Experimental concept. AExample system diagram X Four qubits within a 16-qubit lattice. This subsystem has a volume of 4 (maroon sites) and an area of 8 (orange line). Second, two-dimensional HCBH lattice for superconducting quantum circuit simulation. Each site can be occupied by at most one particle. Cvitality Second Spectra of the HCBH lattice simulated by our device, shown in a rotating frame that resonates with the lattice positions.The energy spectrum is divided into different sectors defined by the total number of particles n. dScaling of entanglement entropy S and subsystem volume V For the eigenstate at the center of the energy spectrum (orange line, corresponding to the energy eigenstate highlighted by the orange ellipse in the figure C) and eigenstates at the edges of the energy spectrum (cyan lines, corresponding to energy eigenstates highlighted by the cyan ellipse in the figure C). eChanges in entanglement behavior, quantified through geometric entropy ratio sV/sAfor those with n=8. FSchematic diagram of a flip-chip sample consisting of 16 superconducting qubits. G,Hoptical image of the qubit layer (G) and intermediary layer (H) is illustrated using qubits and different signal lines in false colors. Scale bar, 1 mm. Credit: nature (2024). DOI: 10.1038/s41586-024-07325-z
Entanglement is a form of correlation between quantum objects, such as atomic-scale particles. The laws of classical physics cannot explain this unique quantum phenomenon, 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 in systems that are more likely to support quantum acceleration. types and those that do not support quantum acceleration.
“Here, we are demonstrating that we can use 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 to extend this technology and methods beyond the scope of classical computing,” said Amir H. Karamlou ’18, MEng ’18, Ph.D. ’23, lead author of the paper.
The study appears 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.
“While we have not yet completely abstracted away the role that entanglement plays 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 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 try to fully track a system with 80 qubits, I need to store more information than we have stored for the entire 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 this experiment 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, we operated the processor as an analog analog device, allowing us to efficiently prepare states with different entanglement structures and operate them as a digital computing device. operation, needed to measure the subsequent entanglement scale,” Rosen said.
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 on entangled Hamiltonian tomography with trapped ions , this work was published in a parallel publication nature 2023,” said 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 this is a great example of how quantum simulations can help,” said Google’s Pedram Roushan, who also was not involved in the work. Research.
“Using two-dimensional arrays of superconducting qubits, Karamlou and colleagues were able to measure the entanglement entropy of various subsystems of different sizes. They measured the contribution of the volume law and the area law to the entropy, revealing the crossover in the energy adjustment of the system’s quantum state. The behavior is 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 did 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.
More information:
Amir Karamlou, detecting entanglement in the two-dimensional hard-core BoseHubbard lattice, nature (2024). DOI: 10.1038/s41586-024-07325-z. www.nature.com/articles/s41586-024-07325-z
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