Part 0: Introduction

A while back I had a thought: Would it be possible for a moderately wealthy experimental physicist to build a superconducting quantum computer in her basement, mad-scientist-style?

I think one could bodge together the necessary control electronics. It wouldn’t be a top-of-the-line system like we’re used to in academic and industry labs, but I think that with an FPGA and some basic microwave components, it could be done. On the fabrication side, one can find Youtube videos where people buy decommissioned scanning electron microscopes – or even build them themselves – and there’s no reason an SEM couldn’t be used for electron beam lithography. So I think that in principle it should be possible to do the nanometer-scale lithography required for Josephson junctions, and the same methods could be used for the larger circuit elements (or one could build/buy a photolithography system). Likewise, there are many examples of homemade sputtering setups that could be used for deposition. If the sputtered film is not of high-enough quality, one could probably build an electron beam evaporator. So with a few years of dedicated work and a considerable but not-completely-insane amount of money, I think a very skilled experimentalist could fabricate a qubit and build a minimal control setup.

In my mind, where this idea falls apart is with the cryogenics. Superconducting circuits require millikelvin temperatures to operate, and in that regime, dilution refrigeration is the only real option. It used to be the case that researchers in this field built their own dilution refrigerators, so I think that constructing one is not out of the question provided that one is a skilled machinist and has the money for secondhand vacuum pumps, valves, sensors, and so on. So on first glance, this task does not appear to be completely intractable.

Then I remembered how much Helium-3 is going for these days. Securing the necessary quantity of Helium-3 is where this will cross into “insane amount of money” territory, if it hasn’t already. Without Helium-3 one does not have a dilution fridge, and without a dilution fridge, there is no good way to reach the temperatures necessary for qubit operation.

In short, the dream of a basement-lab quantum computer is probably better realized in some other platform. And of course, this whole thought exercise is pretty silly in the first place: if you want to work on superconducting quantum processors, get a PhD in this field or an adjacent one and go work in an academic or industry lab.

Still, I think it’s a fun question because it forces us to think about the minimal requirements for a working superconducting qubit, and this is my mindset in writing this series. My goal is to cover, in detail, each of the following (not necessarily in order):

  • Theoretical description of superconducting quantum devices
  • Device fabrication
  • Dilution refrigeration
  • Initial calibration
  • Control and readout
  • Decoherence and error correction
  • Applications, including bits and pieces of my own research

This is admittedly an ambitious goal. There are some things that I will need to leave out – for example, I don’t really plan to discuss superconductivity itself, beyond the bare minimum. I’m a PhD student, and any time spent writing for this blog is time not spent doing research. So progress will be slow.

Nevertheless, I’m excited to dive into this – to share my love of this field with others, to give the interested reader a taste of what it’s really like to do this work. It’s going to be a lot of fun for me, and I hope it will be for you, too.

Prerequisites

My goal is to make this series accessible to those with a basic background in undergraduate-level quantum mechanics. More advanced tools (time-dependent perturbation theory, the theory of open quantum systems, etc.) will be covered as needed, but you should be familiar with Hamiltonians and operators and kets and so on. Maybe some day I’ll write a short “Intro to Quantum Mechanics” series, with the goal of making it accessible to anyone who knows linear algebra and calculus. In the end, though, there is no substitute for actually solving quantum mechanics problems with pen and paper. If you’ve done your time solving the end-of-chapter exercises in your QM textbook of choice, this series should be very approachable.