The RAM in our computers is constantly refreshed to ensure that it maintains the intended information. For most of us, however, a bit flipped somewhere in the memory of our cell phones or laptops is no big deal. But in many data situations, like banking or rocketry data, a flipped bit can be catastrophic. For this, there's error-correcting RAM, which does exactly what its name implies: catches and fixes any errors that occur.

One way to catch an error involves what's called a parity bit. These bits are tacked on to the end of a larger collection of bits (typically a byte) and simply indicate whether the collection sums up to an even or odd number. If the parity and the contents of the byte don't match, then an error must have occurred.

Although the ability to catch and correct errors is very useful in traditional RAM, it may be even more essential in quantum memory, as most of these memory technologies have a fairly short life span before they interact with their environment and lose their contents. In a potential step toward error correcting quantum memory, researchers have created the first quantum parity bit, which keeps track of the number of photons stored in a neighboring optical cavity.

Getting the speedups expected from quantum computing requires placing every bit it uses in a special state, called a superposition, in which the bit simultaneously contains two or more values. The bits used to perform any calculations must also be entangled, meaning the values they hold are correlated. Both of these properties must be preserved if one of the bits is shifted to memory as part of the computation. Unfortunately, these properties are fragile, and they are lost if any of the bits is measured or interacts with its environment.

One approach to dealing with this challenge is to use quantum memories that persist for a relatively long time before environmental noise takes over. Researchers have made some progress in this area using impurities in diamonds for this approach. An alternative is to create a form of error-correcting quantum memory, and diamonds have also proven useful there.

The challenge with using diamonds is that the states of these qubits need to be set using photons that are entangled with the qubits in the processor. None of the transfers of information is 100 percent efficient, so each of these steps is likely to add to a calculation's failure rate. It might be nicer to simply work with the photons themselves, and several quantum computing schemes do just that.

The key feature of the new device is that it acts as a parity qubit for a collection of photons, which can be entangled in what's termed a "cat state" (after Schrödinger's cat). The photons are stored in an optical cavity, and the cavity is linked to a superconducting qubit. As a result of this linkage, the state of the qubit is influenced by the photons that are stored in the cavity. It can't register the state of the photons (that would measure them and destroy the entanglement), but its behavior is influenced by the number of photons present.

Specifically, the device combines a superconducting microwave resonator to trap the photons and a circuit quantum electrodynamic qubit that's linked to the resonator. Previously, researchers have used the storage of photons in the resonator to track changes in the energy state of the qubit. But this team reversed that process: they used the energy state of the qubit to keep track of the photons in the resonator.

Although the qubit didn't track the precise number of photons—that would be observing the quantum state of the resonator, and it would ruin everything—it flipped back and forth between two different states each time a photon escaped the resonator. "These jumps reveal the loss of individual photons without projecting the system onto a state of definite number or energy, but rather into an eigenspace of even or odd photon number," as the authors put it.

The authors say that there's only a four percent chance that the cavity will lose two photons in the time between measurements. And there's a 98.8 percent chance the measurements won't interfere with the quantum states of the photons.

This paper is mostly a demonstration that the parity bit works; the authors have a long list of potential improvements they'd like to test. And there's always the chance that some technology other than photons will end up being used in quantum computing for reasons unrelated to this new error-correction approach. Since we don't know what technology will end up being best at this point, however, the more capabilities of this sort we develop, the better placed we'll be to exploit whatever technology ends up working out.

Nature, 2014. DOI: 10.1038/nature13436 (About DOIs).