Entanglement over large separations because of, not despite dissipation

The inevitable leakage of energy and information from a quantum system into its surrounding environment is the enemy of quantum technology. Now, researchers have demonstrated that it can be exploited to generate entanglement — the “resource” that quantum technologies use to perform tasks inaccessible to standard, classical technologies.

A collaboration between physicists at the University of Illinois Urbana-Champaign and the University of Chicago has realized a theoretical prediction in which an externally driven quantum system achieves entanglement through dissipation. While the original prediction relies on highly idealized settings, the researchers developed a new technique called synthetic squeezing to realize the phenomenon in a laboratory setting with a pair of superconducting qubits.

Moreover, the generated entanglement is in a steady state, meaning that, in principle, it can be maintained indefinitely over arbitrarily large distances. The researchers believe that this technique holds promise as a more robust and reliable alternative to current methods of entanglement generation.

 “In the past, generating entanglement has meant performing a set of operations of different parts of a system then transporting them away from each other,” said Wolfgang Pfaff, Professor of Physics in the University of Illinois Urbana-Champaign Department of Physics, who led the Illinois part of the collaboration. “As you can imagine, it’s in the transport stage where things go wrong and environmental noise spoils the carefully prepared properties. We’ve shown that it’s possible to bypass the transport stage altogether.”

“This idea has attracted theoretical attention for a long time because it runs counter to our experience with quantum entanglement,” said Aashish Clerk, Professor of Molecular Engineering in the University of Chicago Pritzker School of Molecular Engineering, who led the Chicago part of the collaboration. “Rather than preparing it at one instant and watching it decay, it emerges as the natural point of relaxation in this system. It’s almost like having a ‘refrigerator’ that pumps out external influences to maintain entanglement instead of pumping out heat to maintain coldness.”

This research was recently published in the journal Physical Review X, and it is featured as a viewpoint in Physics magazine.

Quantum entanglement is a phenomenon where different parts of a system display correlations that cannot be explained using non-quantum means. Quantum information technology exploits these correlations to perform tasks that are otherwise impossible or impractical. Most experts believe that building this technology into realms where its true potential can be realized will require combining physically separated units. So, there needs to be a good way for units to share entanglement.

Current methods of distributing entanglement prepare different objects in one location then transmit them to different locations. While in transit, the objects experience external influences and unwanted noise that causes the entanglement to erode, a process called decoherence. Managing these effects has been one of the most significant barriers to realizing quantum technology with practical utility.

“The interesting question is whether we need to have this step of transport that is vulnerable to decoherence,” Pfaff said. “Could we have remote entanglement without having to transport particles in delicate states?”

Theorists have identified a way to achieve remote entanglement between separated objects using a construct known as cascading. A set of quantum objects such as atoms (or superconducting qubits) is made to continuously absorb and emit light. Some of this light is dissipated into the surrounding environment. If external light is introduced in a manner that balances the dissipated light, then a steady state emerges. Carefully engineering this steady state can result in the atoms or qubits displaying entanglement.

“Even though the overall system is in contact with an outside environment and is out of equilibrium, it naturally evolves towards a resting point in which select parts of it are entangled,” Clerk said. “What’s more, the quantum particles never need to move. They just need the ability to communicate.”

While cascaded quantum systems have been achieved, the quality of the entanglement is generally lower than other methods because of noise and hardware imperfections. The researchers introduced synthetic squeezing as a framework that accounts for these “real-world” effects, tuning the system so they do not matter.

“I’ve worked with cascaded quantum systems before, but it was by working with professor Clerk and his research group that we could realize this prediction of high-quality steady-state entanglement,” Pfaff said. “Their theoretical proposal of synthetic squeezing says that the idealized setting of the original model can be replicated by adjusting the right experimental settings. It reduces the problem to fine tuning in the lab.”

Having demonstrated synthetic squeezing on a two-qubit system, the Illinois and Chicago groups are now working to extend the process to multi-qubit systems. The researchers believe that this technique holds promise for networking quantum computers without the need to directly transmit quantum information through noisy, lossy channels.

“The work ahead is going to be figuring out how different protocols can be implemented on this kind of system and determining what, if any, advantage is to be gained by doing so,” Pfaff said.

“One especially exciting route for us is entanglement distillation,” Clerk added. “Right now, the degree of entanglement we can achieve is quite good, but it’s still below the theoretical limit. There are protocols in which a collection of qubits with low entanglement can be combined so a few of them have a very high degree of entanglement. Such a protocol would let us start doing actual quantum computing operations with this system.”

This study’s other contributors are Abdullah Irfan, Kaushik Singirikonda, Michael Mollenhauer and Xio Cao of the University of Illinois Urbana-Champaign; and Mingxing Yao and Andrew Lingenfelter of the University of Chicago.

The article, “Autonomous stabilization of remote entanglement in a cascaded quantum network,” is available online. DOI: 10.1103/z6zz-vw5q

Support was provided by the National Science Foundation, the Air Force Office for Scientific Research, the Army Research Office and IBM.

Wolfgang Pfaff is an Illinois Grainger Engineering assistant professor in the Department of Physics. He is affiliated with the Illinois Quantum Information Science and Technology Center, the Materials Research Laboratory and the Holonyak Micro and Nanotechnology Lab.