Date/time: 22nd April 2014 2pm BST/UTC+1

Speaker: Chris Richardson (University of Liege)

Title: On the Uncertainty of the Ordering of Nonlocal Wavefunction Collapse when Relativity is Considered

Abstract: The temporal measurement order and therefore the originator of the instantaneous collapse of the wavefunction of a spatiality entangled particle pair can change depending on the reference frame of an observer. This can lead to a paradox in which its seems that both measurements collapsed the wavefunction before the other. We resolve this paradox by demonstrating how attempting to determine the order of measurement of the entangled pair introduces uncertainty which makes the measurement order impossible to know.

To watch the talk live, go to the event page at the appointed hour.

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]]>Date/time: Tuesday 25th March 2014 2pm GMT/UTC

Speaker: Tobias Fritz (Perimeter Institute)

Title: A Combinatorial Approach to Nonlocality and Contextuality

Abstract:

Most work on contextuality so far has focused on specific examples and concrete proofs of the Kochen-Specker theorem, while general definitions and theorems about contextuality are sparse. For example, it is commonly believed that nonlocality is a special case of contextuality, but what exactly does this mean? After a brief discussion of previous work, I will introduce our “device-independent” approach to contextuality based on the mathematics of test spaces and explain how nonlocality is indeed a special case of contextuality. This work builds on the graph-theoretic approach of Cabello, Severini and Winter by improving on several of its shortcomings and merging it with the work of Foulis and Randall on test spaces. Our results include:

(1) various relationships to graph invariants, similar to CSW;

(2) a proof that our set of quantum models cannot be characterized by a graph invariant;

(3) a proof that the set of all models satisfying the Consistent Exclusivity principle at any number of copies is not convex;

(4) new results on the Shannon capacity of graphs;

(5) an “inverse sandwich conjecture” with ramifications for C*-algebra theory and quantum logic.

This talk is based on arXiv:1212.4084.

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]]>Date/time: Tue. 25th Feb. at 4pm GMT/UTC

Speaker: Nicholas Brunner (University of Geneva)

Title: Dimension of Physical Systems

Abstract: The dimension of a physical system refers loosely speaking to the number of degrees of freedom relevant to describe it. Here we ask how quantum theory compares to more general models (such as Generalized Probabilistic Theories) from the point of view of dimension. This gives insight to information processing and thermodynamics in GPTs.

To watch the talk live, visit the event page at the appointed hour.

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]]>Date: 28th January 2014

Time: 2pm UTC/GMT

Speaker: Troels Frimodt Rønnow (ETH Zurich)

Title: Quantum annealing on 503 qubits

Abastract: Quantum speedup refers to the advantage of quantum devices can over classical ones in solving classes of computational problems. In this talk we show how to correctly define and measure quantum speedup in experimental devices. We show how to avoid issues that might mask or fake quantum speedup. As illustration we will compare the performance of a D-Wave Two quantum annealing device on random spin glass instances to simulated classical and quantum annealers, and other classical solvers.

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

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]]>Date/time: Tue. 26th Nov. 3pm GMT/UTC

Speaker: Mark Wilde (Louisiana State University)

Title: Strong Converse Theorems in Quantum Information Theory

Abstract: One of the main goals in quantum information theory is to establish the capacity of a quantum channel for communicating various kinds of information, whether it be bits or qubits. While several communication capacities of quantum channels are now known, the characterization of capacity in many of these cases is often limited to it being a threshold that determines the rates at which reliable communication is or is not possible. While this characterization might be satisfactory for some purposes, it leaves open the possibility for a trade-off between communication rate and error probability (that is, one might think that it would be possible to send data at a higher rate by allowing for errors to occur some of the time). However, we now know that such a trade-off is not possible for many channels and capacities of interest. That is, many researchers have now established that a strong converse theorem holds for several channels and capacities, so that as soon as the communication rate exceeds capacity, it is guaranteed that the error probability converges to one in the limit of large blocklength, no matter what strategy the sender and receiver employ. These strong converse theorems strengthen the interpretation and our understanding of capacity as a very sharp dividing line between rates for which asymptotically perfect communication is possible and rates for which an error is guaranteed to occur (analogous to a phase transition in statistical physics). This Q+ talk will review much of the progress in establishing strong converse theorems for several channels and their communication capacities in quantum information theory.

