SPEQS-1: Small Photon-Entangling Quantum System

SPEQS-1. This page describes our past project where we launched a quantum light source (SPEQS-1) into Low Earth Orbit aboard Galassia. On this link, you will find videos, and reports about this project which concluded in 2017. Our subsequent project, SPEQS-2 and SpooQy-Sat, will be unveiled once we complete the handover milestone in 2018.

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.

Source Engineering
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.

A major challenge is to understand the effects of space radiation on our components, and to prepare the necessary electronics. Before launch into space, several demonstrations of the instrument were conducted using high altitude balloons.

The SPEQS-1 project suffered a setback in 2014 when a launcher failed, but the photon pair source was recovered from the wreckage in a fully operational condition.

In 2015, a demonstration in LEO was achieved onboard the Galassia satellite.

Our proposal was described, together with work by groups in Europe and Canada, in the Babbage blog of the Economist: 

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).

We have prepared video animations to help you understand the challenge of miniaturising quantum light sources.

  • Daniel Oi (Advanced Concepts)
  • Christophe Wildfeuer (High Altitude Testing)
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.