The smallest unit of information in quantum computers and quantum networks alike is a quantum bit (qubit). Our current computers and the Internet use (classical) bits as their building blocks of information. These bits can be thought of as switches that can be either ON or OFF, representing a 1 or a 0 state. Thus, the language of computers and the Internet is made up of strings of ones and zeros.

Qubits have a few added features that sets them apart from classical bits thanks to their quantum nature. In contrast to classical bits that can be either 0 or 1, a qubit can be in multiple states at once owing to the quantum mechanical principle of superposition. 

The quality of a qubit can be determined by checking how "far" a given quantum state is from a desired target state; we call this measure the fidelity.

The jury is still out

Qubits can be realised in various ways and can be made from various materials from semiconductors, superconductors, and even combinations of different materials. Since each implementation has its own strengths and limitations, there is no clear consensus on which implementation will be the dominating qubit technology. Research institutes and key industry players such as Google, IBM, and Microsoft are all exploring possible qubit implementations. 

Embracing imperfections

Researchers in the quantum internet division at QuTech are working on a qubit implementation that utilises electron spins inside nitrogen vacancy (NV) centers in diamonds.  An NV center is a point-defect in a diamond crystal where a nitrogen (N) atom and a vacancy (V) replace a carbon atom. Such an NV center can trap electrons, granting researchers access and allowing them to control the electron spin. Among the advantages of NV diamond qubits is their long coherence time and the possibility to operate them at a large range of temperatures.

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