[05-05-2016]

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**Quantum superposition**

The QuTech institute at TU Delft is working on the computer of the future: the quantum computer. This computer is based on the counter intuitive laws of quantum mechanics, allowing it to solve important problems that are far beyond the reach of the best classical computers. The building blocks of the quantum computer, qubits, can not only take the values ‘0’ or ‘1’, but also ‘0’ and ‘1’ at the same time, which we call a quantum superposition.

**Power and weakness**

These superpositions provide the power of the quantum computer. They allow for calculations to be performed as if they are executed simultaneously, such that certain problems can be solved very efficiently. At the same time, these superpositions are the weakness of the quantum computer: a superposition is very fragile and just looking at the system is sufficient to disturb it. Therefore it is challenging to detect possible errors in quantum information during computations.

**Error correction**

To be able to perform useful quantum computations, it is nevertheless essential that errors can be corrected. Quantum error correction therefore forms the basis of quantum computing and is the main open challenge for practical quantum computers. In quantum error correction, quantum information is protected by first encoding it. Then errors are detected by careful measurements that do not disturb the fragile quantum information. Finally, the errors are processed by fast classical electronics and actively corrected. Importantly, the quantum system must remain encoded and continuously protected throughout the whole process.

**Diamond**

The QuTech scientists used electronic and nuclear spins in diamond to implement such a complete error-correction process for the first time. These spins can be used to process quantum information with high fidelity and to store it long enough to process and actively correct the errors using classical electronics. This demonstration of active quantum error correction is an important milestone towards more complex error correction systems that will ultimately lead to a quantum computer.

**Paper**: Julia Cramer, Norbert Kalb, M. Adriaan Rol, Bas Hensen, Machiel S. Blok, Matthew Markham, Daniel J. Twitchen, Ronald Hanson, Tim H. Taminiau. Repeated quantum error correction on a continuously encoded qubit by real-time feedback.

DOI: 10.1038/10.1038/NCOMMS11526

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