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Rosario Fazio

Publications and source records attributed to Rosario Fazio.

At least 199 records · Page 11Linked to original sources
arXiv2012

Photon production from the vacuum close to the super-radiant transition: When Casimir meets Kibble-Zurek

The dynamical Casimir effect (DCE) predicts the generation of photons from the vacuum due to the parametric amplification of the quantum fluctuation of an electromagnetic field\cite{casimir1,casimir2}. The verification of such effect is still elusive in optical systems due to the very demanding requirements of its experimental implementation. This typically requires very fast changes of the boundary conditions of the problem, such as the high-frequency driving of the positions of the mirrors of a cavity accommodating the field. Here, we show that an ensemble of two-level atoms collectively coupled to the electromagnetic field of a cavity (thus embodying the quantum Dicke model\cite{dicke}), driven at low frequencies and close to a quantum phase transition, stimulates the production of photons from the vacuum. This paves the way to an effective simulation of the DCE through a mechanism that has recently found an outstanding experimental demonstration\cite{esslinger}. The spectral properties of the emitted radiation reflect the critical nature of the system and allow us to link the detection of DCE to the Kibble-Zurek mechanism for the production of defects when crossing a continuous phase transition\cite{KZ1,KZ2}. We illustrate the features of our proposal by addressing a simple cavity quantum-electrodynamics (cQED) setting of immediate experimental realisation.

quant-ph↗
arXiv2012

Quantum transducer in circuit optomechanics

Mechanical resonators are macroscopic quantum objects with great potential. They couple to many different quantum systems such as spins, optical photons, cold atoms, and Bose Einstein condensates. It is however difficult to measure and manipulate the phonon state due to the tiny motion in the quantum regime. On the other hand, microwave resonators are powerful quantum devices since arbitrary photon state can be synthesized and measured with a quantum tomography. We show that a linear coupling, strong and controlled with a gate voltage, between the mechanical and the microwave resonators enables to create quantum phonon states, manipulate hybrid entanglement between phonons and photons and generate entanglement between two mechanical oscillators. In circuit quantum optomechanics, the mechanical resonator acts as a quantum transducer between an auxiliary quantum system and the microwave resonator, which is used as a quantum bus.

cond-mat.mes-hall↗
arXiv2011

Josephson current in a four terminal superconductor - exciton condensate - superconductor system

We investigate the transport properties of a bilayer exciton condensate that is contacted by four superconducting leads. We focus on the equilibrium regime and investigate how the Josephson currents induced in the bilayer by phase biases applied to the superconducting electrodes are affected by the presence of an exciton condensate in the bulk of the system. As long as the distance between the superconducting electrodes is much larger than the exciton coherence length, the Josephson current depends only on the difference between the phase biases in the two layers. This result holds true in both short- and long-junction limits. We relate it to a novel correlated four-particle Andreev process which occurs at the superconductor - exciton condensate interface. The system we investigate provides an implementation of the supercurrent mirror proposed by Kitaev as a viable way to realize topologically protected qubits.

cond-mat.supr-con↗
arXiv2011

Time-bin entanglement of quasi-particles in semiconductor devices

We propose a scheme to produce time bin entangled pairs of electrons and holes based on high frequency time-resolved single electron emission from a quantum dot coupled to 1D chiral channels. At first order in the tunneling rate, an electron-hole pair is emitted in a coherent superposition state of different time bins that violates a CHSH inequality. in our scheme entanglement can be detected by means of a Franson interferometer in which the electron and the hole are sent towards different Mach-Zehnder interferometers. An analysis of current cross-correlations allows to detect violations of the CHSH inequality.

cond-mat.mes-hall↗
arXiv2011

Controlled coupling of spin-resolved quantum Hall edge states

Topologically-protected edge states are dissipationless conducting surface states immune to impurity scattering and geometrical defects that occur in electronic systems characterized by a bulk insulating gap. One example can be found in a two-dimensional electron gas (2DEG) under high magnetic field in the quantum Hall regime. Based on the coherent control of the coupling between these protected states, several theoretical proposals for the implementation of information processing architectures were proposed. Here we introduce and experimentally demonstrate a new method that allows us to controllably couple co-propagating spin-resolved edge states of a QH insulator. The scheme exploits a spatially-periodic in-plane magnetic field that is created by an array of Cobalt nano-magnets placed at the boundary of the 2DEG. A maximum charge/spin transfer of about 28% is achieved at 250 mK. This result may open the way to the realization of scalable quantum-information architectures exploiting the spin degree of freedom of topologically-protected states.

