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Our team is working actively on entanglement distribution via free-space or optical fiber. Although the wavelengths can be different - for fiber, we prefer something in the telecommunications band (1,550 nm or 1,310 nm) whereas for free-space we currently prefer the Near Infrared (about 800 nm), the design principles for entangled photon pair sources based on Spontaneous Parametric Downconversion (SPDC) are very similar. We also prefer to work in polarization, and to build sources around bulk crystals, because there are many degrees of freedom that can be utilised in a bulk material (see the POMO concept below).
When designing instruments, it certainly helps to have an envelope for Size, Weight and Power (SWAP), and we use this to determine which design choice to optimize. As we like to reduce the footprint of the instrument, we often choose to work on co-linear designs, where all three SPDC fields co-propagate in the same direction.
POsition MOmentum entangled photon pair sources - POMO. See our pre-print.
While the PACES design works very well when using two co-aligned crystals, in some cases this is quite tricky to work with. For example, when using engineered domain materials such as periodically-poled structures, getting two very similar crystals can be a challenge. It would be highly preferable to use only a single structure, and then to find a method to perform coherent superposition of the SPDC decay paths to obtain entanglement.
In bulk crystals, there are a number of ways to achieve this superposition. Momentum correlation, timing correlation, co-propagating paths, counter-propagating paths, or layering multiple domain periods into a single structure are methods that have been investigated. However, position correlation can also be used, and can lead to a very simple design.
The concept was first tested in the setup shown in Figure 1. The interaction volume is conceptually divided into an upper and lower half, and then the SPDC process in the upper and lower parts are coherently super-posed. The first attempt uses a bulk interferometer.
Fig. 1
Fig. 1. Experimental setup for the conversion of position correlation to polarization entanglement. The zoomed-in image of the SPDC interaction volume describes photon-pairs generated at the upper part of the crystal to be having position state |x1 , x1 > while photon-pairs generated on the lower part are marked as |x2 , x2 >. Two plano-convex lenses (f=15mm) are used to image the interaction volume onto a wedge mirror (WM). The photons are coupled into a single mode fiber (SMF) and a wavelength division multiplexer (WDM) is used to split the signal (s) and idler(i) photons. A mirror inside the interferometer is attached to a piezo-actuator to lock the phase while observing the constructive or distructive interference fringe produced by the pump laser.
Once this concept was validated to produced high quality entangled photon-pairs (observed rates of 120,000 pairs/s/mW using standard Silicon GM-APDs), the concept was implemented using minimal parts demonstrated in Figure 2. This concept works because for some distance after the interaction volume, the SPDC photon-pairs still maintain their position correlation. The segmented half-wave plate (SHWP) performs the polarization rotation, before spatial walk-off is utilised to coherently combine the photon-pairs.
Fig. 2
Fig 2. Schematic of the compact polarization entangled photon source. The dashed line represent the distinction between photons generated at different parts of the PPKTP crystal. Red and blue filled circles represent horizontally and vertically polarized photons respectively. The WDM separates the photon-pairs according to its wavelength.
The quality of the entanglement correlation generated using this technique is very good, and the minimal number of components is used. We suggest that such a design would be of great interest to those who need to build compact sources of entangled photon-pairs using bulk crystals.
Fig. 3
Fig 3. Reconstructed density matrix of the entangled photon pair state. The fidelity is 99.1% to the maximally entangled Bell State.
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