A nonlinear interferometer is a device where two nonlinear effects can enhance or suppress each other. For example, if parametric down-conversion occurs in each of the two successive nonlinear crystals, the nonlinear interference affects the mode structure of the output light. Moreover, the output light is very sensitive to the phase introduced between the crystals. This allows one to perform phase measurements with the accuracy exceeding classical limits.
Bright squeezed vacuum is a quantum state of light generated through high-gain parametric down-conversion. Although it contains macroscopic numbers of photons, it manifests pronounced nonclassical properties. In particular, it has strong quadrature squeezing which, however, is difficult to observe: because of the complicated multimode structure, anti-squeezed modes mask the effect. In our experiments we study the quadrature squeezing of a single mode of bright squeezed vacuum. To access this single mode, we shape the local oscillator both in space and in time. Observation of high quadrature squeezing in different modes will enable the use of bright squeezed vacuum in quantum information protocols.
Kirill Spasibko, Denis Kopylov (with Moscow State University and Palacký University Olomouc)
One of the important features of bright squeezed vacuum is extremely enhanced intensity fluctuations. This makes it very efficient for all multiphoton effects, including harmonic generation. We are using it as a pump in the generation of optical harmonics and supercontinuum. We see that it is by orders of magnitude more efficient than coherent light; moreover, the output radiation has even stronger intensity fluctuations and manifests so-called extreme events. In particular, nonlinear effects can be observed on the surfaces of various materials and can involve plasmonic effects.
Andrea Cavanna, Cameron Okoth (with Division Russell)
We aim at observing a new quantum effect, direct decay of a high-frequency photon into photon triplets. This effect can take place in any material with cubic nonlinearity but we think that the best choice is photonic-crystal fibres. In particular, we are going to use gas-filled hollow-core fibres and the all-solid dual-bandgap fibres we developed recently.
Cameron Okoth, Andrea Cavanna, Tomas Santiago
We aim at the observation of a new nonlinear optical effect: direct decay of a photon in three, the inverse to the third harmonic generation. Besides being fundamentally interesting, it promises the obtaining of novel quantum-optics states and the observation of negative Wigner-function (quasiprobability) distribution. The most natural first step is to realize the `seeded' version of this effect in crystals with cubic nonlinearity.
Gaetano Frascella, Sascha Agne
High-gain parametric down-conversion produces bright squeezed vacuum in a large number of spatial modes. Most of these modes carry orbital angular momentum (OAM), an optical phase twist. Moreover, due to the photon-number correlations, each mode with a certain OAM value contains exactly the same photon number as the mode with the opposite OAM value, and these photon numbers are huge, up to hundreds billions. This poses an interesting problem: to filter out these OAM modes, populated with equal photon numbers, and use them for the interaction with matter or for high-precision measurements.
Paula Cutipa, Sascha Agne
Radiation of parametric down-conversion manifests an interesting type of coherence, with the spatial coherence coupled to the temporal one. In particular, under certain conditions the coherence function will have a funny `O'-shape in space and time. We are going to test it in a complex Young-Michelson experiment.
In crystals with strongly aperiodic, rather than periodic, poling, one can generate very broadband pulsed squeezed light. The bandwidth can be so large that the pulses will be `single-cycle', with the pulse duration comparable with the period. This type of light will have ultrafast photon-number correlations, suitable for the sensitive probing of various effects on femtosecond time scale.
Luo Qi (with Kastler-Brossel Laboratory and Palacký University Olomouc)
Colloidal CdS/CdSe nanocrystals, so-called 'dot-in-rods' (DRs), are one of the most promising single-photon emitters due to their ability to work at room temperatures, relatively simple manufacturing, reduced blinking, and high degree of polarization of the emitted radiation. We study the nonclassical features of radiation emitted not only by single DRs but also their small clusters.
If a source of radiation, classical or quantum, is multimode in frequency, how can one filter out a single mode without introducing losses or admixing other modes? So far, no efficient solution has been implemented. We are going to do it using a dispersive element and the projective operation of a planar waveguide.
Multi-photon nonclassical states of light based on high-gain parametric down-conversion
Generation of photon triplets via three-photon parametric down-conversion
Homodyne detection of macroscopic quantum states of light
FP7 project Bright Squeezed Vacuum and its Applications "BRISQ2", 2013-2016, was coordinated by our group.