## New research opens the book on entangled qubits

**Friday, September 3 **

Researchers have developed a new way to measure how quantum information behaves in correlated quantum systems that could be useful for understanding and improving quantum devices and quantum error correction codes.

“We are all excited about the potential of quantum entanglement for quantum information processing,” said Mohamad Niknam, who was a postdoctoral fellow at IQC when he co-authored the paper.

Mohamad Niknam

But to put quantum entanglement to use, researchers need to understand how quantum information behaves when it is encoded in an entangled state of the collection of qubits, known as a many-body quantum system.

Think of a 100-page book. By reading each page, you learn 1% of the content of the book. In the quantum entangled version of the same book, each page by itself does not have any comprehensible information, the information is encoded in the correlation between pages, and it is only after reading all the pages you learn everything in the book.

Entangled qubits in a quantum processor are similar. Looking at individual qubits doesn’t reveal the quantum information, but looking at the correlations of the whole many-body system does. This fascinating feature of quantum systems has practical uses, in particular for quantum error correction. If quantum information is encoded in a highly entangled state, corruption of one of the qubits has a small effect on the integrity of the entire message.

“In this work, we introduced and implemented an experimental protocol for making collective observations of multi-qubit quantum states that reveals the volume of correlations. When applied to an entangled system, this method probes the length of the entangled state or essentially, the number of pages in the quantum entangled book,” said Niknam.

The team, which includes Lea Santos of Yeshiva University and IQC and University of Waterloo Department of Chemistry faculty member David Cory, call the new quantity the “correlation Renyi entropy.”

The researchers developed a protocol to measure this entropy using nuclear magnetic resonance techniques. They found that the correlation Renyi entropy captures information about the long-time dynamics of the quantum system that is usually not captured by existing entropies, such as entanglement entropy.

Providing a better understanding and quantification of the spread of quantum information, the researchers’ new entropy will be a useful tool for scientists looking to measure and control the evolution of quantum systems in the devices that will power the next quantum revolution.

Niknam, now a postdoc at the Center for Quantum Science and Engineering at UCLA, said he enjoyed his time at IQC. “IQC allowed me the opportunity to contemplate fundamental problems in physics and explore them in the lab.

Now, he is excited to work at the intersection of theory and application designing new methods for efficient two-qubit gates and qubit entanglement, as well as quantum control applications.

Experimental Detection of the Correlation Rényi Entropy in the Central Spin Model was published in Physical Review Letters on August 18, 2021

This research was funded thanks in part to the Canada First Research Excellence Fund (CFREF).