The Joint Quantum Institute’s photonic stopwatch helps quantum experimentation move into the realm of metrology.

Just like human drivers, some photons are faster than others depending on road conditions. Nearly identical materials can make a huge difference to travel time, as discovered by Joint Quantum Institute (JQI), College Park, Md., scientists who built a specialized interferometer that can “stop-watch” individual photons.

A partnership of the National Institute of Standards and Technology (NIST), Gaithersburg, Md., and the Univ. of Maryland, College Park, and Georgetown Univ., Washington, D.C., have found a remarkably large difference in the time it takes photons to pass through nearly identical stacks of materials with different arrangements of refractive layers. The technique ultimately could provide an empirical answer to a long-standing puzzle over how fast light crosses narrow gaps that do not permit the passage of conventional electromagnetic waves.

This diagram shows two stack configurations with odd numbers of layers. Blue layers have a high index of refraction, white layers a low. The stacks are nearly identical with the exception of where the extra layer is deposited. Image: NIST.

Alan Migdall, a NIST fellow at JQI, and his colleagues set up a race course using “correlated” pairs of photons—indistinguishable photons created simultaneously. One photon passes through the sample under test while the other is directed along a path of adjustable length. The finish line is a so-called Hong-Ou-Mandel interferometer, a beamsplitter that the photons strike obliquely. Individual photons have a 50-50 chance of either passing through the beamsplitter or bouncing off of it, but when two correlated photons arrive simultaneously, the rules of physics say they both must come out in the same direction.

As a result, this arrangement can detect when the first photon has taken exactly as long to get through the test object as the second photon did to traverse its path. This changes the difficult problem of measuring extraordinarily short intervals of time into the easier one of measuring distances. Through refinements to the design of their interferometer, Migdall and his colleagues can measure simultaneity with sub-femtosecond precision.

The team measured photon transit times through stacks consisting of alternating layers of material with high and low refractive index—the kind of arrangement that makes a light beam bend as it crosses the boundary.

The new experiments verify the theoretical prediction that photon transit time will vary significantly depending on how the stack is arranged. Migdall and his team found that a photon takes about 20 fs less to get through a stack of 31 layers, totaling a few microns across, when the stack begins and ends with high refractive index layers rather than the opposite. The shorter time delay is apparently superluminal, i.e. shorter, than the time needed for light in a vacuum to traverse the same distance. (This is possible because of a loophole in the speed-of-light limit that says that some wave-related phenomena can propagate superluminally if they do not transmit equivalent information faster than the speed of light.)

The team hopes to move on to a more perplexing case. Light striking the boundary between two refractive materials at a sufficiently shallow angle glances off completely as a reflection rather than passing through, but also creates a decaying field known as an evanescent wave on the other side of the boundary. This evanescent wave can reach across a narrow gap and strike up a new light wave in an adjacent medium. Theorists have presented discrepant calculations of how long light takes to traverse such a gap, but Migdall says the new system should be precise enough to measure such transits directly.

The emerging field of photonic metrology

This research by Migdall and his colleagues at JQI is part of a larger basis of experimentation surrounding photon generation. The type used by Migdall and his team for this experiment was a variant of spontaneous parametric down-conversion (SPDC), in which a UV laser beam is typically used to generate a high-energy photon which can be down-converted to two lower energy photons. Although the researchers at JQI have investigated other forms of photon generation, SPDC is being used for an intensive single photon metrology program, called Quantum Optical Metrology with N-Photons. The researchers are obtaining optical measurements that rely on the quantum-engineered states of light to obtain higher precision than can be obtained using classical states of light.

There are a number of practical applications the team is working toward. First, they are part of a multi-team effort with NIST to build a compact robust entangled photon source for use in quantum cryptography. Path-entangled photon states could be used for precision length measurements in astrophysics, photolithography, and navigation (such as optical gyros).

Further over the horizon, this technique could be used in high-resolution optical microscopy, quantum lithography, non-invasive bioimaging, enhanced N-photon microscopy, and even quantum optical coherence tomography.

For now, JQI’s efforts using Heisenberg-limited interferometry have resulted in both confirmation and dismissal of numerous postulations regarding quantum state behavior and should help to resolve further questions in the near future.

—Paul Livingstone