1. 1 S UPERCONDUCTING N ANOWIRES D ETECTING S INGLE P HOTONS FOR I NTEGRATED Q UANTUM P HOTONICS Roberto Leoni IFN-CNR, Istituto di Fotonica e Nanotecnologie, Via Cineto Romano 42, 00156 Roma, Italy Coauthors: - Alessandro Gaggero (IFN-CNR) - Francesco Mattioli (IFN-CNR) - Andrea Fiore (Eindhoven University of Technology, The Netherlands) FRONTIER DETECTORS FOR FRONTIER PHYSICS 13th Pisa Meeting on Advanced Detectors 24-30 May 2015- La Biodola, Isola d'Elba (Italy)

2. WHY SUPERCONDUCTING DETECTORS? 2 Superconducting detectors are the natural choice for ultrasensitive optical detection. Small energy gap compared to semiconductors, Cryogenic temperatures allow low noise, reduction in blackbody radiation and phonon noise. Single Photon Detectors based on Superconducting Nanowires Their acronym is SSPDs or SNSPDs With a smaller  a larger number of quasiparticles are created Cooper pair quasiparticles

3. 3 SSPD S AND I N G A A S APD SPEC S AT TELECOM WAVELENGTH (1550 NM ) http://www.nict.go.jp/en/press/2013/11/05-1.html SSPDs can be integrated in Photonic Circuits

4. 4 Superconductors that have demonstrated capability of single- photon detection are compounds or alloys (classified as Type II superconductors): NbN (niobium nitride) NbTiN (niobium titanium nitride) WSi (tungsten silicide) MoSi (molybdenum silicide) ….. Type II superconductors have short ξ and large (magnetic penetration depth) They can be grown as ultra thin films (thickness ~ ξ ~ 4- 5 nm) Coherence length ~ Cooper pair size

5. w=30-100 nm D ETECTOR CONCEPT 5 I B ≤ I C T<<T C T ≥ T C J B ~ J C V ou t t=4-5 nm RNRN V B RnRn R out - + RBRB + - Bias-T NbN meander V out Goltsman APL (2001)

6. Nanowire are made of a Type-II ultrathin (4-5nm) films. Not many chances for photons to be absorbed SSPD S DESIGNED FOR FREE SPACE COUPLING SSPD on top of a /4 optical cavity (SiO 2 ) to improve photon absorption efficiency  SiO 2 Si

7. 7 Marsili Nature Photonics (2013) http://www.nict.go.jp/en/press/20 13/11/05-1.html quarter-wavelength optical stack to enhance 

8. Q UANTUM E FFICIENCY 8 A Figure of Merit: Detection Quantum Efficiency DQE DQE = (Nc − DCR ) / Nph Nc = count/sec DCR = dark count rate Nph = photons/sec Dark Count rate DCR DCR is due to spontaneous transient resistive states observed even in the absence of incident photons.

9. O RIGIN OF D ARK C OUNTS 9 Semenov et al Physica C Supercond (2008) Yamashita APL (2011) In a 2-D Superconductor (thickness ~ξ, width >>ξ, like the nanowire) vortices are present even at H a =0 in form of vortices- antivortices pairs (VAP) to meet the condition net B=0. The bias current I B exerts a Lorentz force F that can overcome the binding of the VAP. (U VAP ) and break them into single vortices that move in opposite directions. Crossing of the unbound vortices create decoherence in the film that can trigger the superconducting to normal transition (false pulse => DCR) Z. Zhou PhD thesis 2014

10. CRYOPHOBIA? extreme fear of cold? C OOLING THE DETECTORS 10 SNSPDs are operated in a Gifford-McMahon (GM) closed-cycle refrigerator Noisy environment due to the presence of the refrigerator compressor? Operating at telecom wavelength 1550nm, low loss optical fibers allow remote photon detection

11. semiconducting substrates Miller et al. Opt. Express (2011) EFFICIENTLY FIBER COUPLED DETECTORS Labs: NIST, JPL Companies: Single Quantum, Quantum Opus IFN IFN implementation Alignment issue: The diameter of the detector is 10 um approx. the same size of the core of single mode fiber.

12. SYSTEM QUANTUM EFFICIENCY 12 ε optical coupling efficiency accounts for losses in the optical fiber system between the (room- temperature) light input point and the (cold) detector System Quantum Efficiency Marsili et al. Nature Photonics (2013) T= 0.12 K λ= 1550 nm SQE= 93% achieved with : High ε using the self- aligned mounting scheme based on Si micromachining High η using WSi films SQE = ε DQE= εαη

13. Q UANTUM P HOTONIC I NTEGRATED C IRCUITS To combine active (single photon sources / detectors) and passive (grating couplers, waveguides, beam splitters, MMI…) elements to achieve important functionalities of Quantum Information Processing Photonics circuits Sources (Quantum dots, split ring resonators) SNSPDs

14. 14 Sprenger et al. APL (2011) NbN on top of GaAs ridge WG Roberto Leoni selected as influential paper from Appl Phys Lett D ESIGN FOR TRAVELLING WAVE COUPLING GaAs case absorption probability α absorption coefficient κ abs (TE)= 452 cm -1 [V/m ]

15. 15 Roberto Leoni fabrication facility: 250 m 2 Cleanroom Electron-beam lithography (100kV, FEG) 30nm Lines K EY E NABLING TECHNOLOGIES FOR THE FABRICATION OF INTEGRATED DETECTORS dc-magnetron sputtering (NbN) Electron beam lithography (HSQ, PMMA, SU 8) Reactive ion etching (NbN, Si3N4, Si)

16. 16 Ti/Au Electric contacts 1 st step. Definition of TiAu contact pads on top of a ultrathin (4-5 nm) sputtered NbN 2 nd step. Definition and etching nanowires 30  m 100 nm 3 rd step. Definition and etching of waveguides aligned (better than ~100nm) to nanowires 4 th step. Definition and etching of the vias openings through HSQ to contact pads

17. Coll CNR and UBRI NbN t=5nm TRAVELLING WAVE COUPLING silicon-on-insulator case 10um WG 500 nm Grating coupler 300nm ribs

18. 18 6  m 1.85  m MMI M ULTIMODE I NTERFERENCE COUPLER The combination of splitting and detection is at the very heart of linear-optics quantum computing MMI coupler allows redistributing light from the N inputs into the M outputs Coll UBRI 150  m

19. MMI AND WSPD OPTICAL CHARACTERIZATION λ = 1510, P=85 pW Gaggero, to be published (2015) -SQEs of detectors D1 and D2, feeding light alternatively to the two inputs, overlap. -the input light equally splits between the two output channels.

20. CONCLUSIONS CONCLUSIONS 20 Laser ranging (LIDAR) Deep space communications Single photon sources characterization Quantum key distribution (QKD) Linear optics quanum computing (LOQC) Thank you for listening - SSPDs in a stand alone multichannel system can be efficiently fiber coupled to room temperature experiments - SSPDs integrated in waveguides represent an important step towards the realization of fully- functional quantum photonic integrated circuits