First demonstration of universal operations on an error-free quantum computer

For years we have been told that quantum computing is just around the corner, with many advances suggesting that computers based on quantum mechanics will eventually supersede current computing technologies and open an array of new possibilities. Now, new research has developed the first functioning error-free quantum-computing system.

Using the complex and irregular quantum principles of superposition and entanglement to build a functioning computer is not easy. Current quantum machines, though representing huge leaps forward in our understanding and technical abilities, are rife with errors.

Modern, non-quantum computers avoid faults in the processing and storage of information through high-quality manufacturing. But critical applications still require error-correction procedures based on redundancy of data.

More on quantum computing: Quantum computing remains tantalisingly out of reach

Because of the nature of quantum mechanics, quantum computers are more susceptible to errors and data loss. So, quantum machines will always require error correction. But quantum mechanics also forbids the copying of quantum information, so redundancy has to be achieved by scattering logical quantum information into an “entangled state” comprising several physical systems – for example, multiple atoms.

Explained in a paper published in Nature, researchers implemented computational operations on two quantum bits (qubits). The qubits are part of an ion trap quantum computer comprising 16 trapped atoms. Each qubit’s information was distributed over seven other atoms.

The team then executed the first-ever error-free universal computational operation on qubits.

What do they mean by “universal”? Using different permutations and configurations of two particular quantum logical gates, any possible operation is possible. The two gates are the controlled-NOT (CNOT) gate and the T-gate. “For a real-world quantum computer, we need a universal set of gates with which we can program all algorithms,” explains Lukas Postler, an experimental physicist from the University of Innsbruck, Austria.

CNOT is an operation where the second qubit is flipped if – and only if – the first qubit is in the state 1 (instead of 0). The T-gate, which is particularly difficult to implement on fault-tolerant qubits, changes the phase of the target qubit.

“T-gates are very fundamental operations,” says Markus Müller, theoretical physicist from RWTH Aachen University and Forschungszentrum Jülich in Germany. “They are particularly interesting because quantum algorithms without T-gates can be simulated relatively easily on classical computers, negating any possible speed-up. This is no longer possible for algorithms with T-gates.”

Errors caused by the underlying physics when operations are made on qubits are detected and corrected in the researchers’ machine.

“The fault-tolerant implementation requires more operations than non-fault-tolerant operations,” says team leader and University of Innsbruck experimental physicist Thomas Monz. “This will introduce more errors on the scale of single atoms, but nevertheless the experimental operations on the logical qubits are better than non-fault-tolerant logical operations. The effort and complexity increase, but the resulting quality is better.”

The experimental results were checked and confirmed using numerical simulations on classical computers. Now, two qubits does not a functioning, useable quantum computer make – most modern computers are 32- or 64-bit – but the team has demonstrated that error-free quantum computing is possible. The goal now is to expand their set-up to larger and more complicated, and hence more useful, machines.

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