Joint work with Bhaskar Roy Bardhan (LSU Baton Rouge), Manish K. Gupta (LSU Baton Rouge), Naresh Sharma (TIFR Mumbai), Dong Yang (UAB Barcelona), and Andreas Winter (UAB Barcelona).

To watch the talk live, go to http://gplus.to/qplus at the appointed hour. To stay up to date on the latest news about Q+ hangouts you can follow us on:

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

]]>Date/Time: 29th October 2013 2pm GMT

Speaker: Renato Renner (ETH Zurich)

Title: Does freedom of choice imply that the wave function is real?

Abstract:

The question whether the quantum-mechanical wave function is “real” has recently attracted considerable interest. More precisely, the question is whether the wave function of a system is uniquely determined by any complete description of its “physical state”. In this talk I will present a simple and self-contained proof that this is indeed the case, under an assumption that one may term “freedom of choice”. It demands that arbitrary measurements can be applied to the system, and that these can be chosen independently of all parameters available at the time of measurement (with respect to any relativistic frame). A possible interpretation of this result is that the wave function of a system is as “objective or “real as any other complete description of the system’s state.

(This is based on unpublished work in collaboration with Roger Colbeck.)

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

Note that the change from daylight savings time to standard time will have happened in the UK, but not some other countries like the US and Canada. Therefore, your usual timezone calculation may be out by an hour, e.g. the talk is at 10am in East Coast US and Canada. Please check the time conversion for your location.

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]]>- Title: Computing With Quantum Cats: From Colossus To Qubits
- Author: John Gribbin
- Publisher: Bantam, 2013

- Title: Schrödinger’s Killer App: Race To Build The World’s First Quantum Computer
- Author: Jonathan Dowling
- Publisher: CRC Press, 2013

The task of writing a popular book on quantum computing is a daunting

one. In order to get it right, you need to explain the subtleties of

theoretical computer science, at least to the point of understanding

what makes some problems hard and some easy to tackle on a classical

computer. You then need to explain the subtle distinctions between

classical and quantum physics. Both of these topics could, and indeed

have, filled entire popular books on their own. Gribbin’s strategy is

to divide his book into three sections of roughly equal length, one on

the history of classical computing, one on quantum theory, and one on

quantum computing. The advantage of this is that it makes the book

well paced, as the reader is not introduced to too many new ideas at

the same time. The disadvantage is that there is relatively little

space dedicated to the main topic of the book.

In order to weave the book together into a narrative, Gribbin

dedicates each chapter except the last to an individual prominent

scientist, specifically: Turing, von Neumann, Feynman, Bell and

Deutsch. This works well as it allows him to interleave the science

with biography, making the book more accessible. The first two

sections on classical computing and quantum theory display Gribbin’s

usual adeptness at popular writing. In the quantum section, my usual

pet peeves about things being described as “in two states at the same

time” and undue prominence being given to the many-worlds

interpretation apply, but no more than to any other popular treatment

of quantum theory. The explanations are otherwise very good. I

would, however, quibble with some of the choice of material for the

classical computing section. It seems to me that the story of how we

got from abstract Turing machines to modern day classical computers,

which is the main topic of the von Neumann chapter, is tangential to

the main topic of the book, and Gribbin fails to discuss more relevant

topics such as the circuit model and computational complexity in this

section. Instead these topics are squeezed in very briefly into the

quantum computing section, and Gribbin flubs the description of

computational complexity. For example, see if you can spot the

problems with the following three quotes:

“…problems that can be solved by efficient algorithms belong to a

category that mathematicians call `complexity class P’…”

“Another class of problem, known as NP, are very difficult to

solve…”

“All problems in P are, of course, also in NP.”

