Tuning Photons Like A Guitar String

New paper from researchers at the Institute for Quantum Computing (IQC) shows new method that could help connect quantum devices

Institute for Quantum Computing (IQC) researchers at the University of Waterloo have demonstrated a new technique to help connect quantum devices over long distances, a critical component to realizing a quantum internet.

One of the major challenges with quantum communication is how far quantum information can travel because essential quantum properties, like entanglement, can’t be amplified like classical signals without disturbing the information they carry.

Sharing quantum information over long distances is difficult, but possible. By tuning the frequency of individual particles of light (photons) to atoms, those atoms can be used as memory nodes to distribute entanglement with the help of photons between two distant locations.

Using a photon source called a nanowire quantum dot, IQC researchers have demonstrated a new method that fine tunes the source’s frequency to better enable quantum information sharing. By freezing a thin layer of nitrogen gas on the nanowire surface, they managed to tune the quantum dot photons to the frequency of specific atoms called caesium.

“Ours is a new technique that tunes photons from the quantum dot so precisely that they can match specific spectral features of the atoms — similar to hitting exact musical notes when tuning a guitar string.” - Dr. Rubayet Al Maruf, research associate at IQC and the paper’s lead author.

He adds that this new approach, which is performed inside a cryostat (in-situ) — at temperatures almost as cold as outer space — has never been demonstrated before.

Sreesh Venuturumilli, IQC PhD student from the Department of Electrical and Computer Engineering, added that the tuning can be reversed to adjust the ‘off-key’ photons to the required frequency.

“Atoms ‘talk’ to very specific frequencies and being able to adjust and reverse the tuning is crucial. The quantum dots emit special quantum states of light, like single-photons and entangled photon pairs, and making these photons talk with the atoms allows for novel light-matter interactions.” -Sreesh Venuturumilli, IQC PhD student and paper's co-author.

Dr. Michal Bajcsy led the research in collaboration with Dr. Michael Reimer and Dr. Kevin Resch. All three are faculty members at IQC. Bajcsy and Reimer are professors in the Department of Electrical and Computer Engineering and Resch is professor in the Department of Physics and Astronomy. The nanowire quantum dots were fabricated by Drs. Dan Dalacu and Philip J. Poole at National Research Council Canada (NRC).

“These quantum dots are an excellent source of single photons and entangled photon pairs; they are bright and can produce photons repeatedly at a fairly fast rate. The quantum dots solve many challenges that come with single-photon sources but needed a fine-tuning mechanism — like the one that we have demonstrated — to interface with atomic quantum memories.”
-Dr. Michal Bajcsy, IQC faculty, professor, Department of Electrical and Computer Engineering.

The group’s demonstration is a small but important step towards a quantum internet becoming a reality. Quantum information will need to be repeated to travel necessary distances, whether it’s for communication via satellites, between computers, or quantum key distribution.

“To be able to connect quantum devices, information will need to be transmitted over long distances without losing its ‘quantumness’ and our new method is a step towards realizing that,” Al Maruf says.