Quantum hologram pushes back the limits of information density.

A pattern of carbon monoxide molecules (top) creates a quantum hologram (middle). The input image can be accurately read (bottom). Credit: Nature Nanotech.

The ones and zeroes that propel the digital world — the fording of electrons across a transistor, or hard drives reliant on electrons' intrinsic spin — are getting packed into smaller and smaller spaces. The limit was thought to be set: no more than one bit of information could be encoded on an atom or electron.

But now, researchers at Stanford University in Palo Alto, California, have used another feature of the electron — its tendency to bounce probabilistically between different quantum states — to create holograms that pack information into subatomic spaces. By encoding information into the electron's quantum shape, or wave function, the researchers were able to create a holographic drawing that contained 35 bits per electron.

"Our results will challenge some fundamental assumptions people had about the ultimate limits of information storage," says graduate student Chris Moon, one of the authors of the work published in Nature Nanotechnology1.

Pushing the limit

The researchers have built on a tradition of inscribing information in small spaces that began when eminent physicist Richard Feynman asked in 1959, "Why cannot we write the entire 24 volumes of the Encyclopedia Brittanica on the head of a pin?". A benchmark was achieved in 1989, when researchers at IBM manipulated individual xenon atoms on a nickel plate to spell out the letters 'IBM' across a space just a dozen nanometres wide2. Feynman's challenge was met — and then some.

But Moon and his colleagues saw a way to go smaller by using a quantum analogy to the conventional hologram. They would use the quantum properties of electrons, rather than photons, as their source of 'illumination'.

Using a scanning tunnelling microscope, they stuck carbon monoxide molecules onto a layer of copper — their holographic plate. The molecules were positioned to create speckled patterns that would result in a holographic 'S'. The sea of electrons that exists naturally at the surface of the copper layer served as their illumination. Just as water bouncing off stones in a show pond create a rippling wave patterns, these electrons interfere with the carbon monoxide molecules to create a quantum hologram.

The researchers read the hologram using the microscope to measure the energy state of a single electron wave function. They showed they could read out an 'S' — for Stanford — with features as small as 0.3 nanometres.

Quantum circuits

In addition to breaking the atomic limit for information storage, the researchers demonstrated one of the essential features of holography. They stacked two layers, or pages, of information — in this case, an 'S' and a 'U' — within the same hologram. They teased out the individual pages by scanning the hologram for electrons at different energy levels.

This led the Stanford team to think about the creation of quantum circuits. In encoding the 'S', the researchers were concentrating the electron density at certain points and energy levels. And a concentration of electrons in space is, in essence, a wire. That led study co-author Hari Manoharan to think about using the holograms as stackable quantum circuits — which may eventually be needed to wire together a quantum computer. "You would change the energy level and you would have a different set of a wiring," he says.

The result is "very interesting", says Alexander Sergienko, a quantum-optics physicist at Boston University in Massachusetts. But he says that the technique is a long way from having any practical use. Conventional optical holography — which has its own community of researchers who are trying to store information densely — can take advantage of charge-coupled device (CCD) cameras that read out the information in parallel. The quantum electron holography, for now, requires the tunnelling microscope, which traverses the hologram more slowly.

"To make it practical, one needs to think about the future development of read-out systems," Sergienko says. "Right now, it is an excellent proof of principle."

References 1 Moon, C. R., Mattos, L. S., Foster, B. K., Zeltzer, G. & Manoharan, H. C. Nature Nanotech. Advanced online publication doi:10.1038/nnano.2008.415 (2009). 2 Eigler, D. M. & Schweizer, E. K. Nature 344, 524-526 (1990) Download references

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