cond-mat.mes-hall↗
arXiv2011

High-fidelity quantum driving

The ability to accurately control a quantum system is a fundamental requirement in many areas of modern science such as quantum information processing and the coherent manipulation of molecular systems. It is usually necessary to realize these quantum manipulations in the shortest possible time in order to minimize decoherence, and with a large stability against fluctuations of the control parameters. While optimizing a protocol for speed leads to a natural lower bound in the form of the quantum speed limit rooted in the Heisenberg uncertainty principle, stability against parameter variations typically requires adiabatic following of the system. The ultimate goal in quantum control is to prepare a desired state with 100% fidelity. Here we experimentally implement optimal control schemes that achieve nearly perfect fidelity for a two-level quantum system realized with Bose-Einstein condensates in optical lattices. By suitably tailoring the time-dependence of the system's parameters, we transform an initial quantum state into a desired final state through a short-cut protocol reaching the maximum speed compatible with the laws of quantum mechanics. In the opposite limit we implement the recently proposed transitionless superadiabatic protocols, in which the system perfectly follows the instantaneous adiabatic ground state. We demonstrate that superadiabatic protocols are extremely robust against parameter variations, making them useful for practical applications.

quant-ph↗
arXiv2011

Quantum phase transition between cluster and antiferromagnetic states

We study a Hamiltonian system describing a three spin-1/2 cluster-like interaction competing with an Ising-like exchange. We show that the ground state in the cluster phase possesses symmetry protected topological order. A continuous quantum phase transition occurs as result of the competition between the cluster and Ising terms. At the critical point the Hamiltonian is self-dual. The geometric entanglement is also studied. Our findings in one dimension corroborate the analysis of the two dimensional generalization of the system, indicating, at a mean field level, the presence of a direct transition between an antiferromagnetic and a valence bond solid ground state.

quant-ph↗
arXiv2011

Persistent spin oscillations in a spin-orbit-coupled superconductor

Quasi-two-dimensional superconductors with tunable spin-orbit coupling are very interesting systems with properties that are also potentially useful for applications. In this manuscript we demonstrate that these systems exhibit undamped collective spin oscillations that can be excited by the application of a supercurrent. We propose to use these collective excitations to realize persistent spin oscillators operating in the frequency range of 10 GHz - 1 THz.

cond-mat.mes-hall↗
arXiv2011

Errors in quantum optimal control and strategy for the search of easily implementable control pulses

We introduce a new approach to assess the error of control problems we aim to optimize. The method offers a strategy to define new control pulses that are not necessarily optimal but still able to yield an error not larger than some fixed a priori threshold, and therefore provide control pulses that might be more amenable for an experimental implementation. The formalism is applied to an exactly solvable model and to the Landau-Zener model, whose optimal control problem is solvable only numerically. The presented method is of importance for applications where a high degree of controllability of the dynamics of quantum systems is required.

quant-ph↗
arXiv2011

Statistical mechanics of the Cluster-Ising model

We study a Hamiltonian system describing a three-spin-1/2 cluster-like interaction competing with an Ising-like anti-ferromagnetic interaction. We compute free energy, spin correlation functions and entanglement both in the ground and in thermal states. The model undergoes a quantum phase transition between an Ising phase with a nonvanishing magnetization and a cluster phase characterized by a string order. Any two-spin entanglement is found to vanish in both quantum phases because of a nontrivial correlation pattern. Neverthless, the residual multipartite entanglement is maximal in the cluster phase and dependent on the magnetization in the Ising phase. We study the block entropy at the critical point and calculate the central charge of the system, showing that the criticality of the system is beyond the Ising universality class.

quant-ph↗
arXiv2011

Detecting phonon blockade with photons

Measuring the quantum dynamics of a mechanical system, when few phonons are involved, remains a challenge. We show that a superconducting microwave resonator linearly coupled to the mechanical mode constitutes a very powerful probe for this scope. This new coupling can be much stronger than the usual radiation pressure interaction by adjusting a gate voltage. We focus on the detection of phonon blockade, showing that it can be observed by measuring the statistics of the light in the cavity. The underlying reason is the formation of an entangled state between the two resonators. Our scheme realizes a phonotonic Josephson junction, giving rise to coherent oscillations between phonons and photons as well as a self-trapping regime for a coupling smaller than a critical value. The transition from the self-trapping to the oscillating regime is also induced dynamically by dissipation.

