Particle like

True single photon sources

Wave-like during propagation Particle like V+ Single atom or ion (in a trap) Single dye molecule Single colour centre (diamond NV) Single quantum dot (eg InAs in GaAs) www.bris.ac.uk

Quantum cryptography and linear-optics quantum logic needs single-photon sources with

 High generation rate    High efficiency emission into single mode Small multi-photon probability ( g (2) (0)  0 ) Quantum indistinguishability (time-bandwidth limited, single mode, one polarization, etc. )

Possible solution self-assembled InAs/GaAs QDs

 Good approximation to two level atom    No bleaching effect and long-term stability High generation rate (exciton lifetime~1ns) Embedded in microcavity by in-situ growth    Standard semiconductor processing Solid-state source low extraction efficiency (~2%) (GaAs n=3.5)

Cavity effects

Single photon generation of QDs in 3D microcavity can be improved by Cavity Quantum Electrodynamics (CQED) see lecture 2  Enhance spontaneous emission (Purcell effect)  Improve both coupling and extraction efficiency  Couple to a single cavity mode  Toward time-bandwidth limited photon pulse lifetime T 1 << dephasing time T 2



Samples

 cavity between two GaAs/Al 0.9

Ga 0.1

As DBRs, bottom 27 pairs, top 20 or 14 pairs  One layer self-assembled InGaAs QDs at cavity center

FDTD modelling

circular pillar

ICP etching

elliptical pillar FIB etching

0.015

0.010

0.005

0.000

-0.005

-0.010

100 90 80 70 60 50 40 30

FDTD simulations:

Efficiency 0.50μm radius micropillar microcavity: Plane wave resonance=1001 nm 15 mirror pairs on top and 30 bottom 12000 Q-factor 10500

-0.015

0

20

2000000

10 0 12

4000000 6000000 Time(ps)

13

8000000 10000000

14 15 16 Number of top 17 18 mirror pairs (

N t

) 19 20 9000 7500 6000 4500 3000 1500 0

Experimental Setup

Time Interval Analyser Hanbury-Brown Twiss measurement

g

( 2 ) (  )  

n

(

t

)

n

(

t

  )  

n

 2 ~

p

(

t

:

t

  )

p

(

t

)

3  m  3  m FIB 962 963 964 965 966 Wavelength (nm) 50K 48K 46K 44K 42K 40K 38K 36K 34K 32K 30K 28K 26K 24K 22K 20K 15K 10K 7K 4.3K

967 968 1287 1286 1285 1284 1283 QD1 QD2 Cavity mode 1282 0 5 10 15 20 25 30 35 40 45 50 55 Temperature (K)  Single QD emission can be observed in smaller pillar at low excitation power Oct18h4.opj Graph1  QD emission line shifts faster than cavity mode

Single photon generation in circular pillars With increasing excitation power  QD emission intensity turns saturated  Cavity mode intensity develops

Single photon generation in circular pillars

 g (2) photon (0)=0.05

emission indicates is 20 suppressed.

multi times  g (2) ( 0 ) increases with pump power due to the cavity mode

g b

( 2 ) (  )   2 

g

( 2 ) (  )  1   1  

I signal



I I signal



cavity I background g b

( 2 ) ( 0 )  1   2

• • Time bandwidth limited single photons on demand from pillar microcavities 7, 129-136 (2005) .

Beyond this:

(Journal of Optics B Entangled photon pairs from Biexciton-Exciton cascaded emission (see

,

Nature 439, 179-182 ).

Biexciton State

• • • • Linear gates mixing single photons/ entangled states ‘

bright’ m=±1

Entangled solid state/atomic qubits after emission (Knight, Cirac).

‘dark’ m=±2 Ground State

• Strong coupling, evidence of spectral splitting seen Long term Inter-conversion to/from solid state V eff ~(λ/n) 3 non-linear gates.

Exciton States

Co-workers • Universiy of Bristol: Daniel Ho, J. Fulconis, J. Duligall, C. Hu, R. Gibson, O.Alibart, J. O’Brien

Fibres

• University of Bath: W. Wadsworth, P. Russell

Quantum dots and microcavities

• University of Sheffield: M. Skolnick, D. Whittaker, M Fox • Toshiba Europe: A. Shields, A. Bennett • Univ Cambridge: D. Ritchie Linear optics RAMBOQ IST38864 : Vienna, LMU, HP Bristol, Geneva, Erlangen, Queensland, Toshiba, Cambridge, Thales, MPQ, IdQ, ….