{'fraction_non_alphanumeric': 0.02088167053364269, 'fraction_numerical': 0.01160092807424594, 'mean_word_length': 5.132701421800948, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Group velocity control in multiple-beam and Mach-Zehnder interferometers

In this work, we demonstrate the existence of abnormal pulse propagation regimes in linear and passive multiple-beam interferometers, where the group velocity can be tuned from subluminal to superluminal values by simply changing the length of one of the interferometer's arms. Experiments are performed in the radiofrequency range by using coaxial transmission lines. The interferometers group delay is characterized both in the frequency and in the time domain. Group velocities of 2c and tunnelling with negative group velocity of -0.11c were measured for electromagnetic wave packets of 2 μs width travelling through a Mach-Zehnder interferometer. In a 4-beam interferometer, the group velocity of a 5 μs wide pulse was reduced to only 0.3c (compared to the usual value of 0.67c). The system is scalable to other frequency ranges and its implementation for narrowband optical pulses could be feasible by means of micromachining technologies. A scaling approach for advancing/delaying optical pulses at 1.55 μm is discussed. These systems are proposed as an alternative for controlling the group velocity without the need of using photonic crystals or periodically microstructuring, doping or using non-linear media.