
A. Grankin, D. Vasilyev, P. Guimond, B. Vermersch, P. Zoller Freespace photonic quantum link and chiral quantum optics,
Phys. Rev. A 98 3825 (20181012),
http://dx.doi.org/10.1103/PhysRevA.98.043825 doi:10.1103/PhysRevA.98.043825 (ID: 719980)
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We present the design of a chiral photonic quantum link, where distant atoms interact by exchanging photons propagating in a single direction in free space. This is achieved by coupling each atom in a laserassisted process to an atomic array acting as a quantum phasedarray antenna. This provides a basic building block for quantum networks in free space, i.e., without requiring cavities or nanostructures, which we illustrate with highfidelity quantum state transfer protocols. Our setup can be implemented with neutral atoms using Rydbergdressed interactions.

M. Dalmonte, B. Vermersch, P. Zoller Quantum simulation and spectroscopy of entanglement Hamiltonians,
Nature Phys. 14 151 (20180521),
http://dx.doi.org/10.1038/s4156701801517 doi:10.1038/s4156701801517 (ID: 720028)
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The properties of a strongly correlated manybody quantum system, from the presence of topological order to the onset of quantum criticality, leave a footprint in its entanglement spectrum. The entanglement spectrum is composed by the eigenvalues of the density matrix representing a subsystem of the whole original system, but its direct measurement has remained elusive due to the lack of direct experimental probes. Here we show that the entanglement spectrum of the ground state of a broad class of Hamiltonians becomes directly accessible via the quantum simulation and spectroscopy of a suitably constructed entanglement Hamiltonian, building on the Bisognano–Wichmann theorem of axiomatic quantum field theory. This theorem gives an explicit physical construction of the entanglement Hamiltonian, identified as the Hamiltonian of the manybody system of interest with spatially varying couplings. On this basis, we propose a scalable recipe for the measurement of a system’s entanglement spectrum via spectroscopy of the corresponding Bisognano–Wichmann Hamiltonian realized in synthetic quantum systems, including atoms in optical lattices and trapped ions. We illustrate and benchmark this scenario on a variety of models, spanning phenomena as diverse as conformal field theories, topological order and quantum phase transitions.

A. Elben, B. Vermersch, M. Dalmonte, J. I. Cirac, P. Zoller Rényi Entropies from Random Quenches in Atomic Hubbard and Spin Models,
Phys. Rev. Lett. 120 50406 (20180202),
http://dx.doi.org/10.1103/PhysRevLett.120.050406 doi:10.1103/PhysRevLett.120.050406 (ID: 719876)
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We present a scheme for measuring Rényi entropies in generic atomic Hubbard and spin models using single copies of a quantum state and for partitions in arbitrary spatial dimension. Our approach is based on the generation of random unitaries from random quenches, implemented using engineered timedependent disorder potentials, and standard projective measurements, as realized by quantum gas microscopes. By analyzing the properties of the generated unitaries and the role of statistical errors, with respect to the size of the partition, we show that the protocol can be realized in exisiting AMO quantum simulators, and used to measure for instance area law scaling of entanglement in twodimensional spin models or the entanglement growth in manybody localized systems.
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B. Vermersch, A. Elben, M. Dalmonte, J. I. Cirac, P. Zoller Unitary ndesigns via random quenches in atomic Hubbard and Spin models: Application to the measurement of Rényi entropies,
Phys. Rev. A 97 23604 (20180202),
http://dx.doi.org/10.1103/PhysRevA.97.023604 doi:10.1103/PhysRevA.97.023604 (ID: 719928)
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We present a general framework for the generation of random unitaries based on random quenches in atomic Hubbard and spin models, forming approximate unitary ndesigns, and their application to the measurement of R\'enyi entropies. We generalize our protocol presented in [Elben2017: arXiv:1709.05060, to appear in Phys. Rev. Lett.] to a broad class of atomic and spin lattice models. We further present an indepth numerical and analytical study of experimental imperfections, including the effect of decoherence and statistical errors, and discuss connections of our approach with manybody quantum chaos.
