*After John Preskill’s call for more quantum participation at the APS March Meeting, I couldn’t say no to a request for a blurb about quantum info and foundations! The following is a guest post by Giulio Chiribella.*

Following up the previous post by John Preskill, I’d like draw your attention to the focus session “Quantum Information for Quantum Foundations” which will take place at the APS March Meeting 2012.

If you are interested in the conceptual and fundamental aspects of Quantum Information, this is the right session for you to come and present your work! The event promises to be lively and stimulating, and will be a great occasion to advertise your recent results.

On top of that, your participation will give an important support to foundational research.The foundational space at the March meeting is a great opportunity for our community. But it is vital to keep this space alive, responding with a visible participation and presenting talks on the best of the foundational research in Quantum Information. This is not hard to do: Over the past few years there has been an enormous amount of progresses and a burst of new exciting results at the interface between Quantum Information and Foundations. Also, we should not forget of the numerous results in Quantum Information that, even without being explicitly foundational, continue to shed a bright light on the operational features of Quantum Theory. It is enough to have such a vibrant scientific landscape represented next March in Boston to make the foundational session memorable!

This year’s session will start with an invited talk by Valerio Scarani, who will summarize the main ideas and the latest developments on Information Causality. Valerio’s talk will be followed by a lineup of contributed talks, which hopefully would be as many and as lively as the talks of last year’s edition, organized by Chris Fuchs, which has been a very successful event.

To participate to the session, you can submit your abstract at the webpage http://www.aps.org/meetings/abstract/index.cfm. (don’t forget that the deadline for submissions is this Friday November 11th 2011!)

A last remark before concluding: Chatting with colleagues sometimes I noticed that potential speakers are discouraged by the 12 minutes format, which seems too short to present all the relevant details. We should remind, however, that presenting details is not really the point here: The APS Meetings are huge events where the whole physics community meets to highlight advancements and to advertise new ideas across fields, not to address the specialists of one particular field. The format of the March Meeting talks is designed to rapidly advertise new results, and if you discover that you would like to know more about one particular result… well, during the meeting there is a lot of free time where you can interact directly (and more efficiently) with the speaker about the details of her/his work.

So, let us take the event in the right spirit and make the foundational space at the March Meeting a real exciting forum for the exchange of new ideas!

Hope to see many of you in Boston!

## GQI needs you! — John Preskill guest blogs

*The following post was written by John Preskill.*

I’m writing this guest post because I want all you quantum informationists to come to the American Physical Society 2012 March Meeting in Boston next February 27 through March 2. I’m telling you now, because the deadline for submitting your contributed abstract is next Friday. That’s 11/11/11, which is easy to remember. And lucky.

Why come?

Okay, maybe the March Meeting is not for everyone. Yes, last year there were over 8000 physicists at the meeting in Dallas, including over 3200 students. But that’s not everyone. No, not quite.

And those of us who came might not treasure every memory. Standing dutifully in line for what seems like hours, yet never quite reaching that much needed cup of coffee or sandwich. Missing at least 44 of 45 parallel sessions, while wondering what’s the harm in missing all 45? Surrendering to drowsiness after too many 10-minute talks in a row. Wondering who all these people are.

No, it’s not perfect. But the March Meeting is an exhilarating experience, and you really don’t want to miss it. Though you might want to consider bringing your own sandwich.

Quantum information has been well represented at the March Meeting since 2005, thanks to the efforts of the APS Topical Group on Quantum Information (GQI). Each year the membership of GQI has swelled and our participation in the March Meeting has ramped up, which allows GQI to have an even larger presence in the next meeting. Last year 311 GQI members attended and there were 324 contributed talks in the quantum information sessions. I hope we will beat those numbers substantially in 2012.

The March Meeting provides a valuable opportunity for quantum information enthusiasts to exchange ideas with the broader physics community, and to convey the excitement of our field. Naturally, since the March Meeting is dominated by the condensed matter physicists, the interface of quantum information with condensed matter is especially emphasized, but contributions in all areas of quantum information science are welcome.

The 2012 program will be especially exciting. GQI will sponsor or co-sponsor six sessions of invited talks covering important recent developments: topological quantum computing with Majorana fermions, quantum entanglement in many-body systems, quantum simulations, quantum computing with superconducting circuits, quantum information processing in diamond, and silicon spin qubits. In addition, there will be an invited session about career opportunities in undergraduate teaching for quantum information scientists.

