Detail of a QuEra quantum computer based on extremely cold atoms
QuEra
Algorithms called phantom codes could help quantum computers run complex programs without errors, overcoming a major hurdle to making the technology more widely useful.
Early on, some physicists doubted that quantum computers would ever be useful because they expected these devices to be too prone to difficult errors. Today, several types of quantum computers exist and have already been used for scientific discovery and exploration. Still, while progress has been made, researchers have not been able to fully limit the error.
Many popular error-correcting programs enable quantum computers to store information without error, while struggling when it comes to computation, says Shayan Majidy of Harvard University.
In search of a remedy, he and his colleagues focused on calculations that include many computational steps, making them long and inefficient to run, and risking more errors creeping in.
Quantum computers are made of physical units called qubits, but these calculations involve logical qubits, or groups of qubits that share information to reduce the error rate. To make calculations foolproof, devices typically have to manipulate logical qubits — for example, by firing lasers or microwaves at the physical qubits — to make two or more of them entangled or change their quantum properties.
Phantom codes allow many logical qubits to be entangled without any physical action being necessary – hence the name “phantom”. In practice, this means that the entire calculation will require fewer such actions, increasing efficiency and reducing the number of ways errors can occur.
Majidy and his colleagues used computer simulations to test phantom codes on two tasks: preparing a special qubit state often used in calculations and emulating a toy model of a quantum material. They found that because it required fewer physical manipulations, their approach produced up to 100 times more accurate results than more conventional error correction programs.
Phantom codes can’t help with every quantum computing program, says Majidy, but they excel in situations where a computation requires a lot of entanglement already. They do not create complications out of nothing, he says, but rather take advantage of what is already there. “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 of the University of Galway in Ireland says choosing an error-correcting code for a quantum computing task is like choosing a suit of armor – a suit of armor can achieve more protection than chain mail, at the expense of being heavier and less flexible. Phantom codes offer flexibility, but just like chain mail, they also have drawbacks, such as requiring more qubits than some traditional approaches, says Howard. Because of this, they can be used for some targeted quantum computing program subroutines, but they are unlikely to be a complete solution to quantum computers’ error problems, he says.
Dominic Williamson at the University of Sydney in Australia says it is an open question how competitive phantom codes can be with other error correction methods, some of which may depend on future developments in quantum computer hardware.
Majidy says his team is already working closely with colleagues building quantum computers from extremely cold atoms. He expects that the lessons learned from phantom codes, combined with insights into what a qubit can practically do, will lead to a new strategy where quantum computing programs will be more specifically tailored to a particular task and implementation.
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