Tag Archives: Quantum computers

Quantum computing: achievable reality or unrealistic dream

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Michel Dyakonov’s View on QC                                     My view (based on Michel’s drawing*)

Update:

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Alexander Vlasov’s view (based on Michel and Konstantin’s drawing)

It has been a while since I devoted a post to quantum computation. Meanwhile, we had a cozy, almost private, easy-going, and very interesting discussion thread on my previous, March 2014 post (that featured my Simons Institute videotaped lectures (I,II).)

What can we learn from a failure of quantum computers?

Last week we had a workshop on “Quantum computing: achievable reality or unrealistic dream.” This was a joint venture of the  American Physics Society and the Racah Institute of Physics here at HUJI, organized by Professor Miron Ya. Amusia, and it featured me and Nadav Katz as the main speakers. Here are the slides of my lecture: What can we learn from a failure of quantum computers.

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Noise Sensitivity and BosonSampling

Earlier, I gave a lecture in our CS colloquium about a recent work with Guy Kindler on noise sensitivity of BosonSampling. We show that for a constant level of noise, noisy BosonSampling can be approximated by bounded-depth computation, and that the correlation between the noisy outcome and the noiseless outcome tends to zero if the noise level is ω(1/n) where n is the number of bosons.  Here is the paper Gaussian noise sensitivity and BosonSampling, the videotaped lecture  Complexity and sensitivity of noisy BosonSampling, and the slides of the lecture.

Contagious error sources would need time travel to prevent quantum computation

On the positive side, Greg Kuperberg and I wrote a paper  Contagious error sources would need time travel to prevent quantum computation  showing that for a large class of correlated noise, (teleportation-based) quantum fault-tolerance works! Greg and I have had a decade-long email discussion (over 2000 emails) regarding quantum computers, and this work grew from our 2009 discussion (about my “smoothed Lindblad evolution” model), and heavily relies on  ideas of Manny Knill.

Nadav Katz: Quantum information science – the state of the art

Some years ago, two brilliant experimentalists, Hagai Eisenberg and Nadav Katz,  joined  the already strong, mainly theoretical, quantum information group here at HUJI.  Nadav Katz gave the second lecture in the workshop, and here are the slides of Nadav’s  lecture: Quantum information science – the state of the art.

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Experimental progress toward stable encoded qubits

Also very much on the positive side, Nadav mentioned a remarkable recent progress by the Martini’s group showing certain encoded states based on 9 physical qubits which are order-of-magnitude (factor 8.4, to be precise,) more stable than the “raw” qubits used for creating them !!

Here is a link to the paper:  State preservation by repetitive error detection in a superconducting quantum circuit, by J. Kelly, R. Barends, A. G. Fowler, A. Megrant, E. Jeffrey, T. C. White, D. Sank, J. Y. Mutus, B. Campbell, Yu Chen, Z. Chen, B. Chiaro, A. Dunsworth, I.-C. Hoi, C. Neill, P. J. J. O’Malley, C. Quintana, P. Roushan, A. Vainsencher, J. Wenner, A. N. Cleland, and John M. Martinis.

Update:  Further comments on a Shtetl-optimized post (especially a comment by Graeme Smith,) help to place the new achievement of the Martinis group within the seven smilestones toward quantum computers from a 2012 Science paper by Schoelkopf and Devoret, originated by David DiVincenzo’s 2000 paper “The physical implementation of quantum computation“. (You can watch these milestone here also .)

The new achievement of having a very robust realization of certain encoded states can be seen as achieving the 3.5 milestone.   The difference between the 3.5th milestone and the 4th milestone plays a central role in the seventh post of my 2012-debate with Aram Harrow in connection with a conjecture I made in the first post (“Conjecture 1″). Aram made the point that classical error-correction can lead to very stable encoded qubits in certain states (which is essentially the 3.5 milestone). I gave a formal description of the conjecture, which essentially asserts that the 4th milestone, namely insisting that encoded qubits allows arbitrary superpositions, cannot be reached.  As I said many times (see, for example, the discussion in my 2012 Simons Institute videotaped lecture 2), a convincing demonstration of the 4th milestone, namely  implementation of quantum error-correction with encoded qubits which are substantially more stable than the raw qubits (and allow arbitrary superposition for the encoded qubit) will disprove my conjectures. Such stable encoded qubits are  expected from implementations of distance-5 surface code. So we are 0.5 milestones away :)

I will be impressed to see even a realization of distance-3 (or distance-5) surface code that will give good quality encoded qubits, even if the encoded qubits will have a quality which is somewhat worse than that of the raw qubits used for the encoding. These experiments, including those that were already carried out, also give various other opportunities to test my conjectures.

