Physicists revealed spontaneous T-symmetry breaking and exceptional points in cavity QED

September 13, 2018, Science China Press
Schematic illustration of the system and the mechanism of T-symmetry breaking. Credit: ©Science China Press

Spontaneous symmetry breaking (SSB) is a physics phenomenon in which a symmetric system produces symmetry-violating states. Recently, extensive study shows that the parity-time symmetry breaking in open systems leads to exceptional points, promising for novel applications leasers and sensing.

In this work, the researchers theoretically demonstrated spontaneous time-reversal symmetry (T-symmetry) breaking in a system. The system is composed of an ensemble of 2-level atoms inside a cavity. The atoms are kept near their highest excited states and act like an oscillator with a negative mass. The researchers utilize the dipole interaction between the atoms and the cavity mode to induce the T-symmetry breaking and to obtain exceptional points (EPs).

"The dipole interaction provides a linear coupling between the collective motion of the atoms and the cavity mode," said Yu-Kun Lu, who is an undergraduate at Peking University. "For small coupling strength, the system undergoes harmonic oscillation, which is invariant under time-reversal operation. When the reaches a threshold, the system becomes unstable against the pair-production (annihilation) process, and the excitation number of the cavity mode and the will increase (decrease) with time, thus leading to the spontaneous T-symmetry breaking." The critical point between the T-symmetric and T-symmetry broken phase is proved to be an EP.

Demonstration of T-symmetry breaking in the eigenmode dynamics. Credit: ©Science China Press

"To demonstrate the existence of EP, we showed the dependence of the eigenfrequencies as well as the eigenmode on the cavity-atom detuning, and we found they coalesce at the critical point, and thus proved it to be an EP," said Pai Peng, a former undergraduate in Prof. Xiao's group and now a Ph.D. at Massachusetts Institute of Technology. Moreover, due to the singular topology of the EP, the dynamics in the vicinity of the EP is robust.

"After encircling a whole loop around EP, the final state only depends on the direction of the loop but not its shape, and thus the result is topological protected," said Qi-Tao Cao, a Ph.D. at Peking University.

"EPs used to be studied exclusively in open systems. The demonstration of EP in the present system broadens the understanding of SSB and singularities in physics," said Prof. Xiao. "Apart from its fundamental interest, spontaneous T-symmetry breaking without gain or loss also provides a new platform for various applications, such as sensing and quantum information processing."

Explore further: Physicists observe spontaneous symmetry breaking in an optical microcavity

More information: Yu-Kun Lu et al, Spontaneous T-symmetry breaking and exceptional points in cavity quantum electrodynamics systems, Science Bulletin (2018). DOI: 10.1016/j.scib.2018.07.020

Related Stories

Real-time observation of collective quantum modes

December 15, 2017

A cylindrical rod is rotationally symmetric - after any arbitrary rotation around its axis it always looks the same. If an increasingly large force is applied to it in the longitudinal direction, however, it will eventually ...

Recommended for you

Fiber optic sensor measures tiny magnetic fields

September 19, 2018

Researchers have developed a light-based technique for measuring very weak magnetic fields, such as those produced when neurons fire in the brain. The inexpensive and compact sensors could offer an alternative to the magnetic ...

The hunt for leptoquarks is on

September 19, 2018

Matter is made of elementary particles, and the Standard Model of particle physics states that these particles occur in two families: leptons (such as electrons and neutrinos) and quarks (which make up protons and neutrons). ...

Researchers push the boundaries of optical microscopy

September 19, 2018

The field of optical microscopy research has developed rapidly in recent years. Thanks to the invention of a technique called super-resolution fluorescence microscopy, it has recently become possible to view even the smaller ...

Searching for errors in the quantum world

September 19, 2018

The theory of quantum mechanics is well supported by experiments. Now, however, a thought experiment by ETH physicists yields unexpected contradictions. These findings raise some fundamental questions—and they're polarising ...

Extremely small and fast: Laser ignites hot plasma

September 19, 2018

When light pulses from an extremely powerful laser system are fired onto material samples, the electric field of the light rips the electrons off the atomic nuclei. For fractions of a second, a plasma is created. The electrons ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.