When you set a bit in a normal computer, you expect it to stay set; computers rely on a variety of technologies to ensure it does. The challenge of maintaining a bit is substantially harder in a quantum computer, where any interactions between a qubit and its environment can change the value stored in the qubit. As a result, most quantum bits have a lifetime on the order of milliseconds or less.
Now, researchers have figured out how to turn a source of noise into a solution for maintaining quantum memory. By individually addressing some of the atoms surrounding a qubit, they've turned each into an additional form of storage. By storing a single value in the qubit and its neighbors, they've created a form of error correction for quantum memory.
The research team, a collaboration between Delft University of Technology and the University of Iowa, was focusing on one of the standard forms of qubit storage: a nitrogen vacancy in diamond. Since nitrogen can only form three covalent bonds (instead of carbon's four), it ends up with unpaired electrons when present in the regular matrix of a diamond. Those electrons can then be addressed as qubits, with their state set and read using light of the appropriate wavelength.
Since electrons are extremely light and get knocked around by their environment rather easily, they are a poor choice for stable long-term storage. Fortunately, it's possible to couple the spin of these electrons to the spin of the nitrogen nuclei they orbit. The nuclear spin is much more stable, and it works much better for extended storage.
But even storing a qubit in the spin of a nitrogen nucleus isn't stable for that long. In a diamond, the nitrogen sits in what the researchers term a "spin bath," where the spins of all the nearby carbon atoms influence its state. Given time, these influences cause the quantum state of the nitrogen atom to randomize, losing the value stored in it through a process called decoherence.
The new paper is based on the realization that just as you can couple the electron's spin to that of the nitrogen nucleus, you can also couple it to the spin of individual carbon atoms in the spin bath. All you need to do is time the pulses of light you use to control the electron properly, and you'll set up a resonance with one of the carbon atoms. Change the timing, and you can address a different carbon.
Using this approach, the researchers turned a single nitrogen vacancy into three distinct qubits: one in the nitrogen nucleus, two others in nearby carbon nuclei. By encoding the same information in all three, they were able to do a simple error correction in the form of majority voting. Each of the three has a significant probability of developing an error after a short delay, but that can be corrected for by polling the remaining two and seeing if they agree.
The error correction wasn't perfect, but most of the problems came in setting up the various spins in the right state to begin with. If that can be made more efficient, the error rate for the process as a whole should drop. In addition, the authors performed all the work at room temperature; dropping the system into cryogenic conditions should improve the longevity of its storage (into the range of milliseconds).
The only downside is that most of the operations require manipulating the electron involved multiple times, often with a delay in between light pulses. That adds up—the authors note that performing 300 operations takes about 1.8 milliseconds. So even typical operations might start to stress the system even if it were kept under cryogenic conditions. It's also important to note that, while having stable qubits is essential for any quantum computer, entangling multiple qubits will be essential for actually performing computations. And this work doesn't address that at all.