Quantum systems are inherently fragile as any interactions with the outside world can change their state. That makes creating things like quantum memories rather challenging, since it can be hard to know if it actually preserves the information you put into it. To get around this, researchers have been looking into ways of creating error-correcting quantum memory.

Now, researchers have come up with a rather simple scheme for providing quantum error controls: entangle atoms from two different elements so that manipulating won't affect the second. Not only is this highly effective, the researchers show that they can construct quantum logic gates with the setup. And while they were at it, they demonstrate the quantum nature of entanglement with a precision that's 40 standard deviations away from classic physical behavior.

People have managed to entangle different types of particles previously. For example, you can entangle an atom and a photon, which allows the photon to transfer information elsewhere—something that's undoubtedly necessary for a quantum computer.

In the two new papers, the researchers involved lay out the case for entangling different types of atoms. If you use the same types of atoms for memory and backup copies of the bits, then there's always a chance that a photon meant for the memory will scatter off it and hit one of the backups instead. If you use atoms from different elements, then they'll be sensitive to different wavelengths of light. Manipulating one will leave the other unaffected.

As one of the labs involved (from the University of Oxford) put it, "it allows protection of memory qubits while other qubits undergo logic operations or are used as photonic interfaces to other processing units."

The Oxford team went about this by using two different isotopes of calcium. A second team, from the National Institute of Standards and Technology and the University of Washington, used entirely different atoms (beryllium and magnesium). The calcium atoms could maintain their state for roughly a minute, while the beryllium atoms would hold state for a second and a half, making them somewhat stable in qubit terms.

Both teams confirm that their two atoms are entangled with a very high probability; .998 for one, .979 for the other (of a maximum of one). The NIST team even showed that it could track the beryllium atom as it changed state by observing the state of the magnesium atom.

The true test of the quantum nature of these systems involves formulas known as Bell's inequality. Classical behavior would have values below a critical number; quantum ones would be above that limit. The Oxford team managed to show behavior that was 15 standard deviations above what you'd expect from a classical system. The one from NIST went to 40 standard deviations. It's pretty clear that these systems are entangled.

The NIST team also showed that their setup could be used for quantum processing. By arranging a series of Be/Mg atom pairs, the were able to construct two types of quantum logic gates, CNOT and SWAP.

Again, it's critical to recognize that these experiments are limited to demonstrations; they haven't built a working quantum computer, much less one that could perform useful calculations. What they have succeeded in doing is providing a potentially useful component that could be part of a useful quantum computer. And the more options we have for combining these components, the better the chance that we'll eventually be able to put something together.

Nature, 2015. DOI: 10.1038/nature16186, 10.1038/nature16184 (About DOIs).