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A newly proposed «quantum blockchain» could lead to blockchain systems impervious from quantum-computer hacking, a new study finds.

This new quantum blockchain can be interpreted as influencing its own past, making it behave like a time machine, the researchers add.

A
blockchain
is a kind of database that holds records about the past, such as a history of financial or other transactions, that every node in the network can agree on and that does not require a centralized institution to maintain its ongoing accuracy. The most well-known application of blockchains is
Bitcoin
, but a diverse array of startup companies, corporate alliances, and research projects have explored other potential uses for the technology.

«It’s expected that 10 percent of global GDP could be stored on blockchain technology by 2027,»
says lead author Del Rajan, a theoretical physicist at Victoria University of Wellington in New Zealand.

However, blockchains might face trouble from another up-and-coming technology,
quantum computers
. Whereas classical computers switch
transistors
either on or off to symbolize data as ones and zeroes, quantum computers use quantum bits or
qubits
that, because of the surreal nature of quantum physics, can be in a state of
superposition
where they are both 1 and 0 simultaneously.

Superposition lets one qubit perform two calculations at once, and if two qubits are linked through a quantum effect known as
entanglement
, they can help perform 2^2 or four calculations simultaneously; three qubits, 2^3 or eight calculations; and so on. In principle, a quantum computer with 300 qubits could perform more calculations in an instant than there are atoms in the visible universe. A powerful enough quantum computer could successfully break conventional cryptography, including that protecting blockchains.

Now researchers in New Zealand suggest a quantum blockchain could resist hacking attempts from quantum computers. All the components of this system have already been experimentally realized, they add.

«Previous blockchains that worked with quantum operations were presented, but the blockchain itself was never quantum,» Rajan says. «We are presenting the first fully quantum blockchain.»

Quantum blockchains in theory rely on entanglement. When two or more particles such as photons get entangled, they can influence each other simultaneously no matter how far apart they are, a phenomenon Einstein dubbed «spooky action at a distance.»

Quantum computers and other quantum technologies usually rely on entanglement across space. This new quantum blockchain depends on


entanglement in time


—that is, linking two or more particles no matter how far apart they are in time.

Conventional blockchains collect records into blocks of data which cryptography links in chronological order. If a hacker attempts to tamper with a particular block, the cryptography is designed to invalidate all future blocks following the tampered block.

In the quantum blockchain, the records in a block are encoded into a series of photons that are entangled with each other. These blocks are linked in chronological order through entanglement in time.

As the blocks making up a quantum blockchain are transferred within a network of quantum computers, photons encoding each block get created and then absorbed by the nodes making up the network. However, entanglement links these photons across time, even photons that never existed at the same time.

«Records about past transactions are encoded onto a quantum state that is spread across time,» Rajan says.

In this scenario, a hacker cannot tamper with any photon encoding records of the past, since those photons no longer exist in the current time—they already got absorbed. At best, a hacker can attempt to tamper with the most recent photon, the most current block, and successfully doing so would invalidate that block, informing others it got hacked. «This is more desirable than the standard case where an attacker has the ability, in principle, to tamper with any block,» Rajan says.

The researchers say with entanglement in time, measuring the last photon in a block influences the first photon of that block in the past before it got measured. Essentially, current records in a quantum blockchain are not merely linked to a record of the past, but rather a record in the past, one does that not exist anymore. «This work can be viewed as a quantum time machine,» says senior author
Matt Visser, a theoretical physicist at the Victoria University of Wellington.

Rajan and Visser
detailed


their findings


online April 16.