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The quantum zeno effect is one of the stranger and more fascinating consequences of quantum mechanics. It offers a surprising and counterintuitive way of controlling quantum systems that are changing from one state to another.

That sounds useful but the ability to harness the quantum zeno effect has so far eluded physicists. Now Yu-Ping Huang and buddies at Northwestern University in Evanston Illinois, say they’ve worked out how to use the quantum zeno effect to make an “interaction free” switch.

First a little more about the effect itself. Imagine a photon in state 0 which has a certain probability of decaying into state 1. Now carry out a series of periodic measurements on the photon. Between the measurements, the photon evolves into a superposition 0 and 1 states and a measurement will cause it collapse into one or other of these.

However, if the time between the measurements is small, the chances of it collapsing to form a 1 are smaller than the chances of it becoming a 0. And if the periodic measurements are made rapidly enough, the probability of a measurement producing a 1 tends to zero.

In effect, the process of repeated measurement prevents the photon decaying from a 0 to a 1. That’s the quantum zeno effect, sometimes also called the watched-pot-never-boils effect.

Now Huang and co have come up with a scheme that exploits this effect to create a switch. The basic idea is to take a signal wave in state 0 which will decay or evolve into a 1 when it passes it through a nonlinear waveguide.

However, Huang and pals point out that measuring the wave will prevent this evolution. They say they can perform this “measurement” by making the signal wave interact with another “control” wave.

So the presence of the control wave maintains the signal wave in a 0 state while the absence of the control waves causes the signal wave to switch to a 1. And that’s it: an all optical switch that is interaction-free because it is the absence of the control wave that causes the switch.

This device’s potential is interesting because it offers a number of important advantages over conventional all-optical switches. First, this type of switch should operate at extremely low power since there is no signal loss associated with the switching process. That’s in stark contrast to other types of switching where optical losses are an important limitation of switching performance.

Second, the quantum state of the signal wave is preserved. That’s a biggie. If this kind of switch works in practice, it could become the heart of quantum routers that will make a kind of quantum internet possible.

It’s early days yet, however. Huang and co have so far investigated the properties of their device only theoretically and others are hot on their heels and possibly ahead of them.

What’s for sure is that we’re likely to hear a lot more abut zeno switches. This team’s work is being funded by a project called the Zeno-Based Optoelectronics program set up by the Defense Advanced Projects Agency. This paper is one of the first to come out that program. Real devices shouldn’t be far behind.

Ref: arxiv.org/abs/1008.2408: Interaction-Free All-Optical Switching via Quantum-Zeno Effect

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