
D. Vasilyev, A. Grankin, M. Baranov, L. Sieberer, P. Zoller Monitoring Quantum Simulators via Quantum NonDemolition Couplings to Atomic Clock Qubits,
PRX Quantum 1 (20201009),
http://dx.doi.org/10.1103/PRXQuantum.1.020302 doi:10.1103/PRXQuantum.1.020302 (ID: 720493)
Toggle Abstract
We discuss monitoring the time evolution of an analog quantum simulator via a quantum nondemolition (QND) coupling to an auxiliary `clock' qubit. The QND variable of interest is the `energy' of the quantum manybody system, represented by the Hamiltonian of the quantum simulator. We describe a physical implementation of the underlying QND Hamiltonian for Rydberg atoms trapped in tweezer arrays using laser dressing schemes for a broad class of spin models. As an application, we discuss a quantum protocol for measuring the spectral form factor of quantum manybody systems, where the aim is to identify signatures of ergodic vs. nonergodic dynamics, which we illustrate for disordered 1D Heisenberg and Floquet spin models on Rydberg platforms. Our results also provide the physical ingredients for running quantum phase estimation protocols for measurement of energies, and preparation of energy eigenstates for a specified spectral resolution on an analog quantum simulator.

D. Yang, A. Grankin, L. Sieberer, D. Vasilyev, P. Zoller Quantum Nondemolition Measurement of a ManyBody Hamiltonian,
Nat. Commun. 11 (20200207),
http://dx.doi.org/10.1038/s41467020144895 doi:10.1038/s41467020144895 (ID: 720277)
Toggle Abstract
An ideal quantum measurement collapses the wave function of a quantum system to an eigenstate of the measured observable, with the corresponding eigenvalue determining the measurement outcome. For a quantum nondemolition (QND) observable, i.e., one that commutes with the Hamiltonian generating the system's time evolution, repeated measurements yield the same result, corresponding to measurements with minimal disturbance. This concept applies universally to single quantum particles as well as to complex manybody systems. However, while QND measurements of systems with few degrees of freedom has been achieved in seminal quantum optics experiments, it is an open challenge to devise QND measurement of a complex manybody observable. Here, we describe how a QND measurement of the Hamiltonian of an interacting manybody system can be implemented in a trappedion analog quantum simulator. Through a single shot measurement, the manybody system is prepared in a narrow energy band of (highly excited) energy eigenstates, and potentially even a single eigenstate. Our QND scheme, which can be carried over to other platforms of quantum simulation, provides a novel framework to investigate experimentally fundamental aspects of equilibrium and nonequilibrium statistical physics including the eigenstate thermalization hypothesis (ETH) and quantum fluctuation relations.

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)
Toggle Abstract
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.

X. Huang, E. Zeuthen, D. Vasilyev, Q. He, K. Hammerer, E. Polzik Unconditional SteadyState Entanglement in Macroscopic Hybrid Systems by Coherent Noise Cancellation,
Phys. Rev. Lett. 103602 (20180905),
http://dx.doi.org/10.1103/PhysRevLett.121.103602 doi:10.1103/PhysRevLett.121.103602 (ID: 720079)
Toggle Abstract
The generation of entanglement between disparate physical objects is a key ingredient in the field of quantum technologies, since they can have different functionalities in a quantum network. Here we propose and analyze a generic approach to steadystate entanglement generation between two oscillators with different temperatures and decoherence properties coupled in cascade to a common unidirectional light field. The scheme is based on a combination of coherent noise cancellation and dynamical cooling techniques for two oscillators with effective masses of opposite signs, such as quasispin and motional degrees of freedom, respectively. The interference effect provided by the cascaded setup can be tuned to implement additional noise cancellation leading to improved entanglement even in the presence of a hot thermal environment. The unconditional entanglement generation is advantageous since it provides a readytouse quantum resource. Remarkably, by comparing to the conditional entanglement achievable in the dynamically stable regime, we find our unconditional scheme to deliver a virtually identical performance when operated optimally.

D. Yang, D. Vasilyev, C. Laflamme, M. Baranov, P. Zoller Quantum Scanning Microscope for Cold Atoms,
Phys. Rev. A 98 23852 (20180827),
http://dx.doi.org/10.1103/PhysRevA.98.023852 doi:10.1103/PhysRevA.98.023852 (ID: 720027)
Toggle Abstract
We present a detailed theoretical description of an atomic scanning microscope in a cavity QED setup proposed in Phys. Rev. Lett. 120, 133601 (2018). The microscope continuously observes atomic densities with optical subwavelength resolution in a nondestructive way. The superresolution is achieved by engineering an internal atomic dark state with a sharp spatial variation of population of a ground level dispersively coupled to the cavity field. Thus, the atomic position encoded in the internal state is revealed as a phase shift of the light reflected from the cavity in a homodyne experiment. Our theoretical description of the microscope operation is based on the stochastic master equation describing the conditional time evolution of the atomic system under continuous observation as a competition between dynamics induced by the Hamiltonian of the system, decoherence effects due to atomic spontaneous decay, and the measurement backaction. Within our approach we relate the observed homodyne current with a local atomic density, and discuss the emergence of a quantum nondemolition measurement regime allowing continuous observation of spatial densities of quantum motional eigenstates without measurement backaction in a single experimental run.

D. Yang, C. Laflamme, D. Vasilyev, M. Baranov, P. Zoller Theory of a Quantum Scanning Microscope for Cold Atoms,
Phys. Rev. Lett. 120 133601 (20180330),
http://dx.doi.org/10.1103/PhysRevLett.120.133601 doi:10.1103/PhysRevLett.120.133601 (ID: 720010)
Toggle Abstract
We propose and analyze a scanning microscope to monitor “live” the quantum dynamics of cold atoms in a cavity QED setup. The microscope measures the atomic density with subwavelength resolution via dispersive couplings to a cavity and homodyne detection within the framework of continuous measurement theory. We analyze two modes of operation. First, for a fixed focal point the microscope records the wave packet dynamics of atoms with time resolution set by the cavity lifetime. Second, a spatial scan of the microscope acts to map out the spatial density of stationary quantum states. Remarkably, in the latter case, for a good cavity limit, the microscope becomes an effective quantum nondemolition device, such that the spatial distribution of motional eigenstates can be measured backaction free in single scans, as an emergent quantum nondemolition measurement.