Seminars

Quantum Darwinism posits that the emergence of a classical reality relies on the spreading of classical information from a quantum system to many parts of its environment. But what are the essential physical principles of quantum theory that make this mechanism possible? We address this question by formulating the simplest instance of Darwinism — CNOT-like fan-out interactions — in a class of probabilistic theories that contain classical and quantum theory as special cases. We determine necessary and sufficient conditions for any theory to admit such interactions. We find that every non-classical theory that admits this spreading of classical information must have both entangled states and entangled measurements. Furthermore, we show that Spekkens’ toy theory admits this form of Darwinism, and so do all probabilistic theories that satisfy principles like strong symmetry, or contain a certain type of decoherence processes. Our result suggests the counterintuitive general principle that in the presence of local non-classicality, a classical world can only emerge if this non-classicality can be “amplified” to a form of entanglement.

Nonlocality, as demonstrated by the violation of Bell inequalities, enables device-independent cryptographic tasks that do not require users to trust their apparatus. In this presentation, we consider devices whose inputs are spatiotemporal degrees of freedom, e.g. orientations or time durations. Without assuming the validity of quantum theory, we prove that the devices’ statistical response must respect their input’s symmetries, with profound foundational and technological implications. We exactly characterize the bipartite binary quantum correlations in terms of local symmetries, indicating a fundamental relation between spacetime and quantum theory. For Bell experiments characterized by two input angles, we show that the correlations are accounted for by a local hidden variable model if they contain enough noise, but conversely must be nonlocal if they are pure enough. This allows us to construct a “Bell witness” that certifies nonlocality with fewer measurements than possible without such spatiotemporal symmetries, suggesting a new class of semi-device-independent protocols for quantum technologies.

With the study of ever decreasing systems, it’s paramount to understand thermodynamic phenomena in ultra-small scales, where quantum phenomena live. This gave origin to the flourishing research field known as quantum thermodynamics. One of its main branches, which deals with quantum systems undergoing thermodynamic cycles, is called quantum heat engines (QHEs). These are assembled to convert one kind of energy into another, namely heat into work, in the quantum domain. The idea of converting heat into work has been intensively explored since the First Industrial Revolution, but it gains new direction with the many possibilities brought by quantum theory resources, such as coherence and entanglement. For optimizing the output power of these QHEs, one must consider finite-time operation. In this seminar, I will present a new model of finite-time QHEs. By means of using collisional models, a cyclic sequence of pure heat and pure work strokes are applied to a generic quantum chain. This gives rise to a stroboscopic evolution of the state of the quantum chain, which presents a transient regime as well as a limit cycle. Once reached the limit cycle, the results show that the heat exchanged with the heat baths depends solely on the boundary sites of the quantum chain. The model is proved useful for the optimization of the output power of stroke-based QHEs both analytically and numerically. One curious feature of this framework is that, for a given family of models containing a specific kind of internal interactions, there is a universal efficiency value, the Otto efficiency, which remains the same for any cycle period.

We present a framework in which unique games are represented in the form of edge-labelings of graphs. We define an equivalence relation which preserves both classical and quantum values of games. We also show how the classical value of the game depends on the properties of the cycles within the labeled graph. In particular, we relate the problem of computing the classical value of single-party anti-correlation XOR games to finding the edge bipartization number of a graph, and connect the computation of the classical value of XOR-d games to the identification of specific cycles in the graph. In some specific cases this approach allows us to calculate the classical values of games.

A standardly adopted notion of classicality is the following: a system is deemed classical if its set of states is a simplex. Also, it is traditionally excluded that a classical theory may admit of entangled states. This is of course the case for classical theory (CT). An operational probabilistic theory where all systems are classical, and all pure states of composite systems are entangled, will be presented. The theory, called bilocal classical theory (BCT), is endowed with a rule for composing an arbitrary number of systems, and with a nontrivial set of transformations. Moreover, BCT is proved to be well-posed using an exhaustive procedure to construct generic theories, along with a sufficient set of conditions to verify their consistency. Hence, BCT demonstrates that the presence of entanglement is independent of the existence of incompatible measurements. Some phenomena occurring in the theory are compatible with CT or quantum theory (QT)—such as the existence of a universal processor, cloning, entanglement swapping, dense coding, additivity of classical capacities—while others contradict both CT and QT, including: non-monogamous entanglement and hypersignaling. The theory is causal and satisfies the no-restriction hypothesis. At the same time, it violates a number of information-theoretic principles enjoyed by QT, most notably: local discriminability, purity of parallel composition of states, and purification. Some open problems raised by BCT will be pointed out. In particular, a no-go conjecture for the existence of a local-realistic ontological model associated with BCT will be formulated.

REFERENCES
—Classical theories with entanglement. Giacomo Mauro D’Ariano, Marco Erba, and Paolo Perinotti, Phys. Rev. A 101, 042118
—Classicality without local discriminability: Decoupling entanglement and complementarity. Giacomo Mauro D’Ariano, Marco Erba, and Paolo Perinotti, Phys. Rev. A 102, 052216

Because of the approaching deadline of 15/12/2020 for many NCN grant applications, the Director, himself a member of NCN and a co-author in many of its projects, will share his expertise in writing them.

Because of the approaching deadline of 15/12/2020 for many NCN grant applications, the Director, himself a member of NCN and a co-author in many of its projects, will share his expertise in writing them.

The aim of this journal club talk will be the discussion of a topic that is getting more attention in the quantum thermodynamics community. In fact, numerous efforts, both theoretically and experimentally, are devoted to design technologies able to control and to route the heat flow in qubit systems suitable for the realization of quantum circuits.
In particular, the purpose is to discuss the experimental implementation with superconducting qubits in J. Senior at al., arXiv:1908.05574.

What makes quantum theory stand out against general stochastic classical theories? Such a question necessitates a precise evaluation of the assumptions that go into the definition of classicality. To be of foundational, as well as technological significance, these assumptions must be operationally falsifiable. In this seminar, we shall derive operational properties, specifically statistical equalities, for any number of generalized (with unbounded cardinality) classical ensembles. These properties rely exclusively on operationally falsifiable assumptions. We shall then demonstrate the operational quantum violation of the equality for three preparations. This stems from (hidden-variable) incompleteness of quantum formalism, which in-turn powers quantum advantage.
This forms a follow-up to arXiv:1909.07293v2.

We describe the dynamics of a quantum field coupled to a moving heat bath, in the formalism of the Markovian master equation for the field considered as an open system. We apply this to the superradiance of a rotating black hole, which provides a useful paradigm for understanding other irreversible active processes. Fermions can’t superradiate, but work may be extracted from their motion-induced population inversion in the presence of two baths. We argue that this describes the triboelectric effect (the charging of rough surfaces by rubbing). We also apply this formalism to shock waves, fleshing out Zel’dovich’s intuition that in this case “quantum mechanics helps understand classical mechanics”, and Ginzburg’s insight that “radiation during the uniform motion of various sources is a universal phenomenon rather than an eccentricity”. Finally, we argue that our interpretation of the triboelectric effect offers a qualitatively new mechanism for CP violation in fundamental physics.