Petermann-factor limited sensing near an exceptional point

Kavli Affiliate: Kerry Vahala

| First 5 Authors: Heming Wang, Yu-Hung Lai, Zhiquan Yuan, Myoung-Gyun Suh, Kerry Vahala

| Summary:

Non-Hermitian Hamiltonians describing open systems can feature singularities
called exceptional points (EPs). Resonant frequencies become strongly dependent
on externally applied perturbations near an EP which has given rise to the
concept of EP-enhanced sensing in photonics. However, while increased sensor
responsivity has been demonstrated, it is not known if this class of sensor
results in improved signal-to-noise performance. Here, enhanced responsivity of
a laser gyroscope caused by operation near an EP is shown to be exactly
compensated by increasing sensor noise in the form of linewidth broadening. The
noise, of fundamental origin, increases according to the Petermann factor,
because the mode spectrum loses the oft-assumed property of orthogonality. This
occurs as system eigenvectors coalesce near the EP and a biorthogonal analysis
confirms experimental observations. Besides its importance to the physics of
microcavities and non-Hermitian photonics, this is the first time that
fundamental sensitivity limits have been quantified in an EP sensor.

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