Author Archives: mleifer

Q+ hangout: Chris Richardson

Here are the details of the next Q+ hangout.

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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Q+ Hangout: Tobias Fritz

Here are the details of the next Q+ hangout. To watch live, visit the event page at the appointed hour. Be aware of the fact that although North America has switched to DST, the UK has not yet and the time is still listed in GMT/UTC.

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.

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

or visit our website.

Q+ Hangout: Nicholas Brunner

Here are the details of the next Q+ hangout.

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.

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

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

Q+ Hangout: Troels Frimodt Rønnow

Here are the details of the next Q+ hangout

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.

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

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

Q+ Hangout: Mark Wilde

Here are the details of the next Q+ hangout.

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:

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

Q+ Hangout: Renato Renner

Here are the details of the next Q+ hangout.

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.

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

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

Quantum Times Book Reviews

Following Tuesday’s post, here is the second piece I wrote for the latest issue of the Quantum Times. It is a review of two recent popular science books on quantum computing by John Gribbin and Jonathan Dowling. Jonathan Dowling has the now obligatory book author’s blog, which you should also check out.

Book Review

  • 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.