Quantum technologies present new methods to communicate securely, with security guaranteed by fundamental physical principles. The fragile nature of quantum systems, though, means that the losses inherent in ground transmission of quantum signals make it impossible to go distances beyond a few hundred kilometers. New approaches are required to take quantum communications, as well as experiments probing fundamental quantum physics, to global distances and beyond.
One of our group’s main activities is striving towards bringing quantum communication into space to achieve global distances. We are a key part of the Canadian Quantum Encryption and Science Satellite (QEYSSat) mission, working to implement a quantum receiver on a satellite to perform Quantum Key Distribution (QKD) as well as fundamental physics experiments. We have taken the project through several successful phases, involving studies with the Canadian Space Agency (2010-present), experimental proofs-of-concept for QKD over high-loss channels (PRA 2011) and detailed performance analysis studies (NJP 2013). We have also advanced the scientific discussion on fundamental and applied quantum experiments in space with satellites (Cl.Quant.Grav. 2012), and published a general review of quantum satellites (“The Quantum Space Race”). We are now building prototype systems with our partners in space industry (link to Press Release on IQC website), supported by the Canadian Space Agency. We are actively testing our system in outdoor experiments, such as demonstrating QKD to a moving pickup truck moving over 30 km/h (arXiv paper here) and an aircraft (QT 2017). Notably, in these experiments we implemented active tracking of both receiver and transmitter telescopes, photon polarization and time-of-flight.
Entanglement-Based Quantum Communication
We have researched the transmission of short-wavelength entangled photons over telecom fibres, for the purpose of quantum communications. This approach has the great practical benefit of enabling parallel transmission of classical and quantum signals over existing fibre infrastructure (APL 2010, Opt.Expr. 2011), and entanglement transmission over turbulent free space channels (NJP 2012).
Sources of Entangled Photon Pairs and Triplets
To enable future quantum communication networks and photonic quantum computers, single- and entangled-photon sources are needed with high production rates, tailored spectral properties, and high-fidelity quantum states. Our work focuses both on engineering improved photon pair sources and enabling larger photon states, triplets and beyond, using “cascaded down-conversion”.
We have studied the requirements for photon sources needed for quantum computing (JMO 2011), leading to the first experimental implementations of an actively-switched spontaneous parametric down-conversion (SPDC) source (PRA 2011), and how to improve spectral properties of photon sources (Opt.Lett. 2013). We also create photon pairs from four-wave mixing in polarization-maintaining and tapered fibres, showing the incredibly high efficiency that comes with small core size and large nonlinearity (Opt.Expr. 2013, PRA 2014, APL 2015). Our collaborator at IQC, Dr. Rolf Horn, is developing entangled photon sources towards commercialization, focusing on making robust, high-quality devices with sufficient simplicity for drop-in use.
We also employ cascaded down-conversion, where one output of an SPDC source is input to another SPDC source, to produce more than two photons at a time (Nature 2010). Our directly-produced photon triplets have shown tripartite entanglement in continuous and discrete variables, and non-linear optics at the single photon level (Nat.Phys. 2013, Nat.Photonics 2014). Our focus is to expand the types and capabilities of the entangled photon triplets and use the system to herald photonic qubits after lossy transmission, allowing quantum communication where losses would otherwise prevent it.
In addition, we also investigate building and optimizing sources of photon pairs entangled in energy and time. The individual energies of each photon from the pair will always add to that of the pump photon’s energy (Opt.Lett. 2013, Opt.Lett. 2014). We aim to develop a cleverly designed source of photon pairs to be useful in applications dependent on two photons being absorbed together, such as in the commonly used two-photon microscope in biological imaging.
Read-Out System for Signals from a Negative Feedback Avalanche Diode
Single photon detectors (SPDs) are very important for various research fields, including quantum information fields, space communication, spectroscopy, fluorescence lifetime measurements, and light detection and ranging. SPDs at telecom wavelength, around 1550 nm, have found particular interest for long-distance quantum key distribution (QKD) in optical fiber, where 1550 nm exhibits the lowest loss per kilometer of fiber. A new class of InP/InGaAs avalanche diodes (Princeton Lightwave, Inc.), known as negative feedback avalanche diodes (NFADs), are excellent candidates at this wavelength for single photon detection. These diodes generally work in free-running mode (as opposed to the gated mode more common to telecom SPDs) which is very important in applications where the arrival of each single photon at the detector is random in time. Our research focuses on a novel read-out system for signals generated from a free running NFAD (Rev.Sci.Instr. 2012). This system is designed and built in lab and the electronics are based on positive emitter-coupled logic. The system has an important feature of employing an active hold off time across the NFADs to improve the after-pulsing effect to achieve better performance from the detectors, and now it is in use to characterize NFAD devices.
Foundational Experiments of Quantum Mechanics
Our group has been involved in many foundational experiments of quantum mechanics, such as non-local hidden variable models or closing the locality loophole in Bell-tests (PNAS 2010). Most recently we worked on a test of three-photon entanglement which closed the locality loophole, the first experiment of its kind (Nat.Photonics 2014). This work was performed in tight collaboration with Dr. Kevin Resch (project lead), Dr. Raymond Laflamme and Dr. Gregor Weihs, and was the first to realize three-photon entanglement between three widely separated locations (> 600m).
We have also worked on tests of Born’s rule (Science 2010) using triple-slit interferences. Follow-up experiments studied spatial photonic qutrits and their use for quantum protocols (PRA 2012). Our group has also studied time-resolved single-photon interference (Sci.Rep. 2014) using a single-photon avalanche detector (SPAD) array from Dr. Alberto Tosi’s team.