Compressing single photons

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• August 1, 2013
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Coherent conversion of single photons from one frequency to another is nowadays a mature process. It can be achieved on both directions, up, or down, it preserves quantum properties such as time-bin, or polarization entanglement, and internal conversion efficiencies approach 100%.

The main motivation for coherent frequency conversion is to interface photonic quantum bits in optical fibers to quantum repeater nodes: fiber loss is minimal at wavelengths around 1550 nm, but the currently most efficient optical quantum memories—which form the core of quantum repeaters—operate at around 800 nm.

A crucial problem that is often swept under the carpet in high-profile conversion papers is however that there isn’t just a drastic difference in center wavelengths, but also a drastic difference in spectral bandwidths. Single photons used in quantum communication, in particular when produced via the widely popular process of parametric down-conversion, usually have bandwidths of the order of 100 GHz. The acceptance bandwidth of quantum memories relying on atomic transitions can in contrast be as narrow as MHz.

Frequency conversion is therefore necessary, but by no means sufficient for interfacing these technologies. What’s far more important is the ability to match the spectra of the incoming photons and the receiver.

We address this issue in a new paper published earlier this year in Nature Photonics. The concept is relatively straightforward. A single photon is, as usual, up-converted with a strong laser pulse in the process of sum-frequency generation in a nonlinear crystal. However, the frequency components of single photon and pump are carefully synchronized such that every frequency in the photon bandwidth can only convert into one center frequency. To achieve this, we (that is, mostly first author and experimental wizard Jonathan Lavoie, and John Donohue and Logan Wright in Kevin Resch’s lab at IQC in Waterloo) imparted a positive frequency chirp on the photon, and a corresponding negative chirp on the pump pulse before conversion.

The results are quite impressive, we achieved a spectral single-photon compression by a  factor 40. This is still a far cry from the factors of >1000 we would need to really match a GHz photon to a MHz memory, but there is as always room for improvement. The conversion efficiency in our experiment wasn’t stellar either, but it could easily be increased by using techniques which do achieve near-unity efficiency.

Importantly, this type of chirped single-photon up-conversion also serves as a time-to-frequency converter, and the Kevin’s group has now in fact demonstrated this by detecting time-bin qubits with ultra-short time bin delays in the frequency domain.

A special acknowledgment goes to Sven Ramelow, without whom this research wouldn’t have happened at this time and in this form.