Call for candidature : International Volunteer in NanoPhotonics [France2Israel]

PROJECT TITLE : Ultra-Long Fiber lasers for secure key distribution

RESEARCH PROJECT : The objective of this project is to develop a novel, alternative, key-distribution scheme based on establishing laser oscillation between the sender and the receiver. The proposed scheme consists of an ultra-long fiber laser (UFL) with Alice at one end and Bob on the other (see figure). Each of the two parties controls the reflection spectrum of one of the laser end mirrors while the communication link between them serves as the laser cavity. To exchange a bit, each party randomly selects a bit and encodes it (using his/her end mirror) into the laser. The lasing characteristics allow the two parties (and a possible eavesdropper) to determine only whether they chose identical or opposite bits. Since each party knows his/her own bit they can deduce the other party’s choice and to exchange a bit. An adversary can only deduce whether Alice and Bob succeeded in exchanging a bit, but not to determine its value.
Preliminary modelling of the laser output power at 0 for a random sequence of bit selection shows that the power levels distinguish between correlated (A=B) and anti-correlated (A¹B) bits but cannot distinguish between the two anti-correlated bits possibilities. Thus, Eve cannot learn of the choice of mirrors by monitoring the power. A similar analysis of the lasing spectra of the four different states of the system indicates that the (1, 1) and (0, 0) cases can be distinguished by such measurement. These bits cannot be used to attain secure communication and are, therefore, discarded. The spectra of the anti-correlated bits (1,0) and (0,1) are found to be very similar though not identical. Nevertheless, the difference is very small thus compelling Eve to be able to distinguish between signals which are -40dB below the lasing power. These bits are, therefore, retained and are added to the key. Introducing additional filters into the cavity can reduce the difference between these spectra (and, therefore, Eve’s ability to get knowledge of the key) almost arbitrarily.
In addition to simplicity, the UFL scheme also provides enhanced KER (compared to QKDS), especially at long ranges. The maximal key-establishing rate is given by fmax=c/20Ln. Note, that the key-establishing rate decreases as Log(fmax) ~ -Log(L) for the classical KDS while for QKDS this rate decreases as Log(fmax) ~ -αL. Thus, as the length of the link increases, the UFL scheme becomes more attractive.

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