
Quantum Computer Made from Photons Achieves a New Record - bookofjoe
https://www.scientificamerican.com/article/quantum-computer-made-from-photons-achieves-a-new-record/
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abdullahkhalids
They [1] did not make not a quantum computer. They made a BosonSampler, which
is a different way of demonstrating quantum supremacy. A BosonSampler is not a
universal quantum computer because it's circuit depth and/or width would group
exponentially in the size of BQP [2] problems.

Very much like Google's chip, this machine is only good for solving one
useless task: sampling from a probability distribution formed from the
permanants of the nxn submatrices of a mxm unitary matrix, where m>n. This
task is hard for classical computers but easy for quantum devices because you
can just make this chip and each run returns a sample from the distribution.

The initial estimate threshold for showing quantum supremacy via a
BosonSampler was n=30 photons in m=100 modes, but this might have increased
recently as classical simulations methods have improved. These guys have 20
photons in 60 modes, so they are getting close.

[1] [https://arxiv.org/abs/1910.09930](https://arxiv.org/abs/1910.09930)

[2] set of problems solvable by a universal quantum computer efficiently.

