Centre for Quantum Technologies, Singapore QUANTUM OPTICS Entanglement-based Free Space Quantum Cryptography in Full Daylight, Matthew P. Peloso, Caleb Ho, Antía Lamas-Linares and Christian Kurtsiefer
Physics Department, t NUS Quantum Optics Group Single atom light interaction Single quantum emitters Multiphoton entanglement Experimental Tests of non-locality Quantum Key Distribution IThH3, JWA103
Introduction to QKD Quantum Cryptography = Quantum Key Distribution Have a Random Number at two locations One-time pad for encryption 1980ies: Prepare and Send Bennet Brassard, BB84 Send single Photons, non-cloning theorem 1990ies: Entanglement Based why?
Prepare and Send Need for Random Numbers Different Photons, different Colors? Dimensionality of Hilbert space needs to be known for security
Entanglement based QKD Ekert 1991 (E91) Let s measure two qubits of an entangled entity Perfect (Anti)correlation i Bell s inequality violated = eavesdropper knowledge can be eliminated via PA Photons for convenience Freespace: Ad-hoc set-up
Entanglement based QKD Pairsource: BBO, lean & compact Strong temporally correlated Broader than dimmed d lasers 10 19
Detection of Photons Detection: Polarization analyzer λ/2 22.5 PBS 50:50 APD, +45 PBS V H -45 J.G. Rarity et al., J. Mod. Opt. 41, 2345 (1994)
Setup Alice Sending telescope 350 m Receiving telescope Bob WP BS HW PBS BBO CC PBS PA TU Rb 350 m Rb TU PC Classical Channel PC M. Peloso et al., New J. Phys. 11 045007 (2009)
Problems in Daylight Destruction of detectors Non-linearity of the detector Saturation 0.0 Accidental clicks 0 2 4 6 8 10 6 6 Detecte ed Counts [10 cps s] 2.5 2.0 1.5 1.0 0.5 Ideal Counts [10 cps] Confused with real coincidence events (QBER) Fluctuations ti of ~30dB (day/night) M. Peloso et al., New J. Phys. 11 045007 (2009)
Expected Rates at daylight M. Peloso et al., New J. Phys. 11 045007 (2009)
Sun & Solutions sun focusing focusing interference filter 6.7nm blackout shield baffles coating -12 db -3-4 db to detectors pinhole 100µrad FOV=10cm Δt=1ns M. Peloso et al., New J. Phys. 11 045007 (2009)
Setup Alice Bob M. Peloso et al., New J. Phys. 11 045007 (2009)
Measured rates at daylight Sync possible at daylight M. Peloso et al., New J. Phys. 11 045007 (2009)
Experimental results total detection rate at receiver (kcps) total 'local' detector rate at source (kcps) coincidences (cps) raw key rate (s -1 ) key rate after error correction (s -1 ) accidental coincidence rate r a (cps) QBER (%) M. Peloso et al., New J. Phys. 11 045007 (2009)
Conclusions Entanglement based QKD in daylight, 4 days run without interruption Synchronization works at ambient conditions Noise Suppression Spectral Temporal Spatial Spatial Brighter Sources 70 pairs/(s mw MHz) M. Peloso et al., New J. Phys. 11 045007 (2009)
Thanks for your attention http://qolah.org Interested in Single Molecules? IThH1, Thursday, 2:30pm, Room 315
Rates in Daylight Sifting / Signal rate: Accidentals:
Setup Alice Sending telescope 350 m Receiving telescope Bob WP PH IF BS HW PBS BBO CC PA Shields Baffles PBS TU Rb 350 m Rb TU PC Classical Channel PC
Coincidence Counting events [a a.u.] 700 coincidence window 600 ±45 H/V 500 400 300 200 100 0-3 -2-1 0 1 2 3? t [ns]
Spectral filtering fi ilter tran nsmissio on [%] 1 1 0.8 λ 0 = 811.4 nm λ 0 = 813.11 nm Δλ FWHM = 6.7 nm Δλ FWHM = 8.7 nm 0.8 0.6 0.6 0.4 0.4 0.2 0.2 so ource sp pectrum [a.u.] 0 795 800 805 810 815 820 825 830 0 Wavelength [nm]
Dead time detection event time [ µ s]
Correlation Matrix Daylight Correlation H -45 V +45 H 599 22791 34032 18409-45 18647 2894 17512 44841 V 29062 16422 2125 25246 +45 14635 40558 22280 1498
Entanglement Based QKD Source of entanglement (BBO-Crystal) Time Alice Bob 19:35:01: H V 19:35:01: -45 +45 19:35:01: V H 19:35:01: V H 19:35:01: -45 +45 19:35:01: