AUSTRALIA: Tracking a single atom

Scientists working in the field of quantum mechanics must forget the rules of the Newtonian world and enter another universe where the impossibly small behave in the strangest ways. But what is an ordinary Earthling to make of a scene where physicists place a single atom of nitrogen inside the crystal lattice of a 'nano diamond', itself thousands of times smaller than the diameter of a human hair, insert it into a single human cell and follow the atom's movements?

Not only that: if the physicists excite the nitrogen atom so it occupies what they call "a ground state and an excited state" at the same time, the nano-diamond then behaves like a minute magnetic compass that can be aligned while also emitting red "sparks" when struck by a laser light. Under a microscope, the researchers can see the tiny dots of light sparkling like a far-off star and determine the orientation of the nano diamond in which the nitrogen atom has been placed.

This weird scenario might seem improbable yet Professor Lloyd Hollenberg and a multi-disciplinary team from Melbourne University have done just that. In a world first, the researchers have tracked the single atom of nitrogen inserted in a human cell with techniques that could boost the testing and development of new drugs.

Hollenberg says it is the first time a single atom encased in a nano diamond molecule only 50 nanometres across has been used as a biological sensor to explore the "nanoscale environment" inside a living cell. A nitrogen atom is about 0.3 nanometres in size while a nanometre is one billionth of a metre or about the same relative size to a metre as a marble is to the Earth. Wikipedia claims a nanometre is the amount an average man's beard grows in the time it takes him to raise the razor to his face.

A diamond is made up tens of thousands of carbon atoms in a tetrahedral lattice and physicists are able to replace one of the carbon atoms with a nitrogen atom and also create a vacancy where a carbon atom should be next to it in the lattice. Hollenberg says this "nitrogen vacancy defect" gives the nano diamond remarkable properties that are highly important in quantum computing.

"The beautiful thing about this diamond system is that the nitrogen atom fluoresces: if you shine a green light on the nano particle, the nitrogen atom absorbs the photons and re-emits the energy as red light. So if we have one these particles on a glass slide and scan our optical microscope over it with a laser exciting the atom, we can actually see the red fluorescence from a single nitrogen atom as tiny pinpoints of light.

"It's amazing that in a laboratory at room temperature, we can control the state the atom is in, its ground state, excited state or both at the same time, and we can tell from its fluorescence not only where it is but what state it is in. Having achieved this, we then wanted to see if we could perform these measurements while the nano diamond, with the nitrogen atom in its lattice, was in a living cell."

Hollenberg says the nitrogen atom is "incredibly special" from a quantum physics point view because it is the only one known that can exist in two states at the same time, to have "long-lived quantum coherence" at room temperature. Usually to maintain an atom in both states at the same time the atom has to be cooled almost to absolute zero. Diamond is also very special because its atoms do not vibrate markedly so it is not necessary to cool it to absolute zero to see the quantum effects. Also diamond is highly bio-compatible and non-toxic.

Although he and other physicists had been experimenting with nano diamonds as part of an investigation into quantum computing, it was after discussing his work with biologists at Melbourne the idea arose of testing a nano diamond in a human cell. With colleagues from the chemistry and chemical and biomolecular engineering departments, Hollenberg and his team coated a nano diamond carrying its nitrogen passenger with a substance that, when the particle was introduced into a cell culture, allowed one of the cells to envelop it.

Because the nano diamond is so small and non-toxic it was easily incorporated into the cell and the scientists could then experiment to see if they could conduct the quantum measurements by controlling the single atom and track it moving around.

"We used microwaves at very low power to direct the atom without doing any damage to the cell. We also had green lasers flashing on and off at low power to excite the atom and then collected information about the fluorescence to infer what was happening to the atom quantum mechanically," Hollenberg says.

"We were the first to prove we could do those measurements on a moving target and that we weren't blowing the cell up immediately when in fact it lasted some 10 hours. As well, we could infer that we were measuring the quantum coherence of the single atom - that is how long it stayed in the two states at the same time.

"Probably the most tangible results were the measurements we made of the quantum system at the atomic level; that meant we could infer the orientation of the diamond molecule which we were using as a tiny compass needle and that told us how, as a function of time, the orientation of the nano diamond was changing."

Hollenberg says biologists would like to use such fluorescing small markers to create an image of the internal structures in a cell. If many of the little dots were introduced into a cell, they would attach to different parts and the fluorescence would enable the scientist to see the cell's structures, "itself an important step in fluorescence microscopy".

He says the value of the research to nano medicine is that it could eventually mean drugs could be piggybacked on nano particles or encased within them and targeted so they were carried to a certain location within a cell.

"This is the way universities should work by researcgers crossing discipline lines. That will be the hallmark of science in this century - people talking to people outside their areas of expertise and achieving things that no one group could have done alone."

* An account of this research was published in Nature Nanotechnology last month, see here