Let there be cake…

Our weekly cake club has an elevated status. It is now possible to find ‘Cake seminars’ on the Talks@Bham page, which means our cake club agenda is not only publicly available, but anyone from around the University can drop in if they noticed the event….

…which is a bad thing, as I don’t think I baked enough cakes for everyone in the Uni. Anyway, here are the ones I baked for the weekly gathering…

The all new ‘Resources’ tab

The regular reader(s) may have noticed that a shiny new tab, the ‘Resources’ tab has been added to the blog:

Hopefully I’ll put up here any presentations/essays/slides/generally useful information about the stuff that I am interested in and researching, which is basically anything to do with Josephson junction technology, superconducting electronics, experimental quantum computing, quantum neural networks, artificial general intelligence and the brain. I’m currently in the process of getting some new videos edited too, so they will be going on there soon. I’ll put a separate post about those. Until then, you can enjoy perusing slideshow PDFs of several presentations that I have given to a range of audiences.

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Here’s some cake to enjoy with the slides:

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physics and cake

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(This one was from our regular Wednesday post-group-meeting ‘cake club’)

QIP 2010 – Further thoughts and CAKE!

So I’m back from QIP now, and full of chocolate. One might say I am maximally satisfied. However I didn’t have time to post this final update so I’ll do it now.

I really enjoyed 2 talks on Thursday afternoon session. The first was by Roderich Moessner and the second by Julia Kempe. They were entitled:

“Random quantum satisfiability: statistical mechanics of disordered quantum optimization” and “A quantum Lovasz Local Lemma” respectively.

I enjoyed these talks because they weren’t completely theoretically based, even though the titles made them sound like they might have been. In particular, I liked the way that random, average and typical instances were considered.

The bounds of ‘hardness’ (going from always satisfiable (easy) to possibly satisfiable (hard) to unsatisfiable (easy)) as you increase the number of clauses compared to the number of variables in a SAT problem were explored, and what kind of phase transitions occur throughout this process. Entanglement can help make some of the possibly satisfiable ones easier, so effectively utilising quantum mechanics allows you to tighten the boundaries of the ‘region of hardness’.

One final thought that I had about the conference was that I think that QIP people need to think about Physics a bit more. Physics seems to underlie all these processes and ties them to the real world in some way. I found that quite a few people were advocating the point of view that Computer Science underlies Physics, but I believe this to be the wrong way of looking at the problem. Physics is all we are given really, it is fruitful to remember this and perhaps just considering it once in a while might help keep you a little more grounded in reality.

Anyway, enough Physics, lets talk about cake. So I mentioned in a previous post about this cake shop I found in Zurich called Cakefriends. Well now I have pictures.

The cake that I chose was a heterostructure of deliciously thick cream (almost cheesecake thick) with interstitial poppy seed sponge layers. To complete the unit cell there was some raspberry sauce around the outside of each sponge layers. It was served in a glass:

Here is a picture of me enjoying said cake. And yes, there were Physics discussions throughout the cakey experience, which should always be the case.

And a photo from the Cakefriends menu:

Yes. Yes we do.

Also thanks to this conference I finally understand the meaning of the complexity class qpoly. Thanks QIP for clearing this one up for me.

QIP2010 – Preconference musings

I’m now in Zurich at the QIP2010 conference. I’m hoping to do a bit of live-blogging! I’ve never live-blogged from a conference before, so we’ll see how this goes.

I’m looking forward to hearing all about the cutting edge of theoretical quantum information! I’ll probably be the only experimentalist there and be totally confused…

Zurich seems very beautiful, I love the river running through the city and there are several buildings with really spikey spires. (A red one and a green one). If I had any sense of culture or history I would tell you all about these buildings. But at the moment I just think they are cool because they are spikey. Pictures and slightly more useful information may follow soon.

There is also a cake shop. It’s called CakeFriends. It’s like they knew I was coming. I tried to get in there today but it was way too busy. Looks like they do really cool coffee and cake 😀 Gaining access to this confectionary resource is definitely on the agenda for the upcoming week!

Update on the qubit mask

So here is an update on the mask I was designing to do some qubit experiments:

This mask should allow us to do various things. There are some coplanar resonator structures and some time-domain coherence chips here.

I’ve had to put anodisation bridges and anod-layer cuts all over the place. This is because the entire bottom layer of Niobium needs to be anodised anywhere where a junction region is defined. Anodisation requires electrical contact between regions which can be floating (such as where you have qubits). So you must join up these regions to the main bottom wiring layer and then cut them later in the later process step. I really hope I didn’t forget any pieces!

After all that mask designing, some cake is required. Here are some delicious cakes I baked a little while ago:

P&C @ D-Wave Systems – Redux 2009

In case anyone noticed the blog-tumbleweed accumulation, I should explain where I’ve been: I spent the last 3 weeks in Vancouver at D-Wave hanging out with those awesome pioneers of adiabatic quantum optimization.

