The first experimental exploration of quantum phase transitions

Chinese and Australians scientists have published research experimentally exploring how quantum matter changes when it makes a ‘quantum phase transition’. Their research has been published in the journal Nature Communications.

Study co-author Professor Jason Twamley, from Macquarie University, explained that quantum phase transitions are similar to classical phase transitions of matter changing from one type of state to another, such as when ice turns to liquid when heated.

“Researchers have been intrigued for decades by the quantum counterpart - or a quantum phase transition - where the quantum state of matter abruptly changes as you slowly change a quantity,” Professor Twamley said. “A typical case is where the symmetry of the quantum state changes abruptly, for example, when it changes from a configuration with four-fold symmetry to one with six-fold symmetry.”

The researchers stated that quantum phase transitions play “an important role in many-body systems and have been a research focus in conventional condensed-matter physics over the past few decades”. But exploring quantum phase transitions experimentally was, until recently, considered impossible as most theories predict that one requires very large light-matter coupling strengths - so high that experiments had been unable to reach them.

The researchers circumvented this stumbling block by continually driving the system with microwave radiation. They used a superconducting quantum chip containing four quantum bits (qubits), coupled to an integrated microwave superconducting cavity - essentially an electrical quantum superconducting circuit.

By continually driving the electrical circuit and carefully measuring the quantum state of the qubits, the researchers were able to observe the abrupt change from a ‘normal phase’ to a ‘superradiant phase’ of the quantum chip as they swept through the quantum phase transition. The study authors explained that they monitored the four-qubit scaled moments “for a signature of a structural change of the system’s eigenstates”.

“It is suspected that quantum phase transitions … play a crucial role in various materials whose properties we wish to understand - for example, high-temperature superconductors - so we can engineer them again to suit technology,” said co-author Professor Mang Feng from the Chinese Academy of Sciences. “This experiment is a first step towards a deeper understanding of the curious features of the quantum phase transition.”

Source