algorithms.
Parity Detection of Propagating Microwave Fields
The parity of the number of elementary excitations present in a quantum system provides important insights into its physical properties. Parity measurements are used, for example, to
tomographically reconstruct quantum states or to determine if a decay of an excitation has occurred, information which can be used for quantum error correction in computation or communication protocols. Here we demonstrate a versatile parity detector for propagating microwaves, which distinguishes between radiation fields containing an even or odd number n of photons, both in a single-shot measurement and without perturbing the parity of the detected field. We showcase applications of the detector for direct Wigner tomography of propagating microwaves and heralded generation of Schrödinger cat states. This parity detection scheme is applicable over a broad frequency range and may prove useful, for example, for heralded or fault-tolerant quantum communication protocols.