# Nonlinear interferometry with high-gain parametric down-conversion

A nonlinear interferometer is a device where two nonlinear effects can enhance or suppress each other. For example, if high-gain parametric down-conversion occurs in each of the two successive nonlinear crystals, the nonlinear interference affects the mode structure of the output nonclassical light. This has been demonstrated with two spatially separated crystals where generation of bright squeezed vacuum with nearly a single spatial Gaussian mode has been achieved [1]. Similarly, if the two nonlinear crystals are separated by a dispersive material, the frequency mode structure of the output bright squeezed vacuum will be modified and, in particular, single-mode generation can be achieved [2,3]. Under certain conditions, the mode structure at the output of the interferometer can be more interesting. By using spatially-selective or frequency-selective amplification, we are going to produce at the output squeezed vacuum in modes with large optical angular momentum (OAM) or complicated frequency modes. Our first results [4] show that by changing the distance between the crystals or by varying the pump power we can tune the effective number of OAM modes.

Another important property of a nonlinear interferometer formed by two parametric amplifiers is that it provides phase sensitivity overcoming the shot-noise limit and, in principle, reaching the Heisenberg limit. Moreover, the performance of such an interferometer (known in the literature as SU(1,1) interferometer) is robust to detection losses. Especially advantageous is the unbalanced configuration where the parametric gain of the second crystal is higher than the one of the first crystal [5].

## References:

[1] A. M. PĂ©rez, T. Sh. Iskhakov, P. Sharapova, S. Lemieux, O. V. Tikhonova, M. V. Chekhova, and G. Leuchs, Bright squeezed-vacuum source with 1.1 spatial mode. Opt. Lett. 39, 2403 (2014).

[2] T. Sh. Iskhakov, S. Lemieux, A. Perez, R. W. Boyd, G. Leuchs, and M. V. Chekhova, Nonlinear interferometer for tailoring the frequency spectrum of bright squeezed vacuum. Journal of Modern Optics 63, 64 (2015).

[3] S. Lemieux, M. Manceau, P. R. Sharapova, O. V. Tikhonova, R. W. Boyd, G. Leuchs, and M. V. Chekhova, Engineering the Frequency Spectrum of Bright Squeezed Vacuum via Group Velocity Dispersion in an SU(1,1) Interferometer. Phys. Rev. Letters 117, 183601 (2016).

[4] L. Beltran, G. Frascella, A. Perez, R. Fickler, P. Sharapova, M. Manceau, O. Tikhonova, R. Boyd, G. Leuchs, and M. V. Chekhova, Orbital angular momentum modes of high-gain parametric down-conversion. Journal of Optics, accepted (2017).

[5] M. Manceau, F. Khalili and M. Chekhova, Improving the phase super-sensitivity of squeezing-assisted interferometers by squeeze factor unbalancing. New Journal of Physics 19, 013014 (2017).

## Project leaders:

Maria Chekhova (MPL), maria.chekhova@mpl.mpg.de

Robert Boyd (uOttawa), rboyd@uottawa.ca

## Other researchers:

Mathieu Manceau (MPL)

Angela Perez (MPL)

Samuel Lemieux(uOttawa)

Robert Fickler (uOttawa)

Gerd Leuchs (MPL)