Quantum Optics with single Nano-Objects

Outline: Introduction : nonlinear optics with single molecule Single Photon sources Photon antibunching in single quantum dot fluorescence Conclusion

ADVANTAGES No ensemble averaging Statistical correlations Extreme sensitivity to immediate local nanoenvironment No synchronisation needed, time evolution Single quantum system

Inhomogenous spectrum at 2 K Laser detuning (MHz) Fluorescence Saturation of a SM line Laser detuning (GHz) Fluorescence  hom = 20 MHz I ~ 250I s = 620 nm

- Light Shift - Hyper-Raman Resonance Pump-Probe Detuning (MHz) Fluorescence (a. u.) pp ss pp pp 00 ss Pump-probe experiments:

Light shift (  )  = 2.4   = 4.7  Inverse pump detuning (  -1 )

Electro-Optical Effects Linear Stark Shift:  ~ 0.3 D - 1 st case: High RF field, low laser intensity Modulation of the molecular transition frequency Side Bands Modulation of the laser frequency Linear Coupling with the RF field E 0RF cos(  RF t)

 RF = 80 MHz   

2 nd case: High laser intensity, low RF field - Molecule dressed by the laser field - RF coupling between 1,n> and |2,n> states LL 0 LL |g,n+1> |e,n> |1,n> |2,n> LL - Rabi Resonance:  RF =  G fixed  RF, tuned  L

Shift of the Rabi resonance function of the laser field amplitude  L =1.5   L =3.5 

Quantum Cryptography: * Quantum mechanics provide unconditional security for communication * Encoding information on the polarization of single photons Quantum computing: Quantum logic gates based on single photons have been demonstrated Triggered Single Photon Sources Practical need for Single Photons:

Present day sources of single photons - Correlated photon pairs * Atomic cascade * Parametric down conversion - Highly attenuated laser (or LED) pulses Problems: - Random time generation - Average photon’s number <<1 Present day sources of single photons

Use of a single quantum system Yamamoto’s group experiment: - Coulomb Blockade in a mesoscopic double barrier p-n junction - temperature 50 mK (dilution cryostat) - low detection efficiency ~ low e-h recombination rate

Controlled excitation of a single molecule Deterministic generation of single photons Photon antibunching in single molecule fluorescence

Excitation: Rapid adiabatic passage Two conditions: Adiabatic passage T pass > T Rabi Rapid passage T pass <<  f  L >>  |1,n-2> |1,n> |1,n-1> |2,n> |2,n-1> |2,n-2> |e,n> |g,n+1> |g,n> |g,n-1> |e,n-1> |e,n-2> |e,n> |g,n+1> |g,n> |g,n-1> |e,n-1> |e,n-2> +0+0 -0-0 0

Time (ns) Fluorescence Voltage (V) Detuning (  ) RF = 3 MHz,  L = 2.6 

- T = 250 ns,  = 3 ,  0 = 80  - Short rise time - Relaxation time (  -1  8 ns) - Oscillations Detailed shape of a fluorescence burst

How many emitted photons per sweep? Measurement of the autocorrelation function g (2) (  ) Comparaison with Q.M.C. simulations

Histogram of time delays: RF = 3MHz,  L = 3.2  Number of Coincidences Correlation Function (u. a.) Delay (ns)  0 = 25   0 = 44   0 = 65 

- = 6 MHz,  0 = 44  - n av = p(1) = p(n>1) ~ Mandel Parameter Q sour. = Q detc. = Comparaison with a Coherent source

Room Temperature Single photon source Principe de l’expérience - Pulsed excitation to a vibrationnaly excite level - Rapid Relaxation (ps) to the fluorescent state - Emission of a single photon Experimental setup - Inverted Microscope - Piezo-electric Scanner - Coincidence Setup - Detection effeciency 6% T.A.C. & P.H.A. Stop Start Fluorescence Excitation (532nm)

System: Terrylene in p-terphenyl Favorable photophysical parameters and high photostability Confocal fluorescence image(10  m*10  m) of single Terrylene molecules

CW Excitation : Photon Antibunching 20  W 210  W  W 670  W 1600  W Fluorescence autocorrelation function g (2) (  ) proportional to the excited state population,  >0: Signature of a single molecule emission

Pulsed excitation : Triggered single photon emission - Single exponential decay - fluorescence lifetime:  f = 3.8 ns - M.L. 532 nm laser: - Pulse width: 35ps, - Repetition rate: = 6.25 MHz

Fluorescence autocorrelation Function Laser spot positioned on a single molecule (Signal/Background~ 6) (b) Away from any molecule (background coherent emission ) central peak area / lateral peak area = (B 2 +2BS) / (B+S) 2 ~ 0.27

Saturation du taux d’émission Fit: S  =343 kHz, I s =1.2 MW/cm 2 86% of the pulses lead to a single photon emission At the maximum power : S max =310 kHz, p max (1)=0.86 Short laser pulse   p <<  f p(2) ~ 0 p(1) =  (  =  p ) Emission rate: S=   p(1)= S   (  p )

- n av = p(1) = = 6.25 MHz - Mandel Parameter Q sour. = Q detc. = P(n>1) < ! Comparaison with a coherent source

Photon statistics of single quantum dot fluorescence CdSe ZnS - Colloidal CdSe/ZnS quantum dots - 2 nm Radius, 575 peak emission - fluorescence quantum yield ~50%,  ~ 10 5 M -1 cm -1 - QDs bridge the gap between single molecules and bulk solid state - Size-dependent optical properties - Tunable absorbers and emitters - Applications from labeling to nano-devices

Intermittence in single QD Fluorescence - Blinking attributed to Auger ionization - High S/B ratio - Low photobleaching rate  blea < Blinking: t on, Intensity dependence t off, no I dependence, inverse power law

Photon antibunching in single QD fluorescence - Start-Stop setup - Coincidence histogram C(  ) (TAC time window t TAC of 200 ns, bin width t bin of 0.2 ns) Dip at  =0, signature of a strong photon antibunching C(0) ~ 0 for a large range of intensities (0.1 – 100 kW/cm 2 ) High Auger ionization rates (~1/20 ps -1, Klimov et al.) No multi-excitonic radiative recombination

Quantum dot lifetimes measurements Bulk measurement with TCSPC (M. Dahan et al., 2000) Multi-exponential decay - Experimental accuracy ~ ns - Width of  f histogram : heterogenety in the QDs structure !? Single QDs measurements at low intensities

Saturation intensities: I sat ~ kW/cm 2 Cross-section :  abs ~ 2 – cm 2 From the QD state filling equations

Count rate saturation - At high intensities, very short t on, average count rate R av skewed - Use coincidence histogram for accurate value R av in the On state - With large t TAC and high R av  ~ interphoton mean time (1/ R av ) Good agreemeny For the measured I sat

Conclusion -Demonstration of a single photon source based on controlled fluorescence from single molecule -Room temperature operation - Improve the collection efficiency de collection (cavity…) -Other systems: * NV centers (antibunching observed) * Quantum dots : at low T (<5K) antibunching in spectrally selected fluorescence from InAs QDs -Photon antibunching in colloidal CdSe QDs Efficient Auger ionization effect