What good is a fast computer if you can’t trust it? Thanks to half a century of research on getting computers to do their job correctly even in the presence of mechanical errors, our modern machines tend to be pretty reliable.

Unfortunately, the laws of quantum mechanics render all that research useless for quantum computers, the sheer complexity of which leaves them prone to errors. Now, we finally have the first demonstration of a quantum program that can detect data corruption.

Two research groups – one from the University of Maryland and Georgia Tech and the other from IBM – have demonstrated the same quantum error-detecting program, albeit implemented with different hardware.

“Quantum computers can never be practical without error correction,” says at the University of Southern California. As we build bigger quantum computers, “errors add up to the point that they wash out the quantum effects… which obviates the need for the quantum computer,” says Lidar.

The telltale qubit

In classical computers, error detection and correction are done with duplicated data – any mistakes can be remedied by reconstructing the erroneous bits from uncorrupted parts of the machine.

But in quantum computers, it’s impossible to duplicate quantum states without measuring them, and measurement causes loss of information. So, without any means to back up intermediate results, quantum computers simply cannot use classical techniques of error detection and correction.

The solution the teams are proposing consists of five qubits, each of which can be in two states: one or zero. For every two qubits’ worth of information, there are four possible combinations: zero-zero, zero-one, one-zero, and one-one.

The program uses four qubits to record these states, while the fifth ancillary qubit catches errors in the first four qubits. For example, when four qubits represent a two-qubit state that should be zero-zero, the information is in a superposition where the four qubits are either showing four ones or four zeros, or an equal number of each digit. If there’s an error in one qubit, the fifth qubit will note the uneven distribution of ones or zeros and change its state.

Only one error

This verification system reduces the error rate to 0.1 per cent, compared with about 10 to 15 per cent potential error for quantum programs of about this size, says of the University of Maryland. The IBM group’s implementation shows similarly reduced error rates as well.

However, there are limitations to the approach. For example, if one error changes the ancillary qubit from zero to one, and a second changes it back to zero, then the program will not detect these two consecutive errors. Fortunately, experiments suggests such a scenario is rare.

Moreover, the program merely demonstrates the existence of an error. Locating the error precisely requires more qubits. Linke says his group plans to scale up the experiment and implement an error-correction feature, which requires more qubits. of IBM Watson Research Center says his group plans to first perfect the five-qubit program before moving on to error correction.

Journal references: Science Advances, ,Ěý Physical Review Letters,

Embarrassingly parallel

Google has big plans for quantum computing. The company has come up with a strategy for demonstrating While it’s widely assumed that we will eventually reach quantum supremacy, nobody has done it yet because current quantum computers can only run a small number of specialised algorithms.

Their plan is based on simulating coin flips. An ordinary computer does this by storing two numbers and choosing one of them at random each time. To simulate 50 coin tosses, it just selects 50 times in a row.

This is simple with regular coins, but if the coins behave like particles obeying the laws of quantum mechanics, things get more complicated.

In that case, we cannot know whether any individual coin turned up heads or tails without knowing about all the other coins, a phenomenon known as quantum entanglement. The problem of simulating coin tosses with quantum entanglement is called quantum sampling.

Computers work sequentially, so choosing 50 numbers at the same time is not something they can do. For this reason, the Google group argues, quantum sampling would require storing all possible configurations of all 50 coin tosses, so that all of the coins can be thrown simultaneously.

Since one bit – the building block of classical computers – can only store one of two states, heads or tails, covering all possible configurations for 50 coins takes hundreds of terabytes of data storage.

Qubit coins

This is where quantum computers come in. They’re based on qubits, which can be in two states at the same time. This makes it possible to store the probability distribution of all the configurations at once using a single qubit for each coin. For this reason, the Google group argues, quantum sampling would be easy for a quantum computer.

In their proposal, the team demonstrates quantum sampling up to nine coins with high accuracy using their 9-qubit quantum computer. “If similar error rates are achievable in future devices with around 50 qubits, we will be able to explore quantum dynamics that are inaccessible otherwise,” the proposal states. This way, quantum computers of the near future can be used to study physics, a huge step-up from their infancy when they couldn’t do anything practical.

The only remaining task, then, is to build a 50-qubit computer. And the team just might, given their track record

“They’ve been delivering many of the expectations,” says at the University of Texas in Austin. “The truth is, the Google group has such a strong record that if they say they’re going to do it, people pay attention.”

Indeed, the Google group has completed the necessary preparatory work to build large-scale quantum computers, according to of University of Maryland. “They’ve demonstrated that the obvious pitfalls are largely accounted for,” he says.

Pick your problem

Not everyone is convinced quantum sampling is the right problem to tackle in order to demonstrate quantum supremacy, though.

“It is unclear whether what they claim to show is quantum supremacy,” says of the University of Southern California. “You have to prove that classical computers can’t do it.”

He says that many important quantum-mechanical systems can be simulated on present-day computers, because it’s not necessary to save all of the information about the system to simulate it.

Proof that this kind of simulation can’t be done on a classical computer may not come anytime soon, but many concepts in this field are accepted as true without formal proof.

“We know almost certainly there’s not going to be a fast classical algorithm” that solves quantum sampling problems, says Aaronson.

Read more: Revealed: Google’s plan for quantum computer supremacy

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