Tag Archives: online

Q+ Hangout: Dietrich Leibfried (NIST)

Here are the details of the next Q+ hangout. This is our “Nobel Prize” lecture. Dietrich is a long time colleague of David Wineland at NIST and will tell us about the latest research from the Ion Storage Group. Please note the unusual start time of 5pm BST(UTC+1)

To join the hangout or watch the livestream go to http://gplus.to/qplus at the appointed hour.

Date: 23rd April 2013 5pm BST(UTC+1)

Speaker: Dietrich Leibfried (NIST)

Title: Towards scalable quantum information processing and quantum simulation with trapped ions

Abstract:
Quantum information processing (QIP) and Quantum Simulation (QS) can potentially provide an exponential speedup for certain problems over the corresponding (known) algorithms on conventional computers. QIP makes use of the counter-intuitive properties of quantum mechanics, like entanglement and the superposition principle (being in more states than one simultaneously). On the way towards a useful QIP device these properties, mostly subject of thought experiments so far, will have to become a practical reality. I will discuss experiments towards Quantum Information Processing (QIP) and Quantum Simulation (QS) with trapped ions. Most requirements for QIP and QS have been demonstrated in this system, with two big challenges remaining: Improving operation fidelity and scaling up to larger numbers of qubits.

The architecture pursued at the Ion Storage Group at NIST is based on quantum information stored in long lived internal (hyperfine) states of the ions. We investigate the use of laser beams and microwave fields to induce both single-qubit rotations and multi-qubit gates mediated by the Coulomb interaction between ions. Moving ions through a multi-zone trap architecture allows for keeping the number of ions per zone small, while sympathetic cooling with a second ion species can remove energy and entropy from the system.

After a brief introduction to these elements, I will present the current status of experiments and some future perspectives for QIP and QS.

This work has been supported by IARPA, DARPA, ARO, ONR, and the NIST Quantum Information Program.

To keep up to date with the latest news and announcements about Q+ hangouts you can follow us on:

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

Q+ Hangout: Ivette Fuentes

Here are the details of the next Q+ hangout.

Date/time: Tue. 26th March 2pmGMT/UTC

Speaker: Ivette Fuentes (University of Nottingham)

Title: Quantum information processing in spacetime

Abstract:

Cutting-edge experiments in quantum communications are reaching regimes where relativistic effects can no longer be neglected. For example, there are advanced plans to use satellites to implement teleportation and quantum cryptographic protocols. Relativistic effects can be expected at these regimes: the Global Positioning System (GPS), which is a system of satellites that is used for time dissemination and navigation, requires relativistic corrections to determine time and positions accurately.

Therefore, it is timely to understand what are the effects of gravity and motion on entanglement and other quantum properties exploited in quantum information.

In this talk I will show that entanglement can be created or degraded by gravity and non-uniform motion. While relativistic effects can degrade the efficiency of teleportation between moving observers, the effects can also be exploited in quantum information. I will show that the relativistic motion of a quantum system can be used to perform quantum gates. Our results, which will impact future space-based experiments, can be demonstrated in table-top experiments using superconducting circuits.

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

To keep up to date on the latest news about Q+ hangouts, you can follow us on:

or visit our website.

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.

To keep up to date with Q+ hangouts, follow us on:

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:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

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

Q+ Hangout: Caslav Brukner

Here are the details for the next Q+ hangout.

Date: 24th July 2012

Time: 2pm 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.

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

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

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

Q+ Hangout: Scott Aaronson

Here are the details for the next Q+ hangout.

Date: 19th June 2012

Time: 2pm British Summer Time

Speaker: Scott Aaronson (MIT)

Title: Quantum Money from Hidden Subspaces

Abstract:

Forty years ago, Wiesner pointed out that quantum mechanics raises the striking possibility of money that cannot be counterfeited according to the laws of physics. We propose the first quantum money scheme that is (1) public-key—meaning that anyone can verify a banknote as genuine, not only the bank that printed it, and (2) cryptographically secure, under a “classical” hardness assumption that has nothing to do with quantum money. Our scheme is based on hidden subspaces, encoded as the zero-sets of random multivariate polynomials. A main technical advance is to show that the “black-box” version of our scheme, where the polynomials are replaced by classical oracles, is unconditionally secure. Previously, such a result had only been known relative to a quantum oracle (and even there, the proof was never published). Even in Wiesner’s original setting—quantum money that can only be verified by the bank—we are able to use our techniques to patch a major security hole in Wiesner’s scheme. We give the first private-key quantum money scheme that allows unlimited verifications and that remains unconditionally secure, even if the counterfeiter can interact adaptively with the bank. Our money scheme is simpler than previous public-key quantum money schemes, including a knot-based scheme of Farhi et al. The verifier needs to perform only two tests, one in the standard basis and one in the Hadamard basis—matching the original intuition for quantum money, based on the existence of complementary observables. Our security proofs use a new variant of Ambainis’s quantum adversary method, and several other tools that might be of independent interest.

