High-Capacity, Free-Space Quantum Key Distribution based on Spatial and Polarization Encoding
Abstract
We propose a research effort aimed at developing a free-space optical link for secure communication over marine environments. The link will transmit at a rate of at least 100 Mb/s over a path length of 10 to 30 km. A key aspect of our approach is that we will encode simultaneously in the transverse spatial degree of freedom of the light field and in polarization. We use this hybrid approach in order to maximize the amount of information that can be carried by each photon. We will also explore the use of more complex light beams, known as vector beams, in which both the polarization and the complex field amplitude are varying functions of the transverse position in the beam [Bauer, 2015]. Encoding in the orbital angular momentum (OAM) states of light is just one example of transverse spatial encoding that has received much past attention [Mirhosseini, 2015], and it is the primary example that we will use throughout this proposal. We will however consider other types of spatial encoding, such as Hermite-Gauss beams or plane wave encoding [Boyd, 2011a]. We strongly suspect, and will verify this suspicion over the course of this project, that it will be necessary to use adaptive optics (AO) methods in order to achieve our system goals. Much of the background work needed for the present project was developed under a recent DARPA program known as InPho. Because of the success of this program, we can immediately start work to achieve our system goals; there is no need to first develop needed concepts or needed technology. Some of the key recent breakthroughs of our team members are as follows: (1) Willner’s group has demonstrated that OAM multiplexing can successfully be used to transmit classical information at rates exceeding 1 Tb/s [Wang, 2012]. Our task for the proposed work is to utilize this high speed in the context of quantum communications. (2) Boyd’s group has demonstrated a laboratory scale QKD system based on OAM encoding with an information capacity of more than 2 bits of per sifted photon [Mirhosseini, 2015]. As a key part of any OAM-QKD setup, Boyd’s group (in collaboration with Padgett’s group) has developed a photon-efficient mode sorter that has mode discrimination of greater than 92%, largely solving a key roadblock at the beginning of the InPho program [Mirhosseini, 2013]. In broad scope, our approach is as follows. We consider a free-space space link of length 10 to 30 km. We chose a maximum value of 4 for ? because the size of an OAM-carrying beam increases with ?, and we do not want the required size of our apertures to be too large. As we show in detail later in the proposal, for this case telescope diameters of only 40 cm are required for L = 30 km and 20 cm for L = 10 cm. There will thus be N = 9 possible states in our alphabet, or if we use polarization coding simultaneously there will be 18 states. Since log2 9 = 3.2 and log2 18 = 4.2, we see that we can carry as many as 4.2 bits per photon. Thus, we can achieve a bit rate of 100 Mb/s while transmitting only 25 mega symbols per second (Msym/s). It is also known that it is easier to maintain security in the presence of noisy communication channels through use of a larger alphabet [Cerf, 2007]. In concept, one could use an even larger state space to increase still further the efficiency in terms of bits per photon. We next address how we will achieve a symbol transmission rate of 25 Msym/s. One strategy for performing QKD with OAM states [Mirhosseini, 2015] is to use a spatial light modulator (SLM) or digital micro-mirror device (DMD) as a real-time holographic element to encode the desired transverse field distribution onto each transmitted photon. However, the refresh rates of these devices are at most 100 kHz, and thus are not fast enough to achieve the system goal. Instead we will use a sequence of 9 (or 18) static holograms, each of which is fed by a different laser source. Each laser source will be separately modulated.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 12, 2016
- Source ID
- N000141512635
Entities
People
- Robert W. Boyd
Organizations
- Office of Naval Research
- United States Navy
- University of Rochester