SPANGL4Q: A project to connect spinning photons with trapped electrons

SPANGL4Q: An EU Project Funded by the "Future and Emerging Technologies" section of Framework Programme 7.


Individual photons are the quantum particle of choice for physicists and engineers trying revolutionise the world of information processing and communication by using the "quantum technologies". Quantum technologies use the unique quantum properties of single particles (such as "superposition" - being in two places at once) to make ultra-fast computers and achieve absolutely secure communications.


Quantum information scientists have known for a while that a key component of a quantum computer or a quantum communications system will be a set of “quantum repeaters”, devices which don’t amplify the signal, but stores the photon state in a static quantum particle, waiting for the arrival of a second photon, passing the signal down the chain.


A practical quantum repeater should be possible using semiconductor technology. A nanosized semiconductor trap, a quantum dot, is able to trap an electron and store its quantum information for microseconds. This storage time is already enough to increase the present communication distance limit of about 100km, to potentially 1000’s km.


The key to this technology is to combine the quantum dot that traps the electron with a device that also traps the photon in the same place: a nanophotonic cavity. Holding the electron and photon in the same micron-sized region for longer (just a few 100’s picoseconds is long enough), the information is transferred much more efficiently. Without the cavity, only 1 in 10000 photons is trapped, but with it, the transfer may reach almost 100% efficiency.


SPANGL4Q has pushed the frontiers of what these small quantum dot nano-photonic devices could do to their limits. We aimed to push the storage time of the quantum memory from a millionth of a second, to a few seconds. By slowly transferring the electron information to the ultra-isolated nuclei in the quantum dot, a single photon should be able to travel for tens of thousands of km, before the information is lost, enabling truly global quantum communications.


Our approach required a complete rethink of the well-known rules for how light behaves in small structures. Photonic engineering is a well-established field – modern telecommunications relies on nanophotonic devices. However, even in the research laboratory, using nanophotonics to manipulate the photon using polarization is not well-established, and the SPANGL4Q team has gone back to the drawing board, to start polarization engineering in these structures. Improving our nderstanding of how light behaves in nanophotonics should lead to many more ways to manipulate light for a wide variety of new applications in telecommunications, medicine and security.



Summary of Achievements:


Demonstration that photonic design can affect spin

  • Chiral DBR (distributed Bragg reflector) structure.
  • Unidirectional coupling in a photonic crystal waveguide
  • Demonstration of spin-dependent coupling in an S-shaped chiral nanoantenna
  • Comprehensive, general theory of spin-dependent light-matter interactions for any complex photonic structures.


Establishment of design principles to allow guaranteed interaction of a single photon with the spin

  • Established requirement for input-output coupling efficiency >50%.
  • Demonstration of deterministic interactions in a micropillar cavity.
  • Demonstration of Tamm plasmon structures combined with site-controlled QDs.


Demonstration that nuclear spins can indeed act as efficient long-term quantum memory


  • Nuclear spin coherence times of milliseconds observed for As atoms in QDs
  • Strain studies in QDs reveal that areas of strong nuclear quadrupolar interaction should lead to individual spins with even longer coherence time (many seconds)


Feasibility study of QD spin structures for long distance quantum communications

  • Devices designed to achieve millisecond quantum memories could allow low earth orbit (LEO) satellite quantum communications and 1000km links
  • Future devices with 1s quantum memories would be suitable for geostationary satellite links
  • QDs suggested technology to enable key 5 and 10 year goals identified by the European consortium QIPC for quantum communication (including 1000km quantum communication links, satellite quantum communication links)