Quantum computing could someday far surpass the processing power of today’s classical computers. And that thought has some cryptographers, and cryptocurrency users, worried.
But Google’s quantum computing expert John Martinis wants to put their minds at ease.
At a major crypto event at University of California Santa Barbara this week, Martinis talked about why it could take a decade or more to build a quantum computer. “This is really, really hard, way harder than building a classical computer,” he said.
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He went on to explain to the packed room at Crypto 2017, a four-day conference sponsored by the International Association for Cryptographic Research, that the main reason building a quantum computer is so tough is because qubits (quantum bits), the counterpart of bits in classical computers, are unstable. And that creates extra work for physicists trying to solve the problem.
Breaking Public Key Cryptography
Cryptographers are concerned because if researchers build a quantum computer that outperforms classical supercomputers, due to Shor’s algorithm, it could break RSA encryption, a tool used to secure data transmission on the Internet.
In a similar vein, quantum computing could also break the digital signatures used in Bitcoin and other cryptocurrencies. And the upshot of that is not good.
“That would mean you could forge transactions, and steal coins,” explained Bernardo David, a cryptographer at Tokyo Institute of Technology.
But so far, that threat appears to be a long way off.
Unstable Qubits
Holding a cell phone up on stage to represent a qubit tilting and moving in three-dimensional space, Martinis detailed why a quantum computer could take several years to build.
While classic computers use bits of information, such as 1s and 0s, like in a two-dimensional world, a quantum computer uses qubits that can represent both values simultaneously in a wide range of “superpositions” of the two states.
The fact that these qubits are so inherently unstable makes gauging progress in the field less than straightforward.
For instance, some view the work of competing quantum computing labs as a “horse race” based on who has the most qubits. “That’s wrong,” Martinis said. He explains it’s more like a two-dimensional horse race, where both qubits and qubit errors need to be taken into account.
Yet, while most physicists will tell you their best error rate, because that’s what they are most proud of, those who build systems for a living know it is the worst error that always kills you, he said.
Eventually, his group hopes to get their qubit error rate down to some minuscule number. But as of right now, they are ”closer to Kitty Hawk than the moon,” he said.
Quantum Supremacy
Along the path to building a quantum computer, Google plans to hit smaller goals.
For instance, Google’s goal before the end of the year is to reach “quantum supremacy,” a state where a quantum computer can outperform any conventional computer.
Martinis thinks his group can pull it off, but to do it, they will need an array of 50 qubits. They have 22 qubits right now, and he thinks they should be able to scale way beyond that.
In speaking to Martinis later, I asked how long it would take to break RSA encryption schemes. He replied that even if his group could build a quantum computer in 10 years time, it would likely take a lot longer than that to break RSA.
Blockchain’s Reality
In light of that, I also spoke to several cryptographers at Crypto 2017 to get a sense of the threat quantum computing posed to Bitcoin’s digital signatures. Nobody seemed too concerned.
One cryptographer, who wanted to remain nameless, thought breaking a digital signature would require “government level” spending, adding that the first breakage would likely be a stunt pulled off on a $50 million machine.
“Maybe it will take months, maybe it will take minutes, but it will be a very expensive operation to break one signature,” he said. “Once they break one then we’ll start talking how long it will be for a criminal to do it for essentially a dollar.”
While another expert, who also asked to remain nameless, scoffed at the idea completely, waving a hand and saying by the time quantum computers become available, public key cryptology will have advanced too far for it to matter.
“In the grand scheme of risks and threats, it will take 20 to 30 years to be a real threat and by that time, we will have better signature schemes,” he said.
If that is true, Bitcoin and other cryptocurrencies should be reasonably safe from quantum computing for a long time to come.