If our turnout is below expectations, the GQI Program Chair gets the blame. And that’s me. So … don’t make me look bad — come to Boston and contribute a talk!

To register and submit an abstract go to:

http://www.aps.org/meetings/march/

You should flag your abstract with an appropriate quantum information Focus Topic or Sorting Category from the list available at the website, to ensure that your talk is scheduled in the proper session.

And if you are not a member of GQI, please join. Increasing our numbers means more visibility and influence for GQI within APS, and that too is good for the cause of quantum information science.

See you in Boston!

## Dial M for Matter

It was just recently announced that Institute for Quantum Information at Caltech will be adding an extra letter to its name. The former IQI will now be the Institute for Quantum Information and Matter, or IQIM. But it isn’t the name change that is of real significance, but rather the $12.6 million of funding over the next five years that comes with it!

In fact, the IQIM is an NSF funded Physics Frontier Center, which means the competition was stiff, to say the least. New PFCs are only funded by the NSF every three years, and are chosen based on “their potential for transformational advances in the most promising research areas at the intellectual frontiers of physics.”

In practice, the new center means that the Caltech quantum info effort will continue to grow and, importantly, it will better integrate and expand on experimental efforts there. It looks like an exciting new chapter for quantum information science at Caltech, even if the new name is harder to pronounce. Anyone who wants to become a part of it should check out the open postdoc positions that are now available at the IQIM.

## Let Eve do the heavy lifting, while John and Won-Young keep her honest.

Photon detectors have turned out to be an Achilles’ heel for quantum key distribution (QKD), inadvertently opening the door of Bob’s lab to subtle side-channel attacks, most famously

quantum hacking, in which a macroscopic light signal from Eve subverts Bob’s detectors into seeing all and only the “photons” she wants him to see. Recently Lo, Curty, and Qi (“LCQ”) have combined several preexisting ideas into what looks like an elegant solution for the untrusted detector problem, which they call measurement-device-independent QKD. In brief, they let Eve operate the detectors and broadcast the measurement results, but in a way that does not require Alice or Bob to trust anything she says.

Precursors of this approach include device-independent QKD, in which neither the light sources nor the detectors need be trusted (but unfortunately the detectors need to be impractically efficient) and time-reversed Bell-state methods, in which a Bell measurement substitutes for the Bell-state preparation at the heart of most entanglement-based QKD. It has also long been understood that quantum teleportation can serve as a filter to clean an untrusted quantum signal, stripping it of extraneous degrees of freedom that might be used as side channels. A recent eprint by Braunstein and Pirandola develops the teleportation approach into a mature form, in which side channel attacks are prevented by the fact that no quantum information ever enters Alice’s or Bob’s lab. (This paper is accompanied by an unusual “posting statement,” the academic analog of a Presidential signing statement in US politics. This sort of thing ought to be little needed and little used in our collegial profession.) Two more ingredients bring the LCQ proposal to an exciting level of practicality: weak coherent pulse sources, and decoy states. In the LCQ protocol, Alice and Bob each operate, and must trust, a local random number generator and a weak coherent source (e.g. an attenuated laser with associated polarization-control optics) which they aim at Eve, who makes measurements effectively projecting pairs of simultaneously-arriving dim light pulses onto the Bell basis. If Eve lies about which Bell state she saw, she will not be believed, because her reported results will be inconsistent with the states Alice and Bob know they sent. The final ingredient needed to keep Eve honest, the decoy-state technique introduced by W.Y. Hwang and subsequently developed by many others, prevents Eve from lying about the efficiency of her detectors, for example reporting a successful 2-photon coincidence only when she has received more than one photon from each sender. Fitting all the pieces together, it appears that the LCQ protocol would work over practical distances, with practical sources and detectors, and, if properly implemented, be secure against known attacks, short of bugging or eavesdropping on the interior of Alice’s or Bob’s lab.

