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Supplementary Information
Fabrication of turnstile device and generation of single photons
Figure 1 shows the experimental configuration for quantum key distribution as
implementation of the BB84 protocol. The quantum dot (QD) is held in a helium
cryostat at 5–10 K and excited by 3-ps laser pulses from a Ti–sapphire laser.
The emission is collected through a pinhole and coupled to a single-mode (sm)
fibre. 50% of the light is lost because of the fibre coupling, and subsequent
connectors reduce the overall transmission efficiency to 30%. The light is
reflected off a grating with 70% efficiency and focused onto a spectrometer slit
for wavelength filtering; the resolution is measured to be less than 0.2 nm. A
polarizing beam-splitter (PBS) and half waveplate are used to set the initial
polarization of the photons. An electro-optic modulator (EOM) with a switching
time of less than 4 ns then rotates this polarization into one of the four states of
the BB84 protocol. Bob’s detection apparatus uses a 50–50 beam-splitter that
sends each photon to one of two polarization analysers. The reflected port
measures the photon in the H/V basis, while the transmitted port measures in
the R/L basis by using an additional quarter waveplate. Silicon avalanche
photodiodes with quantum efficiences of 0.3 and dark count rates of less than
100 Hz are placed at each port of the polarizers. The detector pulses are
recorded by a time-interval analyser (TIA) and stored in a computer. A separate
TIA in Alice’s apparatus records the setting of the data generator.
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Characterization of the single quantum dot photon turnstile device
The parameter g(2) is defined as
 
<fd> g (2) 

aˆ †  t  aˆ †  t '  aˆ  t '  aˆ  t  dtdt '
0 0
 †

  aˆ  t  aˆ  t  dt 
0

2
,
where the photon is located in a time interval [0,∆], and aˆ( t ) is the bosonic annihilation
operator at time t. In order to measure g(2), we combine the signals in detectors 1 and 2
(shown in Figure 1) via an OR gate. The combined signal serves as a start input to a
time interval analyser. The signals in detectors (Det) 3 and 4 are also combined, then
delayed and used as the stop input to the TIA. This configuration serves as an HanburyBrown Twiss (HBT) intensity interferometer, which can be used to measure the second
order correlation. The result of the measurement is shown in Figure 2. The
measurement features a series of peaks separated by the pulse repetition rate of the
turnstile device. The single photon nature of the light emission is demonstrated by the
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suppression of the central peak relative to the other peaks. The value of g(2) is
calculated by normalizing the area of the central peak by the area of a peak
corresponding to a long time delay. In quantum cryptography we must make the worst
case assumption that all coincidences in the 13-ns signal time window could in principle
result in a photon splitting attack. Thus, the central peak is integrated over this 13 ns
time. Comparison of the central peak to a far off side peak gives us a value of
g(2)=0.14.
200
1.92
Counts
150
1.93
1.38
1.37
1.18
1.18
100
50
0.14
0
-40
-20
0
20
Time Delay (ns)
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Figure 2: Result of second order correlation using HBT intensity interferometer. The area of the central
peak compared to a far-off peak gives us the value of g(2)=0.14.