Superconducting Resonators as Beam Splitters for Linear-Optics Quantum Computation (original) (raw)

Nonlinear Optics Quantum Computing with Circuit-QED

2012

One approach to quantum information processing is to use photons as quantum bits and rely on linear optical elements for most operations. However, some optical nonlinearity is necessary to enable universal quantum computing . Here, we suggest a circuit-QED approach to nonlinear optics quantum computing in the microwave regime, including a deterministic two-photon phase gate. Our specific example uses a hybrid quantum system comprising a LC resonator coupled to a superconducting flux qubit to implement a nonlinear coupling. Compared to the self-Kerr nonlinearity, we find that our approach has improved tolerance to noise in the qubit while maintaining fast operation.

Quantum State Engineering With the Rf-SQUID: A Brief Introduction

Arxiv preprint quant-ph/0307101, 2003

The SQUID, or superconducting quantum interference device, is a highly sensitive instrument employed for the nondestructive measurement of magnetic fields, with a host of applications in both biophysics and materials technology. It is composed of a cooled superconductive metal ring separated by a thin insulating barrier of non-superconducting metal. Electrons tunnel across the barrier to form a Josephson junction; an rf SQUID is essentially a Josephson junction with tunable current and energy. Quantum computers take advantage of the superpositional logic of quantum mechanics to allow for dramatic increases in computational efficiency. rf SQUIDs show potential for quantum computing applications by forming the qubit component of a quantum computer, through simply treating the direction of current, clockwise or counterclockwise, as the value of the bit.

Photon shell game in three-resonator circuit quantum electrodynamics

Nature Physics, 2011

The generation and control of quantum states of light constitute fundamental tasks in cavity quantum electrodynamics (QED) 1-10 . The superconducting realization of cavity QED, circuit QED 11-14 , enables on-chip microwave photonics, where superconducting qubits control and measure individual photon states . A long-standing issue in cavity QED is the coherent transfer of photons between two or more resonators. Here, we use circuit QED to implement a three-resonator architecture on a single chip, where the resonators are interconnected by two superconducting phase qubits. We use this circuit to shuffle one-and two-photon Fock states between the three resonators, and demonstrate qubitmediated vacuum Rabi swaps between two resonators. This illustrates the potential for using multi-resonator circuits as photon quantum registries and for creating multipartite entanglement between delocalized bosonic modes 27 .

Demonstration of two-qubit algorithms with a superconducting quantum processor

Nature, 2009

By harnessing the superposition and entanglement of physical states, quantum computers could outperform their classical counterparts in solving problems of technological impact, such as factoring large numbers and searching databases 1,2 . A quantum processor executes algorithms by applying a programmable sequence of gates to an initialized register of qubits, which coherently evolves into a final state containing the result of the computation. Simultaneously meeting the conflicting requirements of long coherence, state preparation, universal gate operations, and qubit readout makes building quantum processors challenging. Few-qubit processors have already been shown in nuclear magnetic resonance 3,4,5,6 , cold ion trap 7,8 and optical 9 systems, but a solid-state realization has remained an outstanding challenge.

Two-resonator circuit quantum electrodynamics: A superconducting quantum switch

Physical Review B, 2008

We introduce a systematic formalism for two-resonator circuit QED, where two on-chip microwave resonators are simultaneously coupled to one superconducting qubit. Within this framework, we demonstrate that the qubit can function as a quantum switch between the two resonators, which are assumed to be originally independent. In this three-circuit network, the qubit mediates a geometric second-order circuit interaction between the otherwise decoupled resonators. In the dispersive regime, it also gives rise to a dynamic second-order perturbative interaction. The geometric and dynamic coupling strengths can be tuned to be equal, thus permitting to switch on and off the interaction between the two resonators via a qubit population inversion or a shifting of the qubit operation point. We also show that our quantum switch represents a flexible architecture for the manipulation and generation of nonclassical microwave field states as well as the creation of controlled multipartite entanglement in circuit QED. In addition, we clarify the role played by the geometric interaction, which constitutes a fundamental property characteristic of superconducting quantum circuits without counterpart in quantum-optical systems. We develop a detailed theory of the geometric second-order coupling by means of circuit transformations for superconducting charge and flux qubits. Furthermore, we show the robustness of the quantum switch operation with respect to decoherence mechanisms. Finally, we propose a realistic design for a two-resonator circuit QED setup based on a flux qubit and estimate all the related parameters. In this manner, we show that this setup can be used to implement a superconducting quantum switch with available technology.

