Entanglement Echoes in Quantum Computation

We study the stability of entanglement in a quantum computer implementing an efficient quantum algorithm, which simulates a quantum chaotic dynamics. For this purpose, we perform a forward-backward evolution of an initial state in which two qubits are in a maximally entangled Bell state. If the dynamics is reversed after an evolution time $t_r$, there is an echo of the entanglement between these two qubits at time $t_e=2t_r$. Perturbations attenuate the pairwise entanglement echo and generate entanglement between these two qubits and the other qubits of the quantum computer.Comment: 4 pages, 4 figure

Negative differential conductivity in far-from-equilibrium quantum spin chains

We show that, when a finite anisotropic Heisenberg spin-1/2 chain in the gapped regime is driven far from equilibrium, oppositely polarized ferromagnetic domains build up at the edges of the chain, thus suppressing quantum spin transport. As a consequence, a negative differential conductivity regime arises, where increasing the driving decreases the current. The above results are explained in terms of magnon localization and are shown to be structurally stable against breaking of integrability.Comment: 5 pages, 4 figures. Published versio

Classical versus quantum errors in quantum computation of dynamical systems

We analyze the stability of a quantum algorithm simulating the quantum dynamics of a system with different regimes, ranging from global chaos to integrability. We compare, in these different regimes, the behavior of the fidelity of quantum motion when the system's parameters are perturbed or when there are unitary errors in the quantum gates implementing the quantum algorithm. While the first kind of errors has a classical limit, the second one has no classical analogue. It is shown that, whereas in the first case (``classical errors'') the decay of fidelity is very sensitive to the dynamical regime, in the second case (``quantum errors'') it is almost independent of the dynamical behavior of the simulated system. Therefore, the rich variety of behaviors found in the study of the stability of quantum motion under ``classical'' perturbations has no correspondence in the fidelity of quantum computation under its natural perturbations. In particular, in this latter case it is not possible to recover the semiclassical regime in which the fidelity decays with a rate given by the classical Lyapunov exponent.Comment: 8 pages, 7 figure

Thermalization and ergodicity in many-body open quantum systems

We study thermalization in many-body quantum systems locally coupled to an external bath. It is shown that quantum chaotic systems do thermalize, that is, they exhibit relaxation to an invariant ergodic state which, in the bulk, is well approximated by the grand canonical state. Moreover, the resulting ergodic state in the bulk does not depend on the details of the baths. On the other hand, for integrable systems the invariant state does depend on the bath and is different from the grand canonical state.Comment: 4 pages, 4 figures; v2. one new figur

General purpose readout board {\pi} LUP: overview and results

This work gives an overview of the PCI-Express board $\pi$LUP, focusing on the motivation that led to its development, the technological choices adopted and its performance. The $\pi$LUP card was designed by INFN and University of Bologna as a readout interface candidate to be used after the Phase-II upgrade of the Pixel Detector of the ATLAS and CMS experiments at LHC. The same team in Bologna is also responsible for the design and commissioning of the ReadOut Driver (ROD) board - currently implemented in all the four layers of the ATLAS Pixel Detector (Insertable B-Layer, B-Layer, Layer-1 and Layer-2) - and acquired in the past years expertise on the ATLAS readout chain and the problematics arising in such experiments. Although the $\pi$LUP was designed to fulfill a specific task, it is highly versatile and might fit a wide variety of applications, some of which will be discussed in this work. Two 7$^{th}$-generation Xilinx FPGAs are mounted on the board: a Zynq-7 with an embedded dual core ARM Processor and a Kintex-7. The latter features sixteen 12.5$\,$Gbps transceivers, allowing the board to interface easily to any other electronic board, either electrically and/or optically, at the current bandwidth of the experiments for LHC. Many data-transmission protocols have been tested at different speeds, results will be discussed later in this work. Two batches of $\pi$LUP boards have been fabricated and tested, two boards in the first batch (version 1.0) and four boards in the second batch (version 1.1), encapsulating all the patches and improvements required by the first version.Comment: 6 pages, 10 figures, 21th Real Time Conference, winner of "2018 NPSS Student Paper Award Second Prize

Conservative chaotic map as a model of quantum many-body environment

We study the dynamics of the entanglement between two qubits coupled to a common chaotic environment, described by the quantum kicked rotator model. We show that the kicked rotator, which is a single-particle deterministic dynamical system, can reproduce the effects of a pure dephasing many-body bath. Indeed, in the semiclassical limit the interaction with the kicked rotator can be described as a random phase-kick, so that decoherence is induced in the two-qubit system. We also show that our model can efficiently simulate non-Markovian environments.Comment: 8 pages, 4 figure

Robust and efficient generator of almost maximal multipartite entanglement

Quantum chaotic maps can efficiently generate pseudo-random states carrying almost maximal multipartite entanglement, as characterized by the probability distribution of bipartite entanglement between all possible bipartitions of the system. We show that such multipartite entanglement is robust, in the sense that, when realistic noise is considered, distillable entanglement of bipartitions remains almost maximal up to a noise strength that drops only polynomially with the number of qubits.Comment: 4 pages, 4 figures. Published versio