Electromagnetic Radiation: Radio is the only thing known to attract the mechanical monsters that destroyed Earth, so the Federation strongly prohibits any devices that produce EM with a wavelength longer than half a meter. However microwaves are still acceptable for “local” communication, often as part of a “mesh” relay network. While optical lasers are frequently used for long-distance communication within a star system or sometimes with nearby systems as well.
Neutrinos: Neutrinos (which travel at the speed of light) can be safely used for “broad-spectrum” communication similar to radio but without the risk of attracting alien machines. However, the transmitters require a considerable amount of power and detectors are swimming pool-sized tanks of fluid, limiting their use. In addition the signal loses coherence after half a dozen AUs. Starships usually have a transmitter for emergencies but receivers are uncommon on ships due to their mass, generally reserved for stations and planets, though recently some have appeared on specialized “flagships”.
Quantum Entanglement: Quantum Entangled Particles are primarily used in four ways. The first dates back to pre-exodus Earth, when they were used to make “keys” for unbreakable encryption of radio or optical communications, as QEParts are mass-produced these days a lot of companies or government agencies use them for secure internal communications, even a few wealthy individuals. The second is quantum computing, which was only solved a couple years after interplanetary travel began, the teleportation properties of QEParts enabled immense leaps in computing speed and their “fuzziness” allowed computers to “think” beyond binary. The third was discovered while testing claims of paranormal abilities, it seems that there are entangled particles suffusing the universe, but most of them are found in close proximity to one another. However, if two people happened to each have one of a pair of QEParts integrated into the permanent structure of a neuron, it appeared that they could share thoughts. Naturally, the strongest connections turned out to be between monozygotic twins, those who split off from a single fertilized egg. These twins could communicate instantly to each other across light-years it turned out, unfortunately their resolution was poor, most only manage to get across feelings or sensations, a few very lucky pairs have managed to send images. The majority of telepaths used for communication have to enter a state of altered consciousness in which one partner nudges the other’s vocal apparatus or hand to make them speak or write. Due to the inefficiencies of this method of communication the fourth major use of QEParts was eventually invented, commlinks. QEPart commlinks can transmit text, audio, even video by quantum teleportation, unfortunately the instrumentation used to read the particles on the receiving end changes the quantum state of the particle and causes the connection to break, eventually the paired commlinks need a fresh supply of QEParts.