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It is shown that effective quantum-state and entanglement transfer can be obtained by inducing a coherent dynamics in quantum wires with homogeneous intrawire interactions. This goal is accomplished by tuning the coupling between the wire endpoints and the two qubits there attached, to an optimal value. A general procedure to determine such value is devised, and scaling laws between the optimal coupling and the length of the wire are found. The procedure is implemented in the case of a wire consisting of a spin-1 2 XY chain: results for the time dependence of the quantities which characterize quantum-state and entanglement transfer are found of extremely good quality and almost independent of the wire length. The present approach does not require ad hoc engineering of the intrawire interactions nor a specific initial pulse shaping, and can be applied to a vast class of quantum channels.One of the most commonly requested conditions in quantum communication and computation protocols is that two distant parties, typically Alice and Bob, share a couple of entangled qubits. When the physical objects encoding the qubits can travel, as in the case of optical photons, the above goal can be accomplished by creating the entangled couple in a limited region of space and then letting the qubits fly where necessary. On the other hand, when qubits are realized via intrinsically localized physical objects, as in the case of S = 1 2 spins or atomic systems, a different strategy must be adopted (see for instance Ref. 1 and references therein). One such strategy is the following: first, two neighboring qubits (A and A ′ ) are prepared in an entangled state, by means of a shortrange interaction; then, the mixed state of one of the two qubits (say A) is transferred to a third distant qubit via a quantum channel. If state-transfer is perfect the procedure results in a pair of distant entangled qubits A ′ and B, as requested.Aim of this paper is to set a general framework where such strategy can be successful. In particular, (i) we define a procedure for controlling such dynamics, and hence the quality of the transmission process, by specific operational settings; (ii) we show that the quality of the quantum-state and entanglement transfer is not substantially affected by the length of the wire; (iii) we apply the procedure to the spin-1 2 XY chain and show that high-quality quantum-state and entanglement transfer are obtained.Let us first recall that for the strategy depicted above to make sense, one has to equip oneself with a quantum channel capable of transferring mixed states. How to obtain such a channel is the problem to which many authors have proposed different solutions [1][2][3][4][5][6][7], some based on the idea of engineering the channel itself, by the specific design of its internal interactions, others on that of intervening on the initialization process, by preparing the wire in a configuration found to serve the purpose. In both cases, a severe external action on the physical system is required.Here a different poin...

We study the finite-temperature behaviour of two-dimensional S = 1/2 Heisenberg antiferromagnets with very weak easy-axis and easy-plane exchange anisotropies. By means of quantum Monte Carlo simulations, based on the continuous-time loop and worm algorithm, we obtain a rich set of data that allows us to draw conclusions about both the existence and the type of finite-temperature transition expected in the considered models. We observe that the essential features of the Ising universality class, as well as those of the Berezinskii-Kosterlitz-Thouless (BKT) one, are preserved even for anisotropies as small as 10 −3 times the exchange integral; such outcome, being referred to the most quantum case S = 1/2, rules out the possibility for quantum fluctuations to destroy the long or quasi-long range order, whose onset is responsible for the Ising or BKT transition, no matter how small the anisotropy. Besides this general issue, we use our results to extract, out of the isotropic component, the features which are peculiar to weakly anisotropic models, with particular attention for the temperature region immediately above the transition. By this analysis we aim to give a handy tool for understanding the experimental data relative to those real compounds whose anisotropies are too weak for a qualitative description to accomplish the goal of singling out the genuinely two-dimensional critical behaviour.

An overview on the theoretic formalism and up to date applications in quantum condensed matter physics of the effective potential and effective Hamiltonian methods is given. The main steps of their unified derivation by the so-called pure quantum self-consistent harmonic approximation (PQSCHA) are reported and explained. What makes this framework attractive is its easy implementation as well as the great simplification in obtaining results for the statistical mechanics of complicated quantum systems. Indeed, for a given quantum system the PQSCHA yields an effective system, i.e. an effective classical Hamiltonian with dependence on h(cross) and beta and classical-like expressions for the averages of observables, that has to be studied by classical methods. Anharmonic single-particle systems are analysed in order to get insight into the physical meaning of the PQSCHA, and its extension to the investigation of realistic many-body systems is pursued afterwards. The power of this approach is demonstrated through a collection of applications in different fields, such as soliton theory, rare gas crystals and magnetism. Eventually, the PQSCHA allows us also to approach quantum dynamical properties.

