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Quantum Dot Single-Photon Source: Prospects for Applications in Quantum Information Processing

Quantum Dot Single-Photon Source: Prospects for Applications in Quantum Information Processing. A. I mamo g lu Department of Electrical and Computer Engineering, and Department of Physics, University of California, Santa Barbara, CA 93106. Outline 1) Quantum dots

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Quantum Dot Single-Photon Source: Prospects for Applications in Quantum Information Processing

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  1. Quantum Dot Single-Photon Source: Prospects for Applications in Quantum Information Processing A. Imamoglu Department of Electrical and Computer Engineering, and Department of Physics, University of California, Santa Barbara, CA 93106 Outline 1) Quantum dots 2) Properties of quantum dot single photon sources 3) High efficiency photon counters Co-workers A. Kiraz, J. Urayama, B. Gayral, C. Becher, P. Michler, C. Reese, L. Zhang, E. Hu W.Schoenfeld, B. Gerardot, P. Petroff

  2. Linear optical elements: beam-splitters, polarizers, lenses optical delay/memory Single-photon sources: indistinguishable single-photon pulses on demand (with efficiency > 99%) Photon counters: high-efficiency detectors with single-photon discrimination  Appears to avoid the very demanding requirement for large (coherent) photon-photon interactions. Requirements for linear optics quantum computation (LOQC)

  3. Single Photon Sources Single Photon Sources A regulated sequence of optical pulses that contain one-and-only-one photon Single atom in a cavity: Rempe et al. PRL (2002) Single nitrogen vacancy in diamond: H. Weinfurter et al. PRL (2000) P. Grangier et al. PRL (2002) Single Molecule at room temperature: B. Lounis and W.E. Moerner, Nature (2000) Single InAs Quantum Dot in a microcavity: P. Michler et al., Science 290, 2282 (2000) C. Santori et al., PRL 86, 1502 (2001) Z. Yuan et al., Science 295, 102 (2002)

  4. What is the signature of a single-photon source? • Intensity (photon) correlation function:  gives the likelihood of a second photon detection event at time t+t, given an initial one at time t (t=0).

  5. What is the signature of a single-photon source? • Intensity (photon) correlation function: • Experimental set-up for photon correlation [g(2)(t)] measurement:  gives the likelihood of a second photon detection event at time t+t, given an initial one at time t (t=0). Records the waiting-time between the successive photon-detection events at the two detectors (APD).

  6. g(2)(t) t 0 Signature of a triggered single-photon source Signature of a triggered single-photon source • Intensity (photon) correlation function:  gives the likelihood of a second photon detection event at time t+t, given an initial one at time t (t=0). • Triggered single photon source: absenceof a peak at t=0 indicatesthat none of the pulses contain more than 1 photon.

  7. Artificial structures that confine electrons (and holes) in all 3 dimensions. Atoms Quantum dots (QD) Quantized (discrete) eigenstates in both cases ( 0D density of states). DEQD VQD (x) Vatom (x) DEatom  20 - 500 Å  1 Å Quantum Dots DEatom~ 1–10 eV >> kTroom = 26 meV DEQD ~ 1–100 meV ~ kTroom ! Unlike atoms, QDs are sensitive to thermal fluctuations at room temp.

  8. Strongly trapped emitters: QDs do not have random thermal motion. Easy integration in nano-cavity structures. Strong coupling to optical fields: QD oscillator strength f ~ 10 – 300 (collective enhancement). Electrical injection of carriers (electrons and holes). Each QD has a different resonance (exciton) energy. Difficult to tune QDs into resonance with cavity modes. Quantum Dots vs. Atoms

  9. AFM of InAs QDs 2 μm 2 μm Self-Assembled InAs Quantum Dots Quantum dots appear spontaneously due to lattice mismatch, during MBE growth. Each quantum dot is different • Atom-like characteristics of Quantum Dots: • sharp emission lines • photon antibunching •  artificial atom for T < 77 K!

