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Physical Review A

Multiqubit subradiant states in N -port waveguide devices: ε-and-μ-near-zero hubs and nonreciprocal circulators

Iñigo Liberal, Nader Engheta

Published Feb 6, 2018• 24 citations
Publisher

Abstract

Quantum emitters interacting through a waveguide setup have been proposed as a promising platform for basic research on light-matter interactions and quantum information processing. We propose to augment waveguide setups with the use of multiport devices. Specifically, we demonstrate theoretically the possibility of exciting $N$-qubit subradiant, maximally entangled, states with the use of suitably designed $N$-port devices. Our general methodology is then applied based on two different devices: an epsilon-and-mu-near-zero waveguide hub and a nonreciprocal circulator. A sensitivity analysis is carried out to assess the robustness of the system against a number of nonidealities. These findings link and merge the designs of devices for quantum state engineering with classical communication network methodologies.

DOI
10.1103/physreva.97.022309

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Quantum emitters interacting through a waveguide setup have been proposed as a promising platform for basic research on light-matter interactions and quantum information processing. We propose to augment waveguide setups with the use of multiport devices. Specifically, we demonstrate theoretically the possibility of exciting NNN-qubit subradiant, maximally entangled, states with the use of suitably designed NNN-port devices. Our general methodology is then applied based on two different devices: an epsilon-and-mu-near-zero waveguide hub and a nonreciprocal circulator. A sensitivity analysis is carried out to assess the robustness of the system against a number of nonidealities. These findings link and merge the designs of devices for quantum state engineering with classical communication network methodologies.
Multiqubit entanglement is extremely important to perform truly secure quantum optical communication and computing operations. However, the efficient generation of long-range entanglement over extended time periods between multiple qubits randomly distributed in a photonic system remains an outstanding challenge. This constraint is mainly due to the detrimental effects of decoherence and dephasing. To alleviate this issue, we present engineered epsilon-near-zero (ENZ) nanostructures that can maximize the coherence of light–matter interactions at room temperature. We investigate a practical ENZ plasmonic waveguide system, which simultaneously achieves multiqubit entanglement in elongated distances, extended time periods, and, even more importantly, independent of the emitters' positions. More specifically, we present efficient transient entanglement between three and four optical qubits mediated by ENZ with results that can be easily generalized to an arbitrary number of emitters. The entanglement between multiple qubits is characterized by computing the negativity metric applied to the proposed nanophotonic ENZ configuration. The ENZ response is found to be substantially advantageous to boost the coherence between multiple emitters compared to alternative plasmonic waveguide schemes. Finally, the super-radiance collective emission response at the ENZ resonance is utilized to design a high fidelity two-qubit quantum phase gate that can be used in various emerging quantum computing applications.
The radiative properties of quantum emitters are profoundly influenced by their electromagnetic environment. When coupled to a waveguide terminated at one end by a mirror, distant emitters interact strongly via virtual photon exchange, leading to collective superradiant and subradiant states with enhanced or suppressed decay rates. We demonstrate that applying optimal frequency shifts to each emitter enables the formation of perfect single-excitation dark states (i.e. states with zero decay rate) and near-perfect multi-excitation dark states in small ensembles. These collective states can be deterministically prepared with high fidelity using classical driving fields or few-photon pulses propagating along the waveguide. These results, readily implementable in superconducting qubit platforms, open new avenues for quantum information storage, networking and control of light.
Nonreciprocal interaction between two spatially separated subsystems plays a crucial role in signal processing and quantum networks. Here, we propose an efficient scheme to achieve nonreciprocal interaction and entanglement between two qubits by combining coherent and dissipative couplings in a superconducting platform, where two coherently coupled transmon qubits simultaneously interact with a transmission line waveguide. The coherent interaction between the transmon qubits can be achieved via capacitive coupling or via an intermediary cavity mode, while the dissipative interaction is induced by the transmission line via reservoir engineering. With high tunability of superconducting qubits, their positions along the transmission line can be adjusted to tune the dissipative coupling, enabling to tailor reciprocal and nonreciprocal interactions between the qubits. A unidirectional interaction can be achieved when the separation between the two qubits is ( 4 n + 3 ) λ 0 / 4 , where n is an integer and λ 0 is the photon wavelength. This nonreciprocal interaction enables the generation of nonreciprocal entanglement between the two transmon qubits. Furthermore, applying a drive field to one of the qubits can stabilize the system into a nonreciprocal steady-state entangled state. Remarkably, the nonreciprocal interaction in this work does not rely on the presence of nonlinearity or any explicitly nonreciprocal elements, which has more potential applications in designing nonreciprocal quantum devices, processing quantum information, and building quantum networks.
Nonreciprocity is an important property for building interesting and complex quantum systems and for their applications to quantum information processing. In this thesis, we show a low-loss waveguide circulator, a nonreciprocal device, and the understanding about its internal structure as multi-mode hybrid quantum system. Furthermore, we show the circulator integrated with superconducting niobium cavities, and the tunable nonreciprocal interactions between the intra-cavity photons due to coherent coupling to chiral internal modes. %between the intra-cavity photons. which can be presented through effective non-Hermitian dynamics of this system. Based on this platform, we integrate superconducting transmon qubit into the system and explore the nonreciprocal dispersive-type interaction between a transmon qubit and a superconducting cavity, and we introduce a general master-equation model to discribe the observed qubit-cavity dynamics agnostic to the intermediary system. This study demonstrates an example of a new class of quantum nonreciprocal phenomena beyond typical non-Hermitian Hamiltonians and cascaded systems.
Multi-qubit entanglement is of paramount importance to perform truly secure quantum optical communication operations. In this work, we present efficient long-range entanglement mediated by plasmonic waveguides operating at their cut-off frequency, where they exhibit an effective epsilon-near-zero (ENZ) response. Only the excited ENZ mode at plasmonic waveguides has a strong and homogeneous electric field distribution compared to different modes excited at groove and rod waveguides, resulting in an efficient two- and multi-qubit entanglement that can persist over extended time periods and long distances. Efficient multipartite entanglement from three equidistantly arranged two-level quantum emitters (qubits) inside the ENZ waveguide is also demonstrated by using three different multi-qubit entanglement criterions, i.e., pairwise concurrence, negativity, and genuine multipartite entanglement. The findings of this work stress the importance of ENZ plasmonic waveguides as an ideal nanophotonic platform to support efficient quantum optical effects.