ИСТИНА

We demonastrate the experimental technique for generating a spatial single-mode broadband biphoton field. The method is based on a dispersive optical element which precisely tailors the structure of the type-I SPDC frequency angular spectrum in order to shift different spectral components to a single angular mode. Spatial mode filtering is realized by coupling biphotons into a single-mode optical fiber. Progress in preparation and manipulation of quantum states of light is boosted by development of quantum communication and quantum computation. Spontaneous parametric down-conversion (SPDC) is the most efficient and widespread source of correlated photon pairs (biphotons). Spectral broadening of the single spatial mode biphoton field remains a really significant puzzle among a wide list of biphoton spectrum control problems. The broadband biphotons exhibit ultra-short temporal correlations. Their correlation time ∆τ ∼ 1/∆ν. Ultra-correlated biphotons can be utilized in metrological applications such as distant clock synchronization, quantum optical coherence tomography (QOCT) and quantum interferometric optical lithography. Besides, spectral broadening increases the degree of biphoton entanglement, which can be exploited in quantum communication tasks. A quantum information can be directly encoded in frequency bins or the broad spectrum can be used for a wavelength-multiplexed entanglement distribution. Also biphoton spectrum broadening increases a maximum pair generation rate Rmax ∼ ∆ν, enabling an extremely high-speed quantum communication. In spite of the fact that the total biphoton spectrum is broad both in frequency and angle, there is one-to-one correspondence between angles and frequencies due to phasematching. We propose to reconfigure the SPDC spectrum by means of an angular dispersive optical element, shifting different spectral components to a single angular mode. This method doesn’t decrease the spectral intensity of SPDC radiation and significantly increases the integral intensity in the target angular mode. The principal scheme of the experimental setup is depicted in Fig. 1. Different biphoton spectral components propagate in different angular modes. The lens system focuses emission onto an angular dispersive element (diffraction grating). Such a system transforms the SPDC angular spectrum to provide congruence with the grating dispersion curve. After diffracting from the grating, the majority of spectral components correspond to the same angular modes and the radiation is coupled by the objective to a single-mode fiber. The efficiency of this method was demonstrated both theoretically and experimentally. The spectral bandwidth ∆ν > 100 THz and the correlation time ∆τ < 5 fs can be reached. The key feature of our method – spectral intensity invariance – was also demonstrated