The special-purpose quantum computers called quantum simulators will help scientists in identifying materials with new, useful properties. Thanks to a recent collaboration between Google and scientists at universities in Greece, Singapore and California, this exciting future has just taken a step forward.
Photons in Google's quantum chip were used by the international team to replicate the beautiful and surprising pattern of the 'Hofstadter butterfly', a fractal structure that defines the behavior of electrons in powerful magnetic fields. The results of the study have been reported in 1 December in Science, which demonstrate how quantum simulators are beginning to live up to their promise as robust tools.
"We've always had this idea that we can use photons to simulate and better understand nature. Our collaboration puts this into practice," says Dimitris Angelakis at the Centre for Quantum Technologies, National University of Singapore.
Collaborators at Google and the National University of Singapore, the University of California Santa Barbara in the United States, and Technical University of Crete, Greece performed this breakthrough work on Google's set of nine superconducting quantum bits, or qubits. The experiment showed how all kinds of exotic complex quantum behavior can be reproduced by a quantum simulator. This latest breakthrough will allow researchers to replicate and thus design materials with unusual electronic conduction properties, which would pave the way to a wide range of innovative applications.
"With chips similar to the one used in this experiment, we are interested to study problems at the core of condensed matter, statistical mechanics, and non-equilibrium dynamics," says Google's Pedram Roushan, a quantum electronics engineer.
Hofstadter's butterfly initially appeared in 1976, in computations of electrons in a two-dimensional (2D) material in a powerful magnetic field. The butterfly maps the shifts and splits of the energy levels of the electron with changes in the field strength. This is a quantum simulation, where gates on the qubits offered an analogue of the magnetic field and the photons assumed the role of the electrons. The team's measurements yielded the butterfly pattern.
The experiment depended on the researchers’ invention of a new spectroscopy technique which they named 'hit and listen'. The technique can map the energy levels of microwave photons – light particles stored in the nine qubits.
"Our method is like hitting a bell. The sound it makes is a superposition of all the basic harmonics. By hitting it in different positions a few times and listening to the tune long enough, one can resolve the hidden harmonics. We do the same with the quantum chip, hitting it with photons and then following its evolution in time," explains Angelakis.
The researchers first bombarded the qubits with one photon at a time to observe the butterfly, and then bombarded the qubits with two photons concurrently and made the qubits disorderly by programming some randomness into their properties in order to analyze the complicated phenomenon called 'many-body localization'. This is a quantum phase transition that determines whether materials are insulators or conductors, and is similar to the phase change that occurs when water freezes into ice. By applying their 'hit and listen' approach to different regimes of interaction and disorder, the researchers discovered precursors of many-body localization.
Understanding this kind of phenomenon may give another means to engineer new and useful materials with unusual conduction properties, but on the other hand physicists often find it difficult to replicate such complex scenarios. In the 1950s, it was predicted that material disorder could block the electrons from moving through it. That phenomenon is known as localization. However, if the particles interact with one another, the issue becomes 'many-body' - and more difficult to model.
For merely two photons across nine qubits, the researchers could simulate the type of behavior to expect on traditional computers, finding excellent agreement with their experimental results. However, if a few more qubits are added the issue becomes intractable for conventional machines. This makes the prospect of larger quantum simulators attractive for researchers - they could handle problems that are way beyond the reach of current generation of supercomputers.
"Understanding quantum phases is still one of physics' unsolved mysteries" says Roushan.