Bistability is widely exploited to demonstrate all-optical signal processing and light-based computing. The standard paradigm of switching between two steady states corresponding to '0" and '1" bits is based on the rule that a transition occurs when the signal pulse intensity overcomes the bistability threshold, and otherwise, the system remains in the initial state. Here, we break with this concept by revealing the phenomenon of indefinite switching in which the eventual steady state of a resonant bistable system is transformed into a nontrivial function of signal pulse parameters for moderately intense signal pulses. The essential nonlinearity of the indefinite switching allows realization of well-protected cryptographic algorithms with a single bistable element in contrast to software-assisted cryptographic protocols that require thousands of logic gates. As a proof of concept, we demonstrate stream deciphering of the word 'enigma' by means of an indefinitely switchable optical nanoantenna. An extremely high bitrate ranging from ~0.1 to 1 terabits per second and a small size make such systems promising as basic elements for all-optical cryptographic architectures.
Deep Dive into Ultrafast cryptography with indefinitely switchable optical nanoantennas.
Bistability is widely exploited to demonstrate all-optical signal processing and light-based computing. The standard paradigm of switching between two steady states corresponding to ‘0" and ‘1" bits is based on the rule that a transition occurs when the signal pulse intensity overcomes the bistability threshold, and otherwise, the system remains in the initial state. Here, we break with this concept by revealing the phenomenon of indefinite switching in which the eventual steady state of a resonant bistable system is transformed into a nontrivial function of signal pulse parameters for moderately intense signal pulses. The essential nonlinearity of the indefinite switching allows realization of well-protected cryptographic algorithms with a single bistable element in contrast to software-assisted cryptographic protocols that require thousands of logic gates. As a proof of concept, we demonstrate stream deciphering of the word ’enigma’ by means of an indefinitely switchable optical nano
Continuous growth in communication traffic drives new demands in novel cryptographic techniques that can supply reliable security of information exchange at a high rate. Many approaches have been actively proposed and explored. Although the majority of modern solutions focus on development of software algorithms 1,2 , hardware realizations offer an additional and valuable degree of security. Recent examples of such approaches include digital metamaterials 3 and metasurfaces [4][5][6] , which consider two elemental media with a strong contrast in the electromagnetic response as building blocks (treated as 'metamaterial bits'). The direct correspondence between the spatial structuring of such information bits and the electromagnetic properties of the entire system leads to a number of promising functionalities, such as information encoding in the radiation field patterns 5 as well as analogous 7 and digital 4 computations. Another actively developed realm involves securing optical information in images via linear and nonlinear holography 8,9 . However, all of these techniques typically require bulky structures and complicated programming algorithms 8,10,11 , which restrict their performance.
Alternative strategies for all-optical information processing apply bistability. From a physical viewpoint, bistability occurs when a free-energy landscape of the system contains two minima separated by one maximum 12 . To switch between the minima, the system is disturbed with a form of activation energy, which is typically supplied by a large-amplitude input signal. For the all-optical bistable switchers reported thus far, this type of transition appears when the signal pulse intensity slightly overcomes the bistability threshold and the system returns to the initial state for weaker pulses [12][13][14][15] . Here, we show that this paradigm can be changed for sufficiently strong signal pulses that induce the activation energy to a much larger extent than the local maxima of the free energy. As a result, the eventual steady state is transformed to a nontrivial function of the pulse intensity, phase, and duration, as shown in Fig. 1(a). This phenomenon of indefinite switching is used to demonstrate cryptographically secure stream encryption.
Stream ciphers encrypt and decrypt bits individually using a secret key (a string of bits). To make ciphering sufficiently reliable, cryptographic devices must combine the randomness of a securing key and nonlinearity in the relationship between plaintext and ciphertext 1 . Importantly, vanishing nonlinearity results in a weak cryptographical security even when one has a random key with appropriate statistical properties because it is sufficient to know only a small portion of a message to fully unwrap the secret key by solving the linear system of equations (algebraic attack). Software-assisted cryptographic algorithms exploit combinations of many Boolean operations and shift registers to achieve nonlinearity. This approach calls for cascading of thousands of logic gates, which limits compactness and performance for such cryptographic architectures. For example, one of the fastest and most efficient modern stream ciphers, trivium, occupies approximately 4000 logic gates and encrypts at a maximal bitrate of 2 Gb s -1 1 . In contrast, indefinite switching naturally brings nonlinearity into the relationship between plaintext and ciphertext at the level of a single optical bistable nanoantenna. The ultrafast switching time and small size render such systems promising basic elements for novel lightweight cryptographic protocols operating at a Tb s -1 bitrate.
In general, any side in a cryptographically secured data exchange process performs ciphering and deciphering procedures. Although these operations are the mathematical inverse of each other, they can be physically implemented by different devices. Here, we focus on deciphering by indefinite switching in an optical bistable nanoantenna (receiver), assuming data encoding in the intensities and phases of coherent signal pulses generated by an external laser system (sender).
As proof of concept, we consider scattering of a laser beam by a pair of nanoparticles (Fig. 1(b)). We assume that one nanoparticle in the dimer possesses a Kerr-type nonlinearity combined with a resonant response. In practice, such a nanoparticle can be constructed, for example, using any plasmonic material 16,17,18 . A surface plasmon resonance appears together with a strong cubic susceptibility for such materials as silver, gold and graphene [19][20][21] . This configuration allows driving of bistability at a moderate light power due to the strength of the nonlinear response reached via the resonance. The second nanoparticle is assumed to be linear and nonresonant and plays a role of a director to shape the directivity of the dimer scattering pattern.
Continuous-wave background radiation creates the desired nanoantenna energy landscape, giving rise to bistab
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
