It is well known that totally incoherent light cannot exhibit first-order interference with photons that are uncorrelated,
i.e., the normalized first-order correlation function is g(1)(0) = 0, whereas the second-order correlation
function is g(2)(0) = 1. Less familiar is the fact that both chaotic and coherent sources can exhibit first-order interference,
so that merely using the term "interference" is ambiguous. If fact, some previous QIQC presentations
have centered around whether or not two-photon correlations are actually a form of two-photon interference.1
Another area of ambiguity concerns the detection of quantum state coherence using interference.2 In an attempt
to disambiguate the concept of interference, we examine associated photon states using chaotic sources and the
Hanbury Brown and Twiss (HBT) detection of bunched photons. The unambiguous determination of coherent
quantum states has important applications for:
(1) Atomic Bose-Einstein condensate (BEC) determined using scattered laser interference3
(2) Exciton-Polariton BEC determined using emitted photon interference4
(3) Coherent light states.
(4) Characterizing photon statistics.
(5) Characterization of extended sources.
In this paper, we present imaging results for topics 3-5. The difficulties of HBT data acquisition are generally
underappreciated. An advantage of our approach is super-linear speedup through the development of a new
imaging device consisting of a 2-dimensional array of single-photon avalanche detectors.5, 6 A 4 × 4 array
enables 120 HBT coincidence experiments to be run in parallel to generate the 2-dimensional distribution of
g(2)(x) spatial correlations, thus making plausible the term "g2 camera" for this quantum imaging device.
The first implementation of a single photon avalanche diode (SPAD) is reported in 130nm CMOS technology. The
SPAD is fabricated as p+/nwell junction with octagonal shape. Premature edge breakdown is prevented through a guard
ring of p-well around the p+ anode. The dynamics of the new device are investigated using both active and passive
quenching methods. Single photon detection is achieved by sensing the avalanche using a fast comparator. The SPAD
exhibits a maximum photon detection probability of 41% and a typical dark count rate of 100kHz at room temperature.
Thanks to its timing resolution of 144ps (FWHM), the SPAD can be used in disparate disciplines, including medical
imaging, 3D vision, biophotonics, low-light-illumination imaging, etc.
The design and characterization of an imaging sensor based on single photon avalanche diodes is presented. The sensor was fully integrated in a 0.35μm CMOS technology. The core of the imager is an array of 4x112 pixels that independently and simultaneously detect the arrival time of photons with picosecond accuracy. A novel event-driven readout scheme allows parallel column-wise and non-sequential, on-demand row-wise operation. Both time-correlated and time-uncorrelated measurements are supported in the sensor. The readout scheme is scalable and requires only 11 transistors per pixel with a pitch of 25μm. A number of standard performance measurements for the imager are presented in the paper. An average dark count rate of 6Hz and 750Hz are reported at room temperature respectively for an active area diameter of 4μm and 10μm, while the dead time is 40ns with negligible crosstalk. A timing resolution better than 80ps over the entire integrated array makes this technique ideal for a fully integrated high resolution streak camera, thus enabling fast TCSPC experiments. Applications requiring low noise, picosecond timing accuracies, and measurement parallelism are prime candidates for this technology. Examples of such applications include bioimaging at cellular and molecular level based on fluorescence lifetime imaging and/or, fluorescence correlation spectroscopy, as well as fast optical imaging, optical rangefinders, LIDAR, and low light level imagers.
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