Laboratories around the world are racing to develop new sensitive computing devices that work on the principles of quantum mechanics and that could provide significant advantages over conventional devices. However, these technologies still face several challenges, and one of the most important is how to deal with “noise” – simple fluctuations that can eradicate the data contained in these devices.
The method involves fine-tuning the system to deal with the types of noise that are most likely to be installed instead of a wide network in order to try to capture all possible sources of disturbance.
The main problems that MIT researchers are now facing in the development of quantum technologies are small and noisy systems. Noise, an unwanted disturbance of any kind, is particularly disturbing because many quantum systems are essentially very sensitive, a feature that underlies their potential applications.
There is another problem, and that is that quantum systems are affected by any observation. So things are more complex in the quantum world than in the classical one. It is really an aggravating circumstance that if you observe quantum systems, you will be inclined to collapse them.
Classical error correction schemes are based on redundancy. For example, in a noise-prone communication system, instead of sending one bit (1 or 0), three copies of each bit (111 or 000) can be sent. Therefore, if the three bits do not match, it shows that an error has occurred. The more copies of each bit sent, the more efficient the error correction.
The same principle could be applied in quantum bits, or “qubits.” However, if we want to have a high level of protection, we must dedicate a large part of our system to performing these types of checks. This is difficult, because we currently have fairly small systems, we simply do not have the resources to do useful quantum error corrections in the usual way. Therefore, researchers have found a way to target error corrections to the specific types of noise that are most likely.
The quantum systems they work with consist of carbon nuclei located near a special type of defect in a diamond crystal called the nitrogen free center. These defects behave like single, isolated electrons, and their presence allows control of nearby carbon nuclei.
The team found that the vast majority of noise affecting these cores comes from only one source and that is random fluctuations in nearby defects. This noise source can be precisely modeled, and the suppression of its effects could have a great impact, because other noise sources are relatively insignificant.
In fact, we have a good understanding of the main source of noise in these systems, so we don’t have to have a wide network to catch every hypothetical type of noise.
The team devised a different error correction strategy, tailored to counter this particular, dominant source of noise. Noise comes from this central defect, or from this central electron, which tends to move around randomly. That is, it shakes.
This tremor is felt by all these nearby nuclei, in a predictable way that can be corrected. The result of this approach is that we are able to obtain a fixed level of protection using much fewer resources than would otherwise be required. We can also use a much smaller system with this approach.
The work so far is theoretical and the team is actively working on a laboratory demonstration of this principle. If it works as expected, it could be an important part of future quantum-based technologies of various kinds, including quantum computers that could potentially solve previously unsolvable problems: quantum communication systems or highly sensitive sensor systems.
This is a component that can be used in many ways. It’s like we’re developing a key part of the engine, and we’ve made progress in the critical part.
The complexity of quantum error correction codes is daunting, as they require a large number of qubits to efficiently encode quantum information.
This paper shows that the usual type of error can be corrected in a much more efficient way than expected. In order for quantum computers to become practical, more ideas like this are needed.
In a diamond crystal, three carbon atoms (blue) surround an empty space called the nitrogen free center, which behaves similarly to a single electron (red). Carbon nuclei act as quantum bits or qubits, and it turns out that the primary source of noise that disturbs them comes from “electrons” from the middle. By understanding the unique source of that noise, it becomes easier to make up for it.