High-quality quantum-state and entanglement transfer can be achieved in an unmodulated spin bus operating in the ballistic regime, which occurs when the endpoint qubits A and B are coupled to the chain by an exchange interaction j0 comparable with the intrachain exchange. Indeed, the transition amplitude characterizing the transfer quality exhibits a maximum for a finite optimal value j opt 0 (N ), where N is the channel length. We show that j opt 0 (N ) scales as N −1/6 for large N and that it ensures a high-quality entanglement transfer even in the limit of arbitrarily long channels, almost independently of the channel initialization. For instance, the average quantumstate transmission fidelity exceeds 90 % for any chain length. We emphasize that, taking the reverse point of view, should j0 be experimentally constrained, high-quality transfer can still be obtained by adjusting the channel length to its optimal value.

Invasiveness of quantum measurements is a genuinely quantum mechanical feature that is not necessarily detrimental: Here we show how quantum measurements can be used to fuel a cooling engine. We illustrate quantum measurement cooling (QMC) by means of a prototypical two-stroke two-qubit engine which interacts with a measurement apparatus and two heat reservoirs at different temperatures. We show that feedback control is not necessary for operation while entanglement must be present in the measurement projectors. We quantify the probability that QMC occurs when the measurement basis is chosen randomly, and find that it can be very large as compared to the probability of extracting energy (heat engine operation), while remaining always smaller than the most useless operation, namely dumping heat in both baths. These results show that QMC can be very robust to experimental noise. A possible low-temperature solid-state implementation that integrates circuit QED technology with circuit quantum thermodynamics technology is presented. arXiv:1806.07814v3 [cond-mat.stat-mech]

We consider the Heisenberg antiferromagnet on the square lattice with S = 1/2 and very weak easy-plane exchange anisotropy; by means of the quantum Monte Carlo method, based on the continuous-time loop algorithm, we find that the thermodynamics of the model is highly sensitive to the presence of tiny anisotropies and is characterized by a crossover between isotropic and planar behaviour. We discuss the mechanism underlying the crossover phenomenon and show that it occurs at a temperature which is characteristic of the model. The expected Berezinskii-Kosterlitz-Thouless transition is observed below the crossover: a finite range of temperatures consequently opens for experimental detection of non-critical 2D XY behaviour. Direct comparison is made with uniform susceptibility data relative to the S = 1/2 layered antiferromagnet Sr2CuO2Cl2. [3]. However, despite the BKT theory being originally formulated as referred to 2D planar magnets, evidences of XY behaviour in real magnets are weak and limited to very peculiar cases [4]. On the other hand, for S = 1/2 there exist several cuprous oxides that, besides being excellent realizations of 2D antiferromagnets, are known [5] to display a weak exchange easy-plane (EP) anisotropy: unambiguous observations of XY critical behaviour in such systems are not easy to achieve, due to both the weakness of the anisotropy and the existence of finite inter-layer coupling. As for the former point, the EP anisotropy observed in real magnets is usually 10 −4 ÷ 10 −3 times the isotropic exchange coupling, and the signatures of BKT critical behaviour are often either too weak to be extracted from the isotropic thermodynamics or too close to the critical temperature to be experimentally accessible. The residual inter-layer coupling, even if orders of magnitudes smaller than the intra-layer one, drives the system towards a 3D transition, which is actually triggered by the divergence of 2D intra-layer spin correlations. Therefore purely 2D critical behaviour of diverging quantities is most often masked by the onset of 3D long-range order.In this work we show that several non-diverging quantities are sensitive to the presence of EP anisotropy and display an evident and detectable crossover between isotropic and XY behaviour above the expected BKT transition. Such crossover occurs at a temperature which is characteristic of the model and is marked by peculiar features in the temperature dependence of non-critical observables. In particular, we present quantum Monte Carlo (QMC) data for the uniform susceptibility, the finite-size staggered out-of-plane magnetization, the specific heat and the density of in-plane vortices.We consider the S = 1/2 easy-plane antiferromagnet on the square lattice described by the Hamiltonianwhere i = (i 1 , i 2 ) runs over the sites of a square lattice, d connects each site to its four nearest neighbours, J > 0 is the antiferromagnetic exchange coupling and ∆ ∈ (0, 1] is the EP anisotropy parameter. We use the reduced temperature t = T /J. The above model is stud...

Quantum-state transfer with fidelity higher than 0.99 can be achieved in the ballistic regime of an arbitrarily long one-dimensional chain with uniform nearest-neighbor interaction, except for the two pairs of mirror symmetric extremal bonds, say x (first and last) and y (second and last-but-one). These have to be roughly tuned to suitable values x ∼ 2N −1/3 and y ∼ 2 3/4 N −1/6 , N being the chain length. The general framework can describe the end-to-end response in different models, such as fermion or boson hopping models and XX spin chains.