1. Production of Photon Triplets James Cockburn Introduction References MethodsResultsConclusion The method works by having down- conversion sources be pumped by a laser in order to produce a photon pair. This primary source was periodically poled potassium titanyl phosphate, or PPKTP. This produced photons of 775nm and 848nm from the laser photon at 405nm. From this created photon pair, the 775nm one is subjected to another down-conversion at a secondary source, this time of periodically poled lithium niobate (PPLN)., producing two photons at 1510nm and 1590nm. The photon triplets, then, were measured using three different photon counters, D1, D2 and D3. D1 detected the 848nm photon, at a frequency of about 1MHz, and this detection opened a 20ns gate at D2, which detects one of the photons made at the second source, and this opened the gate at D3 for 1.5ns. It then follows that, since D3 can only be opened if D1 and D2 have both detected, an event at D3 would mean a photon triplet was detected. The results of the experiment showed that, indeed, photon triplets were observed. It shows that most of the photons were detected in the period between events at D3 and D1 at 1.2ns. This width of time interval is dominated by detector jitter. Other peaks were observed as well, but are caused by a read detection at D1 and a dark detection at D3. A dark detection is caused by background noise and so is discountable. From the results, it can be showed that there is a raw triplet production rate of 124 +/- 11 events in 20 hours. Using conversion efficiencies, the theoretical model predicts the triplet rate to be 5.6 +/- 1.1 counts per hour, which then agrees very well with the measured value. Hübel, Hamel, Fedrizzi, Ramelow, Resch and Jennewein, Nature, 466, 601-603 (29 July 2010). An experiment was carried out in order to produce entangled photon triplets using a theory proposed 20 years ago but never tested. A down- conversion source is pumped to create a photon pair. Then one of these photons undergoes a second down-conversion process, resulting in the production of another pair. The overall result is a photon triplet. Histograms of detected photon triplets against the time interval between detecting the photon at the first down-conversion processor and the third over a period of twenty hours shows that a sharp peak is observed around where the time interval is zero. This agrees well with what was expected. Conclusions that can be drawn from this experiment include being able to create hyper-entangled photons without elaborate and probabilistic post- selection schemes. The results also confirmed that down-conversion efficiency is independent of the pump power down to the single-photon level, which would allow for new tests of nonlinear optics in the quantum system. a, A down-conversion source (SPDC 1) produces a pair of photons in spatial modes 0 and 1, where the photon in mode 0 creates another photon pair in the second source (SPDC 2) in modes 2 and 3, generating a photon triplet. b, The primary source, pumped by a 405-nm laser, produces photon pairs at 775 nm and 848 nm. The 848-nm photon is directly detected by a silicon avalanche photodiode (D1), and the 775-nm photon serves as input to the secondary source, creating a photon pair at 1,510 nm and 1,590 nm that is detected by two InGaAs avalanche photodiodes (D2 and D3). A detection event at D3 represents a measured photon triplet. BS, beam splitter; F0, F1, band-pass filters; FP, long-pass filter; G, gate; TAC, time acquisition card; PC, computer. a, Measured triple coincidences obtained in 20 h. Each bin corresponds to a 0.8-ns time interval between events at D3 and D1 (Δτ D3–D1 ). The sharp peak indicates a strong temporal correlation between all three detection events, as expected of the C-SPDC process. b, Triple-coincidence histograms with varying delays of τ = 0 and ±0.5 ns between D2 and D3, resulting in a decrease of the coincidence peak. The absolute rate reduction for τ = 0 results from a different setting on the InGaAs detectors for this measurement series. Error bars, 1 s.d.