Steps towards a quantum advantage

Xanadu's latest results on Gaussian BosonSampling attracted quite a bit of media attention. Their essential breakthrough is to combine time multiplexing with number-resolved photon detectors to massively increase the number of modes and photon counts, pushing their device into a regime that seems to be intractable using classical computers. 

It is reassuring that one of the reviewers of the Nature paper was Sergio Boixo, who has also authored work on developing more efficient classical algorithms for spoofing Gaussian BosonSampling. Other advanced classical algorithms can spoof shallow circuits and measurements with too many photons per mode.

The Gaussian BosonSampling device is now accessible on AWS. The price per shot is (\$0.00020) is slightly more than half of that of the superconducting processors (\$0.00035). Note this is not a general-purpose system, which remains challenging to implement, as noted by the Xanadu team in their paper:

If one were to target a universal and programmable interferometer, with depth equal to the number of modes, that covers densely the set of unitary matrices, the exponential accumulation of loss would prohibit showing a quantum advantage. There are then two ways around this no-go result: one can either give up programmability and build an ultralow loss fixed static interferometer, ...., or give up universality while maintaining a high degree of multimode entanglement using long-ranged gates.

Despite the lack of universality, this does seem to be the first classically-intractable programmable quantum processor available for general use via the cloud!

 On a related note, last week Nature published a paper demonstrating repeated error correction, using 17 superconducting qubits to create a single logical qubit. Each error correction cycle took 1.1 us and succeeding with 97% probability (95% without post-selection). Quantum error correction experiments are still at a very early stage; this experiment (and most others) have not yet reached the "break-even" point where the logical qubit lifetime exceeds the lifetime of a single physical qubit.