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ADC Nonlinearity Correction for the Majorana Demonstrator
Imperfections in analog-to-digital conversion (ADC) cannot be ignored when signal digitization requirements demand both wide dynamic range and high resolution, as is the case for the Majorana Demonstrator 76Ge neutrinoless double-beta decay search. Enabling the experiment's high-resolution spectral analysis and efficient pulse shape discrimination required careful measurement and correction of ADC nonlinearities. A simple measurement protocol was developed that did not require sophisticated equipment or lengthy data-taking campaigns. A slope-dependent hysteresis was observed and characterized. A correction applied to digitized waveforms prior to signal processing reduced the differential and integral nonlinearities by an order of magnitude, eliminating these as dominant contributions to the systematic energy uncertainty at the double-beta decay Q value
Spectral broadening and shaping of nanosecond pulses: towards shaping of single photons from quantum emitters
We experimentally demonstrate spectral broadening and shaping of
exponentially-decaying nanosecond pulses via nonlinear mixing with a
phase-modulated pump in a periodically-poled lithium niobate (PPLN) waveguide.
A strong, 1550~nm pulse is imprinted with a temporal phase and used to
upconvert a weak 980 nm pulse to 600 nm while simultaneously broadening the
spectrum to that of a Lorentzian pulse up to 10 times shorter. While the
current experimental demonstration is for spectral shaping, we also provide a
numerical study showing the feasibility of subsequent spectral phase correction
to achieve temporal compression and re-shaping of a 1~ns mono-exponentially
decaying pulse to a 250 ps Lorentzian, which would constitute a complete
spectro-temporal waveform shaping protocol. This method, which uses quantum
frequency conversion in PPLN with >100:1 signal-to-noise ratio, is compatible
with single photon states of light.Comment: 4 pages, 4 figure
Security Proof for Variable-Length Quantum Key Distribution
We present a security proof for variable-length QKD in the Renner framework
against IID collective attacks. Our proof can be lifted to coherent attacks
using the postselection technique. Our first main result is a theorem to
convert a series of security proofs for fixed-length protocols satisfying
certain conditions to a security proof for a variable-length protocol. This
conversion requires no new calculations, does not require any changes to the
final key lengths or the amount of error-correction information, and at most
doubles the security parameter. Our second main result is the description and
security proof of a more general class of variable-length QKD protocols, which
does not require characterizing the honest behaviour of the channel connecting
the users before the execution of the QKD protocol. Instead, these protocols
adaptively determine the length of the final key, and the amount of information
to be used for error-correction, based upon the observations made during the
protocol. We apply these results to the qubit BB84 protocol, and show that
variable-length implementations lead to higher expected key rates than the
fixed-length implementations.Comment: Fixed typos, updated terminology for variable-length protocols,
updated Lemma
Performance of various quantum key distribution systems using 1.55 um up-conversion single-photon detectors
We compare the performance of various quantum key distribution (QKD) systems
using a novel single-photon detector, which combines frequency up-conversion in
a periodically poled lithium niobate (PPLN) waveguide and a silicon avalanche
photodiode (APD). The comparison is based on the secure communication rate as a
function of distance for three QKD protocols: the Bennett-Brassard 1984 (BB84),
the Bennett, Brassard, and Mermin 1992 (BBM92), and the coherent differential
phase shift keying (DPSK). We show that the up-conversion detector allows for
higher communication rates and longer communication distances than the commonly
used InGaAs/InP APD for all the three QKD protocols.Comment: 9 pages, 9 figure
Quantum key distribution with "dual detectors"
To improve the performance of a quantum key distribution (QKD) system, high
speed, low dark count single photon detectors (or low noise homodyne detectors)
are required. However, in practice, a fast detector is usually noisy. Here, we
propose a "dual detectors" method to improve the performance of a practical QKD
system with realistic detectors: the legitimate receiver randomly uses either a
fast (but noisy) detector or a quiet (but slow) detector to measure the
incoming quantum signals. The measurement results from the quiet detector can
be used to bound eavesdropper's information, while the measurement results from
the fast detector are used to generate secure key. We apply this idea to
various QKD protocols. Simulation results demonstrate significant improvements
in both BB84 protocol with ideal single photon source and Gaussian-modulated
coherent states (GMCS) protocol; while for decoy-state BB84 protocol with weak
coherent source, the improvement is moderate. We also discuss various practical
issues in implementing the "dual detectors" scheme.Comment: 22 pages, 9 figure
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