Photons are highly effective for both classical and quantum communication. In Quantum Key Distribution (QKD) applications, we can use a source of entangled photon pairs to grow a secret key between two parties, provided they share an optical channel. These channels can be either through optical fibers or free-space links, and on Earth, both are currently limited to about 140 km. This limitation is practical: fibers are lossy, and line-of-sight communication links are geographically limited.
To extend QKD networks towards continental (& global) scales, satellites in low earth orbit can be used. In this scenario, entangled photon sources on satellites will beam photons to distant locations. The first step in this direction is to demonstrate that entangled photon sources can be operated in space.
To be cost-effective, our team is using the nano-satellite platform called CubeSat. We are currently working on building a complete entanglement system, consisting of the source, detectors and micro-electronics for running the experiment and collecting data. Photon pair correlations are tested onboard the satellite. This will allow us to validate the entanglement system, and provide confidence for future projects where photons will be transmitted away from the satellite. This is a natural continuation of the QKD demonstrations performed with Christian Kurtsiefer.
The entire experiment must fit within 300 mililitres, have a mass less than 300 gm and use 1.5 W (or less) of electrical power. In addition, the system must withstand temperature fluctuations and launch vibrations. To meet these challenges, we are developing Small Photon-Entangling Quantum Systems (SPEQS) that meet the form factor and power limitations in a CubeSat. Essentially, the entire system can rest in the palm of your hand.
How do you build an entangled photon source so that it fits onto the palm of your hand? We started by choosing the really neat source by Trojek & Weinfurter. It is a source geometry that can be readily compactified.
At its core, an entangled photon-pair source based on Spontaneous Parametric Down Conversion (SPDC) is nothing more than an optical pump, inline with a few birefringent crystals. The trick is to keep the pump and crystals aligned so that the photon pairs of interest are generated in the direction we want.
The optical pump can be a semiconductor laser diode (small and robust). Keeping the crystals aligned is trickier; for best performance, angular alignment must be within a few millirad. Fig. 2 shows one method for aligning the crystals relative to the pump source. Once the crystals are glued in place, the tall adjuster is removed so that the source is 80 mm long and 12 mm high. Fits nicely into a regular sized palm.
Please visit again in the future for more updates.
Our proposal was described, together with work by groups in Europe and Canada, in the Babbage blog of the Economist:http://www.economist.com/blogs/babbage/2012/02/quantum-cryptography
Tan Yue Chuan and William Morong presented the SPEQS project at the 2012 AAAS Meeting.
Our work with single photon detectors on satellites was mentioned by the Minister of State for Trade and Industry (Feb 2012).
Fig. 1: SPEQS package for flight on a 1U CubeSat. Optics are mounted in an aluminium tray that fits onto a PC104 board typically used in picosatellites.
Image by Daniel Oi.
Fig. 2: Concept for crystal alignment in the SPEQS package.
Image by Yau Yong Sean.