Hugues de Riedmatten and Hugo Zbinden. GAP Optique. Markus Aspelmeyer, Andreas Poppe and Anton Zeilinger. University of Vienna. John Rarity. Qinetiq.
QuComm
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Long Distance Photonic Quantum Communication QuComm Deliverable D22 Dissemination level: Public
DELIVERABLE D22 (REPORT)
Report on entanglement enhanced quantum cryptography field trials Prepared by Hugues de Riedmatten and Hugo Zbinden GAP Optique Markus Aspelmeyer, Andreas Poppe and Anton Zeilinger University of Vienna John Rarity Qinetiq ABSTRACT We summarise the work within QuComm on the QKD with entangled photons outside a laboratory environment.
Report Version: 1 Report Preparation Date: 2003-06-05 Classification: Public
Project funded by the European Community under the “Information Society Technologies” Programme (19982002)
QuComm
IST-1999-10033
Long Distance Photonic Quantum Communication QuComm Deliverable D22 Dissemination level: Public
1.
Introduction
In deliverable D17, the report on "entanglement enhanced quantum cryptography fibre systems" we summarised the advantages and disadvantages of entanglement based QKD systems with respect to faint laser systems and we shortly reviewed the accomplished work in this field, inside and outside our consortium. (See D17 and references therein). Let us recall the main points: A. The main advantages of entanglement based QKD are the conceptual beauty, the immunity against photon number splitting attacks and the potential lack of empty pulses. Additionally, they offer interesting possibilities in the context of cryptographic optical networks, since a key provider with a few sources of entangled photons can operate such a network with high redundancy B. The disadvantage clearly is the increased experimental difficulty. Recent research showed that faint laser systems can be secure despite the photon splitting number attacks. In particular, a novel protocol has been discovered that resists much better to such eavesdropping strategies [1]. Therefore, faint laser pulses seem to be the first choice for immediate application in a cryptography system. However, in the long run, the preparation of entangled photons might become simpler and the experimental difficulties will thus be reduced to the level of other single-photon setups. For example, state-of-the art technology already allows to completely remove a polarization-entangled photon source from the ideal laboratory environment for outdoor field trials [5]. It should also be mentioned that the techniques developed for this purpose will be relevant for further fundamental experiments and also for novel advanced quantum communication concepts such as long distance teleportation. In our consortium there are two activities that go into this direction. A. Experiments with short wavelength (810 nm), polarization-entangled photons (EXPUNIVIE) B. Experiments with telecom wavelength, energy-time entangled photons (GAP, KTH).
2.
Polarization-entangled photons
Polarization-entangled photons at wavelengths around 800 nm are not well adapted for transmission in optical fibres, due to considerable losses and depolarisation. Nevertheless, in the recent teleportation field experiment by the Vienna group [2], polarization-entangled photons were transmitted using glass fibre in a channel underneath the Danube River in Vienna. Above the river, there was a direct microwave connection for the classical channel. The two stations were physically separated by approx. 600m. The transmission range can be considerably increased sending the photons through free space. Free space QKD experiments have been performed by different groups, notably by the members of our consortium (LANL, QinetiQ, LMU) [3,4]. These experiments used faint laser
QuComm
IST-1999-10033
Long Distance Photonic Quantum Communication QuComm Deliverable D22 Dissemination level: Public
pulses, but there is no reason that this wouldn't work with entangled photons. EXPUNIVIE has demonstrated a step in this direction, namely the distribution of quantum entanglement via optical free-space links in an outdoor environment. Polarization-entangled photon pairs have been transmitted across the Danube River in the city of Vienna via optical free-space links to independent receivers separated by 600m (see Fig. 1). A Bell inequality between those receivers was violated by more than 4 standard deviations confirming the quality of the entanglement [5]. In this experiment, the setup for the source has been miniaturized to fit on a small optical breadboard and it could easily be transported from the laboratory to the outdoor field site, where it was kept in a small shielded container. Electrical power for the source was provided by a gasoline-driven power generator, which makes it completely independent from an ideal laboratory environment.
Figure 1. The outdoor experimental field site in Vienna in a cross-cut. The source was located at the bank of the Danube River, pointing towards the rooftops of two buildings (Alice and Bob).
Free space QKD based on single qubits (photons) needs a line of sight between Alice and Bob. Note that this is not the case for entanglement-based QKD schemes (see [5]); there, a line of sight is necessary between source and each of the communicating parties but not between the parties itself. In general, however, free-space links are limited in range and dependent on the weather conditions. A possible solution to overcome this limitation is to establish earth-satellite links. The feasibility of this strategy has been shown for faint laser pulses [6]. A recent study of the Vienna group proves the feasibility for entangled photons as well [7]. Even if this solution might not be the adopted for QKD, the perspective of a Bell experiment almost around the world is fascinating.
3.
