Paul van Gerven
21 June 2021

A new type of imaging device takes advantage of quantum mechanical entanglement to see finer details.

Ever since their invention in the 17th century, microscopes have revolutionized our understanding of life. As technology advanced, biologists have been able to discern increasingly fine details of tissue and cells. Peeking ever deeper into the nanorealm, however, requires ever more light: today’s best microscopes employ laser light that’s billions of times brighter than the sun. That gets you great pictures, but it also burns the sample within seconds. So tracking biological processes in time, for example, is impossible.

University of Queensland researchers have found a way to image smaller details without cranking up the brightness. By entangling photons in correlated pairs, they managed a 35 percent signal-to-noise improvement, allowing detection of molecular samples at 14 percent lower concentration.

Queensland quantum microscope
Credit: University of Queensland

Shot noise

The new technique builds on a popular bio-imaging technique called stimulated Raman scattering. SRS is based on the Raman effect, which occurs when a photon is inelastically scattered by a molecule. This causes the light particle, which after scattering is called the Stokes photon, to lose an amount of energy that’s characteristic for the molecule it bounced off. By collecting Stokes photons, the position of the molecules of interest can be pinpointed, producing an image.

The Raman effect, however, is quite weak because most photons are elastically scattered without loss of energy. In SRS, this is overcome by ‘amplifying’ the Stokes signal. It works by illuminating a sample with two lasers. A so-called pump laser shoots photons at molecules of interest, prompting them to emit Stokes photons. This emission is encouraged by the second laser, which irradiates the sample at the Stokes frequency – hence the term stimulated Raman scattering. The signal is enhanced even more because of constructive interference occurring between scattered Stokes and laser photons.

While the Stokes signal is increased, it still has to be distinguished from the much larger background of the Stokes laser field. This requires careful amplification while eliminating as many sources of noise as possible. The detection sensitivity, however, is fundamentally limited by the fact that laser light is composed of discrete units. This causes random fluctuations in the number of photons hitting the detector at any given moment, producing what’s called “shot noise.”

Increasing the number of photons smooths out the statistics, but typically, SRS imaging is already carried out at laser intensities just low enough to not damage the sample. So, it seems researchers have run out of options to decrease shot noise any further.

State of the art

Enter some Australian quantum mechanical trickery. By entangling photons in the Stokes laser beam, they’re no longer completely independent of each other. This reduces randomness and therefore shot noise. Since the signal itself is unaffected, the signal-to-noise ratio increases.

The improvement is relatively modest, and the performance in the proof-of-principle experiment in Queensland is still below that of state-of-the-art SRS systems. “Nevertheless, the work underlines the exciting possibilities of using quantum light in optical imaging techniques. Despite the challenges ahead, it’s likely to transform SRS microscopy for the better,” commented Eric Potma of the University of California, Irvine.