Quantum Dot SinglePhoton Source: Prospects for Applications in Quantum Information Processing

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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
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Requirements for linear optics quantum
computation (LOQC)

• Linear optical elements beam-splitters,
  polarizers, lenses
• optical delay/memory
• Single-photon sources indistinguishable
  single-photon pulses
• on demand (with efficiency gt 99)
• Photon counters high-efficiency
  detectors with
• single-photon discrimination
• ? Appears to avoid the very demanding requirement
  for large (coherent) photon-photon interactions.

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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)
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What is the signature of a single-photon source?
Intensity (photon) correlation function
? gives the likelihood of a second photon
detection event at time tt, given an initial one
at time t (t0).
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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 tt, given an initial one
at time t (t0).
Records the waiting-time between the successive
photon-detection events at the two detectors
(APD).
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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 tt, given an initial one
at time t (t0).
Triggered single photon source absence of a
peak at t0 indicates that none of the pulses
contain more than 1 photon.
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Quantum Dots

• 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).

DEatom 110 eV gtgt kTroom 26 meV DEQD 1100
meV kTroom !
Unlike atoms, QDs are sensitive to thermal
fluctuations at room temp.
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Quantum Dots vs. Atoms

• 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.

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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 lt 77 K!

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A single InAs Quantum Dot

• 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
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Photon correlation of a single-photon source

• all peaks in G(2)(t) have the same intensity
• pulsed coherent light

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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 t0 disappears.
• single photon turnstile device with
• at most one photon per pulse

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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
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Microdisk Cavities
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 ! Qgt18000
for 4.5mm diameter microdisk Q11000 for 2mm
diameter microdisk
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A single quantum dot embedded in a microdisk
Larger width of the peaks due to longer lifetime
of the quantum dot
P20W/cm2 T4K
Q 6500
Pump power well above saturation level
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Tuning the exciton into resonance with a cavity
mode
Cavity coupling can provide better collection
T44K

• Small peak appears at t0
• Peaks in G(2)(t) are narrower
• ?Purcell effect ?

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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.

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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 gt 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 gt Gcav gdep ? Purcell
effect is essential for applications in linear
optics quantum computation.
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Linear optics quantum computation (LOQC)
Linear optics quantum computation (LOQC)

• Key step is two-photon interference on a
  beam-splitter

youtgt 20gt 02gt ? g34(2)(t0) 0
E3
E1
yingt 11gt
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)(t0) lt 0.01
• Santori et al. observed g34(2)(t0) 0.3 using
  resonant excitation

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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 gives h ? 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
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Photon counting using stored light

• 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 gt 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 gt 99.

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Storing light using electromagnetically induced
transparency (EIT)
coupling laser
signal pulse
F2, mF2
F1, mF1
of atoms in state F2,mF2gt of initial
signal photons
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Measuring photon number using EIT