Analogue quantum simulator could solve physics mysteries

Physicists from Stanford University in the USA and University College Dublin in Ireland have developed an analogue quantum computer that is able to solve complex physics problems. Research published in Nature Physics has shown that a novel type of highly specialised analogue computer, whose circuits feature quantum components, can solve quantum physics problems that were previously beyond reach. When scaled up, such devices may be able to shed light on some important unsolved problems in physics.

For example, scientists and engineers have long wanted to gain a better understanding of superconductivity, because existing superconducting materials — such as those used in MRI machines, high-speed trains and long-distance energy-efficient power networks — currently operate only at low temperatures, limiting their wider use. Researchers are also looking to find materials that are superconducting at room temperature, which could be used in a host of technologies.

Dr Andrew Mitchell, Director of the UCD Centre for Quantum Engineering, Science and Technology (C-QuEST) and co-author of the paper, said certain problems are too complex for even the fastest digital classical computers to solve. According to Mitchell, the accurate simulation of complex quantum materials such as high-temperature superconductors is 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 specialised analogue 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 analogue devices with quantum components that can solve specific quantum physics problems,” Mitchell said.

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 modelling was done by Mitchell at UCD. Goldhaber-Gordon, a researcher with the Stanford Institute for Materials and Energy Sciences, said the researchers are always making mathematical models that aim to capture the essence of phenomena they’re interested in. “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,” Goldhaber-Gordon said.

The essential idea of these analogue devices is to build a kind of hardware analogy to the problem that needs to be solved, rather than writing some computer code for a programmable digital computer. For example, a mechanical model of the solar system could be constructed in order to predict the motions of the planets in the night sky and the timing of eclipses — it could be operated by turning a crank, with rotating interlocking gears representing the motion of the moon and planets. In fact, such a mechanism was reportedly discovered in an ancient shipwreck off the coast of a Greek island dating back more than 2000 years. This device can be seen as an early analogue computer. Analogous machines were used into the late 20th century for mathematical calculations that were too hard for the digital computers at the time.

To solve quantum physics problems, the devices need to involve quantum computers. 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 almost identically to the others — this is necessary for analogue 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 analogue computer.

The new design offers a unique pathway for scaling up the technology from individual units to large networks capable of simulating bulk quantum matter. Furthermore, the researchers shows 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 analogue quantum computers.

Image caption: Micrograph image of the new Quantum Simulator, which features two coupled nano-sized metal-semiconductor components embedded in an electronic circuit. Image credit: University College Dublin.