Transmission lines and photons

S. Brandt, Hans Dieter Dahmen

Juha Leppäkangas, Göran Johansson, Michael Marthaler +1 more

We investigate electromagnetic radiation emitted by a small voltage-biased Josephson junction connected to a superconducting transmission line. At frequencies below the well-known emission peak at the Josephson frequency (2eV/h), extra radiation is triggered by quantum fluctuations in the transmission line. For weak tunneling couplings and typical Ohmic transmission lines, the corresponding photon-flux spectrum is symmetric around half the Josephson frequency, indicating that the photons are predominately created in pairs. By establishing an input-output formalism for the microwave field in the transmission line, we give further evidence for this nonclassical photon pair production, demonstrating that it violates the classical Cauchy-Schwarz inequality for two-mode flux cross correlations. In connection to recent experiments, we also consider a stepped transmission line, where resonances increase the signal-to-noise ratio.

Alberto Tabarelli de Fatis, Stephanie Matern, Gianluca Rastelli +1 more

We investigate superconducting transmission lines as a novel platform for realizing a quantum fluid of microwave photons in a propagating geometry. We predict that the strong photon-photon interactions provided by the intrinsic nonlinearity of Josephson junctions are sufficient to enter a regime of strongly interacting photons for realistic parameters. A suitable tapering of the transmission line parameters allows for the adiabatic conversion of an incident coherent field into a Tonks-Girardeau gas of fermionized photons close to its ground state. Signatures of the strong correlations are anticipated in the correlation properties of the transmitted light.

S. Brandt, Hans Dieter Dahmen

Деннис Габор

Martijn A. Nagtegaal

I propose an implementation of the quantum private query protocol as described in an article using a photon to encode a question and reflectionor transmission of the photon as answer options. Each question is represented by a photon in a transmission line with both ends returning to the user, and the answer is represented by reflection or transmission of this photon caused by the single photon transistor as described in another article. By solving the quantum Langevin equations for the 32 × 32-dimensional operators describing the single photon transistor the system is analysed. This analysis shows that the user privacy is maintained when the returning transmission lines are under the user's control. The probabilities for reflection and transmission are calculated to verify the behaviour of the answering mechanism. By using pulse trains instead of numbered lines to represent questions, the scalability of the system could be improved.