Quantum computing: Three of diamonds.

From DNA replication to satellite communication, identifying and fixing errors is an essential part of building robust large-scale systems. Common approaches to error correction make use of redundancy (encoding copies of information across more than one entity) and rely on the fact that multiple errors are less likely to occur than a single error. For error correction in quantum information processors, such redundancy requires control over at least three coupled quantum bits (qubits), which is challenging in many systems. Now Gerald Waldherr and co-workers writing in Nature and Ronald Hanson and co-workers writing in Nature Nanotechnology, report the use of the spins of defects in diamond along with trace nuclear spins in the host lattice to demonstrate the control required to implement a quantum error correction protocol1,2. Qubits are extremely sensitive to disturbances from their environment, often leading to quantum information lifetimes in the millisecond range, or less. To make matters worse, performing a measurement on a qubit destroys the information it represents, ruling out error-handling approaches such as continuously measuring and re-setting the qubit (as happens when dynamic random access memory is refreshed). Practical quantum computing therefore only became a real prospect with the advent of quantum error correction protocols3,4, which map the quantum state of a ‘data’ qubit across two further qubits, known as ancillae. Using a procedure analogous to majority voting among the three qubits (Fig. 1), it is possible to correct errors in the data qubit without ever measuring its state, thus leaving the quantum information intact. The errors are effectively filtered off into the ancilla qubits, which are periodically reset. Nitrogen–vacancy (NV) defect centres in diamond (Fig. 2) are among the most promising systems being investigated as potential qubits. They possess an electronic spin whose state can be initialized and read out optically, even at room temperature5. This electron spin interacts strongly with the nuclear spin of the defect’s nitrogen atom (14N); however, to find the three or more spin qubits requisite for quantum error correction, one has to look to the