Alice and Bob still need to trust their lasers, polarization and attenuation optics, and random number generators, and of course their control software. It is hard to see how Alice and Bob can achieve this trust short of custom-building these items themselves, out of mass-marketed commodity components unlikely to be sabotaged. A considerable element of do-it-yourself is probably essential in any practical cryptosystem, classical or quantum, to protect it from hidden bugs. CHB acknowledges helpful discussions with Paul Kwiat, who is however not responsible for any opinions expressed here.

## Entangled LIGO

The quest to observe gravitational waves has been underway for several years now, but as yet there has been no signal. To try to detect gravitational waves, the LIGO collaboration basically uses huge kilometer-scale Michaelson-type interferometers, one of which is seen in the aerial photo to the left. When a gravitational wave from, say, a supernova or in-spiraling pair of black holes arrives at the detector, the wave stretches and shrinks spacetime in the transverse directions, moving the test masses at the ends of the interferometer arms and hence changing the path length of the interferometer, creating a potentially observable signal.

The problem is, the sensitivity requirements are extreme. So extreme in fact, that within a certain frequency band the limiting noise comes from vacuum fluctuations of the electromagnetic field. Improving the signal-to-noise ratio can be achieved by a “classical” strategy of increasing the circulating light power, but this strategy is limited by the thermal response of the optics and can’t be used to further increase sensitivity.

But as we all know, the quantum giveth and the quantum taketh away. Or alternatively, we can fight quantum with quantum! The idea goes back to a seminal paper by Carl Caves, who showed that using squeezed states of light could reduce the uncertainty in an interferometer.

What’s amazing is that in a new paper, the LIGO collaboration has actually succeeded for the first time in using squeezed light to increase the sensitivity of one of its gravity wave detectors. Here’s a plot of the noise at each frequency in the detector.The red line shows the reduced noise when squeezed light is used. To get this to work, the squeezed quadrature must be in phase with the amplitude (readout) quadrature of the observatory output light, and this results in path entanglement between the photons in the two beams in the arms of the interferometer. The fluctuations in the photon counts can only be explained by stronger-than-classical correlation among the photons.

It looks like quantum entanglement might play a very important role in the eventual detection of gravitational waves. Tremendously exciting stuff.

## Stability of Topological Order at Zero Temperature

From today’s quant-ph arXiv listing we find the following paper:

### Stability of Frustration-Free Hamiltonians, by S. Michalakis & J. Pytel

This is a substantial generalization of one of my favorite results from last year’s QIP, the two papers by Bravyi, Hastings & Michalakis and Bravyi & Hastings.

In this new paper, Michalakis and Pytel show that any local gapped frustration-free Hamiltonian which is topologically ordered is stable under quasi-local perturbations. Whoa, that’s a mouthful… let’s try to break it down a bit.

Recall that a local Hamiltonian for a system of n spins is one which is a sum of polynomially many terms, each of which acts nontrivially on at most k spins for some constant k. Although this definition only enforces *algebraic* locality, let’s go ahead and require *geometric* locality as well by assuming that the spins all live on a lattice in d dimensions and all the interactions are localized to a ball of radius 1 on that lattice.

Why should we restrict to the case of geometric locality? There are at least two reasons. First, spins on a lattice is an incredibly important special case. Second, we have very few tools for analyzing quantum Hamiltonians which are k-local on a general hypergraph. Actually, few means something closer to none. (If you know any, please mention them in the comments!) On cubic lattices, we have many powerful techniques such Lieb-Robinson bounds, which the above results make heavy use of [1].

We say a Hamiltonian is *frustration-free* if the ground space is composed of states which are also ground states of *each term separately.* Thus, these Hamiltonians are “quantum satisfiable”, as a computer scientist would say. This too is an important requirement, since it is one of the most general classes of Hamiltonians about which we have any decent understanding. There are several key features of frustration-free Hamiltonians, but perhaps chief among them is the *consistency* of the ground space. The ground states on a local patch of spins are always globally consistent with the ground space of the full Hamiltonian, a fact which isn’t true for frustrated models.

We further insist that the Hamiltonian is gapped, which in this context means that there is some constant γ>0 independent of the system size which lower bounds the energy of any eigenstate orthogonal to the ground space. The gap assumption is extremely important since it is again closely related to the notion of locality. The spectral gap sets an energy scale and hence also a length scale, the correlation length. For two disjoint regions of spins separated by a length L in the lattice, the connected correlation function for any pair or local operators decays exponentially in L.

