Before we go any further, we want to warn you that many of the experiments described are out of the budget of many enthusiasts. This is because applications involving quantum entanglement are in the early stages of development, so the specialized crystals and detectors are not yet mass-produced. However, we feel that it is worthwhile discussing these techniques from a practical perspective, since their development is expected to lead us into a complete revolution in the way in which we compute and communicate. If the equipment is outside your budget range, you can still perform very realistic simulations using software that Mark Beck from Whitman College has kindly made available for free.||
At the moment, the most commonly used source of entangled particles is a crystal of β-barium borate (BBO). A single BBO crystal is able to split UV “pump” photons into so-called “signal” and “idler” photons through a process known as parametric down-conversion. The two down-converted photons have entangled properties, because they need to conserve the energy and momentum of their single parent photon. However, the individual properties of the photons are free to vary as long as their sum agrees with the energy and momentum of the parent photon.
There are two types of down-conversion processes that can be used in BBO crystals to produce entangled photons. As shown in Figure 139a, so-called type I down-conversion produces two down-converted photons with the same polarization, which is opposite to that of the pump photon. On the other hand, in type II down-conversion, the signal and idler photons have orthogonal polarizations, as shown in Figure 139b.
Figure 139 Nonlinear BBO crystals can be manufactured to produce entangled photons through two types of down-conversion processes. (a) The signal and idler photons in type I down-conversion have the same polarization, which is opposite to that of the pump photon. (b) Type II down-conversion produces photons with orthogonal polarization, with the idler photon having the same polarization as the pump photon. (c) and (d) The distribution of down-converted photons depends on the type of down-conversion process. Type II down-conversion produces photons with randomized polarization useful for our work only in line with the intersections between the cones.
The wavelengths of the photons produced by parametric down-conversion do not necessarily have to match. In fact, the wavelengths of the down-converted photons will distribute around an average of twice the pump photon’s wavelength (since on average, the down-converted photons will have one-half the energy of the pump photons). For our entangled-photon source, we will use photons that have the same wavelength λidler = λsignal = 2 × λpump.
We will use BBO crystals that support type I down-conversion, because they can be combined to produce much larger numbers of entangled photons than a type II crystal. Our entangled-photon source is based on a design by Dehlinger and Mitchell from the Physics Department of Reed College in Oregon.45 It must be noted, however, that type II down-conversion is preferred for many applications, because it is easier to collect entangled pairs with high purity—that is, containing much lower numbers of nonentangled, stray photons.
BBO crystals cut for type I down-conversion only convert photons of a specific pump polarization. Photons of the orthogonal polarization simply pass through. For this reason, we use a design46 by Paul Kwiat** and his colleagues at the Los Alamos National Laboratory, which calls for two identical BBO crystals, one next to the other, with one of the crystals rotated by 90°. Photop Technologies†† can supply a mounted, ready-assembled crystal stack for around $1,400. Please note that BBO crystals are very sensitive to humidity. They must be kept in a bag with a desiccator any time they are not in use.47
As shown in Figure 140, a pump photon polarized at 45° going through the BBO crystal stack has equal probability of down-converting in either crystal. For 100-mW pump power, the rate at which polarization-entangled photons are produced can reach up to 6 × 106 entangled pairs per second. This is more than 300 times the number of entangled-photon pairs produced by type II crystals. Another advantage of this method is that the photons are entangled in energy and momentum as well. This hyper-tangled state is very useful when it comes to developing applications such as quantum computing, quantum encryption, and quantum communication.
Figure 140 The nonlinear crystal in our photon entangler comprises two 5 mm × 5 mm × 0.1 mm BBO crystals mounted face-to-face at an angle of 90° to each other. Pump photons polarized at 45° produce two cones of entangled down-converted photons. The down-conversion process is very inefficient, so the vast majority of pump photons pass unimpeded by the nonlinear crystals.
High-frequency pump photons are produced by a violet ~405-nm laser module. These used to cost many thousands of dollars just a few years ago, but their use in TV projectors, Blu ray disks, and laser pointers have dropped their cost to under $100. Many of these lasers produce other wavelengths besides the ~405-nm line, so a band-pass filter should be placed after the laser. We use a filter made of BG3 colored glass (Thorlabs model FGB25) that allows 315- to 445-nm photons to pass through. In addition, two washers with small holes act as baffles to keep unwanted light and reflections away. Our laser is built from a laser diode extracted from a Blu ray disk burner. We use the circuit shown in Figure 141 to drive the laser diode with 160 mA to produce around 100 mW of 405-nm polarized light. As an alternative, you may use an inexpensive (<$20) 405-nm, 5-mW laser pointer based on a Blu ray player laser diode. However, consider that the entangled-photon yield is proportional to the pump laser’s power. A rough estimate for the crystals made by Photop is: number of entangled pairs per second ≈ pump power [mW] × 6 × 104.
Figure 141 This 405-nm laser is based on the laser diode extracted from a Blu ray burner drive. Make sure that the laser diode is thoroughly heat-sinked, and that the current produced by the circuit (measured by replacing the laser diode with a milliammeter) is below 160 mA before connecting the laser diode.
As we discussed, type I parametric down-conversion by a BBO crystal happens only at a very specific polarization. Therefore, we need to place polarization optics between the laser’s output filter and the BBO crystal. A linear glass polarizer (Edmund Optics NT54-926) in a rotatable mount adjusts the polarization angle, and a small quarter-wave quartz waveplate (e.g., 0.5-mm thick x-cut polished quartz plate; Casix WPZ1310 or Edmund Optics NT83-927) is used to adjust the phase of the pump laser light to maximize entangled-photon yield. Finally, a piece of black foam is used as a beam stop for the pump laser beam that goes unimpeded through the BBO crystals. As shown in Figure 142, entangled-photon pairs of approximately the same wavelength exit the crystal at an angle of around 2.5° to 3°.
Figure 142 We use a 100-mW, 405-nm laser module to pump an assembly of two BBO crystals (Figure 140) to produce entangled IR photons at approximately twice the pump wavelength. A 45° polarizer and a thin quartz plate are used to tune the polarization and phase of the pump beam for optimal yield of entangled-photon pairs by the BBO crystals. The BBO crystals are very hygroscopic, so they must be kept in a bag with a desiccator at any time that they are not in use.
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