Manuel Gessner

Detecting nonclassical system-environment correlations by local operations.

We develop a general strategy for the detection of nonclassical system-environment correlations in the initial states of an open quantum system. The method employs a dephasing map which operates locally on the open system and leads to an experimentally accessible witness for genuine quantum correlations, measuring the Hilbert-Schmidt distance between pairs of open system states. We further derive the expectation value of the witness for various random matrix ensembles modeling generic features of complex quantum systems. This expectation value is shown to be proportional to a measure for the quantum discord which reduces to the concurrence for pure initial states.

Experimental generation of quantum discord via noisy processes.

B. P. Lanyon,1, 2 P. Jurcevic,1, 2 C. Hempel,1, 2 M. Gessner,3, 4 V. Vedral,5, 6, 7 R. Blatt,1, 2 and C. F. Roos1, 2 1Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Technikerstr. 21A, 6020 Innsbruck, Austria 2 Institut für Experimentalphysik, Universität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria 3 Department of Physics, University of California, Berkeley, California 94720, USA 4 Physikalisches Institut, Universität Freiburg, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany 5 Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, 117543, Singapore 6 Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. 7Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore (Dated: May 9, 2019)

Local detection of quantum correlations with a single trapped ion

In open quantum systems the correlations between the system and its environment play an important role. A trapped-ion experiment demonstrates that these correlations can be detected without accessing or knowing anything about the environment or its interactions.

Correlations in quantum states and the local creation of quantum discord

Quantum discord is usually referred to as a measure for quantum correlations. In the search of the fundamental resource to gain a quantum advantage in quantum information applications, quantum discord is considered a promising candidate. In this paper we present an alternative view on quantum correlations in terms of the rank of the correlation matrix as introduced by Dakic, Vedral, and Brukner [Phys. Rev. Lett. 105, 190502 (2010)]. According to our analysis, information about the quantum discord does not necessarily determine the amount of quantum correlations but rather the quantumness of the state. Nonzero quantum discord is only a necessary but not a sufficient condition for correlations above the classically achievable limit. This becomes clear when we consider states of nonzero discord, which can be created from zero discord states only by a single local operation. We further show that the set of these states has measure zero.

Robust Asymptotic Entanglement under Multipartite Collective Dephasing.

We derive an analytic solution for the ensemble-averaged collective dephasing dynamics of N noninteracting atoms in a fluctuating homogeneous external field. The obtained Kraus map is used to specify families of states whose entanglement properties are preserved at all times under arbitrary field orientations, even for states undergoing incoherent evolution. Our results apply to arbitrary spectral distributions of the field fluctuations.

Local witness for bipartite quantum discord

Recently, we proposed a method for the local detection of quantum correlations on the basis of local measurements and state tomography at different instances in time [Phys. Rev. Lett. 107, 180402 (2011)]. The method allows for the detection of quantum discord in bipartite systems when access is restricted to only one of the subsystems. Here, we elaborate the details of this method and provide applications to specific physical models. In particular, we discuss the performance of the scheme for generic complex systems by investigating thermal equilibrium states corresponding to randomly generated Hamiltonians. Moreover, we formulate an ergodicity-like hypothesis which links the time average to the analytically obtained average over the group of unitary operators equipped with the Haar measure.

Generating and protecting correlated quantum states under collective dephasing

We study the collective dephasing process of a system of non-interacting atomic qubits, immersed in a spatially uniform magnetic field of fluctuating intensity. The correlation properties of bipartite states are analysed based on a geometric representation of the state space. Particular emphasis is put on the dephasing-assisted generation of states with a high correlation rank, which can be related to discord-type correlations and allow for direct applications in quantum information theory. Finally we study the conditions that ensure the robustness of initial entanglement and discuss the phenomenon of time-invariant entanglement.

Experimental detection of polarization-frequency quantum correlations in a photonic quantum channel by local operations

The measurement of correlations between different degrees of freedom is an important, but, in general, extremely difficult task in many applications of quantum mechanics. Here, we report an all-optical experimental detection and quantification of quantum correlations between the polarization and the frequency degrees of freedom of single photons by means of local operations acting only on the polarization degree of freedom. These operations only require experimental control over an easily accessible two-dimensional subsystem, despite handling strongly mixed quantum states comprised of a continuum of orthogonal frequency states. Our experiment thus represents a photonic realization of a scheme for the local detection of quantum correlations in a truly infinite-dimensional continuous-variable system, which excludes an efficient finite-dimensional truncation.

Observing a quantum phase transition by measuring a single spin

We show that the ground-state quantum correlations of an Ising model can be detected by monitoring the time evolution of a single spin alone, and that the critical point of a quantum phase transition is detected through a maximum of a suitably defined observable. A proposed implementation with trapped ions realizes an experimental probe of quantum phase transitions which is based on quantum correlations and scalable for large system sizes.

Efficient entanglement criteria for discrete, continuous, and hybrid variables

We develop a method to construct entanglement criteria for arbitrary multipartite systems of discrete or continuous variables and hybrid combinations of both. While any set of local operators generates a sufficient condition for entanglement of arbitrary quantum states, a suitable set leads to a necessary and sufficient criterion for pure states. The criteria are readily implementable with existing technology and reveal entanglement that remains undetected by the respective state-of-the-art methods for discrete and continuous variables.