Researchers have developed a quantum key distribution (QKD) system that enables a greater number of secret bits to be packed into a single photon. The new technique allows for two secret bits in a single photon, with a secret key rate of 26.2 megabits per second. Current QKD systems are only able to transmit 10,000 to 100,000 secret bits per second. QKD, an emerging quantum technology that enables the establishment of secret keys between two or more parties in an untrusted network, is widely regarded as a viable solution to the security threats posed by future quantum computers. Although QKD technology is relatively mature, practical QKD systems still face some limitations. One limitation is the secret key throughput, a constraint that is largely due to the choice of a low-dimensional quantum information basis to encode quantum information. The new approach, developed by an international team from the National University of Singapore (NUS), Duke University, Ohio State University and Oak Ridge National Laboratory (ORNL), is based on time and phase bases. According to the team, encoding quantum information in the time and phase bases is a promising approach that is highly robust against typical optical channel disturbances, yet scalable in the information dimension. are only able to transmit 10,000 to 100,000 secret bits per second. In this approach, the secret bits are encoded in the arrival time of single photons and the complementary phase states (for measuring information leakages) are encoded in the relative phases of the time states. This encoding technique, in principle, could allow one to pack arbitrarily many bits into a single photon and generate extremely high secret key rates for QKD. However, implementing such high-dimensional systems is technically challenging, and tools for quantifying the practical security of high-dimensional QKD are limited. The researchers used a novel combination of techniques to overcome these issues. Their QKD system is based on a prepare-and-measure scheme, where a continuous wave laser is randomly modulated and attenuates the outgoing photonic wave packets to the single-photon level. The photonic wave packets are then transmitted via an untrusted quantum channel to a distant receiver that uses single-photon detectors or interferometers coupled to single-photon detectors to measure the wave packets in the time or phase bases, respectively. To deal with so-called photon number-splitting attacks, the researchers used a practical decoy-state method to estimate the number of single-photon wave packets received. The secret key was calculated using the sifted photon time-of-arrival data, and the amount of extractable secret data was determined using the noise level observed in the sifted phase measurement data. To obtain high secret key rates, the researchers used high-dimensional quantum states that transmit more than one secret bit per received photon. Their system was constructed using commercial off-the-shelf components. According to researchers, their protocol could be extended to free-space quantum channels. “Poor secret key rates arising from current QKD implementations have been a major bottleneck affecting the use of quantum secure communication on a wider scale. For practical applications, such systems need to be able to generate secret key rates in the order of megabits per second to meet today’s digital communication requirements,” said professor Charles Lim. “Our newly developed theoretical and experimental techniques have resolved some of the major challenges for high-dimensional QKD systems based on time-bin encoding, and can potentially be used for image and video encryption, as well as data transfer involving large encrypted databases. This will help pave the way for high-dimensional quantum information processing,” Lim said. Moving forward, the team will be exploring ways to generate more bits in a single photon using time-bin encoding. This could help advance the development of commercially viable QKD systems for ultra-high rate quantum secure communication. The research was published in Science Advances (doi: 10.1126/sciadv.1701491).