NTT have realized a quantum cryptography scheme

OREANDA-NEWS. September 16, 2015. Nippon Telegraph and Telephone Corp. (NTT; Head Office: Chiyoda-ku, Tokyo; President: Hiroo Unoura) and The University of Tokyo (Bunkyo-ku, Tokyo; President: Makoto Gonokami, Ph.D.) have realized a quantum cryptography scheme that can assure security without monitoring the error rate of photon transmission.

1.Background

Network security is becoming increasingly important in recent years. Quantum cryptography is a method to provide ultimately secure means of communication based on quantum physics. In quantum cryptography, the sender, Alice, and the receiver, Bob, share a secure key via quantum key distribution (QKD), and they encrypt their communication with it. Since the security of the key shared by QKD is guaranteed by the principle of quantum physics, we can achieve communication that is secure against eavesdropping with any future technology.
The security of conventional QKD relies on the Heisenberg uncertainty principle, which states that observing a quantum state necessarily disturbs it. In QKD, Alice encodes the key information on a quantum state of a photon, the elementary particle of light, and sends it to Bob through a transmission channel. If an eavesdropper (Eve) performs a measurement on the photon during the transmission, the quantum state is altered, and as a result, Alice and Bob probabilistically observe an error in their photon transmission. Therefore in a QKD, Alice and Bob monitor the error rate of key distribution using test bits so that they can estimate the amount of information that can be leaked to an eavesdropper. By compressing the key using the estimated amount of information leakage, they can obtain a quantum-secure key. Thus, in conventional QKD schemes, periodical monitoring of the error rate has been necessary. In 2014, a research team from the University of Tokyo (one of the authors of the release) together with a team from the National Institute of Informatics proposed the RRDPS protocol, whose security is based on wave function collapse, unlike previous QKD schemes. However, the experimental verification of the RRDPS protocol remained to be realized.

2.Achievements

By implementing the RRDPS protocol, NTT Basic Research Laboratories and the University of Tokyo jointly realized, for the first time, a QKD without the need for monitoring the error rate.

In the RRDPS protocol, the amount of information that can be leaked to Eve is bounded by a certain value (Figure 1). The protocol uses a quantum state that consists of multiple pulses as a carriers of the key, and the amount of the leaked information is determined solely by the number of pulses and is independent of the error rate. Therefore, by compressing the key using the estimated amount of the leaked information, we can distribute a secure key without monitoring the error rate.

With the present result, we can omit the periodical monitoring of the error rate between Alice and Bob using test bits in a QKD system and thus significantly simplify the control sequence between the sender and receiver. Furthermore, we can improve the efficiency of key generation since an RRDPS system does not consume keys as test bits. Note too that this is the first experiment where we can achieve unconditionally secure key distribution without relying on the Heisenberg uncertainty principle.

Experiments

1. (1)The setup is shown in Figure 2. Alice prepares an optical packet that consists of five weak coherent pulses whose temporal interval is denoted by T, and modulates the phase of each pulse randomly by 0 or ?. The modulated packet is then sent to Bob through an optical fiber. Bob is equipped with a round-robin phase difference measurement setup, which is composed of a 1 x 4 optical splitter, four delay Mach-Zehnder interferometers (MZI) with delay times of T, 2T, 3T and 4T, and single photon detectors that are connected to the output ports of MZIs. Since the average number of photons per packet is much smaller than 1, the phase difference measurement is performed at one of the four MZIs to which a photon happens to arrive.

   Figure 2: Experimental setup: implementation of round-robin phase difference measurement

2. (2)We experimentally confirmed that secure key distribution is possible with the RRDPS protocol (Figure 3). The maximum tolerable transmission loss between Alice and Bob is 8.7 dB. We also demonstrated secure key distribution over 30 km of fiber.

   Figure 3: Secure key distribution result

3. (3)With our experimental data, we confirmed that we could distribute a secure key with its finite-size effect taken into account (Figure 4). Accordingly, we showed that we can achieve secure key distribution in a realistic situation where the finite-size effect of the key should be considered.

   Figure 4: Results of finite-key analysis

4.Future Plans

In the present experiment, the number of pulses in a packet L was limited to five. However, it is predicted that the performance of an RRDPS system, such as the secure key rate and maximum reachable distance, will be greatly enhanced by increasing L. NTT Laboratories will implement a round-robin phase difference measurement with ~100 delays using NTT’s optical waveguide fabrication technologies so that we can enhance the performance. The University of Tokyo will investigate some expected features of the RRDPS protocol, such as the possibilities of highly efficient secure key generation with a shorter key length and high tolerance against noise as a research program supported by ImPACT.

Publication

H. Takesue, T. Sasaki, K. Tamaki, and M. Koashi, “Experimental quantum key distribution without monitoring signal disturbance,” Nature Photonics (2015) (DOI: 10.1038/nphoton.2015.173).