The last chapter of Gribbin’s book is an tour of the proposed

experimental implementations of quantum computing and the success

achieved so far. This chapter tries to cover too much material too

quickly and is rather credulous about the prospects of each

technology. Gribbin also persists with the device of including potted

biographies of the main scientists involved. The total effect is like

running at high speed through an unfamiliar woods, while someone slaps

you in the face rapidly with CVs and scientific papers. I think the

inclusion of such a detailed chapter was a mistake, especially since

it will seem badly out of date in just a year or two. Finally,

Gribbin includes an epilogue about the controversial issue of discord

in non-universal models of quantum computing. This is a bold

inclusion, which will either seem prescient or silly after the debate

has died down. My own preference would have been to focus on

well-established theory.

In summary, Gribbin’s has written a good popular book on quantum

computing, perhaps the best so far, but it is not yet a great one. It

is not quite the book you should give to your grandmother to explain

what you do. I fear she will unjustly come out of it thinking she is

not smart enough to understand, whereas in fact the failure is one of

unclear explanation in a few areas on the author’s part.

Dowling’s book is a different kettle of fish from Gribbin’s. He

claims to be aiming for the same audience of scientifically curious

lay readers, but I am afraid they will struggle. Dowling covers more

or less everything he is interested in and I think the rapid fire

topic changes would leave the lay reader confused. However, we all

know that popular science books written by physicists are really meant

to be read by other physicists rather than by the lay reader. From

this perspective, there is much valuable material in Dowling’s book.

Dowling is really on form when he is discussing his personal

experience. This mainly occurs in chapters 4 and 5, which are about

the experimental implementation of quantum computing and other quantum

technologies. There is also a lot of material about the internal

machinations of military and intelligence funding agencies, which

Dowling has copious experience of on both sides of the fence. Much of

this material is amusing and will be of value to those interested in

applying for such funding. As you might expect, Dowling’s assessment

of the prospects of the various proposed technologies is much more

accurate and conservative than Gribbin’s. In particular his treatment

of the cautionary tale of NMR quantum computing is masterful and his

assessment of non fully universal quantum computers, such as the D-Wave

One, is insightful. Dowling also gives an excellent account of quantum

technologies beyond quantum computing and cryptography, such as

quantum metrology, which are often neglected in popular treatments.

Chapter 6 is also interesting, although it is a bit of a hodge-podge

of different topics. It starts with a debunking of David Kaiser’s

thesis that the “hippies” of the Fundamental Fysiks group in Berkeley

were instrumental in the development of quantum information via their

involvement in the no-cloning theorem. Dowling rightly points out

that the origins of quantum cryptography are independent of this,

going back to Wiesner in the 1970′s, and that the no-cloning theorem

would probably have been discovered as a result of this. This section

is only missing a discussion of the role of Wheeler, since he was

really the person who made it OK for mainstream physicists to think

about the foundations of quantum theory again, and who encouraged his

students and postdocs to do so in information theoretic terms. Later

in the chapter, Dowling moves into extremely speculative territory,

arguing for “the reality of Hilbert space” and discussing what quantum

artificial intelligence might be like. I disagree with about as much

as I agree with in this section, but it is stimulating and

entertaining nonetheless.

You may notice that I have avoided talking about the first few

chapters of the book so far. Unfortunately, I do not have many

positive things to say about them.

The first couple of chapters cover the EPR experiment, Bell’s theorem,

and entanglement. Here, Dowling employs the all too common device of

psychoanalysing Einstein. As usual in such treatments, there is a

thin caricature of Einstein’s actual views followed by a lot of

comments along the lines of “Einstein wouldn’t have liked this” and

“tough luck Einstein”. I personally hate this sort of narrative with

a passion, particularly since Einstein’s response to quantum theory

was perfectly rational at the time he made it and who knows what he

would have made of Bell’s theorem? Worse than this, Dowling’s

treatment perpetuates the common myth that determinism is one of the

assumptions of both the EPR argument and Bell’s theorem. Of course,

CHSH does not assume this, but even EPR and Bell’s original argument

only use it when it can be derived from the quantum predictions.

