In the global race to build a fully functional quantum computer, Australia has wagered heavily on a company developing a dark-horse technology. The Australian national government and the state of Queensland will invest AU$940 million in PsiQuantum, a 9-year-old Silicon Valley startup,last week.
A fully functional quantum computer could solve problems that would overwhelm any conventional supercomputer. Whereas the bits in a conventional computer must be set to either 0 or 1, a quantum computer’s qubits can be 0 and 1 at the same time, although when measured that state collapses to 0 or 1. Multiple qubits can be “entangled” so that even while the state of each remains completely uncertain, the states of all the qubits are perfectly correlated.
Less helpfully, photons barely interact with one another. They can be made to do so with a beam splitter, a half-silvered mirror set at an angle that will, with equal probability, transmit a photon or deflect it by 90°. When two photons traveling in perpendicular directions strike the mirror from opposite sides, one might expect that half the time the photons will emerge still traveling in perpendicular directions, because both were transmitted or both reflected.
In 2001 Laflamme; Emanuel Knill, now at the U.S. National Institute of Standards and Technology; and Gerard Milburn of the University of Queensland found a way around the problem. “The funny thing is when and I started on this, we wanted to show that it would never work,” Laflamme says.