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John O | February 2017

Scientists observe quantum phase transitions for the first time


A team of scientists at the Institute of Science and Technology Austria (IST Austria) have revealed the first experimental observation of first-order phase transition in a dissipative quantum system, according to a report on the institute’s website.

 

ist_austria_600

Probability distribution showing the equal likelihood for the cavity being transparent and opaque at the critical point.
(IST Austria)

 

As the article noted, phase transitions are visible every day, for example when water freezes or ice melts, but these transitions at the quantum level have been “relatively unexplored.”

 

The scientists were able to experimentally observe a photon-blockade breakdown, which had only been discovered two years ago. A photon blockade occurs when a photon fills a cavity in an optical system and prevents other photons from entering or leaving, but it was predicted that when the photon flux reached a certain level that a phase change would occur and the system would go from opaque to transparent.

 

The IST Austrian scientists were able to meet the necessary requirements for observing this particular phase change.

 

According to the article, “Their setup consisted of a microchip with a superconducting microwave resonator acting as the cavity and a few superconducting qubits acting as the atoms. The chip was cooled to a temperature astoundingly close to absolute zero – 0.01 Kelvin – so that thermal fluctuations did not play a role.”

 

The researchers sent a continuous microwave tone to the input of the chip’s resonator and amplified and measured the transmitted microwave flux through the output. At certain input powers, the scientists recorded a signal switching randomly between zero transmission and full transmission, which demonstrated the phase change in process.

 

“Our experiment took exactly 1.6 milliseconds to complete for any given input power,” said lead scientist Johannes Fink. “The corresponding numerical simulation took a couple of days on a national supercomputer cluster. This gives an idea why these systems could be useful for quantum simulations.”

 

The work was recently published in Physical Review X. The abstract of the report stated:

 

“Nonequilibrium phase transitions exist in damped-driven open quantum systems when the continuous tuning of an external parameter leads to a transition between two robust steady states. In second-order transitions this change is abrupt at a critical point, whereas in first-order transitions the two phases can coexist in a critical hysteresis domain.

 

“Here, we report the observation of a first-order dissipative quantum phase transition in a driven circuit quantum electrodynamics system. It takes place when the photon blockade of the driven cavity-atom system is broken by increasing the drive power. The observed experimental signature is a bimodal phase space distribution with varying weights controlled by the drive strength.

 

“Our measurements show an improved stabilization of the classical attractors up to the millisecond range when the size of the quantum system is increased from one to three artificial atoms. The formation of such robust pointer states could be used for new quantum measurement schemes or to investigate multiphoton phases of finite-size, nonlinear, open quantum systems.”

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