Thus, there is not the option of “uncertainty” for evading the

consequences of these theorems, as Dowling maintains throughout the

book.

However, the worst feature of these chapters is the poor choice of

analogy. Dowling insists on using a single analogy to cover

everything, that of an analog clock or wristwatch. This analogy is

quite good for explaining classical common cause correlations,

e.g. Alice and Bob’s watches will always be anti-correlated if they

are located in timezones with a six hour time difference, and for

explaining the use of modular arithmetic in Shor’s algorithm.

However, since Dowling has earlier placed such great emphasis on the

interpretation of the watch readings in terms of actual time, it falls

flat when describing entanglement in which we have to imagine that the

hour hand randomly points to an hour that has nothing to do with time.

I think this is confusing and that a more abstract analogy,

e.g. colored balls in boxes, would have been better.

There are also a few places where Dowling makes flatly incorrect

statements. For example, he says that the OR gate does mod 2 addition

and he says that the state |00> + |01> + |10> + |11> is entangled. I

also found Dowling’s criterion for when something should be called an

ENT gate (his terminology for the CNOT gate) confusing. He says that

something is not an ENT gate unless it outputs an entangled state, but

of course this depends on what the input state is. For example, he

says that NMR quantum computers have no ENT gates, whereas I think

they do have them, but they just cannot produce the pure input states

needed to generate entanglement from them.

The most annoying thing about this book is that it is in dire need of

a good editor. There are many typos and basic fact-checking errors.

For example, John Bell is apparently Scottish and at one point a D-Wave

computer costs a mere $10,000. There is also far too much repetition.

For example, the tale of how funding for classical optical computing

dried up after Conway and Mead instigated VLSI design for silicon

chips, but then the optical technology was reused used to build the

internet, is told in reasonable detail at least three different times.

The first time it is an insightful comment, but by the third it is

like listening to an older relative with a limited stock of stories.

There are also whole sections that are so tangentially related to the

main topic that they should have been omitted, such as the long anti

string-theory rant in chapter six.

Dowling has a cute and geeky sense of humor, which comes through well

most of the time, but on occasion the humor gets in the way of clear

exposition. For example, in a rather silly analogy between Shor’s

algorithm and a fruitcake, the following occurs:

“We dive into the molassified rum extract of the classical core of the

Shor algorithm fruitcake and emerge (all sticky) with a theorem proved

in the 1760s…”

If he were a writing student, Dowling would surely get kicked out of

class for that. Finally, unless your name is David Foster Wallace, it

is not a good idea to put things that are essential to following the

plot in the footnotes. If you are not a quantum scientist then it is

unlikely that you know who Charlie Bennett and Dave Wineland are or

what NIST is, but then the quirky names chosen in the first few

chapters will be utterly confusing. They are explained in the main

text, but only much later. Otherwise, you have to hope that the

reader is not the sort of person who ignores footnotes. Overall,

having a sense of humor is a good thing, but there is such a thing as

being too cute.

Despite these criticisms, I would still recommend Dowling’s book to

physicists and other academics with a professional interest in quantum

technology. I think it is a valuable resource on the history of the

subject. I would steer the genuine lay reader more in the direction

of Gribbin’s book, at least until a better option becomes available.

Upon reformatting my articles for the blog, I realized that I have reached almost Miguel Navascues levels of crankiness. I guess this might be because I had a stomach bug when I was writing them. Today’s article is a criticism of the recent “Snapshots of Foundational Attitudes Toward Quantum Mechanics” surveys that appeared on the arXiv and generated a lot of attention. The article is part of a point-counterpoint, with Nathan Harshman defending the surveys. Here, I am only posting my part in its original version. The newsletter version is slightly edited from this, most significantly in the removal of my carefully constructed title.