The last property, topological order, can be tricky to define. One of the key insights of this paper is a new definition of a sufficient condition for topological stability that the authors call *local topological order*. Roughly speaking, this new condition says that ground states of the *local* Hamiltonian are not distinguishable by any (sufficiently) local operator, except up to small effects that vanish rapidly in a neighborhood of the support of the local operator. Thus, the ground space can be used to encode quantum information which is insensitive to local operators! Since nature presumably acts locally and hence can’t corrupt the (nonlocally encoded) quantum information, systems with topological order would seem to be great candidates for quantum memories. Indeed, this was exactly the motivation when Kitaev originally defined the toric code.

Phew, that was a lot of background. So what exactly did Michalakis and Pytel prove, and why is it important? They proved that if a Hamiltonian satisfying the above criteria is subject to a sufficiently weak but *arbitrary* quasi-local perturbation then two things are stable: the spectral gap and the ground state degeneracy. (Quasi-local just means that strength of the perturbation decays sufficiently fast with respect to the size of the supporting region.) A bit more precisely, the spectral gap remains bounded from below by a constant independent of the system size, and the ground state degeneracy splits by an amount which is at most exponentially small in the size of the system.

There are several reasons why these stability results are important. First of all, the new result is very general: generic frustration-free Hamiltonians are a substantial extension of frustration-free commuting Hamiltonians (where the BHM and BH papers already show similar results). It means that the results potentially apply to models of topological quantum memory based on subsystem codes, such as that proposed by Bombin, where the syndrome measurements are only two-body. Second, the splitting of the ground state degeneracy determines the dephasing (T2) time for any qubits encoded in that ground space. Hence, for a long-lived quantum memory, the smaller the splitting the better. These stability results promise that even imperfectly engineered Hamiltonians should have an acceptably small splitting of the ground state degeneracy. Finally, a constant spectral gap means that when the temperature of the system is such that kT<<γ, thermal excitations are suppressed exponentially by a Boltzmann factor. The stability results show that the cooling requirements for the quantum memory do not increase with the system size.

Ah, but now we have opened a can of worms by mentioning temperature… The stability (or lack there of) of topological quantum phases at finite temperature is a fascinating topic which is the focus of much ongoing research, and perhaps it will be the subject of a future post. But for now, congratulations to Michalakis and Pytel on their interesting new paper.

##### [1] Of course, Lieb-Robinson bounds continue to hold on arbitrary graphs, it’s just that the bounds don’t seem to be very useful.

## TEDxCaltech Videos

Coming online now: http://tedxcaltech.com/. See the Optimizer pontificate about P versus NP (now who can verify the Feynman quote at the beginning?)

and two prestigious professors goof around:

## TEDxCaltech Streaming Link

## Consequence of the Concept of the Universe as a Computer

The ACM’s Ubiquity has been running a symposium on the question What is Computation?. Amusingly they let a slacker like me take a shot at the question and my essay has now been posted: Computation and Fundamental Physics. As a reviewer of the article said, this reads like an article someone would have written after attending a science fiction convention. Which I think was supposed to be an insult, but which I take as a blessing. For the experts in the audience, the fun part starts at the “Fundamental Physics” heading.

## March Meeting Madness

The 2011 APS March meeting deadline for submission of abstracts is today. Chris Fuchs writes with some stats about current submissions from the topical group on quantum information and in particular the number of quantum foundations talks (a list of foundation-ish talks is listed in the email):

As I write to you, 3200 abstracts have already been submitted for the APS March Meeting, with 140 of those earmarked for the Topical Group on Quantum Information. Very importantly for quantum foundations, however,

34 of those abstracts(culled from all sessions) can be considered with good justificationquantum foundations submissions!! In other words, at the moment, we’ve got 1% of the whole meeting thinking about the foundations of physics!-Have a look at some of the titles and speakers below; there are going to be some very good talks at this meeting. It will be a grand opportunity for everyone in our community to mix and mingle and learn from each other.

Please don’t forget that the abstract submission deadline is tomorrow, November 19, at 5:00 PM EST.

I really encourage everyone who wants to see quantum foundations thrive and be memorable to please submit a talk to this meeting. Encourage your colleagues and students too. Let’s build a critical mass. Your voice will count.

