Speaker: Prof. Per Delsing 
               Department of Microtechnology and Nanoscience, Chalmers University of Technology,  Gothenburg, Sweden 

Place: Physics Building 620 

Date and Time: 10am on Feb 4 

Title:  Generating unusual microwave states using superconducting circuits  

Abstract: 
We will discuss two different experiments. In the first experiment, we observe period-tripling subharmonic  
oscillations, in a superconducting coplanar waveguide resonator operated in the quantum regime, kBT≪ℏω.  
The resonator is terminated by a tunable inductance that provides a Kerr-type nonlinearity. We detect the  
output field quadratures at frequencies near the fundamental mode, ω/2π∼5GHz, when the resonator is  
driven by a current at 3ω with an amplitude exceeding an instability threshold. The output radiation is red  
detuned from the fundamental mode. We observe three stable radiative states with equal amplitudes and  
phase-shifted by 120°. Our experimental results are in quantitative agreement with a model for the driven  
dynamics of two coupled modes. 
In the second experiment we use a broadband, semi-infinite transmission line terminated by a  
superconducting quantum interference device (SQUID) to generate entangled pairs of microwave photons. A  
weak pump signal modulates the SQUID inductance, resulting in a single time-varying boundary condition.  
Through the Dynamical Casimir effect entangled photons are generated. We detect both quadratures of the  
microwave radiation emitted at two different frequencies separated by 0.7 GHz. We show that the radiation  
is entangled. 

1. Period-tripling subharmonic oscillations in a driven superconducting resonator 
Ida-Maria Svensson, et al., Phys. Rev. B 96, 174503 (2017) 
2. Observation of broadband entanglement in microwave radiation from a single time-varying boundary  
condition 
B.H. Schneider, et al. arXiv:1802.05529 

Host: Prof. IoChun Hoi