TY - JOUR T1 - Interacting Qubit-Photon Bound States with Superconducting Circuits JF - Phys. Rev. Y1 - 2019 A1 - Neereja M. Sundaresan A1 - Rex Lundgren A1 - Guanyu Zhu A1 - Alexey V. Gorshkov A1 - Andrew A. Houck AB -

Qubits strongly coupled to a photonic crystal give rise to many exotic physical scenarios, beginning with single and multi-excitation qubit-photon dressed bound states comprising induced spatially localized photonic modes, centered around the qubits, and the qubits themselves. The localization of these states changes with qubit detuning from the band-edge, offering an avenue of in situ control of bound state interaction. Here, we present experimental results from a device with two qubits coupled to a superconducting microwave photonic crystal and realize tunable on-site and inter-bound state interactions. We observe a fourth-order two photon virtual process between bound states indicating strong coupling between the photonic crystal and qubits. Due to their localization-dependent interaction, these states offer the ability to create one-dimensional chains of bound states with tunable and potentially long-range interactions that preserve the qubits' spatial organization, a key criterion for realization of certain quantum many-body models. The widely tunable, strong and robust interactions demonstrated with this system are promising benchmarks towards realizing larger, more complex systems of bound states.

VL - X 9 UR - http://arxiv.org/abs/1801.10167 CP - 011021 U5 - https://doi.org/10.1103/PhysRevX.9.011021 ER - TY - JOUR T1 - Photon pair condensation by engineered dissipation JF - Phys. Rev. Lett. Y1 - 2019 A1 - Ze-Pei Cian A1 - Guanyu Zhu A1 - Su-Kuan Chu A1 - Alireza Seif A1 - Wade DeGottardi A1 - Liang Jiang A1 - Mohammad Hafezi AB -

Dissipation can usually induce detrimental decoherence in a quantum system. However, engineered dissipation can be used to prepare and stabilize coherent quantum many-body states. Here, we show that by engineering dissipators containing photon pair operators, one can stabilize an exotic dark state, which is a condensate of photon pairs with a phase-nematic order. In this system, the usual superfluid order parameter, i.e. single-photon correlation, is absent, while the photon pair correlation exhibits long-range order. Although the dark state is not unique due to multiple parity sectors, we devise an additional type of dissipators to stabilize the dark state in a particular parity sector via a diffusive annihilation process which obeys Glauber dynamics in an Ising model. Furthermore, we propose an implementation of these photon-pair dissipators in circuit-QED architecture. 

VL - 123 UR - https://arxiv.org/abs/1904.00016 CP - 063602 U5 - 10.1103/PhysRevLett.123.063602 ER - TY - JOUR T1 - Scale-Invariant Continuous Entanglement Renormalization of a Chern Insulator JF - Phys. Rev. Lett Y1 - 2019 A1 - Su-Kuan Chu A1 - Guanyu Zhu A1 - James R. Garrison A1 - Zachary Eldredge A1 - Ana Valdés Curiel A1 - Przemyslaw Bienias A1 - I. B. Spielman A1 - Alexey V. Gorshkov AB -

The multi-scale entanglement renormalization ansatz (MERA) postulates the existence of quantum circuits that renormalize entanglement in real space at different length scales. Chern insulators, however, cannot have scale-invariant discrete MERA circuits with finite bond dimension. In this Letter, we show that the continuous MERA (cMERA), a modified version of MERA adapted for field theories, possesses a fixed point wavefunction with nonzero Chern number. Additionally, it is well known that reversed MERA circuits can be used to prepare quantum states efficiently in time that scales logarithmically with the size of the system. However, state preparation via MERA typically requires the advent of a full-fledged universal quantum computer. In this Letter, we demonstrate that our cMERA circuit can potentially be realized in existing analog quantum computers, i.e., an ultracold atomic Fermi gas in an optical lattice with light-induced spin-orbit coupling. 

VL - 122 UR - https://arxiv.org/abs/1807.11486 CP - 120502 U5 - https://doi.org/10.1103/PhysRevLett.122.120502 ER - TY - JOUR T1 - Measurement Protocol for the Entanglement Spectrum of Cold Atoms JF - Phys. Rev. X Y1 - 2016 A1 - Hannes Pichler A1 - Guanyu Zhu A1 - Alireza Seif A1 - Peter Zoller A1 - Mohammad Hafezi AB -

Entanglement, and, in particular the entanglement spectrum, plays a major role in characterizing many-body quantum systems. While there has been a surge of theoretical works on the subject, no experimental measurement has been performed to date because of the lack of an implementable measurement scheme. Here, we propose a measurement protocol to access the entanglement spectrum of many-body states in experiments with cold atoms in optical lattices. Our scheme effectively performs a Ramsey spectroscopy of the entanglement Hamiltonian and is based on the ability to produce several copies of the state under investigation together with the possibility to perform a global swap gate between two copies conditioned on the state of an auxiliary qubit. We show how the required conditional swap gate can be implemented with cold atoms, either by using Rydberg interactions or coupling the atoms to a cavity mode. We illustrate these ideas on a simple (extended) Bose-Hubbard model where such a measurement protocol reveals topological features of the Haldane phase. 

VL - 6(4) UR - https://arxiv.org/abs/1605.08624 CP - 041033 U5 - https://doi.org/10.1103/PhysRevX.6.041033 ER -