Based on http://arxiv.org/abs/1203.4740

Joint work with Paul Christiano

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:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

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

Q+ Hangout: Vlatko Vedral

Date: Tuesday 8th May 2012

Time: 14:00 British Summer Time

Speaker: Vlatko Vedral (University of Oxford/National University of Singapore)

Title: Using Temporal Entanglement to Perform Thermodynamical Work

Abstract: Here we investigate the impact of temporal entanglement on a system’s ability to perform thermodynamical work. We show that while the quantum version of the Jarzynski equality remains satisfied even in the presence of temporal entanglement, the individual thermodynamical work moments in the expansion of the free energy are, in fact, sensitive to the genuine quantum correlations. Therefore, while individual moments of the amount of thermodynamical work can be larger (or smaller) quantumly than classically, when they are all combined together into the (exponential of) free energy, the total effect vanishes to leave the Jarzynski equality intact. Whether this is a fortuitous coincidence remains to be seen, but it certainly goes towards explaining why the laws of thermodynamics happen to be so robust as to be independent of the underlying micro-physics. We discuss the relationship between this result and thermodynamical witnesses of spatial entanglement as well as explore the subtle connection with the “quantum arrow of time”.

Based on http://arxiv.org/abs/1204.5559 and possibly also http://arxiv.org/abs/1204.6168 if time permits

To watch the seminars live, go to http://gplus.to/qplus at the appointed hour. You do not need a Google account to watch, but you do need one if you would like to be able to participate in the question and answer session at the end of the talk.

To stay up to date on the scheduled seminars you can visit our website or follow us on various social networks:

Our website: http://qplus.burgarth.de

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://facebook.com/qplushangouts

We also encourage you to suggest speakers for future talks. You can do so by adding them to the spreadsheet at http://bit.ly/qplussuggestions.

Q+ Hangout: Jonathan Oppenheim

Some of you may be aware that, in collaboration with Daniel Burgarth, I have been organizing a series of online seminars on quantum information and foundations using Google+ hangouts. I have avoided advertising them on this blog so far because there used to be a limit on the number of people who could attend and I didn’t want them to get too oversubscribed. However, recently we have gained the ability to stream the seminars to a large number of people, so I will crosspost the announcements here from now on.

You can watch the seminars live on any computer with an internet connection, or after the fact on YouTube. The details of the next seminar are:

Date: 24th April 2012

Time: 14:00 British Summer Time

Title: Fundamental limitations for quantum and nano thermodynamics

Speaker: Jonathan Oppenheim (University College London)

Abstract:

The relationship between thermodynamics and statistical physics is valid in the thermodynamic limit — when the number of particles involved becomes very large. Here we study thermodynamics in the opposite regime — at both the nano scale, and when quantum effects become important. Applying results from quantum information theory we construct a theory of thermodynamics in these extreme limits. In the quantum regime, we find that the standard free energy no longer determines the amount of work which can be extracted from a resource, nor which state transitions can occur spontaneously. We derive a criteria for thermodynamical state transitions, and find two free energies: one which determines the amount of work which can be extracted from a small system in contact with a heat bath, and the other which quantifies the reverse process. They imply that generically, there are additional constraints which govern spontaneous thermodynamical processes. We find that there are fundamental limitations on work extraction from nonequilibrium states, due to both finite size effects which are present at the nano scale, as well as quantum coherences. This implies that thermodynamical transitions are generically irreversible at this scale, and we quantify the degree to which this is so, and the condition for reversibility to hold. There are particular equilibrium processes which approach the ideal efficiency, provided that certain special conditions are met.

Based on http://arxiv.org/abs/1111.3834.

Biography:

Jonathan Oppenheim is has recently been appointed professor at University College London. He is an expert in quantum information theory and quantum gravity. His Ph.D. under Bill Unruh at the University of British Columbia was on Quantum time. In 2004 he was a postdoctoral researcher under Jacob Bekenstein and then held a University Research Fellowship at Cambridge University. Together with Michał Horodecki and Andreas Winter, he discovered quantum state-merging and used this primitive to show that quantum information could be negative.

To watch the seminars live, go to http://gplus.to/qplus at the appointed hour. You do not need a Google account to watch, but you do need one if you would like to be able to participate in the question and answer session at the end of the talk.

To stay up to date on the scheduled seminars you can visit our website or follow us on various social networks:

Our website: http://qplus.burgarth.de

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://facebook.com/qplushangouts

We also encourage you to suggest speakers for future talks. You can do so by adding them to the spreadsheet at http://bit.ly/qplussuggestions.