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If a practical quantum computer is going to become an everyday thing, qubits have to remain in two states at one time for much longer than they do now. One of the possible candidates for a longer lasting qubit is a copper ion embedded in a large molecule.

A quick primer on qubits is in order. Certain ions have unpaired electrons whose spins can assume either of two spin states, up or down—or in computer speak, 0 or 1. But when struck with a microwave pulse, the unpaired electron can be coerced into assuming both the 0 and 1 state simultaneously. The two states are said to be in superposition. All the qubits created up to now stay in a superposition state for very short periods because the spin states of neighboring atoms quickly destroy the coherent state, making the life of the qubit too short for it to perform the desired number of quantum computations. But researchers have been looking high and low for suitable qubits and for ways to lengthen the periods over which they remain in superposition.

Now, a group at the University of Stuttgart reports, in the 20 October issue of Nature Communications, that it has developed a way to protect the spin of a copper ion by placing it inside a molecule that has relatively few spin-carrying atoms and keeping it far away from hydrogen atoms that carry spins. The copper ion is moved into a neighborhood where it’s surrounded by sulfur and carbon atoms that have no spin, and by nitrogen atoms that have a small magnetic moment. The team reports a coherence time of 68 microseconds at a temperature of 7 K—an order of magnitude better than what can be achieved with similar qubits now. The molecule can still function as a qubit at room temperature, although the coherence time is 1 microsecond. Qubits in nitrogen vacancies in diamonds have scored much longer coherence times, but “you cannot make a quantum computer with a single qubit,” says Joris van Slageren, the chemical physicist at the University of Stuttgart who led the research.

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