A promising quantum state was found during error correction research

Window glass, on a microscopic level, exhibits an odd mix of properties. Like a liquid, its atoms are irregular, but like a solid, its atoms are solid, so a force applied to one atom causes them all to move.

It’s an analogy physicists use to describe a quantum state called “quantum spin glass,” in which the quantum mechanical qubits (qubits) in a quantum computer exhibit both disorder (taking into account seemingly random values) and rigidity (when one qubit flips, so do all the others). A team of researchers at Cornell University unexpectedly discovered the existence of this quantum state while conducting a research project designed to learn more about quantum algorithms and related new error-correction strategies in quantum computing.

“Measuring the position of a quantum particle changes its momentum and vice versa. Similarly, for qubits, there are quantities that change each other when they are measured. And we find that some random sequence of these incompatible measurements leads to the formation of quantum spins,” said Eric Müller, professor of physics in the College of Arts and Sciences. (A&S) “One implication of our work is that some types of information are automatically protected in quantum algorithms that share features of our model.”

“Subsystem symmetry, spin-glass order, and criticality from random measurements in a two-dimensional Bacon-Shore circle.” Published 31 July in physical review b. The lead author is Vaibhav Sharma, PhD student in physics.

Assistant Professor of Physics Zhao Mingjian (A&S) is a co-author with Mueller. All three conduct their research at the Cornell Laboratory for Atomic and Solid State Physics (LASSP).

“We’re trying to understand the general features of quantum algorithms, which are features that are beyond any given algorithm,” Sharma said. “Our strategy for revealing these universal features has been to study random algorithms. We have discovered that certain classes of algorithms lead to hidden ‘spin-glass’ ordering. We are now looking for other forms of hidden order and we think this will lead us to a hidden order. A new classification of quantum states .”

Random algorithms are those that include a degree of randomness as part of the algorithm, for example, random numbers to decide what to do next.

Mueller’s 2021 New Frontiers grant proposal for “autonomous quantum subsystem error correction” aims to simplify quantum computer architectures by developing a new strategy for correcting quantum processor errors caused by environmental noise — that is, any factor, such as cosmic rays or magnetic fields, that would interfere. With the quantum bits of a quantum computer, which leads to information corruption.

Parts of classic computer systems are protected by error-correcting codes, Mueller said. The information is copied so that if one bit “flips”, you can detect the error and fix it. “For quantum computing to be applicable now and in the future, we need to come up with ways to protect qubits in the same way.”

“The key to correcting a mistake is repetition,” Mueller said. “If you send three copies of a bit, you can tell if there is an error by comparing the bits with each other. We borrow language from cryptography to talk about such strategies and refer to the group of repeated bits as a ‘code word.’”

When they made their discovery about the order of spin-glasses, Müller and his team were looking at generalizations, in which multiple codewords are used to represent the same information. For example, in subsystem code, bit “1” might be stored in four different ways: 111; 100; 101; and 001.

“The extra freedom one has in coding the quantum subsystem simplifies the process of error detection and correction,” Mueller said.

The researchers emphasized that they weren’t simply trying to create a better bug-protection system when they began this research. Instead, they were studying random algorithms to see the general properties of all of these algorithms.

“It’s interesting that we found a nontrivial structure,” Mueller said. “What’s even more dramatic is the presence of this spin-glass system, which suggests there’s some additional hidden floating information that must somehow be usable for computing, though we don’t know how yet.”