There is a massive international effort going to develop a computer that can use quantum physics to do computations of unparalleled complexity. While there are still major technological barriers to overcome in order to build such a quantum computer, today’s early prototypes are capable of incredible feats.
For instance, consider the formation of a new phase of matter known as a “time crystal.” A time crystal, like a crystal’s structure in space, repeats in time and, more crucially, does so eternally and without any further energy input – similar to a clock that runs forever without batteries. The desire to understand this phase of matter has been a long-standing struggle in theory and experiment, and it has now finally been realised.
A team of scientists from Stanford University, Google Quantum AI, the Max Planck Institute for Physics of Complex Systems, and Oxford University discuss their fabrication of a time crystal using Google’s Sycamore quantum computing hardware in a paper published in the journal Nature on November 30, 2021.
“The big picture is that we are taking the devices that are meant to be the quantum computers of the future and thinking of them as complex quantum systems in their own right,” said Matteo Ippoliti, a postdoctoral scholar at Stanford and co-lead author of the work. “Instead of computation, we’re putting the computer to work as a new experimental platform to realize and detect new phases of matter.”
“Time-crystals are a striking example of a new type of non-equilibrium quantum phase of matter,” said Vedika Khemani, assistant professor of physics at Stanford and a senior author of the paper. “While much of our understanding of condensed matter physics is based on equilibrium systems, these new quantum devices are providing us a fascinating window into new non-equilibrium regimes in many-body physics.”
Time crystal & quantum computer
Thanks to the quantum computer’s exceptional characteristics, the researchers were able to confirm their claim of a true time crystal. Despite the fact that their experiment was limited in size and duration due to the (imperfect) quantum device’s finite size and coherence time – the time crystal oscillations could only be observed for a few hundred cycles rather than indefinitely – the researchers devised various protocols for assessing the stability of their creation. Running the simulation forward and backward in time, as well as scaling its size, were among them.
The presence of indefinite oscillations in all states is an essential feature of an ideal time crystal. The researchers designed a protocol to probe over a million states of their time crystal in just a single run of the machine, using only milliseconds of runtime, to verify this robustness to choice of states. This is similar to looking at a real crystal from various angles to confirm its repeating structure.
“A unique feature of our quantum processor is its ability to create highly complex quantum states,” said Xiao Mi, a researcher at Google and co-lead author of the paper. “These states allow the phase structures of matter to be effectively verified without needing to investigate the entire computational space – an otherwise intractable task.”
Creating a new phase of matter is unquestionably exciting on a fundamental level. In addition, the fact that these researchers were able to do so points to the increasing usefulness of quantum computers for applications other than computing. “I am optimistic that with more and better qubits, our approach can become a main method in studying non-equilibrium dynamics,” said Pedram Roushan, researcher at Google and senior author of the paper.
