Getting the World Quantum Ready | Hacker News Reader

Getting the World Quantum Ready

ibm.com

36 pointszaphod_ibm13 comments

> What if everyone in the 1960s had a decade to prepare for PCs [...] while they were still prototypes? In hindsight, we can all see that jumping in early would have been the right call.

This is an interesting hypothetical based on the assumption that quantum computers will be available in 10 years time. What if the first practical quantum computers are still 50 years away? Using the same hypothetical, it would be like someone preparing for the PC age in that 1920s. That might be "jumping in" too early.

Am I missing something because D-Wave has 1024 qubit machines available now and 2000 qubit ones soon.

I'm going to repost from a year ago:

D-Wave isn't trying to build a general purpose quantum computer. As it stands, they've been trying to optimize a computer that executes a particular algorithm (and as it stands, it's not faster than optimized classical algorithms). It is an open research question if D-Wave's architecture actually utilizes any quantum effects to give it an advantage over classical architectures. D-Wave hopes that it does, and that there will be enough problems that their machine is good at that their approach will be justified.

D-Wave markets themselves as having hundreds or thousands of qubits, but these qubits aren't easily controllable or measurable in ways that would allow Shor's Algorithm to be executed on them (for example), so at the least, it seems like dishonest marketing. In order to build a general quantum computer, you need to be able to apply gates to arbitrary collections of qubits.

Ah, thanks

This is why people are less excited by D-Wave’s approach:

https://www.theverge.com/2016/9/28/13057414/quantum-computer...

D-Wave isn't a real quantum computer in the way that most people use the term, so that isn't an apples to apples comparison.

Can I run Shors algorithm on this? If not, how many qubits would I need for that? If so, how do I short bitcoin?

According to this paper:

https://arxiv.org/abs/quant-ph/0205095

You can implement Shor's algorithm to factor an N-bit number with just 2n+3 qubits. So the 20 qubit processor would be able to factor any 8 bit number (not necessarily faster than a classical computer could, or even a human could, mind you).

As the question was about bitcoin, I'm not sure how prime factoring is relevant.

> Can I run Shors algorithm on this? If not, how many qubits would I need for that?

The question was about Shor's algorithm. The question relating to bitcoin seemed like an aside.

https://en.wikipedia.org/wiki/Shor%27s_algorithm

Given that the original article was about Quantum Computing, I was more interested in answering questions about Quantum Computing.

Yes, but bitcoin's authentication is via ECDSA. Elliptic curve discrete log is solvable with Shor's algorithm, but the qubit calculation would be different.

Wikipedia seems to indicate that 2330 qubits and 126 billion Toffoli gates would be required for elliptic curves with 256-bit group size, although I wonder if the endomorphism of secp256k1 gives an additional speedup.

1. Probably

2. Several Thousand [0]

3. You can't, at least not easily

[0] https://en.wikipedia.org/wiki/Elliptic-curve_cryptography#Qu...

3. Yes you can easily. The futures contracts have enabled short bets against Bitcoin. A few brokers have set this week or next as a roll-out for that, in the very near future hundreds of different financial companies will enable shorting via futures for their clients.

For example, Interactive Brokers is going live with their shorting this week:

https://www.bloomberg.com/news/articles/2017-12-12/bitcoin-b...

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