Single Photon Generation & Application in Quantum Cryptography

Transcription

1 Single Photon Generation & Application in Quantum Cryptography Single Photon Sources Photon Cascades Quantum Cryptography

2 Single Photon Sources Methods to Generate Single Photons on Demand Spontaneous emission (single emitters) atoms, molecules, quantum dots, defect centers optical, electrical and STIRAP excitation pulsed, (non-resonant) excitation relaxation decay spontaneous single photon emission M: Brunel et al., PRL 83, 2722 (1999) Lounis & Moerner, Nature 47, 491 (2) DC: Kurtsiefer et al., PRL 85, 29 (2) Beveratos et al., PRA 64, 6182(R) (21) QD: Kim et al., Nature 397, 5 (1999) Michler et al., Science 29, 2282 (2) Santori et al., PRL 86, 152 (21) Yuan et al., Science 295, 12 (22)

3 Single Photon Sources Quantum Dots Transmission electron microscope images [11] Atomic force microscope image AFM 1nm [11] 1nm Contains ~1 atoms InP dots grown on GaInP K. Georgsson et al., Appl. Phys. Lett. 67, 2981 (1995)

4 Energy Single Photon Sources Quantum Dots Exciton in a QD: exciton Biexciton: biexciton GaInP InP GaInP n=2 n=1 Biexciton Exciton n=1 n=2 Size: O(1 nm) Ground state (empty QD) Photoluminescence of an ensemble of InAs quantum dots Photoluminescence image of a set of InP quantum dots

5 Single Photon Sources Quantum Dots Specific advantages of single quantum dots Stability Compatible with chip-technology Wide spectral range Electrical Pumping High repetition rate Strong interactions available Specific disadvantages of single quantum dots Low temperature operation Non-uniformity Device production yield Decoherence Efficiency AFM

6 Single Photon Sources Experimental Setup Liquid He Cryostat (4 K) Laser (cw orpulsed) Spectrograph Stop- APD Coincidence counter Filter CCD Dichroic mirror Sample Michelson interferometer Start- APD Hanbury Brown-Twiss correlator SPS g1 g2

7 Single Photon Sources Experimental Setup

8 Single Photon Sources InP Quantum Dots in GaInP Emission around 69 nm maximum detection efficiency of Si detectors) Lifetime around 1 ns Dot density: 1 8 cm -2 through 2 nm bandpass filter Linewidth around 1 µev GaInP (4 nm) InP QDs Al mirror (2 nm) Epoxy Si substrate Intensity (a.u.) with 2-nm bandpass filter without filtering Wavelength (nm)

9 Single Photon Sources Intensity Correlation Measurements Number of coincidences (raw data, a.u.) cw Delay time (ns) Central peak vanishes nearly completely generation of only one photon per pulse Single photon generation observed up to 4 K Number of coincidences (raw data, a.u.) 5 Number of coincidences (arb. units) ZOOM -1 1 Delay time (ns) pulsed Delay time (ns) V. Zwiller, et al., Appl. Phys. Lett. 82, 159 (23)

10 Single Photon Sources Wave and Particle Aspects Stop- APD Coincidence counter Start- APD Correlations (arb. units) Delay time (ns) Pulse counter (DAC) Taylor-experiment (196) T. Aichele, et al., AIP proc. Vol. 75, 35 (25) V. Jacques, et al. Eur. Phys. J. D 35, 561 (25) J. T. Höffges, et al. Opt. Comm., 133, (1997) Number of counts per 1 ms Time after Path starting length the difference measurement (s)

11 Energy Photon Cascades Cascaded Emission Exciton in a QD: exciton GaInP InP GaInP n=2 n=1 Biexciton: biexciton Intensity / a.u Exciton Triexciton Biexciton n=1 n=2 25 Size: O(1 nm) Wavelength / nm biexciton exciton σ+ σ- Different energy of exciton, biexciton, triexciton,... due to Coulomb interaction σ σ+ ground state (empty QD)

12 Photon Cascades Cascaded Emission Spectra and anti-bunching in photon cascades: Intensity / a.u Exciton Wavelength / nm Triexciton Biexciton Correlations / a.u t / ns Triexciton Biexciton Exciton

13 Photon Cascades Cascaded Emission exc. Stop- APD biexc. Coincidence counter Start- APD Coincidences, a.u triexciton biexciton Delay time (ns) Correlation measurements reveal dynamics of multiphoton cascades J. Persson et al., Phys. Rev. B 69, (24) D. V. Regelmann, et al. Phys. Rev. Lett. 87, (21) E. Moreau et al., Phys. Rev. Lett. 87, (21) A. Kiraz et al. Phys. Rev. B 65, (22) Coincidences (a.u.) 1 5 biexciton exciton Delay time (ns)

14 Photon Cascades Single Photon Multiplexing Separating spectral lines using a Michelson interferometer Coincidences (a. u.) ns Delay time (ns) Delay= 1/2 Rep.Rate One quantum emitter acts as two independent single photon sources. Delaying the two photons by half the excitation repetition time doubles the photon rate.

15 Quantum Cryptography The BB84 Protocol Eve Eve cannot copy the photon (no cloning theorem) Alice 1 Bob 1 Quantum channel Classical public channel: Bennett, Brassard, Proc. IEEE Int. Conf. on Computers, Systems & Signal Processing (1984), First realization with QDs: Waks et al., Nature 42, 762 (22)

16 Quantum Cryptography BB84 Protocol Alice sends randomly polarized photons (, 45, 9 or 135 ) to Bob. Bob randomly measures in the straight or diagonal base. Bob keeps his results secret. Bob publically tells his measurement bases (not the results!). Alice publically tells him if he chose the right base. Alice and Bob keep only the results with the common bases. They both have now a common and random key:

17 FromSinglePhoSource FromSinglePhoSource Michelson w1 ton w1 w2 ton Michelson ALICE EOM Michel Quantu son mchanel Miche lson Analyz ALICE w2 EOM MUX DEM UX Delay Michelson Quantu EOM mchanel EOM EOM EOMAnaly Quantum Cryptography Multiplexed Quantum Cryptography From Single Photon Source w 1 Quantum Channel EOM Michelson Michelson Michelson EOM w 2 ALICE MUX DEMUX Analyzer EOM Detection EOM Detection BOB From Single Photon Source w 1 Michelson Delay Michelson EOM Quantum Channel Analyzer EOM Detection w 2 ALICE BOB

18 Quantum Cryptography Multiplexed Quantum Cryptography From single photon source Polarizer Analyzer EOM EOM APD Alice Bob Alice s original data Encoded image Bob s decoded image Transmission to Bob: 3 successfull counts/s at a laser modulation of 2 khz Similarity between Alice s and Bob s keys: 95% T. Aichele, G. Reinaudi, O. Benson, Phys. Rev. B, 7, (24)