First successful control of 'quantum fluctuations' Science
U.S. scientists have demonstrated a new technique for exploiting random energy fluctuations present in the void and biasing the fluctuations with an applied field. The researchers believe that the technique could be applied to sensing and random number generation in probabilistic optical computing.
The research results were published in Science on July 13 under the title "Biasing the quantum vacuum to control macroscopic probability distributions".
Just as Heisenberg's uncertainty principle prohibits particles from having no momentum at all, it also prohibits a system from having no energy at all. Thus, in quantum mechanics, there are tiny electric field fluctuations of random frequency in the vacuum. These fluctuations are called quantum fluctuations; they are usually too small and experimentally irrelevant, but they can become important in specific situations.
For example, in 2021, theoretical physicist Ortwin Hess of Trinity College Dublin and colleagues led by Hui Cao of Yale University in Connecticut used these fluctuations to generate a random number generator from a multimode laser.Hess explains, "In the description of lasers that we were using at the time, we described the many modes of interaction that were generated unpredictability and hopping, but this was a very interesting result that allowed quantum fluctuations (fluctuations) to be captured."
Although sets of truly random numbers are widely used in cryptography and computer simulation, their generation is notoriously difficult. This has made Cao and Hess' work of great interest outside the field of quantum optics.
In this new work, researchers at the Massachusetts Institute of Technology (MIT) have taken the concept a step further by applying an external signal to interfere with quantum fluctuations and measuring the effect of this interference.Yannick Salamin, Charles Roques-Carmes and colleagues placed a lithium niobate crystal in an optical cavity and pumped it with photons from a laser . This created an excited state in the crystal, and the decay of the excited state produced two photons with exactly half the energy of the pumped photons.
"The phases of these photons are completely random because they are triggered by vacuum energy fluctuations." Salamin explains, "But now the photons will circulate within the cavity, which can energize and amplify the same photon when the next one arrives. Due to the physical nature of this effect, only two possible phases can be amplified."
Tuning the probability distribution of a multistable system by biasing vacuum energy fluctuations.
The photon is initially amplified by both phases, but the system undergoes a "bifurcation transition", where one of the modes is selected once the mode has accumulated enough energy to overcome losses. "Once it enters a steady state, the result is fixed." Roques-Carmes explains, "If you want a new sample, you have to restart the whole process, go back to the vacuum distribution, and go through the bifurcation again."
With no external bias applied, the cavity was equally likely to end up in either of the two possible modes, and the relative frequencies of the various combinations of results formed a perfect Gaussian distribution after repeating the experiment. The researchers then applied a pulsed electromagnetic field and attenuated it to a level comparable to the vacuum fluctuations. They found that while the system could still be stabilized in either state, they could bias the probability of the system choosing a state. When they applied a stronger bias, the system always chose the same state.
The team is now looking at possible applications - including probabilistic computing. roques-Carmes says: "The general idea is that by coupling many p bits (probability bits) together, we can build a p computer. In many areas of science, you want to be able to encode uncertainty ...... We plan to take this photonic p-bit and incorporate it into a photonic processing unit." More than that, the researchers are looking at the possibility of using the system's ability to respond to small electric fields to create sensors.
Experimental demonstration of a photonic p-site in a biased OPO.
Measurement of subphoton level ultrashort pulses.
These findings provide a platform for studying quantum dynamics in nonlinearly driven dissipative systems and point to potential applications in probabilistic computing and weak-field sensing.
Reference links:
[1] https://www.science.org/doi/10.1126/science.adh4920
[2] https://physicsworld.com/a/quantum-fluctuations-are-controlled-for-the-first-time-say-optics-researchers/
