Tag Archives: quantum

FQXi Essay Contest

I wrote an essay for the FQXi essay contest.  This year’s theme is “It from bit or bit from it?” and I decided to write about the extent to which Wheeler’s “it from bit” helps us to understand the origin of quantum probabilities from a subjective Bayesian point of view.   You can go here to read and rate the essay and it would be especially great if any fellow FQXi members would do that.

Q+ Hangout: Roger Colbeck

Here are the details of the next Q+ hangout.

Date: Tuesday 29th January 2013

Time: 2pm GMT/UTC

Speaker: Roger Colbeck (ETH Zurich)

Title: No extension of quantum theory can have improved predictive power

Abstract:

According to quantum theory, measurements generate random outcomes, in stark contrast with classical mechanics. This raises the important question of whether there could exist an extension of the theory which removes this indeterminism, as famously suspected by Einstein, Podolsky and Rosen. Under the assumption of free choice within a particular causal structure, Bell’s work showed this to be impossible. However, existing results do not imply that the current theory is maximally informative. Could it be that certain hidden variable theories (for example) allow us to make more accurate predictions about the outcomes?

In this talk, I will discuss this question and show that, under the same free choice assumption, the answer is negative: no extension of quantum theory can give more information about the outcomes of future measurements than quantum theory itself.

I will then show that as a corollary of this result, we can reach the same conclusion as Pusey, Barrett and Rudolph that the wavefunction cannot be thought of as subjective.

(This is based on arXiv:1005.5173, arXiv:1111.6597 and arXiv:1208.4123)

To watch the talk live go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Rob Spekkens

Here are the details of the next Q+ hangout.

Date/time: Tuesday 20th November 2pm GMT/UTC

Speaker: Rob Spekkens (Perimeter Institute)

Title: Quantum correlations from the perspective of causal discovery algorithms

Abstract: If correlation does not imply causation, then what does? The beginning of a rigorous answer to this question has been provided by researchers in machine learning, who have developed causal discovery algorithms. These take as their input facts about correlations among a set of observed variables and return as their output a causal structure relating these variables. We show that any attempt to provide a causal explanation of Bell-inequality-violating correlations must contradict a core principle of these algorithms, namely, that an observed statistical independence between variables should not be explained by fine-tuning of the causal parameters. In particular, we demonstrate the need for such fine-tuning for most of the causal mechanisms that have been proposed to underlie Bell correlations, including superluminal causal influences, superdeterminism (that is, a denial of freedom of choice of settings), and retrocausal influences which do not introduce causal cycles. This work suggests a novel perspective on the assumptions underlying Bell’s theorem: the nebulous assumption of “realism” is replaced with the principle that all correlations ought to be explained causally, and Bell’s notion of local causality is replaced with the assumption of no fine-tuning. Finally, we discuss the possibility of avoiding the fine-tuning by replacing conditional probabilities with a noncommutative generalization thereof.

Based on arXiv:1208.4119.

Joint work with Chris Wood.

To watch the talk live, go to http://gplus.to/qplus at the appointed hour.

Don’t forget to fill out our survey to have your say on who we should invite for future talks.

Q+ Hangout: Markus Mueller

Here are the details of the next Q+ hangout.

Date/time: Tuesday 23rd October 2pm BST

Speaker: Markus Mueller (Perimeter Institute)

Title: Three-dimensionality of space and the quantum bit: an information-theoretic approach

Absract: It is sometimes pointed out as a curiosity that the state space of quantum two-level systems, i.e. the qubit, and actual physical space are both three-dimensional and Euclidean. In this talk, I report on joint work with Lluis Masanes, where we attempt an information-theoretic analysis of this relationship, by proving a particular mathematical result: suppose that physics takes place in d spatial dimensions, and that some events happen probabilistically (not assuming quantum theory in any way). Furthermore, suppose there are systems that behave in some sense as “units of direction information”, interacting via some continuous reversible time evolution. We prove that this uniquely determines spatial dimension d=3 and quantum theory on two qubits. Hence, abstractly postulating the “nice” behavior of a Stern-Gerlach device in information-theoretic terms determines already some important aspects of physics as we know it.

