Minor administrative note: the link indicating the number of comments on a post is not presently updating. Any comments you post will be fine (and will display okay), they just won’t be noted on the front page.

Unfortunately, I won’t be able to fix this until I’m back from Canberra, in a week or so.

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I’m off to Canberra, for my third workshop / conference in three weeks – the Australian Institute of Physics Biennial Congress, which doesn’t exactly qualify as a workshop, with a thousand or so delegates.

I’m not sure I’ll have good internet access, unfortunately, so this blog may be on hiatus for a week or so.

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I’ve changed the design of my site around a bit, mostly to install the links on the sidebar. Let me know if anything looks broken in your browser. I certainly can’t claim any aesthetic merit (I trust that is obvious), but I’d least like all the text to be visible.

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Tony Leggett

At today’s physics colloquium, Tony Leggett gave a rather striking talk about tests of quantum mechanics. It was striking because, if I understood him correctly, about five years ago he thought it likely that quantum mechanics (specifically, the superposition principle) would break down at macroscopic levels. It’s a very interesting, uncommon, and refreshing point of view.

Apparently, he’s much less certain of this now, in part because of the experiments done over the past few years with superconducting qubits. He still regards Bell inequality violations in such systems as important, however.

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QIP talk

Powerpoint of my talk at last week’s QIP Workshop. I’d post pdf as well, but Acrobat seems to be on holiday.

The talk is about a model of quantum computation known as the one-way quantum computer, or cluster-state quantum computer.

The talk reports joint work with Chris Dawson and Henry Haselgrove.

Here’s a slightly modified version of the abstract.

In the one-way quantum computer a quantum computation is performed by first preparing a special entangled state (the cluster state), and then performing a sequence of adaptive single-qubit measurements on that state. It’s thus a radical departure from the standard way of thinking about quantum computers, which is in terms of quantum circuits.

The talk begins with a review of the cluster-state model, following Raussendorf and Briegel.

The talk describes two new contributions. First, I explain a simple argument for why in some physical systems (particularly optics) it may be much easier to build a one-way quantum computer than it is to build a conventional circuit-based quantm computer.

I should say, by the way, that Yoran and Reznik have independently pursued some related ideas.

In the second part of the talk I discuss how to make the one-way quantum computer fault-tolerant, i.e., noise-resistant. I explain why the fault-tolerance is not obvious from standard thresholds for quantum circuits, and how to adapt the threshold proof to the one-way quantum computer.

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Additional travel tip

I arrive in LA from Toronto at one of the domestic terminals (terminal 4), get off, walk to the international terminal (terminal 3) to catch my flight to Australia, go through security, and wonder why my flight isn’t listed on any of the monitors.

Finally, I realize. It’s because it doesn’t leave from the international terminal.

No, bizarrely, it leaves from the domestic part of terminal 4, about 100 meters from where I got off my Toronto flight.

So I go out past security, walk back to terminal 4, and go through security again, to get to the gate.


This story happened about 6 months ago. I just repeated it about 18 hours ago, with “Toronto” replaced by “Boston”. Fortunately, I remembered before getting through security in the international terminal.

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Optimizing travel

Update: See the improved version 2.0 of this post here.

Most researchers travel a lot, and I’m no exception. Recently, I’ve been collecting tips to optimize my travel. Nothing incredibly insightful, but the gradual accumulation of small but consistently applied habits has made a big difference to my travel. I thought I’d post them here as a way of forcing myself to systematize them, in the hope of providing something useful to other people, and in the hope of hearing some other tips in comments!

At home

Buy at least one large or extra battery to enable longer laptop use on the plane.

Buy a collection of travel adaptors which will enable you to plug in anywhere.

Put together a travel drawer at home. This contains foreign money collected on earlier trips (one envelope per currency), my passport, travel adaptors so I can plug in my laptop elsewhere, remote mouse for presentations, the checklists mentioned below, and any other specialized equipment – it’s handy to buy miniature versions of various things (toothpaste, laptop adaptor, and so on), in order to minimize luggage.

Try out some different types of earplugs, and when you’ve found some you like, buy a large supply of high quality earplugs. I find these extremely helpful for sleeping on planes. (Some people like noise-cancelling headphones. I find them okay, but prefer earplugs.)

Join the Qantas club or equivalent. This provides access to Qantas and partner lounges all over the world. Most significantly, this means access to showers and good quality chairs while connecting, as well as faster checkin. It’s usually not possible, but some lounges will have a massage service, which I find helps significantly with recovery at the end of the flight.

