Friday, July 16, 2021

Slow blogging + a couple of articles

Sorry - blogging has been slow in recent days because, despite it being summer, it's been a very busy time for various reasons.

Here are a couple of articles that I've come across that seem interesting.  On the news/popular writing front:

On the science front, there have been several cool things that I haven't had time to look at in depth.  A couple of quantum info papers:

  • In this Nature paper, the google quantum AI team have used their 53 qubit chip to do proof-of-concept demonstrations of two different quantum error correction approaches.  Perhaps someone more knowledgable that me can chime in below in the comments about how the ratio of physical qubits to logical qubits depends on the fidelity and other properties of the physical qubits.  Basically, I'm wondering if, e.g., ion trap-based schemes would be able to make even better advantage out of the 1D error correction approach here.
  • Meanwhile, in China a large group has demonstrated a 66 qubit system similar in design to the google/Martinis approach.  

11 comments:

Anonymous said...

For the last two, perhaps worth pointing out that although you've given the correct number of functioning qubits on each chip, in the actual experiments both groups only use a subset of the total- presumably because of varying fabrication or error rates. The Google experiment uses up to 21 qubits in this paper, while the Chinese experiment uses up to 56 qubits.

Don Monroe said...

The conclusions section of the paper says this:

However, practical quantum computation will require Λ ≈ 10 for a reasonable physical-to-logical qubit ratio of 1,000:1 .... Achieving Λ ≈ 10 will require substantial reductions in operational error rates and further research into mitigation of error mechanisms such as high-energy particles.

(Here Λ is a measure of the fidelity of individual qubits.)
So they are saying that if they can do things much better, it will require "only" 1000 physical qubits for each logical qubit to do reliably error-corrected computations. If they want to compute with 50 logical qubits, they will need 50,000 physical qubits, almost 1000 times the size of systems that have been demonstrated so far, and then only if they can reduce the baseline error rate significantly.
Nice work, but there is a long way for this strategy.

Anonymous said...

Another very interesting paper (to me) was done on superconducting qubits that showed the errors induced by cosmic rays are *correlated*, unlike the usual assumption in error correction that errors are independent and occur on the single qubit level.

Besides pointing out new and interesting hurdles for error correction, this type of correlated error detection could be a very novel detector of solid state correlation physics or novel high energy particle phenomena.

https://www.nature.com/articles/s41586-021-03557-5

Douglas Natelson said...

@Don, yeah. So, my question is, what is \(\Lambda\) for, e.g., trapped ion qubits?

@Anon1, thanks - I should've pointed that out.

@Anon2, thanks for pointing out that paper. That's really neat (though a concern if it means that we really need to worry about cosmic rays as an error source in solid-state qubit implementations). It does make you think about quantum-enhanced sensing for particle detection - can the entanglement buy us enhanced sensitivity?

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