I love working with these guys, they are really good at what they do. Thanks go to all the guys there for making my trip very enjoyable. I played with the latest quantum processors, which gives me a total buzz. I did more Physics than I think I’ve done in months, musing over the Hamiltonians of Ising spin systems and whatnot. The processor technology is really coming along now, as I mentioned a few posts ago.. I totally want a quantum computer for Christmas. Superconducting flux qubits FTW!

Vancouver was cool too. I ate more sushi than I probably should have done. There was lots of rain, but then again I don’t mind the rain, especially when there are accompanying storms. I like Vancouver and the West coast in general as all the people there are so enthusiastic. It beats the perpetually miserable British folks 😉

So although I have been neglecting to post over the last 3 weeks, lament not this absence. There has been tons of cool Physics, lots of fun, and… did I mention… cake. They MADE ME CAKE. Not only can these guys engineer the most advanced superconducting processors in the world, but they also make a mean chocolate cake. Respect. So we had a coffee, cake and donut party 🙂 I had to honour that on here of course.

Check it out:

Where there are quantum computers, let there also forever be cake…

Herding quantum cats

Two interesting arXiv papers this week:

Adiabatic quantum computation along quasienergies

A potentially new model of Quantum Computation, which is a discretized variant of Adiabatic Quantum Computation (AQC). Is it equivalent to the standard model? Is it useful? No-one knows.

This paper also got me thinking:

Electronic structure of superposition states in flux qubits.

How do you measure the cattiness of a flux qubit? Cattiness being defined as the ability of a system to exhibit quantum properties as it approaches a classical limit in terms of mass, size, or some other measure. The name comes from the question of whether or not it is possible to put an entire ‘Schrodinger’s cat’ into a macroscopic superposition of states.

I have been wondering about this problem with regards to flux qubits for a while. You might think it is possible just to ‘count’ the number of electrons involved in the Josephson tunneling, giving around 1^10 particles. But wait, the electrons all form a macroscopic state – do you count the condensate as a single particle instead? This paper argues that the actual cat state is somewhere between these two extremes. This is good news, because although the upper bound would have been cooler in terms of Macroscopic Quantum Coherence, the superconducting flux qubit might still be the ‘cattiest thing in town’.

I’m also wondering about the cattiness of nanomechanical resonators coupled to optical or microwave cavities. This system can be put in a superposition of two mechanical states relating to the position and motion of the atoms in the bar. For example, the ground state can be thought of as the fundamental harmonic of the bar (think of it like a guitar string), with an antinode in the centre, wheras the first excited state has a node in the centre and two antinodes at 1/4 and 3/4 of the way along the bar. But here we find a similar problem to that of the flux qubit: Does the number of atoms in the bar matter?

For fun let’s calculate the number of atoms in a Niobium nanomechanical resonator:

Let’s say the mechanical bar is 20nm x 20nm x 1um.
The volume of the bar is therefore 4e-22m^3
The density of Nb is 8.57g/cm^3
The mass of the bar is therefore 3.428e-17kg
The atomic mass of Niobium is 92.906amu = 1.54e-25kg.
The number of atoms in the bar is ~2.2e8

To check that value:
The atomic radius of a Nb atom: 142.9pm = 0.1429nm
In 20nm there are 139.958 atoms,
and in 1um there are 6997.9 atoms.
Therefore in the bar there are 1.37e8 atoms

which is roughly the same as by the previous method.

So does that mean the ‘cattiness of the bar’ has an upper bound of 2e8? This would make it more catty than the flux qubit. Or do you have to assign more (or less) than one ‘quantum degree of freedom’ per atom? It’s not as simple as tunneling electrons, where the quantum state is determined by the direction of current flow around the loop. If anyone has any thoughts on this they would be appreciated. Just what exactly are the quantum degrees of freedom here?

The bar is obviously constrained by its end points, albeit not ideally. The displacement of the bar may therefore probably behave more classically near the ends, or the wavefunction may extend into the structural supporting region. This may affect the actual number of atoms in the superposition. What fraction of the length of the bar is behaving quantum mechanically?

Note that the mass of both the electron condensate in the case of the flux qubit AND that of the nanomechanical bar are both much lower than Penrose’s quantum mass limit of about 1e-8kg – so we can’t test that hypothesis in the lab yet. Note this relates to a post I wrote a while ago about electrons in a lump of superconductor – there are enough electrons in a bulk sample for the mass to be greater than the Penrose limit, but they aren’t doing any useful quantum computation, you can’t put them into a well defined superposition of states for example. We need to ENGINEER and CONTROL these cat states…

Anyhow, after that complicated Physics we are definitely in need of some cake:

mrkipling_frenchfancies

We had this type of cake yesterday (amongst others) to celebrate a colleague passing his PhD viva 🙂