F. de Melo

Experimental investigation of the dynamics of entanglement: Sudden death, complementarity, and continuous monitoring of the environment

We report on an experimental investigation of the dynamics of entanglement between a single qubit and its environment, as well as for pairs of qubits interacting independently with individual environments, using photons obtained from parametric down-conversion. The qubits are encoded in the polarizations of single photons, while the interaction with the environment is implemented by coupling the polarization of each photon with its momentum. A convenient Sagnac interferometer allows for the implementation of several decoherence channels and for the continuous monitoring of the environment. For an initially entangled photon pair, one observes the vanishing of entanglement before coherence disappears. For a single qubit interacting with an environment, the dynamics of the complementarity relations connecting single-qubit properties and its entanglement with the environment is experimentally determined. The evolution of a single qubit under continuous monitoring of the environment is investigated, demonstrating that a qubit may decay even when the environment is found in the unexcited state. This implies that entanglement can be increased by local continuous monitoring, which is equivalent to entanglement distillation. We also present a detailed analysis of the transfer of entanglement from the two-qubit system to the two corresponding environments, between which entanglement may suddenly appear, and show instances for which no entanglement is created between dephasing environments, nor between either of them and the corresponding qubit: the initial two-qubit entanglement gets transformed into legitimate multiqubit entanglement of the Greenberger-Horne-Zeilinger type.

Entanglement of Identical Particles and the Detection Process

We introduce detector-level entanglement, a unified entanglement concept for identical particles that takes into account the possible deletion of many-particle which-way information through the detection process. The concept implies a measure for the effective indistinguishability of the particles, which is controlled by the measurement setup and which quantifies the extent to which the (anti-)symmetrization of the wave-function impacts on physical observables. Initially indistinguishable particles can gain or loose entanglement on their transition to distinguishability, and their quantum statistical behavior depends on their initial entanglement. Our results show that entanglement cannot be attributed to a state of identical particles alone, but that the detection process has to be incorporated in the analysis.

Experimental observation of environment-induced sudden death of entanglement

We demonstrate, using an all-optical setup, the difference between local and global dynamics of entangled quantum systems coupled to independent environments. Even when the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear.

Quantum nondemolition circuit for testing bipartite complementarity.

We present a quantum circuit that implements a nondemolition measurement of complementary single- and bipartite properties of a two-qubit system: entanglement and single-partite visibility and predictability. The system must be in a pure state with real coefficients in the computational basis, which allows a direct operational interpretation of those properties. The circuit can be realized in many systems of interest to quantum information.

Direct measurement of the quantum state of the electromagnetic field in a superconducting transmission line

We propose an experimental procedure to directly measure the state of an electromagnetic field inside a resonator, corresponding to a superconducting transmission line, coupled to a Cooper-pair box (CPB). The measurement protocol is based on the use of a dispersive interaction between the field and the CPB, and the coupling to an external classical field that is tuned to resonance with either the field or the CPB. We present a numerical simulation that demonstrates the feasibility of this protocol, which is within reach of present technology.

Physical Realizations of Quantum Information

When we want to consider physical realizations of quantum information models and protocols, we have to identify specific (experimental) settings, which allow to implement the fundamental building blocks of quantum information processing. Thus, first we have to identify these building blocks and the requirements they imply for an experimental realization.

Single observable concurrence measurement without simultaneous copies

We present a protocol that allows us to obtain the concurrence of any two-qubit pure state by performing a minimal and optimal tomography of one of the subsystems through measuring a single observable of an ancillary four-dimensional qudit. An implementation for a system of trapped ions is also proposed, which can be achieved with present-day experimental techniques.

Observation of the entanglement sudden death

We demonstrate, using an all-optical setup, the difference between local and global dynamics of entangled quantum systems coupled to independent environments. Even when the decay of each system is asymptotic, quantum entanglement may suddenly disappear. We also demonstrate experimentally that measurements performed in the environment affects the local evolution.