Peter Shor’s challenge #1 and my predictions from the failure of quantum computation

My lecture on predictions from the failure of QC is based on two lengthy recent comments (first, second) regarding predictions from the failure of quantum computers. On April 2014, Peter Shor challenged me with the following: Continue reading

Why Quantum Computers Cannot Work: The Movie!

Here are links to a videotaped lecture in two parts entitled “why quantum computers cannot work” recorded at the Simons Institute for the Theory of Computing on December 2013 and two additional videos: a short talk on topological quantum computers and a twelve minute overview.  Here are the links: OverviewPart IPart IITopological QC.  (Update, Nov 14: BosonSampling.)

Why Quantum Computers Cannot Work:

Overview and Vision.

Why Quantum Computers Cannot Work I:

From the “Trivial Flow” to Smoothed Lindblad Evolutions

Why Quantum Computers Cannot Work II:

Debate, Reasons to Disbelieve, and Experimentation

Why Topological Quantum Computers Cannot Work

The Geometry of Spacetime is Enabled by the Failure of Quantum Fault-Tolerance

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Left: Nick Read; Right The front page of Nick’s 1990 famous paper with Greg Moore on nonabelions, and below his email to me from March 2005 on topological quantum computation. (click for full view.)

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Left: the argument regarding topological QC demonstrated via Harris’ famous cartoon. While not strictly needed I expect the argument to extend from qubits to gates as well. Right: a recent discussion with Nick over Shtetl Optimized (click for full view). Update: We are actually not in an agreement as it seems from the above discussion (see the discussion below). 

Update (Nov’ 2014): A fifth video, this time in front of a live audience

Complexity and Sensitivity of Noisy BosonSampling

Update: A subsequent post by Steve Flammia, Quantum computers can work in principle over The Quantum Pontiff. (July:) See also this post: Quantum future” just beyond our grasp.

Added later (April 18): Let me quote from what Steve wrote about the videos: The surprising part is the superior production value relative to your typical videotaped lecture (at least for the first overview video). Producing the videos was an interesting and demanding experience and I was certainly happy to read Steve’s description of the production value.  (Of course, the main purpose of Steve’s post was to express his disagreement with the content of the videos. See either the post or Co-‘s comment below.) 

Also there are two earlier versions of my lecture (in 1-hour format) that were videotaped. The first was taken by Jesus De Loera in Davis. Very interesting shooting-angle and interesting comments by Greg Kuperberg, Bruno Nachtergaele and other participants.  The second was taken in Seattle in a UW-PIMS colloquium lecture. Again interesting questions by several participants including James Lee and John Sidles.

(July:) The Simons Institite (almost identical) versions of the movies are now linked from the web-page of my November 15 lecture at SI.

(Added nov 2014): The only difference from the HUJI version is that there are no opening slides and that for the closing slides I used two pictures of my department’s administrative staff.

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The administrative crew of the Einstein Institite of Mathematics (click to enlarge)

I thought of it as a nice opportunity to thank our great administrative staff whose part is crucial  in the academic endeavor, and this is a good opportunity to thank the staff in my second academic home – Yale University, in the Simons Institute, in many other places.

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Alistair Sinclair and the Simons Institure friendly and helpful staff (click for full size) 

Following Saharon Shelah: How to watch these videos

(Added Nov 2014)

Saharon Shelah explained in an introduction to one of his books, that instructions on “how to read this book” are actually instruction on “how to not read this book”. If you want to read the book you start on page 1 and read through to the last page.  Instructions for “how to read  this book” rather tell you how to jump to a place that interests you.

So, in a similar spirit, here are direct links to the different parts of the videos.

Continue reading

Pictures from Recent Quantum Months

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A special slide I prepared for my lecture at Gdansk featuring Robert Alicki and I as climber on the mountain of quantum computers “because it is not there.”

It has been quite a while since I posted here about quantum computers. Quite a lot happened in the last months regarding this side of my work, and let me devote this post mainly to pictures. So here is a short summary going chronologically backward in time. Last week – Michel Dyakonov visited Jerusalem, and gave here the condensed matter physics seminar on the spin Hall effect. A couple of weeks before in early January we had the very successful Jerusalem physics winter school on Frontier in quantum information. (Here are the recorded lectures.) Last year I gave my evolving-over-time lecture on why quantum computers cannot work in various place and different formats in Beer-Sheva, Seattle, Berkeley, Davis (CA), Gdansk, Paris, Cambridge (US), New-York, and Jerusalem. (The post about the lecture at MIT have led to a long and very interesting discussion mainly with Peter Shor and Aram Harrow.) In August I visited Robert Alicki, the other active QC-skeptic, in Gdansk and last June I took part in organizing a (successful) quantum information conference Qstart in Jerusalem (Here are the recorded lectures.).

