
Google Engineers Think This 72-Qubit Processor Can Achieve Quantum Supremacy - buu700
https://motherboard.vice.com/en_us/article/pam958/bristlecone-google-quantum-computer-72-qubits
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woliveirajr
If there's something that I began to scratch and easily loose myself is
imagining how do you program something using qubits, how the language looks
like, or even an algorithm. In few seconds I come back to the 0-or-1 kind of
thinking and can't advance.

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gdubs
Don’t feel bad — in a recent WSJ magazine interview with Bill Gates he said,
(paraphrasing), “I keep up with most of the new tech at Microsoft, but if
there’s one area where the math looks like Greek to me, it’s Quantum
Computing.”

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vtomole
The basics of quantum computing are not difficult if you know linear algebra.
The state of a qubit is described using a column vector that holds complex
numbers. Gate operations are performed by multiplication on the states with
unitary matrices.

A good introduction:
[http://pyquil.readthedocs.io/en/latest/intro.html](http://pyquil.readthedocs.io/en/latest/intro.html)

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dudus
That's the greek right there

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AlexCoventry
Is there a paper out about this yet?

The blog post was weird: "Here's this chip we've made, but we don't know
anything about its performance characteristics, yet." Made me skeptical about
this being any kind of breakthrough.

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vtomole
>Made me skeptical about this being any kind of breakthrough.

As you should. Google is planning on sharing the results of this processor
soon: [https://research.googleblog.com/2018/03/a-preview-of-
bristle...](https://research.googleblog.com/2018/03/a-preview-of-bristlecone-
googles-new.html)

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williamstein
> "Qubits will endow quantum computers with the ability to do certain
> tasks—such as querying a database, factoring large _prime_ numbers..."

Why do people make that mistake so often?

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AndrewGaspar
I'm not a mathematician, so excuse my ignorance, but is it wrong to call it
factoring? The definition of a prime number is a number whose only factors are
1 and itself, so you're still factoring the number to see if it fits the
definition.

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beisner
The real threat of quantum computers is not the testing of prime numbers, but
the factoring of semiprime numbers (numbers that have two distinct prime
factors). The difficulty of taking a semiprime number and extracting it’s
prime factors is considered an extremely hard problem, and this assumption of
difficulty underpins nearly all of our modern cryptographic systems. If it
were trivial to factor a large semiprime, one could intercept/hack all credit
card transactions. This is the quintessential problem that quantum supremacy
raises, and the most publicized; OP was remarking that the journalists likely
meant to talk about this threat, but somewhat sloppily wrote prime instead of
semiprime.

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ianai
The lay audience might ignore semi-prime and just interpret it as prime. Or
the author did that.

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mikro2nd
I'll believe it when I see Satoshi's Bticoins draining off to a new wallet...

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Zaak
That would be fun to watch, but to be overly pedantic about it, it will take a
computer with hundreds of qubits to break current public key encryption.

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vtomole
It will take hundreds of fault tolerant qubits. That means it will take
potentially millions of physical qubits of relatively high quality. There is a
long road ahead from 72 relatively medium quality to millions of relatively
high quality qubits.

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ByThyGrace
As a layman, I've never heard of qubit quality. What is it exactly? How do you
determine it? And why does it affect computation?

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vtomole
Qubit quality is defined by the quantum volume [0]. Errors that occur when
gates are applied to qubits need to be below a certain threshold to perform
error correction.

[0]: [https://www.research.ibm.com/ibm-q/resources/quantum-
volume....](https://www.research.ibm.com/ibm-q/resources/quantum-volume.pdf)

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viggity
so at what point does a quantum computer's ability to factor large number
threaten RSA (since RSA relies on prime factorization being extremely cpu
intensive)?

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VikingCoder
Well, at a guess, you need at least as many qubits as there are bits in your
key?

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ianai
For at least one quantum computer you would need a way to encode the number in
the qbit states as well as the algorithm, I think. It seems like something
that may require a specially engineered quantum computer.

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vtomole
That is not how Shor's algorithm works. To factor a number N, Shor's algorithm
uses a classical computer to pick a random number a where a < N. A quantum
computer is used to find the period of f(x) = a^x mod N. This Wikipedia entry
[0] provides a good overview.

[0]:
[https://en.wikipedia.org/wiki/Shor%27s_algorithm#Classical_p...](https://en.wikipedia.org/wiki/Shor%27s_algorithm#Classical_part)

