Single Semiconductor Nanostructures for Quantum Photonics Applications: A solid-state cavity-qed system with semiconductor quantum dots

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1 The 3 rd GCOE Symposium 2/17-19, 19, 2011 Tohoku University, Sendai, Japan Single Semiconductor Nanostructures for Quantum Photonics Applications: A solid-state cavity-qed system with semiconductor quantum dots Wen-Hao Chang Dept. of Electrophysics,, National Chiao Tung University (NCTU), Taiwan 張文豪

2 Introduction: The 3 rd GCOE Symposium Tohoku University, Sendai, Japan Single Semiconductor Nanostructures for Quantum Photonics Applications: A solid-state cavity-qed system with semiconductor quantum dots Solid-state cavity QED Applications: - single photon sources for quantum cryptography - scalable quantum logic gate systems using photons as flying quantum bits Solid-State State Cavity-QED based on QDs in Micro cavities Weak coupling: Purcell effect, single photon sources Strong coupling: Rabi splitting Stress tuning of cavity-qd coupling Photonic Molecules with tunable coupling

Cavity Quantum Electrodynamics (QED) A central paradigm for the study of open quantum systems Cavity: an optical or microwave resonator QED: the interaction of some material systems (usually atomic)

3 Cavity Quantum Electrodynamics (QED) A central paradigm for the study of open quantum systems Cavity: an optical or microwave resonator QED: the interaction of some material systems (usually atomic) with the electromagnetic field (photons) inside the cavity A. Rauschenbeutel et al., Science 288, 2024 (2000) C. J. Hood, T. W. Lynn, A. C. Doherty, A. S. Parkins, H. J. Kimble, Science 287, 1447 (2000).

4 Single atoms/ions as a single qbit: quantum gate operations can be achieved by optical laser field. Challenge: Scalability of atomic qbit system Nonlocal entanglement is hard to achieve. J. I. Cirac, P. Zoller, H. J. Kimble, and H. Mabuchi, Phys. Rev. Lett. 78, 3221 (1997). Site A: a laser beam transfers quantum information from the internal state of an atom in one cavity to a photon state, which travels along an optical fiber. Nonlocal entanglement can be created among the atoms in the two cavities: Quantum Network Site B: the photon enters a second cavity, and the information is transferred to an atom in that cavity.

Solid-State State Cavity-QED A solid-state analogue of atomic cavity-qed system Cavity: an optical or microwave resonator QED: the interaction of some

5 Solid-State State Cavity-QED A solid-state analogue of atomic cavity-qed system Cavity: an optical or microwave resonator QED: the interaction of some material systems (usually quantum dots) with the electromagnetic field (photons) inside the cavity Micro-pillar Micro-disk Photonic crystal microcavity

Self-Assembled Quantum Dot AFM 300 nm Self-assembled

with the state-of-the-art semiconductor technology

into monolithic nanocavity (VCSEL technology, photonic

6 Self-Assembled Quantum Dot AFM 300 nm Self-assembled QDs: stable solid-state artificial atoms compatible with the state-of-the-art semiconductor technology integration with optoelectronic devices incorporate into monolithic nanocavity (VCSEL technology, photonic crystal nanocavity, microdisk cavity etc.) electrically driven device

Solid-State Quantum Networks excitation Fiber

Cavity-QED 1 Cavity-QED 1 Flying Qbit Flying

Long-range Strong coupling between QDs 1.

Two identical cavity-qed systems with QDs 3.

7 Solid-State Quantum Networks excitation Fiber taper Fiber taper Quantum Dots Quantum Dots Cavity-QED 1 Cavity-QED 1 Flying Qbit Flying Qbit Quantum Dots Quantum Dots Cavity-QED 2 Long-range Strong coupling between QDs 1. Strong QD-cavity coupling in each microcavity 2. Two identical cavity-qed systems with QDs 3. Transfer quantum information faithfully via photons Challenges: 1.High-Q microcavity with very small mode volume 2.Unlike atoms, QDs are not identical 3.QD-cavity spectral matching is nearly impossible without post-fabrication processes. Some tuning scheme must be applied

Single Photon Sources A (quantum) light source that emits one and only one photon at a time, which can be used for quantum cryptography, by encoding the information to be transferred onto each single

8 Single Photon Sources A (quantum) light source that emits one and only one photon at a time, which can be used for quantum cryptography, by encoding the information to be transferred onto each single photon Single and isolated two-level system excited state e hv excitation Single photon emission ( sp ) g ground state g (2) ( ) Photon antibunching Time ( ns ) n 2 =1 Photon extraction problems Incorporating QDs into a monolithic microcavity is a promising way to improve the efficiency.