This talk is based on http://arxiv.org/abs/1206.0630

To watch the talk live go to http://qplus.to/qplus at the appointed hour.

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Q+ Hangout: Stephanie Wehner

Here are the details for the next Q+ hangout.

Date: 25th September 2012

Time: 10am British Summer Time

Speaker: Stephanie Wehner (Singapore)

Title: Uncertainty and the generation of randomness

Abstract: We consider the creation of classical randomness by making measurements on a largely unknown quantum system. We show a close relations between measurements that are good at this task and measurements that satisfy strong uncertainty relations. Finally, we mention an application to quantum cryptography.

Please note the unusual start time.

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

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Rerun: Caslav Brukner’s Q+ hangout

We are rerunning Caslav Brukner’s Q+ hangout due to problems with the livestream and video recording in July. Note the earlier than usual starting time.

Date: 18th September 2012

Time: 12noon British Summer Time

Speaker: Caslav Brukner (University of Vienna)

Title: Quantum correlations with indefinite causal order

Abstract:

In quantum physics it is standardly assumed that the background time or definite causal structure exists such that every operation is either in the future, in the past or space-like separated from any other operation. Consequently, the correlations between operations respect definite causal order: they are either signalling correlations for the time-like or no-signalling correlations for the space-like separated operations. We develop a framework that assumes only that operations in local laboratories are described by quantum mechanics (i.e. are completely-positive maps), but relax the assumption that they are causally connected. Remarkably, we find situations where two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operations is before or after the other. The correlations between the operations are shown to enable performing a communication task (“causal game”) that is impossible if the operations are ordered according to a fixed background time.

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

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Q+ Hangout: Francesco Buscemi

Here are the details for the next Q+ hangout.

Date: 28th August 2012

Time: 2pm British Summer Time

Speaker: Francesco Buscemi (Nagoya University)

Title: All entangled quantum states are nonlocal: equivalence between locality and separability in quantum theory

Abstract:

In this talk I will show how, by slightly modifying the rules of nonlocal games, one can prove that all entangled states violate local realism.

As it is well known, Bell inequalities, which are used to test the violation of local realism, can be equivalently reformulated in terms of nonlocal games (namely, cooperative games with incomplete information) played between one referee and two (or more) players, these latter being separated so to make any form of communication between them impossible during the game. Quantum nonlocality is that property of quantum states that allows players sharing them to win nonlocal games more frequently than the assumption of local realism would imply.

However, as Werner proved in 1989, not all quantum states enable such a violation of local realism. In particular, Werner showed the existence of quantum states that cannot be created locally (the so-called “entangled” states) and, yet, do not allow any violation of local realism in nonlocal games. This fact has been since then considered an unsatisfactory gap in the theory, attracting a considerable amount of attentions in the literature.

In this talk I will present a simple proof of the fact that all entangled states indeed violate local realism. This will be done by considering a new larger class of nonlocal games, which I call “semiquantum,” differing from the old ones merely in that the referee can now communicate with the players through quantum channels, rather than being restricted to use classical ones, as it was tacitly assumed before. I will then prove that one quantum state always provides better payoffs than another quantum state, in semiquantum nonlocal games, if and only if the latter can be obtained from the former, by local operations and shared randomness (LOSR). The main claim will then follow as a corollary.

The new approach not only provides a clear theoretical picture of the relation between locality and separability, but also suggests, thanks to its simplicity, new experimental tests able in principle to verify the violation of local realism in situations where previous experiments would fail.

Based on http://arxiv.org/abs/1106.6095

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

To stay up to date on future Q+ hangouts, follow us on:

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or visit our website http://qplus.burgarth.de