Quantum computing is real

Tonight I had the pleasure of visiting Rigetti Computing, a quantum computing start-up in Berkeley, for a Lisp at the Frontier of Computation meet-up. While I'm always happy to attend a talk on Lisp, this time I (and I'm sure many other attendees) was drawn more by the opportunity to see quantum computing's outside of academia.

The night began with a brief tour of the facilities. Whereas most software startups are little more than a collection of desks, chairs, and Macbooks, half of Rigetti's premises is dedicated to serious hardware, the most significant of which are several "fridges" that cool their custom-made¹ quantum processors (QPUs) to 10 milikelvins. It's all very physics-y and impressive–"SCIENCE!", it screams in all caps. We were also shown a quantum processor, which looks more or less like a standard chip, except for the distinct "sunburst" shapes that indicate qubits.

The talk by Robert Smith, director of software at Rigetti, was about using Common Lisp (CL) to build a quantum computing VM (QVM) for Quil², the in-house quantum instruction language³. At this time, real processor time is very valuable, so a good VM is absolutely necessary for testing. Currently, the QVM is even able to simulate more qubits than the real chips, though performance degrades exponentially w.r.t. the number of qubits⁴.

Motivations for using CL include the usual strengths of Lisp, such as expressiveness and rapid iteration, as well as some SBCL specifics, particularly performance. Difficulties include a lack of good libraries (particularly for linear algebra) and no interest on the part of SBCL maintainers to improve performance in key areas⁵.

A few more points from my chat with Robert after the talk:

• Quantum processors are not better for every problem; a general-purpose computer will use the QPU for certain tasks, just like GPUs.
• There's no scientific barrier to quantum computing now⁶; it's an engineering (particularly fabrication) problem
• Adding qubits is not very difficult; within 5 years, 100's of qubits is possible.
  • However, using Shor's algorithm⁷ to crack today's cryptographic security would require 10,000's of qubits, which is still a ways off!
• D-Wave, another quantum computing company, does a more limited sort of computing called quantum annealing
  • Interestingly, D-Wave's also known to use CL, though we don't know for which purposes
• Nielsen and Chuang's Quantum Computation and Quantum Information is the recommended book for learning about quantum computing

All in all, this was a very enlightening night. While quantum computing is quite beyond my grasp at the moment, I'm excited to think where it may be in a decade.