And now some pictures in random ordering

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With Robert Alicki in Gdynia near Gdansk

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With (from left) Connie Sidles, Yuri Gurevich, John Sidles and Rico Picone in Seattle  (Victor Klee used to take me to the very same restaurant when I visited Seattle in the 90s and 00s.) Update: Here is a very interesting post on GLL entitled “seeing atoms” on John Sidles work.

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With Michel Dyakonov (Jerusalem, a few days ago)

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With Michal Horodecki in Sopot  near Gdansk (Michal was a main lecturer in our physics school a few weeks ago.)

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Aram Harrow and me meeting a year ago at MIT.

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Sometimes Robert and I look skeptically in the same direction and other times we look skeptically in opposite directions. These pictures are genuine! Our skeptical face impressions are not staged. The pictures were taken by Maria, Robert’s wife. Robert and I are working for many years (Robert since 2000 and I since 2005) in trying to examine skeptically the feasibility of quantum fault-tolerance. Various progress in experimental quantum error-correction and other experimental works give good reasons to believe that our views could be examined in the rather near future.

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A slide I prepared for a 5-minute talk at the QSTART rump session referring to the impossibility of quantum fault-tolerance as a mild earthquake with wide impact.

GTprod2This is a frame from the end-of-shooting of a videotaped lecture on “Why quantum computers cannot work” at the Simons Institute for the Theory of Computing at Berkeley. Producing a videotaped lecture is a very interesting experience! Tselil Schramm (in the picture holding spacial sets of constant width) helped me with this production.

My Quantum Debate with Aram III

This is the third and last post giving a timeline and some non technical highlights from my debate with Aram Harrow.  

Where were we

After Aram Harrow and I got in touch in June 2011, and decided to have a blog debate towards the end of 2011, the first post in our debate describing my point of view was launched on January, 2012 and was followed by three posts by Aram. The discussion was intensive and interesting.  Here is a link to my 2011 paper that initiated the debate and to a recent post-debate presentation at MIT.

 Happy_Passover  Happy passover, readers!

Back to the debate: Conjecture C is shot down!

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In addition to his three posts, Aram and Steve Flammia wrote a paper refuting one of my Conjectures (Conjecture C).  We decided to devote a post to this conjecture.

Quantum refutations and reproofs

Post 5, May 12, 2012. One of Gil Kalai’s conjectures refuted but refurbished

Niels Henrik Abel was the patron saint this time

The first version of the post started with this heartbreaking eulogy for Conjecture C. At the end most of it was cut away. But the part about Aram’s grandchildren was left in the post.

Eulogy for Conjecture C

(Gil; old version:) When Aram wrote to me, inn June 2011, and expressed willingness to publicly discuss my paper, my first reaction was to decline and propose having just private discussions. Even without knowing Aram’s superb track record in debates, I knew that I put my beloved conjectures on the line. Some of them, perhaps even all of them, will not last. Later, last December, I changed my mind and Aram and I started planning our debate. My conjectures and I were fully aware of the risks. And it was Conjecture C that did not make it.

A few words about Conjecture C

Conjecture C, while rooted in quantum computers skepticism, was a uniter and not a divider! It expressed our united aim to find a dividing line between the pre- and post- universal quantum computer eras.

Aram’s grandchildren and the world before quantum computers


When Aram’s grandchildren will ask him: “
Grandpa, how was the world before quantum computers?” he could have replied: “I hardly remember, but thanks to Gil we have some conjectures recording the old days, and then he will state to the grandchildren Conjectures 1-4 and the clear dividing line in terms of Conjecture C, and the grandchildren will burst in laughter about the old days of difficult entanglements.” Continue reading

Mittag-Leffler Institute and Yale, Winter 2005; Test your intuition: Who Played the Piano?

This is a little “flashback” intermission in my posts about my debate with Aram Harrow. This time I try to refer to Cris Moore’s question regarding  the motivation for my study. For the readers it gives an opportunity to win a $50 prize! 

Let me also bring to your attention an interesting discussion (starting here) between Peter Shor and me regarding smoothed Lindblad evolutions.

(Cris Moore, the debate’s very first comment!) I am also a little confused by Gil’s motivation for his conjectures.  (My response:)  To the best of my memory, my main motivation for skeptically studying quantum fault-tolerance was that I thought that this is a direction worth pursuing and that I had a shot at it.