  10. A single InAs Quantum Dot phonon emission - - - non-resonant laser excitation exciton emission (1X) + + + GaAs GaAs InAs • Two principal emission lines from lowest energy s shell • 1X radiative recombination of a single e-h pair in the s-shell (exciton) • 2X radiative recombination when there are two e-h pairs in the s-shell (biexciton) Due to carrier-carrier interaction Typically h1X = h2X + 3 meV

  11. Photon correlation of a single-photon source • all peaks in G(2)(t) have the same intensity • pulsed coherent light

  12. Photon correlation of a single-photon source Photon correlation of a single-photon source Pump power well above saturation level • all peaks in G(2)(t) have the same intensity • pulsed coherent light • the peak at t=0 disappears. • single photon turnstile device with at most one photon per pulse

  13. Turnstile Device at Different Pump Powers Turnstile Device at Different Pump Powers well above saturation onset of saturation well below saturation Lower pumping power has the same effect as loss in the optical path

  14. Microdisk Cavities Q>18000 GaAs GaAs GaAs GaAs AlGaAs AlGaAs GaAs GaAs substrate substrate Photoluminescence from a high-QD density sample Fundamental whispering gallery modes cover a ring with width ~ l/2n on the microdisk No roughness on the sidewall up to 1nm ! Q>18000 for 4.5mm diameter microdisk Q=11000 for 2mm diameter microdisk

  15. A single quantum dot embedded in a microdisk Larger width of the peaks due to longer lifetime of the quantum dot P=20W/cm2 T=4K Q = 6500 Pump power well above saturation level

  16. Tuning the exciton into resonance with a cavity mode Cavity coupling can provide better collection T=44K • Small peak appears at t=0 • Peaks in G(2)(t) are narrower: • Purcell effect ?

  17. Quantum dot lifetime measurement • Time-correlated single-photon counting experiments show no temperature dependence for exciton lifetime. • First direct measurement of Purcell effect (FP 2) for a single quantum dot.

  18. Purcell Effect: cavity-induced decay • When an emitter is placed inside a high-Q, low volume cavity, there are two channels for radiative decay: • i) spontaneous emission into vacuum modes (Gspon) • ii) irreversible emission into the cavity mode (g2/ Gcav) – scales as Q/Veff •  Purcell effect:g2/ Gcav > Gspon Purcell effect in a single photon source i) Fast emission  reduced jitter in photon emission time. ii) Emission predominantly into a single cavity mode  high collection efficiency. iii) Reduced sensitivity to dephasing  Transform limited (indistinguishable) photons in the good-cavity limit:g2 > Gcavgdep  Purcell effect is essential for applications in linear optics quantum computation.

  19. Linear optics quantum computation (LOQC) Linear optics quantum computation (LOQC) • Key step is two-photon interference on a beam-splitter: |yout> = |20> + |02>  g34(2)(t=0) = 0 E1 E3 |yin> = |11> No coincidence detection for indistinguishable photons E2 E4 • Requires that the two incident photons have the same spatio-temporal profile: single photon pulses have to be transform-limited . For LOQC we need (?) g34(2)(t=0) < 0.01 [ Santori et al. observed g34(2)(t=0) = 0.3 using resonant excitation]

  20. Can we use QD single-photon source in LOQC? Can we use QD single-photon source in LOQC? • High single-photon collection efficiency (h ~ 44%) has been demonstrated using the Purcell effect (Gerard et al., Pelton et al.): •  FP ~ 10 givesh 90% and photon emission time tsp ~ 100 psec. • Even under resonant p-shell excitation, we have jitter in photon emission time ~ 10 psec: •  Coincidence count-rate in two-photon • interference will be ~ 10%, since • information about the single-photon • pulse can be obtained from the • emitted phonon(s). •  The requirements for high collection efficiency and complete indistinguishability are incompatible (even in the good-cavity limit)! phonon emission resonant laser excitation

  21. It is possible to map the quantum state of a propagating light pulse onto metastable collective excitations of an atomic gas, using electromagnetically induced transparency (EIT).  # of incoming photons = # of atoms in the (hyperfine) excited state. State-selective fluorescence measurements (developed for trapped ions) can achieve efficiencies > 99% in measuring the number of atoms/ions in a given state – without requiring high efficiency photon detection.  By combining these two techniques, we could realize a photon counter with efficiency > 99%. Photon counting using stored light

  22. i) coupling laser on EIT medium signal pulse ii) coupling laser on; signal pulse inside the medium signal pulse iii) coupling laser off Stored signal pulse (dark polariton) Storing light using electromagnetically induced transparency (EIT) coupling laser signal pulse F=2, mF=2 F=1, mF=1 # of atoms in state |F=2,mF=2> = # of initial signal photons

  23. F=3, mF=3 scattered photons detection laser EIT medium stored singal pulse (dark polariton) detection laser F=2, mF=2 F=1, mF=1 Measuring photon number using EIT

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