SQUID systems for macroscopic quantum coherence and quantum computing

Applied …, 2001

Among the various devices proposed as elements of a quantum computer, the rf-SQUID is a very promising candidate. In fact, systems based on this element can be adjusted in situ, can be coupled by means of superconducting transformers, can be prepared individually and measured with superconducting electronics. Moreover, many progresses were made in these years which showed quantum effects in this system. The present paper describes a complete device developed in order to get a direct measurement of the quantum coherent oscillation. The knowledge of this time, together with its limiting factors, is a prerequisite for fabricating a qubit based on rf- SQUIDS.

Microwave photonics with superconducting quantum circuits

Physics Reports

In the past 20 years, impressive progress has been made both experimentally and theoretically in superconducting quantum circuits, which provide a platform for manipulating microwave photons. This emerging field of superconducting quantum microwave circuits has been driven by many new interesting phenomena in microwave photonics and quantum information processing. For instance, the interaction between superconducting quantum circuits and single microwave photons can reach the regimes of strong, ultra-strong, and even deep-strong coupling. Many higher-order effects, unusual and less familiar in traditional cavity quantum electrodynamics with natural atoms, have been experimentally observed, e.g., giant Kerr effects, multi-photon processes, and single-atom induced bistability of microwave photons. These developments may lead to improved understanding of the counterintuitive properties of quantum mechanics, and speed up applications ranging from microwave photonics to superconducting quantum information processing. In this article, we review experimental and theoretical progress in microwave photonics with superconducting quantum circuits. We hope that this global review can provide a useful roadmap for this rapidly developing field.

Potential Characterization of a Double SQUID Device for Quantum Computing Experiments

IEEE Transactions on Applied Superconductivity, 2007

We report on experiments performed on a system consisting of a double SQUID (superconducting quantum interference device) built with gradiometer geometry. Two single-turn coils provide two independent control fluxes: one of these allows biasing the device and tilting the potential, while the other changes the barrier height of the potential. When the dynamics of the inner dc SQUID can be neglected, the free energy of the double SQUID, as a function of the internal magnetic flux, is just the corrugated parabola of an rf SQUID whose local minima represent metastable states for the system. Our analysis instead is substantially concerned with the interesting phenomenology generated by the static configurations of an internal two-junction interferometer and by the tunability of the internal loop inductance. Two readout systems are employed to thoroughly characterize the dynamics of our system. We investigate the dynamical response at temperatures low enough (tens of mK) to minimize the effects of thermal fluctuations concentrating the analysis on the aspects that could be relevant for macroscopic quantum coherence and computing. The results indicate that from the finite inductance of the inner loop originates a potential well generating competing processes with the tunneling between the two main wells of the rf-SQUID potential.

Introduction to quantum electromagnetic circuits

International Journal of Circuit Theory and Applications, 2017

The article is a short opinionated review of the quantum treatment of electromagnetic circuits, with no pretension to exhaustiveness. This review, which is an updated and modernized version of a previous set of Les Houches School lecture notes, has 3 main parts. The first part describes how to construct a Hamiltonian for a general circuit, which can include dissipative elements. The second part describes the quantization of the circuit, with an emphasis on the quantum treatment of dissipation. The final part focuses on the Josephson non-linear element and the main linear building blocks from which superconducting circuits are assembled. It also includes a brief review of the main types of superconducting artificial atoms, elementary multi-level quantum systems made from basic circuit elements.

ac Stark Shift and Dephasing of a Superconducting Qubit Strongly Coupled to a Cavity Field

Physical Review Letters, 2005

We have spectroscopically measured the energy level separation of a superconducting charge qubit coupled non-resonantly to a single mode of the electromagnetic field of a superconducting on-chip resonator. The strong coupling leads to large shifts in the energy levels of both the qubit and the resonator in this circuit quantum electrodynamics system. The dispersive shift of the resonator frequency is used to non-destructively determine the qubit state and to map out the dependence of its energy levels on the bias parameters. The measurement induces an ac-Stark shift of 0.6 MHz per photon in the qubit level separation. Fluctuations in the photon number (shot noise) induce level fluctuations in the qubit leading to dephasing which is the characteristic back-action of the measurement. A cross-over from lorentzian to gaussian line shape with increasing measurement power is observed and theoretically explained. For weak measurement a long intrinsic dephasing time of T2 > 200 ns of the qubit is found.