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Physical Review A

Generation of Dicke states in the ultrastrong-coupling regime of circuit QED systems

Chunfeng Wu, Chu Guo, Yimin Wang, Gangcheng Wang, Xun‐Li Feng, Jing‐Ling Chen

Published Jan 27, 2017• 40 citations
Publisher

Abstract

Ultrastrong coupling in circuit quantum electrodynamics makes enhanced fast quantum operations possible. We present a scheme to realize controllable qubit-resonator interactions in ultrastrong circuit quantum electrodynamics with the aid of a unitary transformation. The controllable qubit-resonator interaction is just one type of the so-called selective resonant interactions. From the effective dynamics, it is possible to create the multipartite Dicke states at nanoseconds with high fidelity if the large detuning constraint of the selective resonant interaction is fulfilled. We also investigate the performance of our scheme in the presence of decoherence. Our results present a promising way toward achieving enhanced fast generation of the Dicke states.

DOI
10.1103/physreva.95.013845

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Ultrastrong coupling in circuit quantum electrodynamics makes enhanced fast quantum operations possible. We present a scheme to realize controllable qubit-resonator interactions in ultrastrong circuit quantum electrodynamics with the aid of a unitary transformation. The controllable qubit-resonator interaction is just one type of the so-called selective resonant interactions. From the effective dynamics, it is possible to create the multipartite Dicke states at nanoseconds with high fidelity if the large detuning constraint of the selective resonant interaction is fulfilled. We also investigate the performance of our scheme in the presence of decoherence. Our results present a promising way toward achieving enhanced fast generation of the Dicke states.
We investigate theoretically the performance of resonant two-qubit gates in the crossover from the strong to the ultrastrong coupling regime of light–matter interaction in circuit quantum electrodynamics. Two controlled-phase (CPHASE) gate schemes—which work well within the rotating wave-approximation—are analysed while taking into account the effects of counter-rotating terms appearing in the Hamiltonian. Our numerical results show that the fidelity of the gate operation is above 96% when the ratio between the coupling strength and the resonator frequency, g / ω r , is about 10%. Novel schemes are required in order to implement ultrafast quantum gates when increasing the ratio g / ω r .
We analyze a multiqubit circuit QED system in the regime where the qubit-photon coupling dominates over the system's bare energy scales. Under such conditions a manifold of low-energy states with a high degree of entanglement emerges. Here we describe a time-dependent protocol for extracting these quantum correlations and converting them into well-defined multipartite entangled states of noninteracting qubits. Based on a combination of various ultrastrong-coupling effects, the protocol can be operated in a fast and robust manner, while still being consistent with experimental constraints on switching times and typical energy scales encountered in superconducting circuits. Therefore, our scheme can serve as a probe for otherwise inaccessible correlations in strongly coupled circuit QED systems. It also shows how such correlations can potentially be exploited as a resource for entanglement-based applications.
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.
The latest technological progress of achieving the ultrastrong-coupling regime in circuit quantum electrodynamics (QED) systems has greatly promoted the developments of quantum physics, where novel quantum optics phenomena and potential computational benefits have been predicted. Here, we propose a scheme to accelerate the nontrivial two-qubit phase gate in a circuit QED system, where superconducting flux qubits are ultrastrongly coupled to a transmission line resonator (TLR), and two more TLRs are coupled to the ultrastrongly-coupled system for assistant. The nontrivial unconventional geometric phase gate between the two flux qubits is achieved based on close-loop displacements of the three-mode intracavity fields. Moreover, as there are three resonators contributing to the phase accumulation, the requirement of the coupling strength to realize the two-qubit gate can be reduced. Further reduction in the coupling strength to achieve a specific controlled-phase gate can be realized by adding more auxiliary resonators to the ultrastrongly-coupled system through superconducting quantum interference devices. We also present a study of our scheme with realistic parameters considering imperfect controls and noisy environment. Our scheme possesses the merits of ultrafastness and noise-tolerance due to the advantages of geometric phases.
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.
Circuit quantum electrodynamics, consisting of superconducting artificial atoms coupled to on-chip resonators, represents a prime candidate to implement the scalable quantum computing architecture because of the presence of good tunability and controllability. Furthermore, recent advances have pushed the technology towards the ultrastrong coupling regime of light-matter interaction, where the qubit-resonator coupling strength reaches a considerable fraction of the resonator frequency. Here, we propose a qubit-resonator system operating in that regime, as a quantum memory device and study the storage and retrieval of quantum information in and from the Z(2) parity-protected quantum memory, within experimentally feasible schemes. We are also convinced that our proposal might pave a way to realize a scalable quantum random-access memory due to its fast storage and readout performances.