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arXiv:1601.06006 · Quantum Science and Technology

Parity-preserving light-matter system mediates effective two-body interactions

Thi Ha Kyaw, S. Allende, L. C. Kwek, Guillermo Romero

Published Apr 28, 2017• 10 citations
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Abstract

Abstract We study the equilibrium and non-equilibrium physics of two qubits interacting through an ultrastrong coupled qubit-cavity system. By tuning the qubits energy gap while keeping the ultrastrong coupling system to its ground state, we demonstrate a strong two-qubit interaction as well as an enhanced excitation transfer between the two qubits. Our proposal has twofold implications: a means to attain multipurpose parity-protected quantum information tasks in superconducting circuits and a building block for ultrastrong coupled cavity-enhanced exciton transport in disordered media.

DOI
10.1088/2058-9565/aa701c

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Abstract We study the equilibrium and non-equilibrium physics of two qubits interacting through an ultrastrong coupled qubit-cavity system. By tuning the qubits energy gap while keeping the ultrastrong coupling system to its ground state, we demonstrate a strong two-qubit interaction as well as an enhanced excitation transfer between the two qubits. Our proposal has twofold implications: a means to attain multipurpose parity-protected quantum information tasks in superconducting circuits and a building block for ultrastrong coupled cavity-enhanced exciton transport in disordered media.
We study the equilibrium and nonequilibrium properties of two qubits interacting through a polariton bus with an ultrastrong coupled qubit-cavity system. By tuning the energy gap of the qubits maintaining the polariton within the vacuum, we demonstrate an improvement of two-qubit gate operation time and excitations transfer as the ultrastrong coupling strength is increased. Our proposal might pave the way for multipurpose parity protected quantum information tasks, and for the enhancement of excitations transfer in disordered systems.
We study the feasibility of reaching the ultrastrong (USC) and deep-strong coupling (DSC) regimes of light-matter interaction, in particular at resonance condition, with a superconducting charge qubit, also known as Cooper-Pair box (CPB). We show that by shunting the charge qubit with a high-impedance LC-circuit, one can maximally reach both USC and DSC regimes exceeding the classical upper bound ∣g∣≤ωqωr/2|g|\leq \sqrt{\omega_q\omega_r}/2∣g∣≤ωq​ωr​​/2 between two harmonic systems with frequencies ωq\omega_qωq​ and ωr\omega_rωr​. In our case, the fundamental model corresponds to an enhanced quantum Rabi model, which contains a displacement field operator that breaks its internal parity symmetry. Furthermore, we consider a multipartite device consisting of two CPBs ultrastrongly coupled to an oscillator as a mediator and study a quantum state transfer protocol between a pair of transmon qubits, all of them subjected to local incoherent noise channels with realistic parameters. This work opens the door for studying light-matter interactions beyond the quantum Rabi model at extreme coupling strengths, providing a new building block for applications within quantum computation and quantum information processing.
Reaching the strong coupling regime of light-matter interaction has led to an impressive development in fundamental quantum physics and applications to quantum information processing. Latests advances in different quantum technologies, like superconducting circuits or semiconductor quantum wells, show that the ultrastrong coupling regime (USC) can also be achieved, where novel physical phenomena and potential computational benefits have been predicted. Nevertheless, the lack of effective decoupling mechanism in this regime has so far hindered control and measurement processes. Here, we propose a method based on parity symmetry conservation that allows for the generation and reconstruction of arbitrary states in the ultrastrong coupling regime of light-matter interactions. Our protocol requires minimal external resources by making use of the coupling between the USC system and an ancillary two-level quantum system.
We present a technique to store and retrieve single- and two-qubit states into a parity-protected quantum memory implemented on a state-of-the-art superconducting circuit architecture. Our proposal relies upon a specific superconducting circuit design that permits a tunable qubit-resonator coupling strength. The latter allows us to adiabatically tune from the weak coupling to the ultrastrong coupling regime of light-matter interaction, where a controllable and well-protected effective two-level system is defined due to the Z2 parity symmetry. Storage and retrieval time of the qubit are in a few nanoseconds timescale, which is far below the effective qubit coherence time.
We study quantum state transfer between two qubits coupled to a common quantum bus that is constituted by an ultrastrong coupled light-matter system. By tuning both qubit frequencies on resonance with a forbidden transition in the mediating system, we demonstrate a high-fidelity swap operation even though the quantum bus is thermally populated. We discuss a possible physical implementation in a realistic circuit QED scheme that leads to the multimode Dicke model. This proposal may have applications on hot quantum information processing within the context of ultrastrong coupling regime of light-matter interaction.
We study effective light-matter interactions in a circuit QED system consisting of a single LCLCLC resonator, which is coupled symmetrically to multiple superconducting qubits. Starting from a minimal circuit model, we demonstrate that, in addition to the usual collective qubit-photon coupling, the resulting Hamiltonian contains direct qubit-qubit interactions, which have a drastic effect on the ground- and excited-state properties of such circuits in the ultrastrong-coupling regime. In contrast to the superradiant phase transition expected from the standard Dicke model, we find an opposite mechanism, which at very strong interactions completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings resolve previous controversies over the existence of superradiant phases in circuit QED, but they more generally show that the physics of two- or multiatom cavity QED settings can differ significantly from what is commonly assumed.