A newly developed quantum simulator merges digital and analog techniques, vastly improving the ability to model complex systems. With applications ranging from material science to black hole research, this advance opens new doors in computational physics. (Artist’s concept.) Credit: SciTechDaily.com
By blending digital control with analog simulations, scientists have created a powerful new quantum simulator that pushes beyond traditional limitations.
This hybrid system allows precise manipulation of quantum states while naturally modeling real-world physics, enabling breakthroughs in fields like magnetism, superconductors, and even astrophysics.
Breakthrough in Quantum Simulation
Physicists working in Google’s laboratory have developed a new type of digital-analog quantum simulator, capable of studying complex physical processes with unprecedented precision and adaptability. Two researchers from PSI’s Center for Scientific Computing, Theory, and Data played a crucial role in this breakthrough.
Consider the simple act of pouring cold milk into hot coffee — how does it spread and mix? Even the most advanced supercomputers struggle to model this process with high accuracy because the underlying quantum mechanics are incredibly complex. In 1982, Nobel Prize-winning physicist Richard Feynman proposed an alternative: instead of using classical computers, why not build quantum computers that can directly simulate quantum physical processes? Now, with rapid advancements in quantum computing, Feynman’s vision is closer than ever to becoming reality.
A Milestone in Quantum Computing
Together with researchers from Google and universities in five countries, Andreas Läuchli and Andreas Elben, two theoretical physicists at PSI, have built and successfully tested a new type of digital-analog quantum simulator. This represents a milestone because their simulator calculates physical processes not only with unprecedented precision; their concept is also particularly flexible, meaning that it can be applied to many different problems – from solid-state physics to astrophysics. Their findings were published today in the renowned scientific journal Nature.
The two PSI physicists Andreas Läuchli (left) and Andreas Elben were involved in developing a new type of digital-analogue quantum simulator. Credit: © Paul Scherrer Institute PSI/Mahir Dzambegovic
Combining Analog and Digital
A key aspect of the new quantum processor is that the 69 superconducting quantum bits (qubits) on the quantum chip developed by Google permit both digital and analog operating modes. Digital quantum computers perform their operations using universal quantum gates, similar to the logic gates in classical computers. The difference is that, thanks to quantum mechanical superposition, qubits can not only assume the states 0 and 1 but also a multitude of intermediate states.
Although such purely digital quantum computers are already very powerful, their potential as quantum simulators is still limited. Analog quantum simulators, on the other hand, rely on the direct simulation of physical processes, realistically modeling the interactions between the different particles, for example to study magnetic properties in solids. These two approaches – digital and analog – have now been successfully combined for the first time in an experiment that brings together the strengths of both worlds.
Simulating Complex Physical Processes
To do this, the physicists define discrete initial conditions, such as introducing heat into a solid – this is the digital mode. This allows the starting conditions to be defined precisely and flexibly. In the coffee-cup analogy, for example, this would be a milk jug pouring drops of milk in a specified and controlled manner in a hundred different places, all at the same time. The subsequent process by which the milk spreads out in the coffee corresponds to the analog mode. The interaction between the qubits simulates the physical dynamics, such as heat propagation or the formation of magnetic domains, as they occur in real solids.
“We can watch the quantum simulator as it reaches thermal equilibrium – or in the coffee analogy: the milk is distributed in the coffee and the temperature is equalized in the process,” says Andreas Elben, a tenure-track scientist at PSI. “Our research demonstrates that it is possible to create superconducting analog-digital quantum processors on a chip and that these are suitable as quantum simulators,” Andreas Läuchli points out.
Heading Towards a Universal Quantum Simulator
However, thermalization – the process of reaching thermal equilibrium – is just one of many exciting questions that can be answered using the new quantum simulator. The concept demonstrated here paves the way for a universal quantum simulator and is to be used in a wide range of different areas of physics. It extends beyond the capabilities of existing analog quantum simulators, each of which is only suitable for a specific physical problem.
One topic that can be studied in this way is magnetism, Läuchli’s speciality. The qubits in Google’s quantum chip are arranged in the shape of a rectangle, and in the initial state the directions of their magnetic fields alternate strictly. But what happens if the chip is triangular? This could disrupt the tidy arrangement because the qubits are unable to adjust their magnetic orientation in the regular pattern they naturally adopt. This phenomenon is known as frustrated magnetism and is of interest, for example, in connection with computer chips that switch and store bits based not on the charge of the electrons but on their magnetic spins. This leads to a much higher memory density and a higher computational speed.
Expanding Applications: From Superconductors to Black Holes
Further applications are opening up in the development of new materials, such as high-temperature superconductors, and even medicines that can be used more precisely and cause fewer side effects. Quantum simulators are even in demand in astrophysics. One example is the so-called information paradox, which states that no information may be lost in quantum physics. However, astrophysicists believe that black holes do in fact destroy information about their formation – new types of quantum simulators might clarify the situation.
The Future of Quantum Simulators
“Our quantum simulator opens the door to new research,” promises Andreas Läuchli. Although the project with Google has come to an end, many other physical questions await him and his team at PSI. At the Quantum Computing Hub of ETHZ and PSI and beyond, quantum computers and quantum simulators are being developed on various technological platforms, including trapped ions, superconducting qubits, and Rydberg atoms. These systems will soon make it possible to study exciting questions posed by quantum physics at PSI.
Andreas Läuchli: “We also serve as a source of ideas for new experiments at PSI’s large research facilities. And we offer our support to researchers carrying out their experiments at the facilities when it comes to interpreting surprising results. And in the future, we will increasingly be using quantum simulators for this purpose.”
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DOI: 10.1038/s41586-024-08460-3
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