Scientists tweak the entangled structure in an array of qubits

Experimental concept. a,Schematic diagram of the example subsystem X for four qubits in a 16-qubit lattice. The volume of this subsystem is 4 (maroon sites) and 8 regions (orange line). b, Two-dimensional HCBH lattice simulated by superconducting quantum circuits, each site can be occupied by up to one particle. c, the energy E spectrum of the HCBH lattice simulated by our device, shown in a rotating frame resonating with the lattice site. The energy spectrum is divided into different sectors defined by the total number of particles n. d, scaling of the entangled entropy S with the volume V of the subsystem for the eigenstates located at the center of the energy spectrum (orange line, corresponding to the energy eigenstates highlighted by the orange ellipse in c) and the eigenstates located at the edge of the energy spectrum (blue-green line, corresponding to the energy eigenstates highlighted by the blue-green ellipse in c). e, the variation of the entanglement behavior, by the geometric entropy ratio sV/s a, for the state of n = 8. f, Schematic diagram of a flip chip sample consisting of 16 superconducting qubits. Optical images of g, h, qubit layer (g) and interposer layer (h) are represented by false-colored qubits and different signal lines. Scale bar, 1 mm. Image Credit: Nature (2024). DOI： 10.1038/s41586-024-07325-z

Entanglement is a form of association between quantum objects, such as particles on the atomic scale. The laws of classical physics cannot explain this unique quantum phenomenon, but it is one of the properties that explain the macroscopic behavior of quantum systems.

Because entanglement is central to how quantum systems work, a better understanding of it can give scientists a deeper understanding of how information is effectively stored and processed in such systems.

Qubits or qubits are the building blocks of a quantum computer. However, it is extremely difficult to make specific entangled states in multi-qubit systems, let alone study them. There are also various entangled states, and it can be challenging to distinguish between them.

Now, researchers at the Massachusetts Institute of Technology have demonstrated a technique that can efficiently create entanglement between a series of superconducting qubits that exhibit specific types of behavior.

Over the past few years, researchers in the Engineered Quantum Systems (EQuS) group have developed techniques that 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 type of entanglement to another, including between those that are more likely to support quantum acceleration and those that do not.

"Here, we are demonstrating that we can leverage emerging quantum processors as a tool to further our understanding of physics. While everything we did in this experiment was done on a scale that can still be simulated on classical computers, we have a good roadmap to extend this technique and approach beyond classical computing," says Amir H. Karamlou '18, MEng '18, Ph.D. '23, lead author of the paper.

The study was published in the journal Nature.

Assess entanglement

In a large quantum system consisting of many interconnected qubits, entanglement can be thought of as the amount of quantum information shared between a given qubit subsystem and the rest of the larger system.

Entanglement within a quantum system can be categorized as either the area law or the volume law, depending on how this shared information is proportional to the geometry of the subsystem. In volume-law entanglement, the amount of entanglement between the qubit subsystem and the rest of the system grows in proportion to the total size of the subsystem.

Area-law entanglement, on the other hand, depends on how much shared connectivity exists between the qubit subsystem and the larger system. As the subsystem expands, the amount of entanglement will only grow along the boundary between the subsystem and the larger system.

Theoretically, the formation of the entangled law of volume is related to the reason why quantum computing is so powerful.

"While we haven't fully abstracted the role of entanglement in quantum algorithms, we do know that generating volume-law entanglement is a key factor in achieving quantum advantage," Oliver said.

However, volume-law entanglement is also more complex than area-law entanglement, and the scale of simulation using classical computers is almost prohibitive.

"As the complexity of a quantum system increases, it becomes increasingly difficult to simulate it with a conventional computer. For example, if I'm trying to fully track a system with 80 qubits, then I need to store more information than we've stored in the entire history of mankind," Karamlou said.

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

Their processors include superconducting circuits for designing artificial atoms. Artificial atoms are used as qubits that can be controlled and read with high accuracy using microwave signals.

The equipment used for this experiment contains 16 qubits, which are arranged in a two-dimensional grid. The researchers carefully tuned the processor so that all 16 qubits had the same conversion frequency. They then applied an additional microwave driver to all the qubits at the same time.

If this microwave driver is at the same frequency as the qubits, it will produce quantum states that exhibit the entangled laws of volume. However, as the microwave frequency increases or decreases, the qubits exhibit less volumetric law entanglement, eventually crossing over the entangled states, which increasingly follow area-law scaling.

Careful control

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

To achieve this control, the team has put in years of work to carefully build the infrastructure around quantum processors.

By proving the intersection from the law of volume to the law of area entanglement, the researchers experimentally confirmed the predictions of the theoretical study. More importantly, this method can be used to determine whether entanglement in general-purpose quantum processors is an area 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 nicely complements our work on entangled Hamiltomy scans of trapped ions in a parallel publication published in Nature in 2023," says Peter Zoller, 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 quantum simulations are a good example of how they can help," said Google's Pedram Roushan, who was also not involved in the study.

Using a two-dimensional array of superconducting qubits, Karamlou and colleagues were able to measure the entangled entropy of various subsystems of various sizes. They measured the contribution of the law of volume and the law of area to entropy, revealing the crossover behavior of the system's quantum state energy when it is adjusted. It is a powerful demonstration of the unique insights that quantum simulators can provide.

In the future, scientists can use this technique to study the thermodynamic behavior of complex quantum systems, which are too complex to be studied using current analytical methods and cannot be simulated 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," Karamlou said.

更多信息：Amir Karamlou，《探测二维硬核玻色-哈伯德晶格中的纠缠》，《自然》（2024）。 DOI： 10.1038/s41586-024-07325-z.www.nature.com/articles/s41586-024-07325-z

Journal Information: Nature