Vladimir Timofeev/Getty By Mark Kim WHAT good is a fast computer if you can’t trust it? Half a century of research on getting computers to work even when errors pop up makes our modern machines 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 quantum program that detects 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 Daniel Lidar 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. In classical computers, error detection and correction are done with duplicated data – mistakes can be remedied by rebuilding 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. Without any means to back up intermediate results, quantum computers cannot use classical error detection methods. “The more complex the computers, the more errors add up and wash out the quantum effects” The solution the teams propose 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 qubit catches errors in the first four. For example, when four qubits represent a two-qubit state that should be zero-zero, it turns out that the four qubits must either show 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 number of ones or zeros and change its state. 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 Norbert Linke of the University of Maryland (Science Advances, doi.org/gcgjbz). The IBM group shows similarly reduced error rates (Physical Review Letters, doi.org/cfxq). However, there are limitations. For example, if one error changes the fifth 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 suggest such a scenario is rare. Moreover, the program merely notes 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,