G. Massimo Palma

Detection of geometric phases in superconducting nanocircuits

When a quantum-mechanical system undergoes an adiabatic cyclic evolution, it acquires a geometrical phase factor in addition to the dynamical one; this effect has been demonstrated in a variety of microscopic systems. Advances in nanotechnology should enable the laws of quantum dynamics to be tested at the macroscopic level, by providing controllable artificial two-level systems (for example, in quantum dots and superconducting devices). Here we propose an experimental method to detect geometric phases in a superconducting device. The setup is a Josephson junction nanocircuit consisting of a superconducting electron box. We discuss how interferometry based on geometrical phases may be realized, and show how the effect may be applied to the design of gates for quantum computation.

Entanglement-enhanced information transmission over a quantum channel with correlated noise

We show that entanglement is a useful resource to enhance the mutual information of the depolarizing channel when the noise on consecutive uses of the channel has some partial correlations. We obtain a threshold in the degree of memory above which a higher amount of classical information is transmitted with entangled signals.

Berry Phase for a Spin 1=2 Particle in a Classical Fluctuating Field

The effect of fluctuations in the classical control parameters on the Berry phase of a spin 1/2 interacting with an adiabatically cyclically varying magnetic field is analyzed. It is explicitly shown that in the adiabatic limit dephasing is due to fluctuations of the dynamical phase.

Fidelity and Leakage of Josephson Qubits

The unit of quantum information is the qubit, a vector in a two-dimensional Hilbert space. On the other hand, quantum hardware often operates in two-dimensional subspaces of vector spaces of higher dimensionality. The presence of higher quantum states may affect the accuracy of quantum information processing. In this Letter we show how to cope with {\em quantum leakage} in devices based on small Josephson junctions. While the presence of higher charge states of the junction reduces the fidelity during gate operations we demonstrate that errors can be minimized by appropriately designing and operating the gates.

Entanglement detection in hybrid optomechanical systems

We study a device formed by a Bose-Einstein condensate (BEC) coupled to the field of a cavity with a moving end mirror and find a working point such that the mirror-light entanglement is reproduced by the BEC-light quantum correlations. This provides an experimentally viable tool for inferring mirror-light entanglement with only a limited set of assumptions. We prove the existence of tripartite entanglement in the hybrid device, persisting up to temperatures of a few milli-Kelvin, and discuss a scheme to detect it.

Entanglement between two superconducting qubits via interaction with nonclassical radiation

Control of the dynamics of a complex quantum system requires a trade-off between tunability and protection against noise. To this end one can be interested in processes where some physical properties of a subsystem are reliably transferred onto the state of a second one (of perhaps different nature) where information can be manipulated. The connection between the two subsystems is effectively realized via a physical interface. An interface is a communication channel used to connect the elements of a quantum register to perform quantum information processing or a physical mechanism that gives full access to the system under investigation and allows to manipulate it. To investigate this problem, in this paper we describe the coupling between a nanoelectronic circuit implementing a pair of quantum bits and a two-mode electromagnetic field. We discuss a mechanism for the transfer of entanglement from a two-mode squeezed state to the pair of qubits. Here, the information sheltered in the electromagnetic medium may be manipulated, using just single-qubit operations, when transferred to the solid-state subsystem. This may offer advantages with respect to integrability and scalability. In particular, we consider the field modes to interact with a pair of (initially independent) superconducting quantum interference devices (SQUIDs) that embody two charge qubits. 1 Direct experimental evidence of the use of these systems as controllable coherent two-level systems has already been provided. 4,5 We find that a nearly maximally entangled state of two qubits can be tailored, with our interaction model, via an effective process of transfer of quantum correlations. The entanglement poured into the joint state of the qubits can be regulated controlling the interaction times between qubits and field modes. At the interaction time corresponding to the maximum of the transferred entanglement, the qubits are in an almost pure state that may be used for efficient quantum information processing. This work is organized as follows. In Sec. II we introduce the system we consider and derive the effective model for the coupling between a superconducting charge qubit and a field mode. Section III is devoted to the study of the process of transfer of quantum correlations from the two-mode field to the qubits. The joint state of these latter, once the field modes are traced out, turns out to be entangled. We quantify the amount of entanglement between the qubits and find the corresponding degree of mixedness of the state. Finally, in Sec. IV, we investigate about the variations in the amount of transferred entanglement as the initial preparation of the qubits is changed. We find that the transfer process is optimized if the qubits are initially in their computational ground state.

Quantum cloning in spin networks

We introduce an approach to quantum cloning based on spin networks and we demonstrate that phase covariant cloning can be realized using no external control but only with a proper design of the Hamiltonian of the system. In the 1! 2 cloning we find that the XY model saturates the value for the fidelity of the optimal cloner and gives values comparable to it in the general N! M case. We finally discuss the effect of external noise. Our protocol is much more robust to decoherence than a conventional procedure based on quantum

Transition behavior in the channel capacity of two-quibit channels with memory

We prove that a general upper bound on the maximal mutual information of quantum channels is saturated in the case of Pauli channels with an arbitrary degree of memory. For a subset of such channels we explicitly identify the optimal signal states. We show analytically that for such a class of channels entangled states are indeed optimal above a given memory threshold. It is noteworthy that the resulting channel capacity is a non-differentiable function of the memory parameter.

Dynamical entanglement transfer for quantum-information networks

A key element in the architecture of a quantum-information processing network is a reliable physical interface between fields and qubits. We study a process of entanglement transfer engineering, where two remote qubits respectively interact with an entangled two-mode continuous-variable (CV) field. We quantify the entanglement induced in the qubit state at the expenses of the loss of entanglement in the CV system. We discuss the range of mixed entangled states which can be obtained with this setup. Furthermore, we suggest a protocol to determine the residual entangling power of the light fields inferring, thus, the entanglement left in the field modes which, after the interaction, are no longer in a Gaussian state. Two different setups are proposed: a cavity-QED system and an interface between superconducting qubits and field modes. We address in detail the practical difficulties inherent in these two proposals, showing that the latter is promising in many aspects.

Shortcut to Adiabaticity in the Lipkin-Meshkov-Glick Model

We study transitionless quantum driving in an infinite-range many-body system described by the Lipkin-Meshkov-Glick model. Despite the correlation length being always infinite the closing of the gap at the critical point makes the driving Hamiltonian of increasing complexity also in this case. To this aim we develop a hybrid strategy combining a shortcut to adiabaticity and optimal control that allows us to achieve remarkably good performance in suppressing the defect production across the phase transition.