Spatially-entangled 4-photon states (Michiel de Dood)

Figure: Joined spatial density of four-photon states produced by parametric down-conversion of blue laser light in a periodically poled KTP crystal. The false color image shows additional counts produced by stimulated parametric down-conversion of a second photon pair. The image shows the strong correlations between the photons produced.

Spontaneous parametric down-conversion or SPDC refers to a non-linear optical process that may take place in a non-linear crystal when excited by intense laser light. We use so-called periodically poled KTP (PPKTP) crystals pumped by intense blue laser light at 413 nm. At these intensities a non-linear process that splits a single photon into a pair of photons with equal energy becomes possible. This process is said to be spontaneous because there is no clear, classical, reason for this process to occur.

 

Quantum mechanics explains SPDC as stimulated emission of vacuum fluctuations and allows to understand the strong correlation that exist between the two photon produced in the same event. In our experiments we distinguish the photons based on their emission direction and the position in the crystal where they are created, leading to what is called ‘spatial entanglement’.

 

An interesting situation is created when using pulsed laser light because this increases the probability to create more than one pair at the same time. The correlations between these multiple photons can be measured and the amplitude of the stimulated process relative to the background can be explored. For 4-photons in 2 different modes the photon generation rate is enhanced by a factor 2 compared to what is expected from the random, spontaneous, process. An open question remains on how these probabilities change when 4 photons are emitted in the same mode. For these multiple photons to be useful for future quantum information applications we need to know how many optical modes are involved in order to optimize both the purity and brightness of the source.