MIT scientists tune entanglement structure in qubit array

Entanglement is a form of correlation between quantum objects, such as atomic-scale particles. This unique quantum phenomenon cannot be explained by the laws of classical physics, but it is one of the properties that explains the macroscopic behavior of quantum systems.

Since entanglement is at the heart of how quantum systems work, a better understanding of it could give scientists greater insight into how information is efficiently stored and processed in such systems.

Qubits, or qubits, are the building blocks of quantum computers. However, it is extremely difficult to create specific entangled states in multi-qubit systems, let alone study them. There are also many kinds of entangled states, and distinguishing them can be challenging.

Now, MIT researchers have demonstrated a technique that can effectively create entanglement between a series of superconducting qubits that exhibit a specific type of behavior.

Over the past few years, researchers in the Engineered Quantum Systems (EQuS) group have developed techniques to use microwave technology to precisely control quantum processors composed of superconducting circuits. In addition to these control techniques, the methods introduced in this work enable processors to efficiently generate highly entangled states and transfer these states from one entanglement type to another, including those that are more likely to support quantum acceleration. types and those that do not support quantum acceleration.

Here, we show that we can exploit emerging quantum processors as tools to deepen our understanding of physics. While everything we did in this experiment was at a scale that can still be simulated on a classical computer, we have a good roadmap for extending this technology and method beyond the confines of classical computing, Amir H. Karamlou 18, MEng 18, Ph.D. 23, is the first author of the paper.

The senior author is William D. Oliver, Henry Ellis Warren Professor of Electrical Engineering, Computer Science, and Physics, Director of the Center for Quantum Engineering, and EQuS Group Leader, Deputy Director, Electronics Research Laboratory. Karamlu and Oliver were joined by research scientist Jeff Grover, postdoc Ilan Rosen and MIT Departments of Electrical Engineering and Computer Science and Physics, MIT Lincoln Laboratory, Wellesley College, along with others at the University of Maryland.The research was published today in nature.

Evaluate entanglement

In large quantum systems containing many interconnected qubits, one can think of entanglement as the amount of quantum information shared between a given qubit subsystem and the rest of the larger system.

Entanglement within quantum systems can be classified as area law or volume law, depending on how the shared information scales with the geometry of the subsystem. In volume-law entanglement, the amount of entanglement between a qubit subsystem and the rest of the system grows proportionally to the total size of the subsystem.

Area-law entanglement, on the other hand, depends on how many shared connections there are between the qubit system and the larger system. As a subsystem expands, the amount of entanglement only grows along the boundary between the subsystem and the larger system.

Theoretically, the formation of volume-law entanglement is related to what makes quantum computing so powerful.

Oliver said that while the role of entanglement in quantum algorithms has not yet been fully abstracted, we do know that generating volume law entanglement is a key factor in achieving quantum advantage.

However, volume law entanglement is also more complex than area law entanglement, and is actually difficult to simulate on a large scale using classical computers.

As the complexity of a quantum system increases, it becomes increasingly difficult to simulate it with conventional computers. For example, if I tried to fully track a system with 80 qubits, I would need to store more information than we have stored in all of human history, Karamlu said.

The researchers created a quantum processor and control protocol that allowed them to efficiently generate and detect both types of entanglement.

Their processor contains superconducting circuits for engineering artificial atoms. Artificial atoms are used as qubits, which can be controlled and read out with high precision using microwave signals.

The device used for the experiments contained 16 qubits arranged in a two-dimensional grid. The researchers carefully tuned the processor so that all 16 qubits had the same transition frequency. They then applied additional microwave drive to all qubits simultaneously.

If this microwave driver has the same frequency as the qubit, it will produce quantum states that exhibit volume-law entanglement. However, as the microwave frequency increases or decreases, the qubits exhibit less volume-law entanglement, eventually crossing over to entangled states that increasingly follow area-law scaling.

careful control

Our experiment is a masterpiece of the capabilities of superconducting quantum processors. In one experiment, Rosen said, we operated the processor as an analogue device, allowing us to efficiently prepare states with different entanglement structures and as a digital computing device to measure subsequent entanglement scaling.

To achieve this kind of control, the team has spent years carefully building the infrastructure around the quantum processor.

By demonstrating the entangled crossover from volume laws to area laws, the researchers experimentally confirmed the predictions of theoretical studies. What’s more, the method can be used to determine whether entanglement in a universal quantum processor is area law or volume law.

The MIT experiment highlights the difference between area-law and volume-law entanglement in two-dimensional quantum simulations using superconducting qubits.This perfectly complements our work in a parallel publication on entangled Hamiltonian tomography using trapped ions nature That could happen in 2023, said Peter Zoller, a professor of theoretical physics at the University of Innsbruck who was not involved in the work.

Quantifying entanglement in large quantum systems is a challenging task for classical computers, but this is a good example of where quantum simulations can help, said Google’s Pedram Roushan, who also was not involved in the study. Using two-dimensional arrays of superconducting qubits, Karamlou and colleagues were able to measure the entanglement entropy of various subsystems of various sizes. They measured the contribution of the volume law and the area law to entropy, revealing the crossover behavior as the system’s quantum state energy adjusts. It provides a powerful demonstration of the unique insights that quantum simulators can provide.

In the future, scientists could use this technique to study the thermodynamic behavior of complex quantum systems that are too complex to study using current analytical methods and nearly impossible to simulate even on the world’s most powerful supercomputers.

“The experiments we do in this work can be used to characterize or benchmark larger-scale quantum systems, and we can also learn more about the nature of entanglement in these many-body systems,” Karamlu said.

Other co-authors of the study are Sarah E. Muschinske, Cora N. Barrett, Agustin Di Paolo, Leon Ding, Patrick M. Harrington, Max Hays, Rabindra Das, David K. Kim, Bethany M. Niedzielski, Meghan Schuldt, Kyle Serniak, Mollie E. Schwartz, Jonilyn L. Yoder, Simon Gustafsson, and Yariv Yanai.

This research was funded in part by the U.S. Department of Energy, the U.S. Defense Advanced Research Projects Agency, the U.S. Army Research Office, the National Science Foundation, the STC Center for Integrated Quantum Materials, and the Wellesley College Samuel and Hilda Levitt Fellowship , NASA and the Oak Ridge Institute of Science and Education.