Q1. Which of the following questions is best resolved by taking a straw

poll of physicists attending a conference?

A. How long ago did the big bang happen?

B. What is the correct approach to quantum gravity?

C. Is nature supersymmetric?

D. What is the correct way to understand quantum theory?

E. None of the above.

By definition, a scientific question is one that is best resolved by

rational argument and appeal to empirical evidence. It does not

matter if definitive evidence is lacking, so long as it is conceivable

that evidence may become available in the future, possibly via

experiments that we have not conceived of yet. A poll is not a valid

method of resolving a scientific question. If you answered anything

other than E to the above question then you must think that at least

one of A-D is not a scientific question, and the most likely culprit

is D. If so, I disagree with you.

It is possible to legitimately disagree on whether a question is

scientific. Our imaginations cannot conceive of all possible ways,

however indirect, that a question might get resolved. The lesson from

history is that we are often wrong in declaring questions beyond the

reach of science. For example, when big bang cosmology was first

introduced, many viewed it as unscientific because it was difficult to

conceive of how its predictions might be verified from our lowly

position here on Earth. We have since gone from a situation in which

many people thought that the steady state model could not be

definitively refuted, to a big bang consensus with wildly fluctuating

estimates of the age of the universe, and finally to a precision value

of 13.77 +/- 0.059 billion years from the WMAP data.

Traditionally, many physicists separated quantum theory into its

“practical part” and its “interpretation”, with the latter viewed as

more a matter of philosophy than physics. John Bell refuted this by

showing that conceptual issues have experimental consequences. The

more recent development of quantum information and computation also

shows the practical value of foundational thinking. Despite these

developments, the view that “interpretation” is a separate

unscientific subject persists. Partly this is because we have a

tendency to redraw the boundaries. “Interpretation” is then a

catch-all term for the issues we cannot resolve, such as whether

Copenhagen, Bohmian mechanics, many-worlds, or something else is the

best way of looking at quantum theory. However, the lesson of big

bang cosmology cautions against labelling these issues unscientific.

Although interpretations of quantum theory are constructed to yield

the same or similar enough predictions to standard quantum theory,

this need not be the case when we move beyond the experimental regime

that is now accessible. Each interpretation is based on a different

explanatory framework, and each suggests different ways of modifying

or generalizing the theory. If we think that quantum theory is not

our final theory then interpretations are relevant in constructing its

successor. This may happen in quantum gravity, but it may equally

happen at lower energies, since we do not yet have an experimentally

confirmed theory that unifies the other three forces. The need to

change quantum theory may happen sooner than you expect, and whichever

explanatory framework yields the next theory will then be proven

correct. It is for this reason that I think question D is scientific.

Regardless of the status of question D, straw polls, such as the three

that recently appeared on the arXiv [1-3], cannot help us to resolve

it, and I find it puzzling that we choose to conduct them for this

question, but not for other controversial issues in physics. Even

during the decades in which the status of big bang cosmology was

controversial, I know of no attempts to poll cosmologists’ views on

it. Such a poll would have been viewed as meaningless by those who

thought cosmology was unscientific, and as the wrong way to resolve

the question by those who did think it was scientific. The same is

true of question D, and the fact that we do nevertheless conduct polls

suggests that the question is not being treated with the same respect

as the others on the list.

Admittedly, polls about controversial scientific questions are

relevant to the sociology of science, and they might be useful to the

beginning graduate student who is more concerned with their career

prospects than following their own rational instincts. From this

perspective, it would be just as interesting to know what percentage

of physicists think that supersymmetry is on the right track as it is

to know about their views on quantum theory. However, to answer such

questions, polls need careful design and statistical analysis. None

of the three polls claims to be scientific and none of them contain

any error analysis. What then is the point of them?