cond-mat.mes-hall↗
arXiv2011

Stiffness in 1D Matrix Product States with periodic boundary conditions

We discuss in details a modified variational matrix-product-state algorithm for periodic boundary conditions, based on a recent work by P. Pippan, S.R. White and H.G. Everts, Phys. Rev. B 81, 081103(R) (2010), which enables one to study large systems on a ring (composed of N ~ 10^2 sites). In particular, we introduce a couple of improvements that allow to enhance the algorithm in terms of stability and reliability. We employ such method to compute the stiffness of one-dimensional strongly correlated quantum lattice systems. The accuracy of our calculations is tested in the exactly solvable spin-1/2 Heisenberg chain.

quant-ph↗
arXiv2011

Generating topological order from a 2D cluster state using a duality mapping

In this paper we prove, extend and review possible mappings between the two-dimensional Cluster state, Wen's model, the two-dimensional Ising chain and Kitaev's toric code model. We introduce a two-dimensional duality transformation to map the two-dimensional lattice cluster state into the topologically-ordered Wen model. Then, we subsequently investigates how this mapping could be achieved physically, which allows us to discuss the rate at which a topologically ordered system can be achieved. Next, using a lattice fermionization method, Wen's model is mapped into a series of one-dimensional Ising interactions. Considering the boundary terms with this mapping then reveals how the Ising chains interact with one another. The relationships discussed in this paper allow us to consider these models from two different perspectives: From the perspective of condensed matter physics these mappings allow us to learn more about the relation between the ground state properties of the four different models, such as their entanglement or topological structure. On the other hand, we take the duality of these models as a starting point to address questions related to the universality of their ground states for quantum computation.

quant-ph↗
arXiv2011

Spin-supersolid phase in Heisenberg chains: a characterization via Matrix Product States with periodic boundary conditions

By means of a variational calculation using Matrix Product States with periodic boundary conditions, we accurately determine the extension of the spin-supersolid phase predicted to exist in the spin-1 anisotropic Heisenberg chain. We compute both the structure factor and the superfluid stiffness, and extract the critical exponents of the supersolid-to-solid phase transition.

cond-mat.str-el↗
arXiv2011

Quantum Quenches, Thermalization and Many-Body Localization

We conjecture that thermalization following a quantum quench in a strongly correlated quantum system is closely connected to many-body delocalization in the space of quasi-particles. This scenario is tested in the anisotropic Heisenberg spin chain with different types of integrability-breaking terms. We first quantify the deviations from integrability by analyzing the level spacing statistics and the inverse participation ratio of the system's eigenstates. We then focus on thermalization, by studying the dynamics after a sudden quench of the anisotropy parameter. Our numerical simulations clearly support the conjecture, as long as the integrability-breaking term acts homogeneously on the quasiparticle space, in such a way as to induce ergodicity over all the relevant Hilbert space.

cond-mat.stat-mech↗
arXiv2011

Spatially-resolved analysis of edge-channel equilibration in quantum Hall circuits

We demonstrate an innovative quantum Hall circuit with variable geometry employing the moveable electrostatic potential induced by a biased atomic force microscope tip. We exploit this additional degree of freedom to identify the microscopic mechanisms that allow two co-propagating edge channels to equilibrate their charge imbalance. Experimental results are compared with tight-binding simulations based on a realistic model for the disorder potential. This work provides also an experimental realization of a beam mixer between co-propagating edge channels, a still elusive building block of a recently proposed new class of quantum interferometers.

cond-mat.mes-hall↗
arXiv2011

Quantitative entanglement witnesses of Isotropic- and Werner-class via local measurements

Quantitative entanglement witnesses allow one to bound the entanglement present in a system by acquiring a single expectation value. In this paper we analyze a special class of such observables which are associated with (generalized) Werner and Isotropic states. For them the optimal bounding functions can be easily derived by exploiting known results on twirling transformations. By focusing on an explicit local decomposition for these observables we then show how simple classical post-processing of the measured data can tighten the entanglement bounds. Quantum optics implementations based on hyper-entanglement generation schemes are analyzed.

quant-ph↗