Entanglement based QKD at telecom wavelength
QuComm
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Long Distance Photonic Quantum Communication QuComm Deliverable D22 Dissemination level: Public
GAP studied the feasibility of a quantum key distribution experiment with photon pairs over a distance of ~30 km between Geneva and Nyon in the commercial standard optical fiber network has been studied. We focused on the scheme shown in Fig 4 using photon pairs entangled in energy and time [8]. The two-photon source (a KNbO3 crystal pumped by a cw doubled Nd-YAG laser) produces photons at non-degenerate wavelengths - one around 810 nm, the other one centred at 1550 nm. This choice allows detecting the photon of the lower wavelength with high efficiency silicon based single photon counters. The high transmission loss at this wavelength in optical fibres doesn't matter as the distance between the source and the corresponding analyser is kept very short. The other photon, at the wavelength where fibre losses are minimal, is sent via an optical fiber to Bob's interferometer and is then detected by InGaAs APD's. The BB84 protocol was implemented with 2 birefringent interferometers, with polarisation multiplexing of the two bases. Interference visibilities were typically 92% over.
33 km
Interferometer
Class. Det.
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Classical channel (1550 nm) λ/4 plate PBS
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Figure 2: Experimental setup for quantum key distribution over large distance
Energy-time entanglement is very robust in optical fibres. However, the chromatic dispersion (CD) encountered by the 1550 nm photon proofs to be the limiting factor in long fibre links. More precisely, in the presence of a large amount of CD, the different peaks associated with different path in the interferometers start to overlap. Two solutions can be envisaged: the first one is to reduce the bandwidth of the photon using an interference filter, and the second is to compensate passively the CD. However, in both cases the count rate is reduced.. We showed that adding 3 km of dispersion compensating fibre (inducing ~3dB losses) efficiently compensates CD for 30km of standard optical fibre (see Fig.3). We also performed quantum key distribution over 30km (in the lab) with a raw key rate of ~45Hz. The QBER still has to be reduced, in order to implement secure transmission. Actually, we're improving the stability of the interferometers. For measurements outside the lab, an active stabilisation would be preferable. However, we don't expect any fundamental complication for performing a key exchange over installed telecom fibres and Alice and Bob physically separated.
QuComm
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Long Distance Photonic Quantum Communication QuComm Deliverable D22 Dissemination level: Public
1
[u.a]
30 km without compensation
30 km with compensation 0.5
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Figure 3. Chromatic dispersion compensation at 1550nm for 30 km of standard optical fibre visualised with the time histogram of the arrival times of the photons. Without compensation, the central peak, which represents the interfering processes is considerably broadened by CD. With compensation, the width of the peak is almost equal to the one at 0 km.
5.
Conclusions
We studied the feasibility of entanglement based QKD in the field. QKD distribution has been performed in the lab over 30 km of optical fibres. Conservation of polarization-entanglement in a free-space field trail has been proven using a miniaturized source of entangled photons outside the shielded lab environment. Although faint laser-pulses is currently the system of choice for an imminent application in a quantum cryptography setup, recent field experiments demonstrate the technological improvements towards stand-alone systems delivering entangled photons, thus reducing the experimental difficulties to a comparable level of complexity. Additionally, these experiments are of interest from a fundamental point of view, as they would allow for instance Bell experiments over larger distances than before and they would also allow advanced novel quantum communication concepts such as long distance teleportation.
6.
References
[1]
V. Scarani, A. Acin, N. Gisin, G.. Ribordy, Quantum cryptography protocols robust against photon number splitting attacks for weak laser pulses implementations, quantph/0211131
[2]
R. Ursin, T. Jennewein, M. Aspelmeyer, R. Kaltenbaek, M. Lindenthal, M. Taraba, P. Walter, and A. Zeilinger, in preparation.
QuComm
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Long Distance Photonic Quantum Communication QuComm Deliverable D22 Dissemination level: Public
[3]
W. T. Buttler et al., “Daylight quantum key distribution over 1.6 km”, Physical Review Letters 84, 5652 (2000).
[4]
C. Kurtsiefer et. al., A step towards global key distribution, Nature 419, 450 (2002).
[5]
M. Aspelmeyer, H. R. Böhm, T. Gyatso, T. Jennewein, R. Kaltenbaek, M. Lindenthal, G. Molina-Terriza, A. Poppe, K. Resch, M. Taraba, R. Ursin, P. Walther, A. Zeilinger, Long-Distance Free-Space Distribution of Quantum Entanglement, Science (in press).
[6]
J G Rarity, P R Tapster, P M Gorman and P Knight, Ground to satellite secure key exchange using quantum cryptography, New J. Phys. 4, 82 (2002).
[7]
Markus Aspelmeyer, Thomas Jennewein, Martin Pfennigbauer, Walter Leeb, Anton Zeilinger, Long-Distance Quantum Communication with Entangled Photons using Satellites, quant-ph/0305105, submitted to IEEE Journal of Selected Topics in Quantum Electronics, special issue on "Quantum Internet Technologies"
[8]
G. Ribordy, J. Brendel, J.D. Gautier, N. Gisin, H. Zbinden, Long distance entanglement based quantum key distribution, Phys. Rev. A 63, 012309 (2001).
