The microscope used to image the quantum dot microcavity samples. The cryostat is fixed below a movable baseplate containing the imaging optics. The white light source and camera are used to image the sample prior to measurements. Optical excitation of the sample is provided by the 784 nm wavelength laser diode which is reflected by a BK7 plate glass beamsplitter and a gold-faced mirror to the sample via a x50 microscope objective with a numerical aperture (NA) of 0.42. The photons emitted by the dot in the microcavity are collected into a core diameter optical fiber which can be connected to additional characterization experiments [e.g., for measurements] or to the polarization modulator for QKD. This system coupled of the photons emitted from the microcavity into the acceptance cone angle of the microscope objective into the core diameter fiber.
A normalized spectrum obtained from the quantum dot microcavity single-photon source at an excitation power of 1 μW (measured at the cryostat window) which gives a of 0.38, corresponding to a photon emission rate from the quantum dot microcavity (after correction for detection efficiency) of 480 kHz. The inset shows a subset of the same spectrum before spectral filtering (dark gray) and after (light gray). These measurements were performed using a liquid nitrogen cooled front illuminated CCD connected to a 0.5 m imaging triple grating monochromator. The background level of is due to the dark noise level of the CCD which was approximately 750 counts/s per pixel, significantly higher than the 300 counts/s dark count rate of the Si-SPADs used to measure the autocorrelation value and QKD bit-rates.
The photon flux exiting Alice against the excitation power measured at the cryostat window. The right-hand axis indicates the photon flux emitted from the microcavity into the acceptance cone angle of the microscope objective, assuming 40% detection efficiency for the Si-SPADs and an coupling efficiency in the microscope. The values specified next to the data points denote the value of the second order autocorrelation function at these excitation powers.
A normalized TRPL trace of the quantum dot at an excitation power of 1 μW, which gives a of 0.39 (gray line) and a normalized instrumental response (dark gray line). The photon emission rate of the microcavity, after correction for detection efficiency, was 480 kHz. An iterative reconvolution technique (Ref. 26) was used to measure the primary PL lifetime of this emission as 464 ps. The inset shows the same quantum dot TRPL result over a long timescale, clearly showing the long decay tail.
A schematic diagram of the short wavelength QKD test-bed. The box labeled “single-photon source” contains the custom microscope. The static polarization controllers (denoted by SPC) in the figure operate through mechanically induced birefringence to align the polarization states with the transmission or reflection axis of the polarization dependent beamsplitters (PBS). The silicon single-photon avalanche diodes (Si-SPAD) detect the incident photons and generate an electrical signal for use with the time-stamping electronics. The gray crosses denote the points at which the core diameter fiber in Alice and Bob is spliced to the core diameter standard telecommunications fiber which comprises the quantum channel.
The QBER, expressed as a percentage, against excitation power measured at the cryostat window for the quantum dot microcavity single-photon source in a BB84 QKD system. The black triangles ▲ denote a transmission distance of 0 km while the gray circles ● denote a transmission distance of 2 km. The values specified next to the data points denote the value of the second order autocorrelation function at these excitation powers.
The filled points denote the net bit-rate against excitation power for the quantum dot microcavity single-photon source in a BB84 QKD system at distances of 0 and 2 km analyzed using the Cascade error correction protocol. (Ref. 33) The black triangles ▲ denote a transmission distance of 0 km while the gray circles ● denote a transmission distance of 2 km. The unfilled points denote the transmission conditions which were found to be secure against the PNS attack (Ref. 9) when analyzed using the GLLP (Ref. 36) technique with triangles △ denoting a transmission distance of 0 km and circles ○ a transmission distance of 2 km.
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