Simulating Chemistry using Quantum Computers

Nice preprint from the Harvard group introducing quantum computing for chemical simulation, including a great deal about AQC and how to apply it to such systems, e.g. lattice protein folding and small molecules. Includes references to some experimental and simulation work done at D-Wave (write-up for that in progress).

Simulating Chemistry using Quantum Computers

AQC / AQO video talk

Here is a video lecture that I gave a while ago about Adiabatic Quantum Computing and Adiabatic Quantum Optimization (specifically describing some cool things that you can do with D-Wave hardware) to my former colleagues at the University of Birmingham. This is a slightly higher level talk than the previous ones I have posted. Thanks again to my kind colleague and good friend (soon to be Dr.) Dominic Walliman for editing and posting these videos!

The talk is entitled ‘Playing with adiabatic hardware: From designer potentials to quantum brains’ although it certainly isn’t quite as ‘brain’ focused as some of the previous talks I have given, heh 🙂


Here are the other parts (they should be linked from that one, but just in case people can’t find them):

AQC Part 2
AQC Part 3
AQC Part 4
AQC Part 5
AQC Part 6

P.S. I wasn’t trying to be mean to the gate model (or computer scientists for that matter) – it just kinda happened…

P.P.S Some of the notation is a bit off – the J’s should be K’s to be consistent with the literature I believe…

Clash of the superconducting gate-model giants…

Flux qubits have been basking in the limelight recently…. two papers appearing on the ArXiv within a day of each other. Both papers investigate initialising, controlling and probing entangled states of a 3-qubit system:

Preparation and Measurement of Three-Qubit Entanglement in a Superconducting Circuit

Generation of Three-Qubit Entangled States using Superconducting Phase Qubits

Are Yale and SB going head-to-head in a gate-model phase/flux qubit battle? Looks like it…But who will win? Who has the highest T2?! Place your bets now!

More interestingly, this means we have yet another datapoint on the graph of number of successfully entangled sc gate model qubits versus time (in years). Presumably with 3 data points we can roughly gauge the scaling. I feel a plot may be required…and probably a log-lin scale.

MQT Paper update… now in colour

Oh my goodness, this MQT paper is becoming a TOME….

So yesterday we had the red ink debacle which spurred me to write the Paper Algorithm:

1.) Write paper
2.) Give to colleague
3.) Get returned
4.) Shake off excess red ink.
5.) Rework.

Repeat steps 3-5 and hope for convergence.
Note this algorithm may have bad runtime scaling due to T(step 4) -T(step 3).

A friend of mine tried to suggest some further steps involving the journal submission process, but unfortunately those kind of delightful games are beyond my event horizon at the moment!

Here is a picture of the red ink debacle (which by the way looks worse now as I’m covered it in my own rebuttals of and agreements with the arguments – in black ink I hasten to add).

Anyway, the new version of the document is better for the corrections now but I fear it may have to be streamlined as it’s packing 7 pages of Physics awesomeness already… and I’m wondering about going further into the details of thermal escape from the washboard potential. Maybe I shouldn’t do that.

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:

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.

Paper woes

I’m trying to write a journal paper. It’s really hard. It’s the first paper I’ve ever written. Even if it does not get published, it is very good practice. It would be good to be first author.

The paper is about using the experimental technique of switching measurements (aka Macroscopic Quantum Tunneling) to assess the quality of Josephson junctions for qubit applications (I can’t give too much away though).

It’s quite frustrating though: Just silly things like ordering of sections, what concepts to introduce in what order, which figures to put in, how to present them (that’s actually not too hard to work out, you just look at similar things in the literature), and how mathematical to be when you are describing the model to fit to the data, and pulling together several ideas from the literature to support your work (this is the hardest bit).

There’s also the worry that something exactly the same exists already, somewhere in the literature, and you just haven’t found it.

New experiments on the Heliox fridge, and cake.

Here is a picture of the new experimental setup:


In this picture we are testing that the apparatus cools down OK. This type of fridge is an Oxford Instruments Heliox, which is a He-3 based cooling system. The principle of operation is that you cool a ‘plate’ of metal to 1.5K by pumping a small amount of liquid from the main He-4 bath. The low pressure causes the He-4 to rest at this lower temperature instead of 4.2K. The cold plate is then brought into contact with He-3 gas, which condenses and drips down into a ‘pot’. Once you have condensed all the gas, you then pump on the He-3 pot itself (in the same way that you did with the He-4), which causes the liquid to slowly evaporate and cool as its tries to equilibrate its vapour pressure with the partial vacuum you create. However, unlike He-4, He-3 cools to much lower than 1.5K. In fact the temperature can be maintained at ~250mK for up to 90 hours, and you can supply extra heat to get a stable temperature anywhere up to around 1K.

This apparatus will be used to measure further interesting properties of unconventional pi-Josephson junctions at low temperatures. These junctions will (hopefully) eventually be of use in quantum computing applications: A built in phase shift across the junction gives rise to a degenerate ground state at zero applied flux, removing the need for application of a flux bias to obtain the qubit working point (a superposition of macroscopic quantum states).

With this one going too, it means I’ll be running 2 experiments (MQT on the dilution fridge and conductance on the CF), helping run this one (300mK heliox system), and building another (second dil fridge wiring for co-planar resonator coupled to qubit stuff) all at the same time. Argh! Still, lots of fun to be had.

For additional food for thought here is a delightful cake. A chocolate orange cake in fact. I made this one at the weekend. Excuse the quality of the photos, I only had a phone-cam at the time.