Solving Previously Unsolvable Problems: New Type of Analog Quantum Computer

A groundbreaking study published in Nature Physics by a team of scientists from Stanford University in the United States and University College Dublin (UCD) in Ireland has revealed that a new type of highly specialized analog computer, equipped with quantum components in its circuits, can solve complex problems in quantum physics that were previously beyond reach. If these devices can be scaled up, they have the potential to provide insights into some of the most significant unresolved issues in physics.

For instance, scientists and engineers have been seeking a deeper comprehension of superconductivity for a long time. Currently, superconducting materials, like those utilized in MRI machines, high-speed trains, and energy-efficient long-distance power networks, only function at extremely low temperatures, hindering their broader applications. The ultimate goal of materials science is to discover materials that exhibit superconductivity at room temperature, which would revolutionize their use in a host of technologies.

Andrew Mitchell is the Director of the UCD Centre for Quantum Engineering, Science, and Technology (C-QuEST), a theoretical physicist at the UCD School of Physics, and a co-author of the paper. He said: “Certain problems are simply too complex for even the fastest digital classical computers to solve. The accurate simulation of complex quantum materials such as the high-temperature superconductors is a really important example – that kind of computation is far beyond current capabilities because of the exponential computing time and memory requirements needed to simulate the properties of realistic models.

“However, the technological and engineering advances driving the digital revolution have brought with them the unprecedented ability to control matter at the nanoscale. This has enabled us to design specialized analog computers, called ‘Quantum Simulators,’ that solve specific models in quantum physics by leveraging the inherent quantum mechanical properties of its nanoscale components. While we have not yet been able to build an all-purpose programmable quantum computer with sufficient power to solve all of the open problems in physics, what we can now do is build bespoke analog devices with quantum components that can solve specific quantum physics problems.”

The architecture for these new quantum devices involves hybrid metal-semiconductor components incorporated into a nanoelectronic circuit, devised by researchers at Stanford, UCD, and the Department of Energy’s SLAC National Accelerator Laboratory (located at Stanford). Stanford’s Experimental Nanoscience Group, led by Professor David Goldhaber-Gordon, built and operated the device, while the theory and modeling were done by Mitchell at UCD.

Prof Goldhaber-Gordon, who is a researcher with the Stanford Institute for Materials and Energy Sciences, said, “We’re always making mathematical models that we hope will capture the essence of phenomena we’re interested in, but even if we believe they’re correct, they’re often not solvable in a reasonable amount of time.”

With a Quantum Simulator, “we have these knobs to turn that no one’s ever had before,” Prof Goldhaber-Gordon said.

The essential idea of these analog devices, Goldhaber-Gordon said, is to build a kind of hardware analogy to the problem you want to solve, rather than writing some computer code for a programmable digital computer. For example, say that you wanted to predict the motions of the planets in the night sky and the timing of eclipses. You could do that by constructing a mechanical model of the solar system, where someone turns a crank, and rotating interlocking gears represent the motion of the moon and planets. In fact, such a mechanism was discovered in an ancient shipwreck off the coast of a Greek island dating back more than 2000 years. This device can be seen as a very early analog computer.

Not to be sniffed at, analogous machines were used even into the late 20th century for mathematical calculations that were too hard for the most advanced digital computers at the time.

But to solve quantum physics problems, the devices need to involve quantum components. The new Quantum Simulator architecture involves electronic circuits with nanoscale components whose properties are governed by the laws of quantum mechanics. Importantly, many such components can be fabricated, each one behaving essentially identically to the others. This is crucial for analog simulation of quantum materials, where each of the electronic components in the circuit is a proxy for an atom being simulated and behaves like an ‘artificial atom’. Just as different atoms of the same type in a material behave identically, so too must the different electronic components of the analog computer.

The new design, therefore, offers a unique pathway for scaling up the technology from individual units to large networks capable of simulating bulk quantum matter. Furthermore, the researchers showed that new microscopic quantum interactions can be engineered in such devices. The work is a step towards developing a new generation of scalable solid-state analog quantum computers.

To demonstrate the power of analog quantum computation using their new Quantum Simulator platform, the researchers first studied a simple circuit comprising two quantum components coupled together.

The device simulates a model of two atoms coupled together by a peculiar quantum interaction. By tuning electrical voltages, the researchers were able to produce a new state of matter in which electrons appear to have only a 1/3 fraction of their usual electrical charge – so-called ‘Z3 parafermions’. These elusive states have been proposed as a basis for future topological quantum computation, but have never before been created in the lab in an electronic device.

“By scaling up the Quantum Simulator from two to many nano-sized components, we hope that we can model much more complicated systems that current computers cannot deal with,” Dr. Mitchell said. “This could be the first step in finally unraveling some of the most puzzling mysteries of our quantum universe.”

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