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While listening with Ravi Kannan to this 2002 lecture by Michel Devoret at Yale, I wondered if there are enough scientists working on the “mirage” side.

Flashback: Mittag-Leffler 2005

I started systematically thinking about quantum fault-tolerance in February 2005. There were several things that triggered my interest to the question in the previous fall and I decided to spend some time learning and thinking about it in our winter break.  One of those triggers was something Dorit Aharonov told me a few months earlier: she said that once, when she was telling her students about quantum computers, she suddenly had a feeling that maybe it was all just nonsense. Another trigger came from a former student who told me about a Polish scientist (whose name he could not remember) who wrote an article about impossibility of quantum error-correction. I thought that the lack of a quantum analog of the repetition code, and the unique properties of the majority function  in terms of sensitivity to noise that I studied with Itai Benjamini and Oded Schramm earlier could be a good starting point for looking skeptically at quantum computers.  

In our 2005 winter break, I spent two weeks at Yale and then additional two weeks at the Mittag-Leffler institute near Stockholm.  At Yale, I only had little time to think about quantum computers. I had to finish a survey article with Muli Safra about threshold phenomena (To a volume that Cris Moore and Allon Perkus were among the editors).  One of the last days in Yale we went to dinner with two guests, Chris Skinner who gave the colloquium talk, and Andrei Okounkov who visited me and gave a talk about partition enumeration and mirror symmetry. At the dinner Andrew Casson asked, out of the blue, if we think that quantum computers can be built and it almost seemed as if that Andrew was reading my mind on what I plan to work on the weeks to come. My answer there was the same as my answer now, that I tend to find it implausible.

Mittag-Leffler Institute February 2005 with Xavier Viennot and Alain Lascoux

In Sweden I spent most of my time on quantum fault-tolerance. I was jet-lagged and being jet-lagged in the Mittag-Leffler institute already worked for me once, when finding my subexponential randomized variant of the simplex algorithm was a substitute for sleeping some night in fall 1991 . In 2005 it was not as bad, I just came to my office very early in the morning and started working. And very early in the morning somebody was already playing the piano.

And who was playing the piano at the institute in the cold Swedish mornings of February 2005? The first reader to guess correctly, and convince me in a comment that she or he knows the answer without revealing it to everybody else will get $50. Continue reading

My Quantum Debate with Aram II

This is the second of three posts giving few of the non-technical highlights of my debate with Aram Harrow. (part I)

After Aram Harrow and I got in touch in June 2011, and decided to have a blog debate about quantum fault-tolerance towards the end of 2011, the first post in our debate was launched on January 30, 2012.  The first post mainly presented my point of view and it led to lovely intensive discussions. It was time for Aram’s reply and some people started to lose their patience.

(rrtucky) Is Aram, the other “debater”, writing a dissertation in Greek, as a reply?

Flying machines of the 21st century

Post II, February 6, 2011. First of three responses by Aram Harrow

Dave Bacon was the patron saint for Aram’s first post.

(Aram) There are many reasons why quantum computers may never be built…  The one thing I am confident of is that we are unlikely to find any obstacle in principle to building a quantum computer.

(Aram) If you want to prove that 3-SAT requires exponential time, then you need an argument that somehow doesn’t apply to 2-SAT or XOR-SAT. If you want to prove that the permanent requires super-polynomial circuits, you need an argument that doesn’t apply to the determinant. And if you want to disprove fault-tolerant quantum computing, you need an argument that doesn’t also refute fault-tolerant classical computing.

From the discussion

Why not yet? Boaz set a deadline

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(Boaz Barak could [you] explain a bit about the reasons why people haven’t been able to build quantum computers with more than a handful of qubits yet? Continue reading

My Quantum Debate with Aram Harrow: Timeline, Non-technical Highlights, and Flashbacks I

How the debate came about

   HKD1

(Email from Aram Harrow, June 4,  2011) Dear Gil Kalai, I am a quantum computing researcher, and was wondering about a few points in your paper

(Aram’s email was detailed and thoughtful and at the end he proposed to continue the discussion privately or as part of a public discussion.)

(Gil to Aram) Thank you for your email and interest. Let me try to answer the points you raised. …   (I gave a detailed answer.) …  Right now, I don’t plan on initiating a public discussion. How useful such public discussions are (and how to make them useful) is also an interesting issue. Still they were useful for me, as two of my conjectures were raised first in a discussion on Dave Bacon’s blog and another one is partially motivated by a little discussion with Peter Shor on my blog. Continue reading