Neutrinos have been in the news recently, and although it appears that they probably do not travel faster than light, they still hold court as three of the strangest of the known subatomic particles. Undeterred by these arcane particles, Fermilab scientists have succeeded in communicating with neutrino pulses through 240 meters of rock at a rate of 0.1 bits per second.
Although only capable of sending one alphanumeric character every minute, this is still an experimental tour de force that demonstrates the feasibility of using neutrino beams to provide a low-rate communications link independent of any electromagnetic radiation. However, given the limited range, low data rate, and extreme technologies required to achieve this demonstration, significant improvements in neutrino beams and detectors will be required for “practical” applications of neutrino communications.
Neutrinos are only affected by the two weakest of the four forces, the weak nuclear force and gravity (and they're not exactly sure about gravity). Their mass is smaller than a millionth of that of an electron, the next lightest particle. They are difficult to generate, and even harder to detect. About 50 billion neutrinos pass through your body every second, leaving no sign of their passage, and they will pass through light-years of lead before being stopped. There are three types of neutrinos, and they rapidly transform into each other. As a final bit of strangeness, they are likely their own antiparticles. Despite the difficulties presented in experimental neutrino physics and engineering, this field of work has its own fascination, and has resulted in the award of three Nobel Prizes.
Fermilab's Main Injector neutrino beam is one of the most intense high energy neutrino beams in existence. A particle accelerator directs pulses of 120 GeV (billion electron volt energy) protons onto a carbon target every 2.2 seconds. The pulses are about 8 microseconds long, and the collision produces huge quantities of highly energetic mesons (quark-antiquark pairs). These mesons decay into muon neutrinos and other particles, which continue to travel in roughly the same direction as did the decaying mesons. The particle beam is then passed through 240 meters of rock, which absorbs all particles except the neutrinos. The average energy of the neutrinos is about 3 GeV. The detector is in a cavern about a kilometer from the carbon target, and at the detector the beam spreads out to a few meters in diameter.
The MINERvA (Main INjector ExpeRiment for v-A) detector is designed to provide new information on nuclear reactions involving neutrinos and on neutrino oscillations. It observes the trajectories of particles emitted when neutrinos interact with particles in nuclei of the material making up the detector. MINERvA, combined with the Main Injector neutrino beam, is designed to measure more than 16 million neutrino events during four years of beam time.
Sixteen million events over 4 years is about one event every 8 seconds. During the neutrino communications experiment, the accelerator beam was being run at half intensity in preparation for scheduled downtime, so one would expect a detection rate of about one every 16 seconds.
Deciding that on-off keying (e.g. Morse code) would provide the clearest signal, the experimenters designed their signal to send the word "neutrino" in a 5-bit subset of conventional ASCII code. This gave a message of 40 bits, which was encoded using the NASA/ESA Planetary Standard, and then combined with a 64-bit pseudo-noise synchronization sequence, forming a 156 bit transmission which was transmitted by the neutrino beam for over 3000 cycles. The data showed 15 frames to clearly be in correct synchronization, and the results from those frames were combined for the purpose of extracting the message. If any events were detected during a particular 8 microsecond pulse, it was counted as a 1, and otherwise as a zero bit.
Information theoretic analysis of the synchronized data showed considerable robustness of the message transmission. When the data was refined using the embedded error-correcting code, the effective information transmission rate was found to be about 0.37 bits/pulse. Sending the message only three times would produce an accurate copy at the receiving end. When corrected for the fact that the 156 bit message contained only 40 bits of information, the experimental data rate was estimated as roughly 0.1 bits per second.
Fermilab has shown that long-distance communication using neutrinos is possible, but only just, with current technology. More sensitive detectors and particle accelerators optimized for neutrino communications will be needed for the promise of this early demonstration to approach practicality.
Source: University of Rochester
