Decoy state


Decoy state quantum key distribution (QKD) protocol is the most widely implemented QKD scheme. Practical QKD systems use multi-photon sources, in contrast to the standard BB84 protocol, making them susceptible to photon number splitting (PNS) attacks. This would significantly limit the secure transmission rate or the maximum channel length in practical QKD systems. In decoy state technique, this fundamental weakness of practical QKD systems is addressed by using multiple intensity levels at the transmitter's source, i.e. qubits are transmitted by Alice using randomly chosen intensity levels (one signal state and several decoy states), resulting in varying photon number statistics throughout the channel. At the end of the transmission Alice announces publicly which intensity level has been used for the transmission of each qubit. A successful PNS attack requires maintaining the bit error rate (BER) at the receiver's end, which can not be accomplished with multiple photon number statistics. By monitoring BERs associated with each intensity level, the two legitimate parties will be able to detect a PNS attack, with highly increased secure transmission rates or maximum channel lengths, making QKD systems suitable for practical applications.

Origin

In QKD protocols, such as BB84, a single photon source is assumed to be used by the sender, Alice. In reality, a perfect single photon source does not exist. Instead, practical sources, such as weak coherent state laser source, are widely used for QKD. The key problem with these practical QKD sources is the multi-photon components which they contain. A serious security loophole exists when Alice uses multi-photon states as quantum information carriers. In order to minimize the effects of multi-photon states, Alice has to use an extremely weak laser source, which results in a relatively low speed of QKD. Decoy state QKD is proposed to solve this multi-photon issue by using a few different photon intensities instead of one. With decoy states, the practical sources, such as a coherent-state source or heralded parametric down-conversion (PDC) source, perform almost as well as a single photon source.

Protocol

The first protocol was proposed by Hwang.[1] Then a 3-intensity protocol was proposed.[2] In this protocol, a tightened formula was given and also the effects of statistical fluctuation were considered, therefore the method becomes practically useful. The method was also studied with an infinite number of decoy states.[3]

Experimental implementation

The decoy state method using 3 intensities was demonstrated by a number of experiments.[4][5][6][7][8][9]

Decoy-state QKD using non-coherent-state sources

Decoy state QKD protocols with non-coherent-state sources have also been analyzed. Passive decoy state protocol, where the decoy states are prepared passively, is proposed as a parametric down-conversion source.[10][11]

See also

References

  1. ↑ W.-Y. Hwang, Phys. Rev. Lett. 91, 057901(2003).
  2. ↑ X. B. Wang , "Beating the photon-number-splitting attack in practical quantum cryptography, Phys. Rev. Lett. 94, 230503 (2005).
  3. ↑ "Decoy State Quantum Key Distribution", Physical Review Letters, 94, 230504 (2005)
  4. ↑ Yi Zhao, Bing Qi, Xiongfeng Ma, Hoi-Kwong Lo, and Li Qian, "Experimental Quantum Key Distribution with Decoy States", Phys. Rev. Lett. 96, 070502 (2006)
  5. ↑ Danna Rosenberg et al., "Long-Distance Decoy-State Quantum Key Distribution in Optical Fiber", Phys. Rev. Lett. 98, 010503 (2007)
  6. ↑ Tobias Schmitt-Manderbach et al., "Experimental Demonstration of Free-Space Decoy-State Quantum Key Distribution over 144 km", Phys. Rev. Lett. 98, 010504 (2007)
  7. ↑ Cheng-Zhi Peng et al., "Experimental Long-Distance Decoy-State Quantum Key Distribution Based on Polarization Encoding", Phys. Rev. Lett. 98, 010505 (2007)
  8. ↑ Z. L. Yuan, A. W. Sharpe, and A. J. Shields, "Unconditionally secure one-way quantum key distribution using decoy pulses", Appl. Phys. Lett. 90, 011118 (2007)
  9. ↑ S. H. Shams Mousavi, P. Gallion, "Decoy-state quantum key distribution using homodyne detection", Phys. Rev. A 80, 012327 (2009)
  10. ↑ Yoritoshi Adachi, Takashi Yamamoto, Masato Koashi, and Nobuyuki Imoto, "Simple and Efficient Quantum Key Distribution with Parametric Down-Conversion", Phys. Rev. Lett. 99, 180503 (2007)
  11. ↑ Xiongfeng Ma and Hoi-Kwong Lo, "Quantum key distribution with triggering parametric down-conversion sources", New J. Phys. 10 073018 (2008)
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