Engineering steady Knill-Laflamme-Milburn state of Rydberg atoms by dissipation

Decoherence-free-subspace-based deterministic conversions for entangled states with heralded robust-fidelity quantum gates

The decoherence-free subspace (DFS) serves as a protective shield against certain types of environmental noise, allowing the system to remain coherent for extended periods of time. In this paper, we propose two protocols, i.e., one converts two-logic-qubit Knill-Laflamme-Milburn (KLM) state to two-logic-qubit Bell states, and the other converts three-logic-qubit KLM state to three-logic-qubit Greenberger-Horne-Zeilinger states, through cavity-assisted interaction in DFS. Especially, our innovative protocols achieve their objectives in a heralded way, thus enhancing experimental accessibility. Moreover, single photon detectors are incorporated into the setup, which can predict potential failures and ensure seamless interaction between the nitrogen-vacancy center and photons. Rigorous analyses and evaluations of two schemes demonstrate their abilities to achieve near-unit fidelities in principle and exceptional efficiencies. Further, our protocols offer progressive solutions to the challenges posed by decoherence, providing a pathway towards practical quantum technologies.

Nonreciprocal optomechanically induced transparency and enhanced ground-state cooling in a reversed-dissipation cavity system

We explore the prospects of phase-modulated optical nonreciprocity and enhanced ground-state cooling of a mechanical resonator for the reversed-dissipation system, where the dissipative coupling between two cavities is realized through the adiabatic elimination of a low-Q mechanical mode, while a high-Q mechanical mode interacts with two mutually coupled cavities, forming a closed-loop structure. This unique system facilitates the nontrivial phenomenon of optomechanically induced transparency (OMIT), which exhibits asymmetry due to the frequency shift effect. We also observe the emergence of parity-dependent unidirectional OMIT windows (appearing under the phase-matching condition), which can be dynamically modulated by both the phase factors and the strength of the dissipative coupling. Furthermore, our study delves into the ground-state cooling effect operating within the reversed-dissipation regime. Intriguingly, the cooling effect can be significantly enhanced by carefully engineering dissipative complex coupling, such as in the phase-matching condition. The potential applications of this scheme extend to the fabrication of ideal optical isolators in optical communication systems and the manipulation of macroscopic mechanical resonators at the quantum level, presenting exciting opportunities in quantum technologies.

Direct conversion of Greenberger-Horne-Zeilinger state to Knill-Laflamme-Milburn state in decoherence-free subspace

Several schemes are proposed to realize the conversion of photonic polarized-entangled Greenberger-Horne-Zeilinger state to Knill-Laflamme-Milburn state in decoherence-free subspace (DFS) via weak cross-Kerr nonlinearity and X-quadrature homodyne measurement with high fidelity. DFS is introduced to decrease the decoherence effect caused by the coupling between the system and the environment. Optimizations to improve the success rate and utilization of residual states are further investigated. This study indicates important applications for quantum information processing in the future.

In-phase and anti-phase entanglement dynamics of Rydberg atomic pairs

We study the correlated evolutions of two far-spaced Rydberg atomic pairs with different resonant frequencies, interacting via van der Waals (vdW) potentials and driven by a common laser field. They are found to exhibit in-phase (anti-phase) beating dynamics characterized by identical (complementary) intra-pair entanglements under a specific condition in regard of inter-pair vdW potentials and driving field detunings. This occurs when each atomic pair just oscillates between its ground state and symmetric entangled state because its doubly excited state and asymmetric entangled state are forbidden due to rigid dipole blockade and perfect destructive interference, respectively. More importantly, optimal inter-pair overall entanglement can be attained at each beating node corresponding to semi-optimal intra-pair entanglements, and inevitable dissipation processes just result in a slow decay of intra-pair and inter-pair entanglements yet without destroying in-phase and anti-phase beating dynamics.

One-way quantum state transfer in a lossy coupled-cavity array

Quantum state transfer plays an important role in quantum information processing, and it has been obtained many of the theoretical and experimental triumphs. But designing a dissipation-assisted scheme to transfer a quantum state is still by no means trivial. Here we put forward an easier scheme to dissipatively transfer an arbitrary quantum state from a sender to a receiver with two four-level atoms and three lasers in a lossy coupled-cavity array, and make the quantum state stable at the receiver via the photon loss of optical cavities. Owing to the assistance of the dissipation, the target state becomes the steady state of the whole process. Thus there is no requirement on external time-dependent controls. Furthermore, the atomic spontaneous emission can be significantly suppressed as the adiabatic elimination of the excited states. We also discuss the experimental feasibility of this scheme with the current experimental technologies and a high fidelity of the transferred state in the receiver can be above 98%.