The Amazing Power of Quantum Memory

The first version of this story appeared in the middle Quanta Magazine.

It is not easy to study quantum systems—collections of particles that obey the contradictory laws of quantum mechanics. Heisenberg’s uncertainty principle, the basis of quantum theory, states that it is impossible to simultaneously measure a particle’s exact position and its velocity—the most important information for understanding what is happening.

To study, say, a particular collection of electrons, researchers have to be smart about it. They might take a box of electrons, poke it in various directions, and take a snapshot of what it looks like in the end. By doing so, they hope to reconstruct the quantum energy within the work.

But there’s a catch: They can’t measure all system properties at once. So they repeat. They will start with their plan, click, and rate. Then they will do it again. Each time they iterate, they will measure a new set of properties. Build enough abstractions together, and machine learning algorithms can help reconstruct the full features of the original system—or at least get really close.

This is a tedious process. But in theory, quantum computers could help. These machines, which operate according to quantum laws, have the potential to be much better than conventional computers at modeling the workings of quantum systems. They can also store information not in classical binary memory, but in a more complex form called quantum memory. This allows richer and more accurate descriptions of particles. It also means that a computer can store multiple copies of a quantum state in its working memory.

A few years ago, a team based at the California Institute of Technology showed that certain algorithms that use quantum memory require significantly fewer iterations than algorithms that don’t. Their method was a major advance, but it required a large amount of quantum memory.

That’s a deal-breaker, because as a practical matter, quantum memory is hard to come by. A quantum computer is made of interconnected quantum bits called qubits, and qubits can be used for computation or memory but not both.

Now, two independent groups have come up with ways to bypass the smallest quantum memory. In the first paper, Sitan Chen, a computer scientist at Harvard University, and his colleagues showed that just two copies of a quantum state can significantly reduce the number of times you need to take a snapshot of your quantum system. Quantum memory, in other words, is always worth the investment.