QUIE2T Quantum Information Entanglement-Enabled Technologies

Physical approaches and perspectives

All photonic approaches to quantum information technology rely upon an efficient detection technology. Although single photon detectors are commercially available, these are simple digital devices, which detect the presence or absence of one or more photons. Future detector technologies will not only have to have a dramatically higher detection efficiency but also considerable lower dark count rates as well as a timing jitter that does not limit the transmission rates. The commercial detection systems are based on semiconductor avalanche photodiodes (APDs) such as Si (400-1000nm) and InGaAs/InP (1100-1700nm). These are robust and generally only require electric cooling. Recent alternatives include superconducting devices, either transition-edge sensors (TES) that have shown efficiencies > 90% but remain relatively slow, or superconducting nanowire single photon detectors (SNSPD) that are faster (both low jitter and high count rates) but have only realised efficiencies ~ 25%. Both of these have demonstrated photon number resolution capability. The need for cryogenic cooling is offset by the potentially high performance. For continuous variable (CV) measurements single photon resolution is not needed. There, apart from the quantum efficiency and bandwidth, the signal to noise ratio of the detector module is important. This is not an extensive list, but focuses on the most advances or promising technologies in the context of quantum communication.

European groups working in this field include – for APDs: S. Cova (Milan, I), A. Shields, (TREL, UK), H. Zbinden (Geneva, CH), J. Rarity (Bristol, UK), G. Buller (Heriot-Watt, UK), A. Giudice (Micro Photon devices, I), G. Ribordy (id Quantique, CH):- for superconducting devices: G. Gol`tsman (Moscow, RU), A. Fiore (Eindhoven, NL), V. Zwiller & T.M. Klapwijk (Delft, NL), R. Leoni & S. Pagano (CNR Rome, I), J-C. Villegier & J-Ph. Poizat (CEA Grenoble, F).

State of the art

A severe limitation of today’s photon detection technology is the maximum count rate. For example, InGaAs/InP APDs have been traditionally operated in a gated mode with a maximum repetition frequency of 1-10 MHz and a maximum count rate of 100 kcps. However, this field has recently been reinvigorated with novel work on the operating electronics providing advances in rapid gating (GHz) [1] and continuous (free-running) [2] operation opening up new regimes of operation and performance. The superconducting devices have demonstrated photon number resolution capability and high efficiency: TES > 90% [3]; SNSPD ~24% [4]. The later capable of a significantly higher count rate (potentially GHz) and lower timing jitter (<100ps). In the continuous variable regime, several groups report quantum efficiencies approaching 100% using commercially available PIN diodes with increasing bandwidth (> 100 MHz) and signal-to-noise ratios. Conceptually, the strict separation between discrete and continuous detection schemes is complemented by hybrid detection approaches [5].

Challenges

Europe and Japan are currently leading the way for the APD detection schemes, while the US is a clear leader for superconducting devices. The main challenges for APDs are:

• Explore these new operating regimes – faster (> 2 GHz), higher efficiency (> 25% for InGaAs/InP);
• Adapt devices (semiconductor & electronics) for specific applications, e.g. peak efficiency wavelengths;
• Transfer these recent advances to the commercial sector.

For CV detection schemes

• Faster, compact and stable homodyne, heterodyne and hybrid detectors that can be integrated in all-fibre systems;
• Local oscillator phase retrieval techniques for weak coherent states have to be developed for homodyne measurements after fibre channels;
• The optical detection of the signal has to be optimised for free space systems to prevent losses that degrade the CV states (high overall quantum efficiency).

For superconducting detectors

• Fabrication of detectors in cavity structures for high efficiency;
• Improve fabrication and coupling to increase the efficiency and robustness;
• Demonstrate the detectors: lower dark counts (<1Hz), increased detection efficiency (> 70%), low jitter (< 100ps) and photon number resolving capabilities;
• All of these characteristics in one device.

[1] Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, and A. J. Shields, Appl. Phys. Lett. 91, 041114 (2007)
[2] R. T. Thew, D. Stucki, J.-D. Gautier, and H. Zbinden, A. Rochas, App. Phys. Lett., 91, 201114 (2007)
[3] A. E. Lita, A. J. Miller, and S-W. Nam, Opt. Exp., 16, 3032 (2008)
[4] X. Hu et al., Opt. Lett., 34, 3607 (2009)
[5] C. Wittmann, M. Takeoka, K.N. Cassemiro, M. Sasaki, G. Leuchs, and U.L. Andersen, Phys. Rev. Lett., 101, 210501 (2008)