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B. Vogell, B. Vermersch, T. E. Northup, B. P. Lanyon, C. A. Muschik Deterministic quantum state transfer between remote qubits in cavities,
Quantum Sci. Technol. 2 45003 (20170908),
http://dx.doi.org/10.1088/20589565/aa868b doi:10.1088/20589565/aa868b (ID: 719794)
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Performing a faithful transfer of an unknown quantum state is a key challenge for enabling quantum networks. The realization of networks with a small number of quantum links is now actively pursued, which calls for an assessment of different state transfer methods to guide future design decisions. Here, we theoretically investigate quantum state transfer between two distant qubits, each in a cavity, connected by a waveguide, e.g., an optical fiber. We evaluate the achievable state transfer fidelities for two different protocols: standard wave packet shaping and adiabatic passage. The main loss sources are transmission losses in the waveguide and absorption losses in the cavities. While special cases studied in the literature indicate that adiabatic passages may be beneficial in this context, it remained an open question under which conditions this is the case and whether their use will be advantageous in practice. We answer these questions by providing a full analysis, showing that state transfer by adiabatic passage  in contrast to wave packet shaping  can mitigate the effects of undesired cavity losses, far beyond the regime of coupling to a single waveguide mode and the regime of lossless waveguides, as was proposed so far. We also clarify that neither protocol can avoid losses in the waveguide and discuss how the cavity parameters should be chosen.
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A. Mazloom Shahraki, B. Vermersch, M. Baranov, M. Dalmonte Adiabatic state preparation of stripe phases with strongly magnetic atoms,
Phys. Rev. A 96 33602 (20170901),
http://dx.doi.org/10.1103/PhysRevA.96.033602 doi:10.1103/PhysRevA.96.033602 (ID: 719755)
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We propose a protocol for realizing the stripe phase in two spin models on a twodimensional square lattice, which can be implemented with strongly magnetic atoms (Cr, Dy, Er, etc.) in optical lattices by encoding spin states into Zeeman sublevels of the ground state manifold. The protocol is tested with clustermeanfield timedependent variational ans\"atze, validated by comparison with exact results for small systems, which enable us to simulate the dynamics of systems with up to 64 sites during the statepreparation protocol. This allows, in particular, to estimate the time required for preparation of the stripe phase with high fidelity under real experimental conditions.
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B. Vermersch, P. Guimond, H. Pichler, P. Zoller Quantum State Transfer via Noisy Photonic and Phononic Waveguides,
Phys. Rev. Lett. 118 133601 (20170327),
http://dx.doi.org/10.1103/PhysRevLett.118.133601 doi:10.1103/PhysRevLett.118.133601 (ID: 719696)
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We describe a quantum state transfer protocol, where a quantum state of photons stored in a first cavity can be faithfully transferred to a second distant cavity via an infinite 1D waveguide, while being immune to arbitrary noise (e.g. thermal noise) injected into the waveguide. We extend the model and protocol to a cavity QED setup, where atomic ensembles, or single atoms representing quantum memory, are coupled to a cavity mode. We present a detailed study of sensitivity to imperfections, and develop a quantum error correction protocol to account for random losses (or additions) of photons in the waveguide. Our numerical analysis is enabled by Matrix Product State techniques to simulate the complete quantum circuit, which we generalize to include thermal input fields. Our discussion applies both to photonic and phononic quantum networks.
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C. Dlaska, B. Vermersch, P. Zoller Robust quantum state transfer via topologically protected edge channels in dipolar arrays,
Quantum Sci. Technol. 2 15001 (20170105),
http://dx.doi.org/10.1088/20589565/2/1/015001 doi:10.1088/20589565/2/1/015001 (ID: 719597)
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We show how to realize quantum state transfer between distant qubits using the chiral edge states of a twodimensional topological spin system. Our implementation based on Rydberg atoms allows to realize the quantum state transfer protocol in state of the art experimental setups. In particular, we show how to adapt the standard state transfer protocol to make it robust against dispersive and disorder effects.
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