BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d4b3d64af06 DTSTART:20250227T093000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:ICFO Auditorium SUMMARY:ICFO | LAURA ZARRAOA SARDÓN CLASS:PUBLIC DESCRIPTION:This thesis studies the use of a single trapped neutral 87Rb at om as a photon counter. Detection of quantum jumps (QJs)\, i.e.\, abrupt c hanges between atomic states observable by a change in atomic fluorescence \, is used to infer the arrival of single photons. This is referred to as the quantum jump photodetection (QJPD) technique.\nThe thesis first situat es QJPD in the context of photodetection. Compared to traditional detector s\, QJPD technique has lower speed and quantum efficiency (QE)\, but has e xceptional performance in other figures of merit: QJPD is intrinsically na rrowband\, has strong out-of-band rejection\, and very low dark counts (DC s). These features make the QJPD interesting for applications that detect weak optical signals in the presence of a strong broadband background.\nEx perimental methods to study QJPD are described. A 87Rb atom is loaded from a magneto-optical trap (MOT) into a far-off resonance trap (FORT) at the center of four orthogonal\, co-focal\, high numerical aperture lenses. The se lenses create the FORT\, couple probe light onto the atom and collect t he atomic fluorescence\, which is used to identify the atomic state. A typ ical QJPD sequence is presented\, which consists of trapping and cooling a n atom in the FORT\, optically pumping it into the dark state\, illuminati ng with probe light\, illuminating with readout light and collecting fluor escence photons\, and checking that the atom has not left the FORT during the sequence.\nStatistical methods for measurement of QE and DC contributi ons are introduced. These compare the observed fluorescence count distribu tion against measured hyperfine-state fluorescence distributions. A QE of (2.4±\;0.1)×\;10&minus\;3 is demonstrated\, a record for single photon absorption by a single atom in free space.\nDark count contributio ns are measured. To produce low DC\, the QJPD technique is implemented in two time windows: an exposure time for the single photon absorption follow ed by a short fluorescence time to read out the atomic state. This implies distinct acquisition and readout DC contributions\, similarly to CCD and CMOS detectors. A dark jump rate (analogous to CCD/CMOS dark current) is m easured of (5 ±\; 10) ×\; 10&minus\;3 jumps/s\, consistent with zero and limited by measurement statistics. The measured readout contribu tion is (4.0 ±\; 0.4) ×\; 10&minus\;3 jumps per ms of fluoresce nce readout. For a 1 Hz readout rate\, with 1 ms readout pulses\, a net da rk count rate of (15 ±\; 10) ×\; 10&minus\;3 counts/acquisition is demonstrated\, which is already competitive with any non-cryogenic det ector.\nThe background rejection capabilities of the system are tested by measuring quantum jump rates when the atom is illuminated with direct sunl ight\, and with light scattered by the atmosphere (skylight). A rate equat ion model is developed to describe QJ probabilities in the presence of bot h intense broadband background and weak resonant probe light. This model i s used to interpret experiments in which a weak signal beam competes with strong broadband background and validated using direct sunlight. Measureme nts where the atom is illuminated with skylight show no observable backgro und-induced QJs. Finally\, measurements of sky brightness and its fluctuat ions are presented\, showing large fluctuations even on mostly clear days\ , a factor that further increases the need for background rejection.\nA nu mber of contemporary applications of extreme photodetection\, including fr ee-space quantum communication in daylight\, classical optical communicati ons in space\, and fundamental physics experiments\, are discussed as poss ible applications of the QJPD technique. A realistic scenario where the de monstrated QJPD capabilities surpass the current performance of commercial single photodetectors is presented.\nFinally\, potential improvements are discussed. It is shown that existing atomic and optical technologies coul d be applied to reach different wavelength ranges\, narrower bandwidths\, higher quantum efficiency\, and lower dark counts.\n \;\nThursday Febr uary 27\, 10:30 h. ICFO Auditorium \nThesis Director: Prof. Dr. Morgan Mit chell and Dr. Romain Veyron DTSTAMP:20260407T073550Z END:VEVENT END:VCALENDAR