We’ve been assured for 29 years that quantum crypto is secure, and for 19 years that quantum computing is set to make public-key cryptography obsolete. Yet despite immense research funding, attempts to build a quantum computer that scales beyond a few qubits have failed. What’s going on?

In a new paper Why quantum computing is hard – and quantum cryptography is not provably secure, Robert Brady and I try to analyse what’s going on. We argue that quantum entanglement may be modelled by coupled oscillators (as it already is in the study of Josephson junctions) and this could explain why it’s hard to get more than about three qubits. A companion paper of Robert’s on The irrotational motion of a compressible inviscid fluid presents a soliton model of the electron which shows for the first time how spin-1/2 symmetry, and the Dirac equation, can emerge in a completely classical system. There has been a growing amount of work recently on classical models of quantum behaviour; see for example Yves Couder’s beautiful experiments.

The soliton model challenges the Bell tests which purport to show that the wavefunctions of entangled particles are nonlocal. It also challenges the assumption that the physical state of a quantum system is entirely captured by its wavefunction Ψ. It follows that local hidden-variable theories of quantum mechanics are not excluded by the Bell tests, and that in consequence we do not have to believe the security proofs offered for EPR-based quantum cryptography. We gave a talk on this at the theoretical physics seminar at Warwick on January 31st; here are the slides and here’s the video, parts 1, 2, 3, 4 and 5.