Detection of entanglement in ultracold lattice gases

We propose the use of quantum polarization spectroscopy for detecting multi-particle entanglement of ultracold atoms in optical lattices. This method, based on a light-matter interface employing the quantum Farady effect, allows for the non destructive measurement of spin-spin correlations. We apply it to the specific example of a one dimensional spin chain and reconstruct its phase diagram using the light signal, readily measurable in experiments with ultracold atoms. Interestingly, the same technique can be extended to detect quantum many-body entanglement in such systems.Comment: Submitted to the Special Issue: "Strong correlations in Quantum Gases" in The Journal of Low Temperature Physic

Genuine quantum correlations in quantum many-body systems: a review of recent progress

Quantum information theory has considerably helped in the understanding of quantum many-body systems. The role of quantum correlations and in particular, bipartite entanglement, has become crucial to characterise, classify and simulate quantum many body systems. Furthermore, the scaling of entanglement has inspired modifications to numerical techniques for the simulation of many-body systems leading to the, now established, area of tensor networks. However, the notions and methods brought by quantum information do not end with bipartite entanglement. There are other forms of correlations embedded in the ground, excited and thermal states of quantum many-body systems that also need to be explored and might be utilised as potential resources for quantum technologies. The aim of this work is to review the most recent developments regarding correlations in quantum many-body systems focussing on multipartite entanglement, quantum nonlocality, quantum discord, mutual information but also other non classical measures of correlations based on quantum coherence. Moreover, we also discuss applications of quantum metrology in quantum many-body systems.Comment: Review. Close to published version. Comments are welcome! Please write an email to g.dechiara[(at)]qub.ac.u

Realism, Human Action and Political Life. On the Political Dimension of Individual Choices

This essay considers the consequences of recent debates on realism in epistemology and ethics on political philosophy, and for political action. It is argued here that reference to the good in political discourse is both unavoidable and recommendable. Further, it is argued that considerations of the role of the good in individual and political action suggest that a political community has a relative ontological substantiality, half-way between those affirmed by individualistic and organicist views

Thermometry Precision in Strongly Correlated Ultracold Lattice Gases

The precise knowledge of the temperature of an ultracold lattice gas simulating a strongly correlated system is a question of both, fundamental and technological importance. Here, we address such question by combining tools from quantum metrology together with the study of the quantum correlations embedded in the system at finite temperatures. Within this frame we examine the spin-$1/2$ XY chain, first estimating, by means of the quantum Fisher information, the lowest attainable bound on the temperature precision. We then address the estimation of the temperature of the sample from the analysis of correlations using a quantum non demolishing Faraday spectroscopy method. Finally, we demonstrate that for sufficiently low temperatures the proposed measurements are optimal to estimate accurately the temperature of the sample.Comment: 16 pages, 5 figure

Long-range multipartite entanglement close to a first order quantum phase transition

We provide insight in the quantum correlations structure present in strongly correlated systems beyond the standard framework of bipartite entanglement. To this aim we first exploit rotationally invariant states as a test bed to detect genuine tripartite entanglement beyond the nearest-neighbor in spin-1/2 models. Then we construct in a closed analytical form a family of entanglement witnesses which provides a sufficient condition to determine if a state of a many-body system formed by an arbitrary number of spin-1/2 particles possesses genuine tripartite entanglement, independently of the details of the model. We illustrate our method by analyzing in detail the anisotropic XXZ spin chain close to its phase transitions, where we demonstrate the presence of long range multipartite entanglement near the critical point and the breaking of the symmetries associated to the quantum phase transition.Comment: 6 pages, 3 figures, RevTeX 4, the abstract was changed and the manuscript was extended including the contents of the previous appendix

Value, Transcendence and Analogy

Current naturalistic accounts of value face the problem of explaining the normative constraints that value impose on agents. Attempts to solve this problem have progressively relaxed the strictness of naturalistic requirements, up to the point of seeking theistic solutions. However, appeals to God are also problematic, since it is questionable that a relevant notion of God is conceivable at all: if God is wholly other He cannot matter for our choices and if He is a being among natural beings He cannot explain our normative constraints. Engaging a discussion with Fiona Ellis\u2019 treatment of the problem, this essay sketches an account of transcendence, which vindicates the conceivability of a notion of God suitable for the explanation of value. The proposal rests on the possibility of transcendental arguments based on analogy

Formal causation and mental representation : a thomistic proposal

In the past years, the relevance of Thomas Aquinas's theory of cognition for contemporary debates on epistemology has been widely discussed. That theory claims that mind and world are formally identical and that this relationship overcomes various problems associated with scepticism concerning mental representation. The proposal, however, is grounded on the idea that the world can act on the mind through a relation of formal causation. This thesis attempts to develop a Thomistic theory of formal causation which may be suitable for a realist account of mental representation and which may meet the requirements prompted by current discussions. The suggested view is grounded on Aquinas's metaphysics, according to which the world is constituted of substances. The claim that change is possible since substances are hylomorphically constituted (viz., metaphysically composed of form and matter) is defended. Aquinas's claim that some substances have forms which may act independently of matter is also supported. The paradigmatic examples are human souls, i.e. the forms of human beings, whose higher cognitive capacity, i.e. thinking, can be in principle carried on without the need of any material organ. A Thomistic theory of causation is subsequently proposed. It is argued that hylomorphism explains the distinction among four species of causes (material, formal, final and efficient). Aquinas's attempt to explain causal relations conditionally is developed along the lines suggested by John Mackie's INUS conditional analysis. Jaegwon Kim's implementation of Mackie's proposal through an object-based metaphysics of events is then adapted to the hylomorphical account of substances. On these grounds, a theory of formal causation can be proposed and applied to Aquinas's theory of mental representation. The ensuing proposal is offered not in the spirit of historical exegesis but as a substantive philosophical account and it is Thomistic only in the broad sense that it is built on Aquinas's metaphysics and is consistent with his claims on causation