Researchers at the State University of New York at Stony Brook (SUNY Stony Brook) and the University of Oregon (Eugene, OR) have devised a wave-particle correlator to detect a photon and then to measure the magnetic field, thereby experimentally connecting the wave and particle aspects of light for the first time.1
Individual photons were generated in a cavity quantum electrodynamic (QED) system by an optically pumped beam of rubidium (Rb) atoms passing through a Fabry-Perot cavity, driven by a 780-nm, Ti:sapphire laser. "Only a few thousand photons emerge from it every second as compared to the trillions that are emitted from a dim household flashlight," said Luis Orozco, research-team leader at SUNY Stony Brook.
The high finesse cavity consisted of a 10-ppm (parts per million) transmission input mirror and a 285-ppm transmission output mirror. "The two mirrors are of such high quality that there are more than 20,000 reflections inside the optical cavity compared to the 10 reflections that are visible when two common bathroom mirrors face each other," he said.
The relatively sparse emission of single photons provided a nonclassical light source for the wave-particle correlation system based on a Mach-Zehnder interferometer with a local oscillator (LO) and a balanced homodyne detector (BHD) in one arm, and the cavity QED system in the other (see figure on p. 22).
Detection of a single photon by an avalanche photodetector (APD) primed a digital oscilloscope to measure field fluctuations caused by subsequent photon emissions. It also enabled selective averaging of the conditionally measured interference pattern between the weak optical signal from the cavity, in which a second photon was only emitted about 10% of the time, and the much stronger signal of the LO. Consequently, the photon field measurements produced a signal on the order of one-tenth of a photon.
"The main idea was to look at the evolution of a wave function in time, and the state produced in the cavity QED system does show a time evolution, which is not a simple decay," Orozco said. "So it shows basically the exchange of excitation between the two normal modes of the cavity and the atom."
The conditional measurement produced by the correlator also produced an unexpected observation. Howard Carmichael's research group at the University of Oregon showed that the conditional measurement was actually the Fourier Transform of the spectrum of squeezing, in which quantum mechanical uncertainties are redistributed between the two variables that define the quantum-mechanical state.
"So you prepare a state and then look at the second photon that is emitted," Orozco said. "And it is these correlated pairs of photons that squeeze the light."
Unlike the particle-to-particle correlations, used to measure stars as well as microscopic dimensions in high-energy physics that are too small for conventional imaging, wave-to-particle correlations preserve wave information.
Particle-to-particle correlations have been used for 40 years to measure stars, and in the same sense, high-energy physicists use them for measurement when they try to make little fireballs with a few quarks, Orozco said. "If you have two detectors pointing in the same direction and they detect two particles in coincidence, then most likely there was a little fireball where the two detectors were pointing." The procedure gives great resolution, he said. But it simply measures size and does not image the object.
Orozco believes particle-to-wave correlation can be used to learn more about the material, such as the index of refraction. "Maybe we can use them in microscopy for intensity-field correlations," he said. "Then we can look at the phase of the field, and it is the phase that allows you to then do a hologram, get the third dimension or get extra information about what material the light is passing through."
Reference
- G. T. Foster et al., Phys. Rev. Lett. 85(15), 3149.