qBitcoin: A Way of Making Bitcoin Quantum-Computer Proof?

A golden circle on a black background has the letters "qb" at its center. The letters are surrounded by the symbol of an atom.
Illustration: Erik Vrielink

A new quantum cryptography-based Bitcoin standard has been proposed that could harden the popular cryptocurrency against the advent of full-fledged quantum computers. Bitcoin as it now exists involves traditional public key cryptography and thus could conceivably be hacked by a future quantum computer strong enough to break it. However, quantum cryptography, which is based not on difficult math problems but the fundamental laws of physics, is expected to be strong enough to withstand even quantum computer-powered attacks.

The proposal, dubbed “qBitcoin,” posits transmission of quantum cryptographic keys between a remitter and a receiver of the eponomous named cryptocurrency, qBitcoin. The system would use provably secure protocols such as the BB84 quantum key distribution scheme.

To exchange qBitcoin, then, requires that there be a transmission network in place that can send and receive bits of quantum information, qubits. And that is no mean feat, considering it typically involves preserving the polarization states of individual photons across thousands of kilometers. To date, there are five known quantum key distribution networks in the United States, Switzerland, Austria, and Japan. China is working on their own massive 2000-km link, as well. And a number of satellite-to-satellite and satellite-to-ground quantum key distribution networks are also being developed and prototyped.

Which is to say that qBitcoin or something like it could not be scaled up today. But if the quantum computer singularity is approaching, in which a powerful enough machine could threaten existing cryptography standards, quantum cryptography would be an essential ingredient of the post-“Y2Q” age. So existing quantum key distribution networks might at least serve as outposts in a burgeoning global quantum network, like Western Union stations in the early days of the telegraph.

Some things about qBitcoin might appear the same to any Bitcoin user today. “Bitcoin is a peer to peer system, and qBitcoin is also peer to peer,” says Kazuki Ikeda, qBitcoin’s creator and PhD student in physics at Osaka University in Japan. He says compared to Bitcoin, qBitcoin would offer comparable or perhaps enhanced levels of privacy, anonymity, and security. (That said, his paper that makes this claim is still under peer review.)

However, the lucrative profession of Bitcoin mining, under Ikeda’s protocol, would be very different than what it is today. Transactions would still need to be verified and secured. But instead of  today’s system of a cryptographic puzzles, qBitcoin’s security would rely on a 2001 proposal for creating a quantum digital signature. Such a signature would rely on the laws of quantum physics to secure the qBitcoin ledger from tampering or hacking.

Ikeda's proposal is certainly not the first to suggest a quantum-cryptographic improvement on classical-cryptography-based digital currencies. Other proposals in 20102016 ,and even earlier this year have also offered up variations on the theme. All work to mitigate against the danger large-scale quantum computers would represent to Bitcoin.

Of course, not every solution to the quantum singularity is as promising as every other. A person going by the handle “amluto” criticized Ikeda’s qBitcoin proposal on a prominent message board last week. (amluto claimed to be author of one of a previous quantum currency proposals from 2010—presumably the 2010 proposal’s co-author Andrew Lutomirski, although IEEE Spectrum was unable to confirm this supposition at press time.)

“This is nonsense… It's like saying that you can transmit a file by mailing a USB stick, which absolutely guarantees that you, the sender, no longer have the original file. That's wrong—all that mailing a USB stick guarantees is that you don't have the USB stick any more, not that you didn't keep a copy of the contents. Similarly, quantum teleportation eats the input state but says nothing about any other copies of the input state that may exist.”

Ikeda says he disagrees with the analogy. The point, he says, is that there are no other copies of the “input state” as it's called above—in other words of the quantum keys that secure qBitcoin. So, Ikeda says, qBitcoin is safe just like Bitcoin is safe today.

But one day, thanks to quantum computers, Bitcoin, will no longer be safe. Someone will need to save it. And, no matter who devises the winning protocol, the thing that threatens Bitcoin may in fact also be the thing that comes to its rescue: The cagey qubit.

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