The three recent polls are based on a set of questions designed by

Schlosshauer, Kofler and Zeilinger, who conducted the first poll at a

conference organized by Zeilinger [1]. The questions go beyond just

asking for a preferred interpretation of quantum theory, but in the

interests of brevity I will focus on this aspect alone. In the

Schlosshauer et al. poll, Copenhagen comes out top, closely followed

by “information-based/information-theoretical” interpretations. The

second comes from a conference called “The Philosophy of Quantum

Mechanics” [2]. There was a larger proportion of self-identified

philosophers amongst those surveyed and “I have no preferred

interpretation” came out as the clear winner, not so closely followed

by de Broglie-Bohm theory, which had obtained zero votes in the poll

of Schlosshauer et al. Copenhagen is in joint third place along with

objective collapse theories. The third poll comes from “Quantum

theory without observers III” [3], at which de Broglie-Bohm got a

whopping 63% of the votes, not so closely followed by objective

collapse.

What we can conclude from this is that people who went to a meeting

organized by Zeilinger are likely to have views similar to Zeilinger.

People who went to a philosophy conference are less likely to be

committed, but are much more likely to pick a realist interpretation

than those who hang out with Zeilinger. Finally, people who went to a

meeting that is mainly about de Broglie-Bohm theory, organized by the

world’s most prominent Bohmians, are likely to be Bohmians. What have

we learned from this that we did not know already?

One thing I find especially amusing about these polls is how easy it

would have been to obtain a more representative sample of physicists’

views. It is straightforward to post a survey on the internet for

free. Then all you have to do is write a letter to Physics Today

asking people to complete the survey and send the URL to a bunch of

mailing lists. The sample so obtained would still be self-selecting

to some degree, but much less so than at a conference dedicated to

some particular approach to quantum theory. The sample would also be

larger by at least an order of magnitude. The ease with which this

could be done only illustrates the extent to which these surveys

should not even be taken semi-seriously.

I could go on about the bad design of the survey questions and about

how the error bars would be huge if you actually bothered to calculate

them. It is amusing how willing scientists are to abandon the

scientific method when they address questions outside their own field.

However, I think I have taken up enough of your time already. It is

time we recognized these surveys for the nonsense that they are.

[1] M. Schlosshauer, J. Kofler and A. Zeilinger, A Snapshot of

Foundational Attitudes Toward Quantum Mechanics, arXiv:1301.1069

(2013).

[2] C. Sommer, Another Survey of Foundational Attitudes Towards

Quantum Mechanics, arXiv:1303.2719 (2013).

[3] T. Norsen and S. Nelson, Yet Another Snapshot of Foundational

Attitudes Toward Quantum Mechanics, arXiv:1306.4646 (2013).

Speaker: Steven Flammia (University of Sydney)

Title: Thermalization, Error-Correction, and Memory Lifetime for Ising Anyon Systems

Abstract:

We consider two-dimensional lattice models that support Ising anyonic excitations and are coupled to a thermal bath, and we propose a phenomenological model to describe the resulting short-time dynamics, including pair-creation, hopping, braiding, and fusion of anyons. By explicitly constructing topological quantum error-correcting codes for this class of system, we use our thermalization model to estimate the lifetime of quantum information stored in the code space. To decode and correct errors in these codes, we adapt several existing topological decoders to the non-Abelian setting: one based on Edmond’s perfect matching algorithm and one based on the renormalization group. These decoders provably run in polynomial time, and one of them has a provable threshold against a simple iid noise model. Using numerical simulations, we find that the error correction thresholds for these codes/decoders are comparable to similar values for the toric code (an Abelian sub-model consisting of a restricted set of allowed anyons). To our knowledge, these are the first threshold results for quantum codes without explicit Pauli algebraic structure. Joint work with Courtney Brell and Simon Burton.

To watch the talk live go to the Q+ page at the appointed hour.

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

- Google+: http://gplus.to/qplus
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or visit our website at http://qplus.burgarth.de

]]>