Fabio Altomare

Entanglement in a quantum annealing processor

Abstract : Entanglement lies at the core of quantum algorithms designed to solve problems that are intractable by classical approaches. One such algorithm, quantum annealing (QA), provides a promising path to a practical quantum processor. We have built a series of architecturally scalable QA processors consisting of networks of manufactured interacting spins (qubits). Here, we use qubit tunneling spectroscopy to measure the energy eigen spectrum of two- and eight-qubit systems within one such processor, demonstrating quantum coherence in these systems. We present experimental evidence that, during a critical portion of QA, the qubits become entangled and entanglement persists even as these systems reach equilibrium with a thermal environment. Our results provide an encouraging sign that QA is a viable technology for large scale quantum computing.

Architectural Considerations in the Design of a Superconducting Quantum Annealing Processor

We have developed a quantum annealing processor, based on an array of tunable coupled rf-SQUID flux qubits, fabricated in a superconducting integrated circuit process. Implementing this type of processor at a scale of 512 qubits and 1472 programmable interqubit couplers and operating at ~ 20 mK has required attention to a number of considerations that one may ignore at the smaller scale of a few dozen or so devices. Here, we discuss some of these considerations, and the delicate balance necessary for the construction of a practical processor that respects the demanding physical requirements imposed by a quantum algorithm. In particular, we will review some of the design tradeoffs at play in the floor planning of the physical layout, driven by the desire to have an algorithmically useful set of interqubit couplers, and the simultaneous need to embed programmable control circuitry into the processor fabric. In this context, we have developed a new ultralow-power embedded superconducting digital-to-analog flux converter (DAC) used to program the processor with zero static power dissipation, optimized to achieve maximum flux storage density per unit area. The 512 single-stage, 3520 two-stage, and 512 three-stage flux DACs are controlled with an XYZ addressing scheme requiring 56 wires. Our estimate of on-chip dissipated energy for worst-case reprogramming of the whole processor is ~ 65 fJ. Several chips based on this architecture have been fabricated and operated successfully at our facility, as well as two outside facilities (see, for example, the recent reporting by Jones).

Thermally assisted quantum annealing of a 16-qubit problem

Efforts to develop useful quantum computers have been blocked primarily by environmental noise. Quantum annealing is a scheme of quantum computation that is predicted to be more robust against noise, because despite the thermal environment mixing the systems state in the energy basis, the system partially retains coherence in the computational basis, and hence is able to establish well-defined eigenstates. Here we examine the environments effect on quantum annealing using 16 qubits of a superconducting quantum processor. For a problem instance with an isolated small-gap anticrossing between the lowest two energy levels, we experimentally demonstrate that, even with annealing times eight orders of magnitude longer than the predicted single-qubit decoherence time, the probabilities of performing a successful computation are similar to those expected for a fully coherent system. Moreover, for the problem studied, we show that quantum annealing can take advantage of a thermal environment to achieve a speedup factor of up to 1,000 over a closed system.

Evidence for Macroscopic Quantum Tunneling of Phase Slips in Long One-Dimensional Superconducting Al Wires

Quantum phase slips have received much attention due to their relevance to superfluids in reduced dimensions and to models of cosmic string production in the early universe. Their establishment in one-dimensional superconductors has remained controversial. Here we study the nonlinear current-voltage characteristics and linear resistance in long superconducting Al wires with lateral dimensions approximately 5 nm. We find that, in a magnetic field and at temperatures well below the superconducting transition, the observed behaviors can be described by the nonclassical, macroscopic quantum tunneling of phase slips, and are inconsistent with the thermal activation of phase slips.

Measurement crosstalk between two phase qubits coupled by a coplanar waveguide

We investigate measurement crosstalk in a system with two flux-biased phase qubits coupled by a resonant coplanar waveguide cavity. After qubit measurement, the superconducting phase undergoes damped oscillations in a deep anharmonic potential producing a frequency chirped voltage or crosstalk signal. We show experimentally that a coplanar waveguide cavity acts as a bandpass filter that can significantly reduce the propagation of this crosstalk signal when the qubits are far off resonance from the cavity. The transmission of the crosstalk signal

Low-loss superconducting resonant circuits using vacuum-gap-based microwave components

\ensuremath{\propto}{({\ensuremath{\omega}}_{q}{C}_{x})}^{2}

Decoherence, Autler-Townes effect, and dark states in two-tone driving of a three-level superconducting system

can be further minimized by reducing the qubit frequencies and the coupling capacitance to the cavity. We model the large amplitude crosstalk signal and qubit response classically with results that agree well with the experimental data. We find that the maximum energy transferred by the crosstalk generating qubit roughly saturates for long energy relaxation times

rf-SQUID-Mediated Coherent Tunable Coupling between a Superconducting Phase Qubit and a Lumped-Element Resonator

({T}_{1}g100\text{ }\text{ns})

Tripartite interactions between two phase qubits and a resonant cavity

while the delay time necessary for the crosstalk signal to propagate to the cavity scales linearly with

Dynamical Autler-Townes control of a phase qubit

{T}_{1}