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Quantum effect improves tomography

27 Aug 2003

Using a pair of entangled photons significantly improves the resolution of optical coherence tomography.

Scientists at Boston University in the US have improved the axial resolution of optical coherence tomography (OCT) by a factor of five (Physical Review Letters 91 083601).

Malvin Teich and colleagues used a two-photon source in what they claim is the first experimental demonstration of ‘quantum OCT’.

OCT is an imaging technique that uses a broadband light source to generate an optical cross-section of biological tissue. It is becoming increasingly widespread in commercial medical applications, particularly for ophthalmology.

But unlike the conventional method, the new quantum technique uses two ‘entangled’ photons that are produced when a 406 nm beam from a krypton-ion laser strikes a lithium iodate (LiIO3) crystal.

Down-converted pairs of photons exiting the nonlinear crystal share the same polarization and central frequency around 812 nm, and are said to be entangled. One photon is directed along a beam path towards the sample and the other towards a mirror. These beams are then recombined via a beamsplitter onto a pair of photon-counting detectors to generate an interferogram.

In this way, the technique resembles a two-photon interferometer, with the sample placed in one arm of the apparatus.

Teich and colleagues imaged a piece of fused silica sandwiched between two zinc selenide windows to demonstrate the effect on resolution. With standard OCT, the silica windows could be imaged to a resolution of 92 µm. However, QOCT improved this to 18.5 µm.

Using entangled photons improves the resolution in two ways, explains Teich. First, there is an automatic improvement simply by using two photons for imaging instead of the normal one. When the photons are entangled, it improves axial resolution by a factor of two.

The second improvement is due to the elimination of dispersion effects. In conventional OCT, resolution is enhanced by increasing the bandwidth of the illuminating light source. However, the broader bandwidth introduces more group-velocity dispersion, which has a detrimental effect on resolution.

Using entangled photons automatically cancels out the dispersion effects, and combined with the two-photon advantage it improves resolution by a factor of five.

This can be seen by the effect of the zinc selenide windows on QOCT resolution, say the researchers. With normal OCT, the introduction of these dispersive windows degraded the technique’s resolving power, while the resolution of QOCT was unaffected.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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