13 dB squeezed vacuum states at 1550 nm from 12 mW external pump power at 775 nm

Recent advancement in technology has led to the vision of secure fiber-based quantum telecommunication networks operated at the wavelength of 1550nm. However, much is still required in this field to ensure reliable, efficient and cost-effective light sources. One of the major necessities for the efficient operation of those devices is the reduction of the optical pump power at 775nm needed for generating the required quantum states of light at 1550nm. Power reduction leads to smaller and cheaper light sources. However, it is evident that other important factors, such as lowest optical loss, are essential as well.

Professor Roman Schnabel, Ph.D. candidate Axel Schönbeck at the University of Hamburg in Germany in collaboration with Ph.D. candidate Fabian Thies at Leibniz University addressed the emerging challenges by investigating the power requirements for the generation of continuous-wave light at 1550nm with a squeezed quantum uncertainty. In the experiment, they used periodically poled potassium titanyl phosphate (PPKTP) in a doubly-resonant squeezing cavity and observed a variance of the light’s electric field strength 13dB below that of the light’s ground state, i.e. a squeeze factor of 13dB. Such ‘squeezed light’ can be used to make optical communication networks save against eaves dropping. Their research work is currently published in the journal, Optics Letters.

Axel Schönbeck and his colleagues successfully achieved the phase matching in the PPKTP crystal and double resonance of the cavity around it simultaneously, which significantly contributed to the low pump power requirement of only 12mW.

As a significant contribution of their study, the authors observed the highest squeeze factor at a telecommunication wavelength so far and showed how the pump power requirement can be further reduced without sacrificing the squeeze factor. If the reflectivities of the cavity mirrors were optimized with respect to the absorption and scattering in PPKTP, the squeeze factor of 13dB would have been achieved by an external pump power of just 3mW. Other factors were also considered to contribute to a further reduction of the external pump power in the experiment, such as optical loss in the anti-reflection coating of the PPKTP crystal.

The research team is optimistic that this research will see more reduction in the pump power. For further advances, they recommend the use of detector electronics having minimal dark noise. Currently, the detection of the squeezed light requires an optical local oscillator at 1550nm of several milliwatts. Detector electronics with lower dark noise will mitigate this requirement. The proposed technologies should enable efficient, compact and low-cost squeezed light sources in future.