Phantom codes could help quantum computers avoid errors

Algorithms called phantom codes could help quantum computers run complex programs without errors, overcoming a major hurdle to the technology’s wider use.

At first, some physicists doubted that quantum computers would ever be useful because they expected the devices to be too prone to hard-to-fix errors. Today, there are several types of quantum computers that have already been used for scientific discovery and exploration. Although progress has been made, researchers have not been able to fully reduce the problem of error generation.

Many popular error-correcting programs allow quantum computers to store information without errors, but still have computational problems, he says Shayan Majida at Harvard University.

In their search for a fix, he and his colleagues focused on calculations that involve many computational steps, making them tedious and inefficient and risking additional creeping errors.

Quantum computers are made of physical units called qubits, but these calculations involve logical qubits, or groups of qubits, that share information to reduce error rates. To make computations fault-tolerant, devices typically have to manipulate logical qubits—for example, by shooting lasers or microwaves at physical qubits—to entangle two or more of them or change their quantum properties.

Phantom codes allow many logical qubits to be entangled without the need for any physical action – hence the name “phantom”. In practice, this means that the entire calculation would require fewer such actions, increasing its efficiency and reducing the number of ways in which errors can occur.

Majidy and his colleagues used computer simulations to test the phantom codes on two tasks: preparing a special qubit state often used in computing, and emulating a toy model of a quantum material. They found that because it required less physical manipulation, their approach produced up to 100 times more accurate results than more conventional error correction programs.

Phantom codes can’t help with every single quantum computing program, Majidy says, but they excel in situations where the computation already requires a lot of entanglement. They don’t create entanglement out of thin air, he says, but rather use what already exists. “It’s not a free lunch. It’s just a lunch that was already there and we didn’t eat it,” he says.

Mark Howard at the University of Galway in Ireland, says that choosing an error-correcting code for a quantum computing task is like choosing a suit of armor—a suit of plate armor can achieve more protection than chain mail, at the cost of being heavier and less flexible. Phantom codes offer flexibility, but like chainmail, they also have drawbacks, such as requiring more qubits than some traditional approaches, Howard says. Because of this, they could be used for some targeted subroutines of quantum computing programs, but are unlikely to be a complete solution to the error problems of quantum computers, he says.

Dominic Williamson at the University of Sydney in Australia, says it is an open question how phantom codes can be competitive with other error correction methods, some of which may depend on future developments in quantum computing hardware.

Majidy says his team is already working closely with colleagues who are building quantum computers from extremely cold atoms. He expects that lessons learned from phantom codes, combined with insights into what a qubit can do in practice, will lead to a new strategy where quantum computer programs are more specifically tailored to a specific task and implementation.

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