Abstract
A significant limitation of practical quantum key distribution (QKD) setups is currently their limited operational range. It has recently been emphasized (Ma et al 2007 Phys. Rev. A 76 012307) that entanglement-based QKD systems can tolerate higher channel losses than systems based on weak coherent laser pulses (WCP), in particular, when the source is located symmetrically between the two communicating parties, Alice and Bob. In the work presented here, we experimentally study this important advantage by implementing different entanglement-based QKD setups on a 144 km free-space link between the two Canary Islands of La Palma and Tenerife. We established three different configurations where the entangled photon source was placed at Alice's location, asymmetrically between Alice and Bob and symmetrically in the middle between Alice and Bob, respectively. The resulting quantum channel attenuations of 35, 58 and 71 dB, respectively, significantly exceed the limit for WCP systems (Ma et al 2007 Phys. Rev. A 76 012307). This confirms that QKD over distances of 300 km and even more is feasible with entangled state sources placed in the middle between Alice and Bob.
Highlights
Quantum cryptography, which promises to solve the problem of secure key distribution for the encryption of messages, is the most mature technical application in the field of quantum information and quantum communication
We demonstrate the feasibility of entanglement-based quantum key distribution (QKD) in loss regimes, where secure communication is no longer possible using weak coherent laser pulses (WCP) systems
Our results demonstrate that in the symmetric case secure keys can be generated up to a channel attenuation of 70 dB, a regime in which asymmetric schemes fail
Summary
Quantum cryptography, which promises to solve the problem of secure key distribution for the encryption of messages, is the most mature technical application in the field of quantum information and quantum communication. Ma et al [1], where the authors conclude that state-of-the-art pulsed entanglement based QKD systems with the source placed symmetrically in the middle between the receivers can cover up to twice as much attenuation as WCP systems. In their particular example, the maximal attenuation was evaluated to be an astonishing 70 dB (in the case of optimized mean photon number in the limit of an infinite number of key bits) when using experimental parameters as in [17].
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