Topological states of matter are particularly robust, since they exploit global features insensitive to local perturbations. In this work, we describe how to create a Chern insulator of phonons in the solid state. The proposed implementation is based on a simple setting, a dielectric slab with a suitable pattern of holes. Its topological properties can be wholly tuned in-situ by adjusting the amplitude and frequency of a driving laser that controls the optomechanical interaction between light and sound. The resulting chiral, topologically protected phonon transport along the edges can be probed completely optically. Moreover, we identify a regime of strong mixing between photon and phonon excitations, which gives rise to a large set of different topological phases. This would be an example of a Chern insulator produced from the interaction between two physically very different particle species, photons and phonons.

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Topological Phases of Sound and Light

We construct shortcuts to adiabatic passage to achieve controllable and fast quantum-information transfer (QIT) between arbitrary two distant nodes in a two-dimensional (2D) quantum network. Through suitable designing of time-dependent Rabi frequencies, we show that perfect QIT between arbitrary two distant nodes can be rapidly achieved. Numerical simulations demonstrate that the proposal is robust to the decoherence caused by atomic spontaneous emission and cavity photon leakage. Additionally, the proposed scheme is also insensitive to the variations of the experimental parameters. Thus, the proposed scheme provides a new perspective on robust quantum information processing in 2D quantum networks.

Controllable and fast quantum-information transfer between distant nodes in two-dimensional networks.

We study an open quantum system of atoms with a long-range Rydberg interaction, laser driving, and spontaneous emission. Over time, the system occasionally jumps between a state of low Rydberg population and a state of high Rydberg population. The jumps are inherently collective, and in fact, exist only for a large number of atoms. We explain how entanglement and quantum measurement enable the jumps, which are otherwise classically forbidden.

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Collective quantum jumps of Rydberg atoms

We report on the direct measurement of the van der Waals interaction between two isolated, single Rydberg atoms separated by a controlled distance of a few micrometers. By working in a regime where the single-atom Rabi frequency of the laser used for excitation to the Rydberg state is comparable to the interaction energy, we observe a \emph{partial} Rydberg blockade, whereby the time-dependent populations of the various two-atom states exhibit coherent oscillations with several frequencies. A quantitative comparison of the data with a simple model based on the optical Bloch equations allows us to extract the van der Waals energy, and to observe its characteristic $C_6/R^6$ dependence. The magnitude of the measured $C_6$ coefficient agrees well with an \emph{ab-initio} theoretical calculation, and we observe its dramatic increase with the principal quantum number $n$ of the Rydberg state. Our results not only allow to test an important physical law, but also demonstrate a degree of experimental control which opens new perspectives in quantum information processing and quantum simulation using long-range interactions between the atoms.

Direct measurement of the van der Waals interaction between two Rydberg atoms (‘Hot Topics session’) (Orale)

Controlling quantum entanglement between parts of a many-body system is the key to unlocking the power of quantum information processing for applications such as quantum computation, highprecision sensing, and simulation of many-body physics. Spin degrees of freedom of ultracold neutral atoms in their ground electronic state provide a natural platform given their long coherence times and our ability to control them with magneto-optical fields, but creating strong coherent coupling between spins has been challenging. We demonstrate a Rydberg-dressed ground-state blockade that provides a strong tunable interaction energy (∼1 MHz in units of Planck’s constant) between spins of individually trapped cesium atoms. With this interaction we directly produce Bell-state entanglement between two atoms with a fidelity ≥ 81(2)%, excluding atom loss events, and ≥ 60(3)% when loss is included.

Entangling Atomic Spins with a Strong Rydberg-Dressed Interaction

We propose a scheme for realizing quantum phase gates for two atoms trapped in separate cavities connected by an optical fiber. In the scheme, the atoms, cavities, and fiber mode are in resonance. In contrast to the original scheme [A. Serafini, S. Mancini, and S. Bose, Phys. Rev. Lett. 96, 010503 (2006)], the gates are performed within the null- and single-excitation subspaces in virtue of the asymmetric encoding for two atomic qubits and can function beyond the limit that the atom-cavity coupling is much smaller than the cavity-fiber coupling. We study the stability of the gates and the influences of dissipation and show that such gates are robust.

Quantum phase gates for two atoms trapped in separate cavities within the null- and single-excitation subspaces

We study reservoir-engineered entanglement for a cascaded bosonic system consisting of three modes, where the adjacent pairs couple to each other via both the beam-splitter interaction and the coherent parametric interaction with the interaction strengths being tunable. We focus on an optomechanical realization of the model by combining a nondegenerate parametric amplifier and an auxiliary cavity. A great steady-state cavity-mechanical entanglement can be achieved by optimizing the ratio of the interaction strengths, where the optomechanical cavity enacts the cold reservoir, simultaneously laser cooling the pair of hybrid modes delocalized over the auxiliary cavity and the mechanical oscillator. In comparison with the case of cooling a single delocalized mode, the dual-mode cooling approach allows one to obtain a greater amount of entanglement with higher cooling efficiencies and to explore strong entanglement in much broader parameter regions, where the rotating-wave approximation fails for the single-mode cooling case. Moreover, we show that the steady-state cavity-mechanical entanglement is robust to the mechanical thermal noise of the high temperature. The improved reservoir engineering approach can potentially be generalized to other bosonic systems with asymmetric beam-splitter and parametric interactions.

Engineering optomechanical entanglement via dual-mode cooling with a single reservoir

We propose an effective scheme for realizing a Jaynes-Cummings (J-C) model with the collective nitrogen-vacancy center ensembles (NVE) bosonic modes in a hybrid system. Specifically, the controllable transmon qubit can alternatively interact with one of the two NVEs, which results in the production of $N$ particle entangled states. Arbitrary $N$ particle entangled states, NOON states, N-dimensional entangled states and entangled coherent states are demonstrated. Realistic imperfections and decoherence effects are analyzed via numerical simulation. Since no cavity photons or excited levels of the NV center are populated during the whole process, our scheme is insensitive to cavity decay and spontaneous emission of the NVE. The idea provides a scalable way to realize NVEs-circuit cavity quantum information processing with current technology.

Arbitrary control of entanglement between two nitrogen-vacancy-center ensembles coupling to a superconducting-circuit qubit

We report an efficient mechanism to generate mechanical entanglement in a two-cascaded cavity optomechanical system with optical parametric amplifiers (OPAs) inside the two coupled cavities. We use the especially tuned OPAs to squeeze the hybrid mode composed of two mechanical modes, leading to strong macroscopic entanglement between the two movable mirrors. The squeezing parameter as well as the effective mechanical damping are both modulated by the OPA gains. The optimal degree of mechanical entanglement therefore depends on the balanced process between coherent hybrid mode squeezing and dissipation engineering. The mechanical entanglement is robust to strong cavity decay, going beyond simply resolved sideband regime, and is resistant to reasonable high thermal noise. The scheme provides an alternative way for generating strong macroscopic entanglement in cascaded optomechanical systems.

Wan-Jun Su

Papers

Quantum phase gates for two atoms trapped in separate cavities within the null- and single-excitation subspaces

Engineering optomechanical entanglement via dual-mode cooling with a single reservoir

Arbitrary control of entanglement between two nitrogen-vacancy-center ensembles coupling to a superconducting-circuit qubit

Improving macroscopic entanglement with nonlocal mechanical squeezing

Probabilistic teleportation of arbitrary superposition of three states of an atom with a single measurement