The place to go is:

http://www.aps.org/meetings/abstract/instructions.cfm

You must have an APS membership before submitting ($128 regular, $64 for recently completed PhDs, and $0 for students first joining), but you can still submit an abstract even if you don’t have your membership number yet–the instructions at the link explain how to do it. (It is not necessary, but please do spend the extra $8 to join the Topical Group on Quantum Information, the official home within the APS for quantum foundations research.)

Sincerely,

Chris Fuchs

Long Talks:A Brief Prehistory of Qubits

Benjamin SchumacherQuantum Information and the Foundations of Quantum Mechanics: A Story of Mutual Benefit

Anton ZeilingerToward a Conceptual Foundation of Quantum Information Processing

Giulio Chribella

On Mutually Unbiased Bases

Berthold-Georg EnglertQuantum States as Probabilities from Symmetric Informationally Complete

Measurements (SICs)

Åsa EricssonThe Lie Algebraic Significance of Symmetric Informationally Complete Measurements

Steven T. FlammiaReport on the Zeilinger Group SIC and MUB Experiments

Christophe SchaefStates with the Same Probability Distribution for Each Basis in a Complete Set of MUBs

William K. Wootters

Short Talks:

Physics as Information

Giacomo Mauro D’Ariano

Quantum theory cannot be extended

Roger Colbeck,Renato Renner

The quantal algebra and abstract equations of motion

Samir LipovacaScaling of quantum Zeno dynamics in thermodynamic systems

Wing Chi Yu,Li-Gang Wang,Shi-Jian Gu

Mathematical Constraint on Realistic Theories

James Franson

Uncertainty Relation for Smooth Entropies

Marco Tomamichel,Renato Renner

Quaternions and the Quantum

Matthew Graydon

A Linear Dependency Structure Arising from Weyl-Heisenberg Symmetry

Hoan Bui Dang,Marcus Appleby,Ingemar Bengtsson,Kate Blanchfield,Asa Ericsson,Christopher Fuchs,Matthew Graydon,Gelo Tabia

Proofs of the Kochen-Specker theorem based on the 600-cell

P.K. Aravind,Mordecai Waegell,Norman Megill,Mladen Pavicic

Proofs of the Kochen-Specker theorem based on two qubits

Mordecai Waegell,P.K. Aravind

Quantum Theory for a Total System with One Internal Measuring Apparatus

Wen-ge Wang

The thermodynamic meaning of negative entropy

Lidia del Rio,Renato Renner,Johan Aaberg,Oscar Dahlsten,Vlatko Vedral

Pseudo-unitary freedom in the operator-sum representation

Yong Cheng Ou,Mark S. Byrd

Quantum Computational Geodesic Derivative

Howard Brandt

Hardy’s paradox and a violation of a state-independent Bell inequality in time

Alessandro Fedrizzi,Marcelo P. Almeida,Matthew A. Broome,Andrew G. White,Marco Barbieri

Topos formulation of History Quantum Theory

Cecilia Flori

Quantum Darwinism in an Everyday Environment: Huge Redundancy in Scattered Photons

Charles Riedel,Wojciech Zurek

Redundant imprinting of information in non-ideal environments: Quantum Darwinism via a noisy channel

Michael Zwolak,Haitao Quan,Wojciech Zurek

Foundational aspects of energy-time entanglement

Jan-Åke Larsson

A Bigger Quantum Region in Multi-Party Bell Experiments

Matty Hoban,Dan Browne

Qutrits under a microscope

Gelo Noel Tabia

Quantum systems as embarrassed colleagues: what do tax evasion and state tomography have in common?

Chris Ferrie,Robin Blume-Kohout

Modal Quantum Theory

Michael Westmoreland,Benjamin Schumacher

On the Experimental Violation of Mermin’s High-Spin Bell Inequalities in the Schwinger Representation

Ruffin Evans,Olivier Pfister

Measurement backaction and the quantum Zeno effect in a superconducting qubit

Daniel H. Slichter,R. Vijay,Irfan Siddiqi

A derivation of quantum theory from physical requirements

Markus Mueller,Lluis Masanes

And that’s just the “foundation”-ish talks.