D-Wave goes public with open-source quantum-classical hybrid software
Search the universe with qbsolv
Want to fool around with some quantum-ish computing? D-Wave has open sourced a software tool that prepares optimisation problems to run on its hardware.
You can think of the software, qbsolv, as a D-Wave-specific compiler: in the white paper it's posted along with the tool at GitHub, the company's Michael Booth, Steven Reinhardt and Aidan Roy explain its role.
Qbsolv is “a tool that solves large quadratic unconstrained binary optimisation (QUBO) problems” for execution on a D-Wave computer, a task that has to be handled with care because the problem has to be partitioned to match the number of qubits on the target chip.
So qbsolv maps the problem to the hardware, partitioning a QUBO into “subQUBOs”, solves them, and recombines them into a solution to “the original instance”. As well as being able to run the problem on a D-Wave device, the software can run on a classical tabu search solver.
So what's a QUBO? Wikipedia describes it as an NP-hard pattern matching technique used in machine learning, and D-Wave's paper says it's well-suited to quantum annealing.
The white paper notes that qbsolv is an iterative solver: each trial first calls the D-Wave hardware as the subQUBO solver “for global minimisation”, followed by “a call to tabu search for local minimisation”.
Readers may remember that minimisation problems are what D-Wave does best: a “solution” is represented by the system being in the lowest possible energy state for a given problem.
However, quantum computing is difficult, and the paper notes that its systems have “limited precision” and might not find the optima for subQUBOs. Hence the hybrid quantum-classical approach qbsolv uses: the quantum computer returns various possible answers, and a tabu search picks out the “best” result from those.
With the software now published, the company hopes tinkerers will experiment with better ways to handle subQUBOs, and different ways to partition the QUBO problem.
Linux or OS X users can play with qbsolv without access to a quantum computer, and D-Wave notes that to run it on a quantum machine needs additional software that's not part of the release. ®