Booking flights

Construct a list of standard routes and flight times that work best for you, and request those. To be effective, you need to either memorize the list, or file it in a location very easily accessible (< 30 second) from where you make travel bookings. My preferred seats are forward in the plane, on the right hand side (get off faster), window seat (people don’t climb over you when you’re asleep), exit row or bulkhead. Having exit row or bulkhead seats is particularly important, as most aeroplane seats are sufficiently small that it’s very difficult to use a laptop onboard unless you’re in an exit or bulkhead (or business). The exception seems to be American Airlines, who have extra legroom. Frequent flyers can set up seating preferences which are then applied automatically to every trip.

Preparing for the trip

Create a checklist for choosing a hotel. Here’s mine:

  • Is the hotel near the conference venue?
  • Is there a supermarket nearby, so one can purchase good food to eat during the day?
  • Are there kitchen facilities in the hotel rooms?
  • Do the hotel rooms have high quality internet access?
  • Do the hotel rooms have a restaurant on premises?
  • Do the hotel rooms have a laundry service?
  • Do the hotel rooms have heating and cooling?

For beating jetlag, I find one of the most important things is to be relaxed. A corollary is that if I’m to speak publicly on a trip, I prefer to speak as early as possible. No matter how well prepared I am, I find anticipation of a public talk creates a little nervous tension. It’s best to get it out of the way, so I can relax. I haven’t done it yet, but in future when speaking at a conference where I expect a lot of jetlag, I plan to ask the organizers if they’d mind scheduling my talk for early on.


Have a checklist. For me this is: passport, tickets, do a ticket check (am I going at the right time?), laptop, pda, keys, remote mouse, a generous supply of clothing (I usually find I need more changes than I think – something always gets spilled, or ripped, or whatever), toiletries.

Make sure you can carry all luggage onboard, especially on long flights. It makes it less likely that you’ll miss connections, you won’t lose your luggage, and you’re not lugging huge quantities of stuff around. Most importantly, you’re likely to save about half an hour per connection in waiting time. That doesn’t sound like a lot, but when you’re traveling for 20 or 30 hours, every bit counts.

Departing at the airport

Buy some water. You’re rarely served enough onboard.

Buy some healthy snacks to eat onboard.

Get some foreign currency. It’s tempting not too – you can usually get by without it – but it’s usually useful to have at least a couple of hundred dollars for emergencies.


Take walks in the morning sun to reset your body clock, and relax.

Treat yourself well. As tempting as it is to go out late and party, or drink lots, you just make yourself sick, and increase the effects of jetlag. All things in moderation and all that.

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Quantum Diaries

Quantum Diaries is a new website, containing 25 weblogs of people in the particle physics community, set up to celebrate the World Year of Physics, 2005.

(Via Peter Woit.)

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The dihedral hidden subgroup problem

I’m at the QIP (quantum information processing) 2005 workshop in Boston, which is being hosted by MIT.

I’m not going to be blogging the workshop, but might mention a few tidbits here and there.

In this morning’s session, Andrew Childs gave a talk about the dihedral hidden subgroup problem (DHSP).

This is an important problem: it’s likely to be hard on a classical computer, and it’s widely regarded as a good candidate for efficient solution on a quantum computer. It’s also connected to important problems in computer science, like finding the shortest vector in a lattice, which have cryptographic applications.

It’s a little difficult to give a brief and accessible overview of what the DHSP is about, but I can at least sketch what progress Andrew and his collaborators have made toward a fast quantum algorithm.

In the first part of his talk, Andrew showed that the DHSP can be reduced to performing a measurement to distinguish certain easily-prepared quantum states.

The difficulty of DHSP then lies in (a) figuring out whether there even exists a measurement which is capable of distinguishing those states, (b) if one does exist, figuring out what the best measurement is (or at least a pretty good one), and (c) figuring out whether or not that measurement can be implemented efficiently on a quantum computer.

Andrew answered part (a) and (b), explicitly constructing a measurement (the “pretty good measurement”) which he can prove is the best possible measurement for distinguishing those states.

Andrew has also made some progress on part c, but no efficient implementation yet exists. Nonetheless, solving part (a) and (b) does represent encouraging progress on this important problem.

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Quantum computing for dummies

Riley Perry has written a free introductory online book on quantum computing that he described to me as “quantum computing for dummies”.

It’s essentially an undergraduate text, and contains many of the topics (intro to quantum mechanics, basics of the quantum circuit model, Shor and Grover algorithms) that are essential to an understanding of quantum computing, as well as some more advanced topics.

I’ve only thumbed through it, but it looks to present many topics in a very clear and comprehensible way.

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