# Fridge wiring diagrams, ick.

Drafting out ideas for where everything goes:

The fridge in question will be used for several different ‘interchangeable’ experiments, which always makes it tricky to wire. The majority of the lines need to be coaxial, as they will be used for either microwave transmission or fast pulse control of qubits.

# Some nice D-Wave info

I suspect this may be redundant information as my readership is probably entirely contained in the superset of D-Wave’s blog readership, but… For anyone who didn’t see it, there is a series of new posts over at Geordie’s blog about D-Wave’s technology, aims, results, fabrication and mostly anything else you could wish to know about the company’s quantum computing efforts. There are several links to presentations containing plenty of data to mull over.

What D-Wave are trying to build

The posts are still being added at the moment, so make sure you check back often to look for updates. For my colleagues who read this blog (I know who you are…) you should definitely check it out.

# Designing qubit circuits

It’s hard work being the only postdoc in the village. One day I’m fixing wiring on the fridge, the next I’m analysing the effect of spin-flip scattering on my superconductor-ferromagnet data. Today I’m being the local RSFQ/SQUID layout afficionado.

I’m designing some qubit circuits. Process design rules are a pain, there are about 10 layers in a Standard Niobium process and you have to get all the holes and structures spaced correctly (do I hear a tiny violin?). Luckily I have several helpful guides such as Ustinov’s group website, which contains information (mostly in the doctoral theses) on their structures which were fabricated by VTT and HYPRES.

Here are some pictures of what I’m doing:

They are very preliminary designs at the moment, I haven’t even got all the layers in there yet.
I also have to calculate the mutual inductances between the structures using finite element techniques, which lets you know how well your qubit couples to your readout circuitry (In this case, DC SQUIDs and microwave resonators, depending on the design). It’s quite fun to do circuit layout though. These circuits will probably be realised at the European FLUXONICS foundry at IPHT.

# 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:

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

# Second Campus?

So the freshers have arrived on campus and the new University term has started. To mark this occasion, the University have decided to adorn the campus with these boards, demonstrating some of the more visual results of research undertaken in the college of Engineering and Physical Sciences. I’m sure the idea is to convey the fun, excitement and beauty of research science.

However there is one problem. As I’m walking through campus, I keep thinking I’m in Second Life. In SL, people convey information on these giant boards that look identical to these ones… apart from they take longer to load in SL. Unless I haven’t had enough coffee in the mornings, then it’s a pretty close call. So you see, I’m now having even more difficulty separating my real and virtual lives…

In SL, you can click on the boards and they give you information. I’m wondering if we can write an iPhone app with one of the augmented reality frameworks that does the same thing with these boards. Perhaps it could take you to the group’s website, etc…

And yes, the students do behave in a remarkably similar fashion to SL avatars. Sometimes they just stand around like they are totally AFK.

# It’s just too cool to be a scientist these days.

Via Pharyngula:

I was thinking after the LHC rap that a QC rap video would be cool to do, but I don’t think anything I could do could possibly match the standards set by the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO):

I also now want a giant gold medallion with a $\Psi$ on it…

# In response to Ray Kurzweil’s comment on Quantum Computing and the brain

I thought I’d make a little note about this because quite a lot of people have been talking about this issue.

Ray Kurzweil addressed the Singularity Summit on Sunday and gave a brief summary of his opinions on some of the other preceding talks. He specifically answered criticism from others of our ability to ever model the brain using classical computing due to the presence of quantum effects in the brain. I don’t know of any supporters of this hypothesis other than Penrose and Stuart Hameroff, but maybe they are out there. He supported his viewpoint by saying that ‘The brain doesn’t factor large numbers’.

I agree with the statement that the brain is not necessarily ‘quantum computing’, but I disagree with this particular argument, because the brain does do lots of other things which quantum computers might ALSO be good at, such as pattern recognition, image processing and memory retrieval (database searching). So I think any argument as to why the brain isn’t quantum computing needs to be a bit more watertight (start by explaining decoherence for example) if you’re going to tackle this issue.

As a secondary effect, it perpetuates the myth that factoring is the only thing QCs will ever be used for. Which is sad, because a lot of smart people might have taken that impression away with them.

# Quantum happenings 081009

So to get me out of the futurist frame of mind and back into the Quantum Physics frame of mind, here are some things to read today:

An introduction to measurement based quantum computing:
Measurement-based quantum computation

Preprint of the latest offering from Martinis’ group showing tomography of two entangled gates after performing various operations:
Quantum Process Tomography of a Universal Entangling Gate Implemented with Josephson Phase Qubits
It’s amazing what you can demonstrate with just 2 qubits.

The published version of Seth Lloyd’s Quantum Algorithm for linear systems of equations:
Quantum Algorithm for Linear Systems of Equations
(and here’s the preprint of that one).