Seminar: Questions and progress in quantum information and causality (summer 2019)

Every Thursday, 10:00-11:30, IQOQI Kitchen (Boltzmanngasse 3).

Contact: Markus Müller

Goals and contents: Presentation of very recent research in quantum information theory and causality. There will be an alteration between invited international speakers, and student talks on current research. Note that "quantum information theory" is construed very broadly, including topics like e.g. quantum thermodynamics. For more detailed bureaucratic information, please see u:space.


List of talks

* March 14: Fabien Clivaz, Unifying paradigms of quantum refrigeration: A universal and attainable bound on cooling (Paper)
Cooling quantum systems is arguably one of the most important thermodynamic tasks connected to modern quantum technologies and an interesting question from a foundational perspective. It is thus of no surprise that many different theoretical cooling schemes have been proposed, differing in the assumed control paradigm, complexity and operating either in a single cycle or in steady state limits. Working out bounds on quantum cooling has since been a highly context dependent task with multiple answers and sometimes obscured assumptions. In this work we derive a universal bound for cooling quantum systems in the limit of infinite cycles (or steady state regimes) that is valid for any control paradigm and machine size. The bound only depends on a single parameter of the refrigerator and is theoretically attainable in all control paradigms. For qubit targets we prove that this bound is achievable in a single cycle and by autonomous machines.

* March 21: Ilya Kull, A spacetime area law bound on quantum correlations
Area laws are a far-reaching consequence of the locality of physical interactions, and they are relevant in a range of systems, from black holes to quantum many-body systems. Typically, these laws concern the entanglement entropy or the quantum mutual information of a subsystem at a single time. However, when considering information propagating in spacetime, while carried by a physical system with local interactions, it is intuitive to expect area laws to hold for spacetime regions. In this work, we prove such a law for quantum lattice systems. 
We consider two agents interacting in disjoint spacetime regions with a spin-lattice system that evolves in time according to a local Hamiltonian. In their respective spacetime regions, the two agents apply quantum instruments to the spins. By considering a purification of the quantum instruments, and analyzing the quantum mutual information between the ancillas used to implement them, we obtain a spacetime area law bound on the amount of correlation between the agents' measurement outcomes. Furthermore, this bound applies both to signaling correlations between the choice of operations on the side of one agent, and the measurement outcomes on the side of the other; as well as to the entanglement they can harvest from the spins by coupling detectors to them.

* March 28: Miguel Navascues, Theoretical research without projects (Paper)
We propose a research funding scheme by which each research unit (be it a single scientist, a group leader or a whole institute) applies for funding, but does not specify how much. The decision of how much funds (if any) must be awarded to each unit is taken by the funding agency, based on the recent scientific activity of the unit and its prior funding. To analyze the performance of this scheme, we introduce a mathematical model of the research system. In this model, each research unit possesses a “scientific productivity function” that relates how much science it can produce with the funds it holds to conduct research. These functions are unknown, i.e., neither the research agency nor the scientists themselves can tell how they look like. Relying on this model, we show that there exist systematic procedures to decide the budget distribution at each grant call, with the property that the total scientific production of the research community will be frequently not far off its maximum possible value. In contrast, "excellence schemes" concentrating funds in the hands of a few are shown to converge to configurations where the total scientific productivity is an arbitrarily small fraction of the maximum achievable.
    Joint work with Costantino Budroni

* April 4: David Trillo Fernandez, Remote time manipulation (Paper)
Harnessing the flow of proper time of arbitrary external systems over which we exert little or no control has been a recurring theme in both science and science-fiction. Unfortunately, all relativistic schemes to achieve this effect beyond mere time dilation are utterly unrealistic. In this work, we find that there exist non-relativistic scattering experiments which, if successful, freeze out, speed up or even reverse the free dynamics of any ensemble of quantum systems present in the scattering region. This "time warping" effect is universal, i.e., it is independent of the particular interaction between the scattering particles and the target systems, or the (possibly non-Hermitian) Hamiltonian governing the evolution of the latter. The protocols require careful preparation of the probes which are scattered, and success is heralded by projective measurements of these probes at the conclusion of the experiment. We fully characterize the possible time translations which we can effect on n target systems through a scattering protocol of fixed duration; the core result is that time can be freely distributed between the systems, and reversed at a small cost. For high n, our protocols allow one to quickly send a single system to its far future or past.

* April 11: Philipp Schüttelkopf, Quantum-coherent mixtures of causal relations (Slides: PDF, PowerPoint)
This talk gives an introduction to the paper by MacLean, Ried, Spekkens, and Resch (arXiv:1606.04523). Abstract of the paper:
Understanding the causal influences that hold among parts of a system is critical both to explaining that system’s natural behaviour and to controlling it through targeted interventions. In a quantum world, understanding causal relations is equally important, but the set of possibilities is far richer. The two basic ways in which a pair of
time-ordered quantum systems may be causally related are by a cause-effect mechanism or by a common cause acting on both. Here, we show a coherent mixture of these two possibilities. We realize this nonclassical causal relation in a quantum optics experiment and derive a set of criteria for witnessing the coherence based on a quantum
version of Berkson’s effect, whereby two independent causes can become correlated upon observation of their common effect. The interplay of causality and quantum theory lies at the heart of challenging foundational puzzles, including Bell’s theorem and the search for quantum gravity.

* May 2: TBA

* May 9: Miles Blencowe, The Wigner Current for Open Quantum Systems
We extend the Wigner current vector field (Wigner current) construct to single bosonic mode quantum systems interacting with an environment. In terms of the Wigner function quasiprobability density and associated Wigner current, the open system quantum dynamics can be concisely expressed as a continuity equation. Through the consideration of the harmonic oscillator and additively driven Duffing oscillator in the bistable regime as illustrative system examples, we show how the evolving Wigner current vector field on the system phase space yields useful geometric insights concerning how quantum states decohere away due to interactions with the environment, as well as how they may be stabilized through the counteracting effects of the system anharmonicity (i.e., nonlinearity).

* May 16: Karim Fabian Osman, Beyond Bell's Theorem: Correlation Scenarios
This will be an introduction to the framework as formulated by Tobias Fritz in this paper. The abstract of the paper is as follows:
Bell's Theorem witnesses that the predictions of quantum theory cannot be reproduced by theories of local hidden variables in which observers can choose their measurements independently of the source. Working out an idea of Branciard, Rosset, Gisin and Pironio, we consider scenarios which feature several sources, but no choice of measurement for the observers. Every Bell scenario can be mapped into such a correlation scenario, and Bell's Theorem then discards those local hidden variable theories in which the sources are independent. However, most correlation scenarios do not arise from Bell scenarios, and we describe examples of (quantum) nonlocality in some of these scenarios, while posing many open problems along the way. Some of our scenarios have been considered before by mathematicians in the context of causal inference.

* May 23: TBA

* June 6: Veronika Baumann and Flavio del Santo, TBA

* June 13: TBA

* June 27: TBA

Suggestions for further topics

Quantum Reference Frames
Born rule result