Download Fibre-based Quantum Key Distribution using phase randomized

Fibre-based Quantum Key Distribution using
phase randomized homodyne detection
Christoffer Wittmann (1), Josef Fürst (1,2), Carlos H. Wiechers (1,3),
Dominique Elser (1), Denis Sych (1), and Gerd Leuchs (1)
1: Institut für Optik, Information und Photonik, Max-Planck-Forschungsgruppe, Universität Erlangen-Nürnberg,
Günther-Scharowsky-Str. 1 / Bau 24, 91058 Erlangen, Germany ([email protected])
2: Physikalisches Institut, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
3: Instituto de Física de la Universidad de Guanajuato, Lomas del Bosque 103, 37150 León, Guanajuato, México
Abstract
We present a fibre-based quantum key distribution (QKD) system, employing optical coherent states at
telecommunication wavelength. The QKD systems detection part consists of two homodyne detections, which
measure two conjugate quadratures with random phase. We present preliminary results for quantum channel of
20km standard telecom fibre.
Introduction
Experimental Setup
In QKD systems the quantum mechanical properties
of light fields are used to establish a secret shared
key between two honest parties, usually named Alice
and Bob [1, 2].
A schematic of the setup is shown in Fig. 1. The light
source is a current pulsed laser diode at 1550nm. The
signal pulses (SP) and local oscillator pulses (LO) are
generated in an asymmetric fibre interferometer. The
signal pulses are phase modulated and attenuated to
weak coherent states. Currently we choose a binary
modulation (producing the coherent states |±α〉) and
concatenate a series of strong, classical signal pulses
as pilot signals. Subsequently, the SP and LO are
time multiplexed into the fibre channel using different
polarization modes. We tested fibre channels of 20km
length. The demultiplexing stage is a polarizing beam
splitter, precisely separating SP and LO. The
detection part consists of two homodyne detection
systems, measuring conjugate quadratures with
random absolute phase. Finally, the interferometric
phase of the pilot signals is considered in the
postprocessing step.
Since continuous-variables QKD was first proposed
[3], several “prepare and measure” systems using
discrete [4] and continuous Gaussian modulation [5,
6] were demonstrated.
We present a QKD-experiment following the
experiments by S. Lorenz et al. [4]. Instead of a short
free space link we adapt our protocol for a quantum
channel of 20km optical fibre. We use a time
multiplexing of signal and local oscillator as shown in
[5], however utilize homodyne detection of both
conjugate quadratures as presented by [4,7]. The
new aspect of the experiment is a phase randomized
homodyne detection. We measure conjugate
quadratures, therefore their relative optical phase is
fixed, while the absolute interferometric phase is
freely drifting. We show that the stabilisation of the
interferometer phase can be substituted by attaching
a pilot or calibration signal. A low noise transmission
with excess noise of approximately 2% is possible.
We will present preliminary results for a two state
alphabet.
Results
Up to now we were able to send signals through a
20km fibre quantum channel. We have investigated
the effect of interferomertric phase drift and recovered
the signal phase using the pilot signal.
Figure 1: Schematics of the setup.
Detection part is a phase randomized
heterodyne
detection
using
the
polarisation degree of freedom.
Conclusions
By homodyne detection of two random conjugate
quadratures the Q-function was reconstructed. The
system worked with a realistic 20km quantum
channel, which introduced only little noise of 2%.
X/SNU
P/SNU
Figure 2: Reconstructed Q-function of binary signal.
We reconstructed the Q-function of the signal states
measured in the receiver station. As an example we
show the Q-function of two signal states in Fig. 2. By
this, we characterized the excess noise (including
channel and phase recovery step) and analyzed the
effects of phase randomized detection on the
postselection process. An excess noise measurement
is shown in Fig. 3. It indicates noiseless signal state
preparation and very low excess noise due to signal
transmission.
Further studies of the excess noise and losses in the
system due to the fibre channel and other
experimental imperfections will provide us with key
rate estimations and allow for verification of effective
entanglement in the system [8, 9]. Multi letter
alphabets using a postselection process [10] are
currently investigated and will be demonstrated with
the setup.
References
1. S. Wiesner, Sigact News 15, 78 (1983).
2. C. H. Bennett and G. Brassard, in International
Conference on Computers, Systems and Signal
Processing (1984). p. 175
3. T. C. Ralph, Phys. Rev. A 61, 010303(R) (1999).
4. S. Lorenz et al. Appl. Phys. B 79, 273 (2004).
5. J. Lodewyck et al., Phys Rev. A 76, 042305
(2007).
6. A. Lance et al., Phys. Rev. Lett. 95, 180503
(2005)
7. C. Weedbrook et al., Phys. Rev. A 73, 022316
(2006).
8. S. Lorenz et al., Phys. Rev. A 74, 042326 (2006).
9. J. Rigas et al., Phys. Rev. A 73, 012341 (2006).
10. Ch. Silberhorn et al., Phys. Rev. Lett. 89,167901
(2002).
Figure 3: Excess noise (in shot noise units) for
increasing attenuation in the signal generation. The
estimated mean photon number in the signal is also
shown.