PCF based photon sources for quantum information
Correlated photon pairs and heralded single photons are both key resources for many quantum information systems. By pumping a photonic crystal fiber (PCF) with a picosecond pulsed laser operating at 1064nm we are able to generate photons at new frequencies in the guided mode of the fiber through the non-linear process of four-wave mixing. This process involves the annihilation of two pump photons to generate a pair of photons, a signal photon at a higher frequency and an idler photon at a lower frequency than the pump. The frequencies of the signal photon and idler photon are determined by energy and phase matching conditions related to the structure of the fiber. The zero-dispersion wavelength (ZDW) of our PCF is at 1087nm so that we are operating in the normal dispersion region and signal and idler wavelengths are widely spaced  at around 810nm and 1550nm respectively. As both photons are generated simultaneously, the 810nm photons can be detected and this can be used to signal the arrival of a 1550nm photon. This then forms a source of heralded 1550nm single photons. By combining two such sources and adjusting the time delays in the system to ensure that the 1550nm photons are output simultaneously, we can form a source of photon pairs. The two photons that make up these pairs must be indistinguishable in spatial and temporal modes, and in energy. The main advantage of using a fiber based source rather than a bulk crystal is that the fiber has a single guiding mode so each output photon is guaranteed to be in the same spatial mode.
For a photon source of this type to be useful in quantum information applications, it is essential that there is a high probability that both the signal and idler photons that are detected come from the same generated pair. For this reason the input power driving the nonlinear process must be low enough that we expect much less than one pair to be produced for each pulse of the laser in order to avoid generating multiple pairs in a single pulse. As we are limited to the low power regime, the lumped collection efficiency (the combined efficiency of the optical components and detectors) becomes the main limitation on the brightness of the source that we can achieve. By splicing the required filtering and wavelength selection devices to the four-wave mixing PCF we aim to produce an all fiber source of photon pairs with superior collection efficiency and brightness than can be achieved in free space.
Our previous studies of correlated photon pair sources from four-wave mixing in PCF (in collaboration with the University of Bristol) have already demonstrated that high brightness can be achieved through systems of this type. In work published in 2005  using a slightly different PCF and free-space filtering optics we were able to observe a net coincidence count rate (after subtracting the background count rate) between the two output channels of 320,000 counts per second with an average input pump power of 0.54mW. This was the brightest source of heralded single photons that was known at the time. The PCF used in this work had a ZDW of 715nm and was pumped in the normal dispersion region by a Ti:Sapphire laser generating 4ps pulses at 708.4nm. The signal and idler wavelengths for this configuration were 587nm and 897nm respectively. These wavelengths were chosen as they are both in the range where they can be measured by a silicon detector with high efficiency (30-60%). In our current work we are forced to use an InGaAs detector with lower efficiency (<30%) in order to detect the photons at 1550nm. However this wavelength is of great interest from an engineering perspective as it is the wavelength of minimum attenuation in optical fibers and is used extensively in telecommunications.
The coincidence rate of 3.2×105 per second was achieved with a measured lumped efficiency of 21% in the signal arm and 11% in the idler arm. Significant contributions to the loss in these arms comes from the loss in the optical components and coupling loss of around 60% due to the need for free space filtering. By utilising high quality fiber optic filtering components we will improve upon the loss in optical components and also avoid the need for free space coupling.
After the demonstration of high brightness this source was also used to demonstrate interference between two heralded photons and showed a high visibility Hong-Ou-Mandel dip . In this experiment two separate sources of heralded single photons were used (both similar to what was demonstrated in ) and both were pumped by the same laser. A mirror on a translation stage was placed before one of the PCFs to introduce a variable time delay between the pump light arriving at the two PCFs. This then gave a corresponding time delay between the two pairs of output photons. The signal photons from the two PCFs were then passed into the two input arms of a 50:50 fiber coupler. The outputs of the fiber coupler and the two idler arms were then connected to four avalanche photodiodes. A four-fold coincidence measurement apparatus was then used to determine the rate of four-fold coincidences from the photodiode outputs. When the time delay is varied there is a sharp dip in the coincidence count rate at a certain delay time which corresponds to the temporal overlap of the two signal photons at the 50:50 coupler. This is referred to as the Hong-Ou-Mandel dip after the researchers who first performed an interference experiment of this type . The dip is observed because the two signal photons that are incident on the 50:50 coupler (or a 50:50 free space beamsplitter as in the original Hong-Ou-Mandel experiment) are identical in terms of spatial and temporal modes and in energy. General quantum mechanical arguments show that if the two photons are indistinguishable then they must both leave the coupler by the same output port. This means that there should be no photons output simultaneously from both outputs of the coupler and the measured four-fold coincidence rate drops as observed. This experiment achieved a visibility of 95% for the Hong-Ou-Mandel dip, demonstrating the suitability of the device as a source of pair photons. At 80 counts per second, the four-fold coincidence rate was also high and comparable to other recently demonstrated pair photon sources.
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 J. Fulconis, O. Alibart, W. J. Wadsworth, P. St. J. Russell, J. G. Rarity, “High brightness single mode source of correlated photon pairs using a photonic crystal fiber,” Opt. Express 13, 7572-7582 (2005)
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