9 Cavity QED: A Simple Picture Cavity mirror Atom state damping two-level system photon Cavity mirror Cavity photon damping [( ) 2 i( 1 2)] i g Normal mode splitting g At zero detuning (ii) Weak coupling g ( ) i g : Purcell effect Spontaneous decay is modified by the cavity (i) Strong coupling 2 g0 4 2g Rabi splitting : Rabi oscillation

Cavity QED effects Weak coupling regime: Purcell effects leak cav Enhanced emission rate free space cavity Cavity mode Q on-resonance QD Suppressed emission rate off-resonance QD

10 Cavity QED effects Weak coupling regime: Purcell effects leak cav Enhanced emission rate free space cavity Cavity mode Q on-resonance QD Suppressed emission rate off-resonance QD Emission rate ~ optical density of state F ( ) Purcell enhancement factor: 3 3 cav P The spontaneous emission rate becomes controllable, if cavity-qd detuning is controllable. Q V

pattern Single mode coupling efficiency ~92% Purcell

11 Cavity- Single QD Coupling Spontaneous emission dynamics Modified L3 cavity ŷ ẑ xˆ Shallow donor modes Far-field pattern Single mode coupling efficiency ~92% Purcell enhancement : x3 (~x11) W.-H. Chang et al., Phys. Rev. Lett. (2006)

12 Single Photon Emissions Hanbury-brown and Twiss stop start t 1 t 2 Electronics t = t 2 -t 1 n(t ) CW TAC MCA 1.5 Photon antibunching pulse g (2) ( ) Coincident counts Time ( ns ) Time ( ns )

13 Reithmaier et al, Nature (2004) Cavity QED effects Strong coupling regime: Rabi oscillation Yoshie et al, Nature (2004) Hennessy et al, Nature (2007)

14 Microdisk cavity Whispering Gallery Mode (WGM) TE 9,1 D ~ 2 m TE 10,1 TE 8,2 TE 11,1 TE 7,2 3D FDTD w MD cavity w/o cavity Wavelength ( m )

15 SNOM Mapping µm (15,1) V µm (14,1) V µm (13,1) V µm µm µm Intensity(a.u.) MicroPL spectrum (15,1) (14,1) (13,1) µm Ground state V µm Wavelength(nm) SNOM spectral resolution=11nm

16 High-Q Microcavity GaAs Microdisk with embedded InAs QDs 2 m strong coupling between QD and cavity is possible For high-q microcavity, both spectral and spatial matching between the QD and cavity emissions become very difficult.

17 Fiber taper Fiber taper NCTU Photonic molecules Cavity-QED 1 Cavity-QED 1 anti-bonding mode bonding mode Cavity-QED 2 Cavity-QED 2 d=200nm d=250nm Intensity d=300nm d=350nm d=400nm 1 m Fabrication difficulty: keep both high-q and narrow gap d=450nm d=500nm Single MD Wavelength(nm)

18 Photonic molecules A controllable way to fabricate photonic molecule 1. Overetching 2. Transfer to sapphire plate 3. Positioning using a fiber tip

) 1100 1150 1200 1250 1300 Wavelength(nm)

20 Tunable Coupling Heating laser Probe laser Intensity(a.u.) Wavelength(nm) Anti-crossing: Coupled Microcavity Heating laser power Wavelength(nm)

21 Tunable Coupling Energy ( ev ) H. Lin et al., Optics Express (2010)

22 Coherent Energy Transfer Coupled mode equation a 1 i1 1 g a1 a g i a Energy ( ev )

23 Summary Cavity QED Weak coupling: Efficient single photon sources based on QDs in microcavities Strong coupling: A new scheme based on stress tuning of cavity-qd coupling is demonstrated. Photonic molecule: We propose a feasible way to build photonic molecules with very high degree of freedom and yields. Acknowledgement NSC M MY2 NSC M NSC M

24 Group Members

Photonic devices for quantum information processing:

Outline Photonic devices for quantum information processing: coupling to dots, structure design and fabrication Optoelectronics Group, Cavendish Lab Outline Vuckovic s group Noda s group Outline Outline