
Engineering tour de force births programmable optical quantum computer - rbanffy
https://arstechnica.com/science/2018/09/engineering-tour-de-force-births-programmable-optical-quantum-computer/
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Strilanc
> _The [two qubit] gates are about as reliable as any others you will find in
> the quantum computing world, which is to say that operations complete
> successfully around 93 percent of the time. For comparison, ion-based
> quantum computers are at 95 to 99 percent and superconducting quantum
> computers are around 90 to 95 percent. [...]_

Those comparison numbers are incorrect. Ion-based and superconducting-based
groups have reported two-qubit gate fidelities of 99.9% [1] and 99.4% [2]
respectively.

[1]: [https://arxiv.org/abs/1512.04600](https://arxiv.org/abs/1512.04600)

[2]: [https://arxiv.org/abs/1402.4848](https://arxiv.org/abs/1402.4848)

~~~
jessriedel
I don't know if there's a sensible way to define "typical" gate fidelities,
and I agree "90 to 99 percent" is misleadingly low, but picking some of the
highest reported numbers (possibly achieved in specialized non-scalable set
ups) is also not very representative. The engineering of gate quality is too
messily intertwined with the rest of the device.

~~~
Strilanc
The article is also reporting numbers for a one-off setup that has scaling
challenges, so it seems appropriate to compare against that type of number.

~~~
jessriedel
Fair enough.

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foxes
I like optical quantum computing, it feels hands on, like it's almost
something you could build in your garage. Linear optical quantum computers
just need waveguides, beam splitters, phase shifters and mirrors (no
disrespect - I know it's not trivial to make work). You set up your circuit
and fire your light through it and measure the system at the end.

It's a nice way to think about quantum computing. You aren't allowed to do any
measurements half way or you will destroy your superposition. It's a bit like
a pure function, no IO allowed until the very end.

Also I know three of the people on the paper, good to see them getting some
mainstream attention for a nice result.

~~~
vtomole
> You aren't allowed to do any measurements half way or you will destroy your
> superposition.

This is true of all quantum computations. It doesn't matter what the
underlying hardware is.

~~~
Strilanc
It's actually extremely common for quantum algorithms to have measurement
operations halfway through. But they apply to individual qubits, not the whole
system.

For example, error corrected quantum computation involves continuously
measuring particular stabilizers in order to catch when they flip. Another
example: measurement can reduce the cost of uncomputation (e.g. [1]).

[1]: [https://quantum-journal.org/papers/q-2018-06-18-74/](https://quantum-
journal.org/papers/q-2018-06-18-74/)

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Soundest
Bingo, straight across the middle. See that, I never thought I was going to
get optical and quantum in the same headline but there you go. You never know
with bullshit bingo.

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biswaroop
A cool implementation of a deep ANN using a similar system:
[https://arxiv.org/pdf/1610.02365.pdf](https://arxiv.org/pdf/1610.02365.pdf)

(the nonlinear units and the backprop are done on a classical computer).

The paper also points out that thermal cross-talk between the thermo-optical
phase shifters can limit gate fidelities or conversely the smallest spacing
between the waveguides.

