High-dimensional quantum cryptography with twisted light

Abstract

Quantum key distribution (QKD) systems often rely on polarization of light for encoding, thus limiting the amount of information that can be sent per photon and placing tight bounds on the error rates that such a system can tolerate. Here we describe a proof-of-principle experiment that indicates the feasibility of high-dimensional QKD based on the transverse structure of the light field allowing for the transfer of more than 1 bit per photon. Our implementation uses the orbital angular momentum (OAM) of photons and the corresponding mutually unbiased basis of angular position (ANG). Our experiment uses a digital micro-mirror device for the rapid generation of OAM and ANG modes at 4 kHz, and a mode sorter capable of sorting single photons based on their OAM and ANG content with a separation efficiency of 93%. Through the use of a seven-dimensional alphabet encoded in the OAM and ANG bases, we achieve a channel capacity of 2.05 bits per sifted photon. Our experiment demonstrates that, in addition to having an increased information capacity, multilevel QKD systems based on spatial-mode encoding can be more resilient against intercept-resend eavesdropping attacks. IntroductionFirst introduced in 1984 by Bennett and Brassard, quantum key distribution (QKD) is a method for distributing a secret key between two parties [1,2]. Due to a fundamental property of quantum physics known as the nocloning theorem, any attempt made by a third party to eavesdrop inevitably leads to errors that can be detected by the sender and receiver [3,4]. Modern QKD schemes conventionally use a qubit system for encoding information, such as the polarization of a photon. Such systems are easily implemented because technology for encoding and decoding information in a qubit state-space is readily available today, enabling system clock rates in the GHz regime [5][6][7]. Recently, the spatial degree of freedom of photons has been identified as an extremely useful resource for transferring information [8,9]. The information transfer capacity of classical communication links has been increased to more than one terabit per second using spatial-mode multiplexing [10]. In addition, it has been theoretically shown that employing multilevel quantum states (qudits) can increase the robustness of a QKD system against eavesdropping [11][12][13][14]. Although the majority of high-dimensional QKD schemes so far have employed time-bin encoding for increasing the alphabet size [15][16][17][18], it is expected that spatial-mode encoding can be alternatively used to enhance the performance of a QKD system considering the recent advances in free-space orbital angular momentum (OAM) communication, .The feasibility of high-dimensional QKD in the spatial domain has been previously demonstrated by encoding information in the transverse linear momentum and position bases [19,20]. While such encoding schemes provide a simple solution for increasing the information capacity, they are not suitable for long-haul optical links due to the cross-talk caused by diffraction. D...

Single-photon imager based on a superconducting nanowire delay line

“…For example, to enhance the information-carrying capacity of a quantum channel 9 and improve security in quantum key distribution 10,11 , information is typically encoded in the position and arrival time of individual photons.…”

Symmetric encryption algorithm using “twisted” light

“…In works [2,3] showed the modification scheme of the quantum key distribution (QKD), the transmission of information with superposition of states with non-zero OAM values of photons [4]. Many works are related to the generation of the light beams with OAM [5,6,7,8].…”

Functional quantum computing: An optical approach

“…This approach has the advantage that the dimensionality of the target qubit will not be expanded in the time, a factor which could ease practical constraints on the time-scales of system stability and lead to a significant reduction in computation time compared to our proposal. The disadvantage to this approach is that current state-of-the-art OAM mode sorters have slower update rates (∼4 kHz) and lower extinction (93%) [31] than the switching technology which can update as fast as 10 GHz [18] with 99% mode extinction [16,17].…”