Pasi Lähteenmäki

Coherence from vacuum fluctuations

  1. Pasi Lähteenmäki,
  2. G. S. Paraoanu,
  3. Juha Hassel,
  4. and Pertti Hakonen
The existence of vacuum fluctuations is one of the most important predictions of modern quantum field theory. In the vacuum state, fluctuations occurring at different frequencies are
uncorrelated. However, if a parameter in the Lagrangian of the field is modulated by an external pump, vacuum fluctuations stimulate spontaneous downconversion processes, creating squeezing between modes symmetric with respect to half of the frequency of the pump. Here we show that by double parametric pumping of a superconducting microwave cavity in the ground state, it is possible to generate another fundamental type of correlation, namely coherence between photons in separate frequency modes that are not directly connected through a single downconversion process. The coherence is tunable by the phases of the pumps and it is established by a quantum fluctuation that takes simultaneously part in creation of two photon pairs. Our analysis indicates that the origin of this vacuum-induced coherence is the absence of „which-way“ information in the frequency space.
arxiv pdf

Advanced Concepts in Josephson Junction Reflection Amplifiers

  1. Pasi Lähteenmäki,
  2. Visa Vesterinen,
  3. Juha Hassel,
  4. G. S. Paraoanu,
  5. Heikki Seppä,
  6. and Pertti Hakonen
Low-noise amplification atmicrowave frequencies has become increasingly important for the research related to superconducting qubits and nanoelectromechanical systems. The fundamental
limit of added noise by a phase-preserving amplifier is the standard quantum limit, often expressed as noise temperature Tq=ℏω/2kB. Towards the goal of the quantum limit, we have developed an amplifier based on intrinsic negative resistance of a selectively damped Josephson junction. Here we present measurement results on previously proposed wide-band microwave amplification and discuss the challenges for improvements on the existing designs. We have also studied flux-pumped metamaterial-based parametric amplifiers, whose operating frequency can be widely tuned by external DC-flux, and demonstrate operation at 2ω pumping, in contrast to the typical metamaterial amplifiers pumped via signal lines at ω.
arxiv pdf