Superconducting Nanowire Single-photon Detectors: A Brief Introduction By Omar Alshehri Waterloo, ON Fall 2014

Outlines History. The device mechanism. Applications (potential). Advantages. Limitations. Improvements. The grand device!

History SSPDs or SNSPDs. First generation single-photon detectors (1995 – 2000) were just detecting infrared photons. In 2001, the Niobium Nitride nanowire was a turning point [1]. It enabled optical photon detection too.

Device mechanism: how does it work?

Emitter Medium Receiver Ref [2] Ref [3] 1- Individual atom. 2- III-V semi-conductor quantum dot. 3- Ni vacancy defects in diamond. 4- Single molecule.

Emitter considerations Medium considerations Receiver considerations

1 st : Photon generator considerations [2] “Ideal” SPD generates electric signal only upon absorbing a photon. In reality, there must be noise. Detection efficiency depends strongly on the wavelength of the incident photon.

2 nd : Medium considerations [2] Photons can be lost before reaching detector due to the following: 1- Absorption. 2- Scattering. 3- Reflection. This loss of photons can be defined as “coupling efficiency”.

3 rd : Receiver considerations Photon absorption by the detector depends on the following (in addition to the aforementioned photon wavelength) [2]: 1- The material of the detector. 2- The geometry of the detector or of the whole device. 2.1 To shorten the recovery time (dead time for the detector to be ready for another detection) we must either (a) reduce the inductance of the receiver (by either shorten or thickens the nanowire [10]), or (b) add a resistance in series with the device to reduce the decay time [4]. 3- The operating temperature.

Final results after taking care of the three

Advantages High efficiency. Low dark counts. Excellent time resolution (also called time jitter). Operate at relatively high temperature (4.2 K).

Limitations Low fabrication yield.

Improvements (done) [2] The science part is to further enhance the performance of the device while the engineering part is to suitably implement it in an application. Several device performance enhancements has been made/tried: 1- Large area meander SNSPDs (essential but low fabrication yield). 2- Cavity and waveguide integrated design. 3- Multi-pixel design. 4- Ultra-narrow wires (20 to 30nm ones are more responsive to low energy photons). 5- Finding alternative superconducting materials (NbTiN [good], Nb [slow energy relaxation], or MgB 2 [defects]).

Improvements (to be done) The origin of the exponential behaviour of dark count rate with bias is poorly understood and hence awaits some focus [2].

Potential applications [2] 1.Quantum key distribution (QKD). 2.Optical quantum computing. 3.Characterization of quantum emmitters. 4.Classical space-to-ground communications. 5.Integrated circuit testing. 6.Fiber temperature sensing. 7.Time-of-flight depth ranging.

Conclusion: The grand device!!! Make one device that detects infrared/microwave photon, then convert it to an optical one. According to the theoretical model by Kadin et al [5], the best size of the device (infrared detection part) is from few nanometers to tens of nanometers. The ground-breaking Gol’tsman et al [1] work did it with 200nm width, 5nm thick, and 1 micron length. There should be no lattice mismatch between the NbN wire and the substrate.

The grand device (cont) The nanowire should be an order of magnitude less than the wavelength (sub-100 nm wire). The dimension for thinness of the wire is 20 or 30nm [6]. Any device must be a meander based with a best dimension of 3µm x 3µm [7]. The relaxation time (cooling time) should not be too long nor too short [9]. This meander must not have sharp corners (e.g. like the spiral meander fabricated at TU Delft) [8].

The grand device (cont) This meander must not have sharp corners (e.g. like the spiral meander fabricated at TU Delft) [8].

References 1.Gol’tsman G N, Okunev O, Chulkova G, Lipatov A, Semenov A, Smirnov K, Voronov B, Dzardanov A, Williams C and Sobolewski R 2001 Picosecond superconducting single-photon optical detector Appl. Phys. Lett –7. 2.Natarajan, C. N., Tanner, M. G., & Hadfield, R. H., 2012 Superconducting nanowire single-photon detectors: physics and applications Supercond. Sci. Technol Gol’tsman G, Okunev O, Chulkova G, Lipatov A, Dzardanov A, Smirnov K, Semenov A, Voronov B, Williams C and Sobolewski R 2001 Fabrication and properties of an ultrafast NbN hot-electron single- photon detector IEEE Trans. Appl. Supercond –7. 4.Yang J K W, Kerman A J, Dauler E A, Anant V, Rosfjord K M and Berggren K K 2007 Modeling the electrical and thermal response of superconducting nanowire single-photon detectors IEEE Trans. Appl. Supercond –5. 5.Kadin A M and Johnson M W 1996 Nonequilibrium photon-induced hotspot: a new mechanism for photodetection in ultrathin metallic films Appl. Phys. Lett –40. 6.Marsili F, Najafi F, Dauler E, Bellei F, Hu X, Csete M, Molnar R J and Berggren K K 2011 Single-photon detectors based on ultranarrow superconducting nanowires Nano Lett –53. 7.Kerman A J, Dauler E A, Yang J K W, Rosfjord K M, Anant V, Berggren K K, Gol’tsman G N and Voronov B M 2007 Constriction-limited detection efficiency of superconducting nanowire single-photon detectors Appl. Phys. Lett Official TU Delft account at youtube, 9.Kerman A J, Yang J K W, Molnar R J, Dauler E A and Berggren K K 2009 Electrothermal feedback in superconducting nanowire single-photon detectors Phys. Rev. B Kerman A J, Dauler E A, Keicher W E, Yang J K W, Berggren K K, Gol’tsman G and Voronov B 2006 Kinetic-inductance-limited reset time of superconducting nanowire photon counters Appl. Phys. Lett