Noise is a lethal enemy of quantum information systems and even the slightest amount of it could prevent a quantum computer from working. But now, Seth Lloyd from the Massachusetts Institute of Technology (MIT) in the US has proposed a way to exploit this sensitivity to noise to create a novel quantum imaging system. He believes that the system could offer an exponential improvement in signal-to-noise over conventional optical imaging techniques, although he admits that implementing it in practice will not be easy.

Conventional optical imaging systems such as a microscope work by shining light onto an object and detecting the light that is reflected back. This is a simple process as long as the system isn’t operating in a noisy environment in which photons from random sources get mixed up with the reflected light.

Discriminating detector

While a conventional detector has no way of discriminating between a photon of reflected light and a similar photon of noise, Lloyd believes that the quantum principle of entanglement could be harnessed to filter out the noise.

Lloyd’s system involves creating pairs of photons that are entangled quantum states. Entanglement is a feature of quantum mechanics that allows particles to share a much closer relationship than classical physics allows. An important feature of entanglement is that the photons retain a “memory” of being created as a pair.

In Lloyd's scheme, one entangled photon (called the signal) is directed at the object of interest, while the other (the ancilla) is retained at the imaging device for future reference (arXiv: 0803.2022). If a signal photon reflects from the object and returns to the imager, it can be compared to the ancilla, which retains a memory of its entangled partner. If the partnership is verified the photon is used to build an image of the object. However, if the ancilla has no memory of the photon, it is rejected as noise.

According to Lloyd’s calculations, an imaging device using his scheme would have a signal-to-noise ratio that is 2e times that of a conventional system. Where 2e is a measure of the degree of entanglement of the two photons in terms of the number of modes of the electromagnetic field that are entangled between the two photons.

Photonic crystal

The challenge, however, is how to compare the signal and ancilla photons. In principle, Lloyd believes that this could be allowing the two photons to recombine to create a single high-energy photon by firing them into a photonic crystal — a special material that contains regularly alternating regions with high and low refractive indices.

This is the reverse of the “down-conversion” technique that is commonly used to make entangled photons in the first place. According to Lloyd, the two photons are more likely to recombine to make a single higher-energy photon, which can be detected, if they retain a memory of entanglement.

But this requires a system that can put both reflected signal and ancilla at the same place at the same time, which Lloyd admits is no mean feat. However, he points out that there is no reason why it couldn’t be done — and that it should be relatively easy to achieve compared to other quantum-information processes that physicists are currently trying to develop.

Lloyd told that a possible application of the system is enhancing the performance of optical communications systems.