There is a famous scene in Star Wars in which Princess Leia appears in the air, projected as a flickering 3-D hologram. "Help me, Obi-Wan," she says. "You're my only hope."

"It looks like you could just reach out and touch the image. That's the beauty of it," says Nasser Peyghambarian of the University of Arizona. But he's not talking about the cinematic trickery of George Lucas. He's referring to the real thing: a series of ghostly 3-D projections that hang in the air in his laboratory. A human skull, a brain, a molecule - they leap out of the screen into the air, hovering in grainy monochrome.

Onlookers can't quite believe what they're seeing with their naked eyes. They tilt their head around, but no matter which angle they look at it, the skull is there in 3-D. "People look confused, like a cat in front of a mirror," says Peyghambarian. "They are seeing things in the air that do not exist."

His team have given holography an added dimension, by creating the first updatable 3-D holographic display. Instead of projecting a single, static image, it refreshes and updates. Very slowly, an animation can develop.

"We have broken a technological barrier," he says. "Your credit card has a 3-D hologram which is permanently printed there - it cannot be changed. But this new display gives the image dynamic capability. You can write a new 3-D image, and then another, and then another in sequence."

In the not-too-distant future, Peyghambarian envisages his screens in department-store windows, dazzling customers with 3-D adverts of new clothing lines. Ultimately, he believes, they could take us into a new era of 3-D movies and television - without those rubbish coloured specs. Football matches and rock concerts would leap out into your front room.

But it's not Sky Sports or MTV who are funding holographic research. It's the US Department of Defence. Specifically, the Air Force Office of Scientific Research.

"They are interested in battlefield simulations," says Peyghambarian. "Commanders could sit around a table and look at a 3-D image of the battleground, then move things around and see the effect of that."

The technology could also have medical applications: surgery, for example. "Right now, brain surgeons have to take a whole load of 2-D brain scans and lay them out on a table. Using this technology, all of that information could be put into a 3-D image which they can all look at simultaneously. If you give the surgeon a proper look at what they're cutting, it could become a lot more efficient."

So what is the secret of this 3-D display technology? It turns out to be a new form of photorefractive polymer - a type of plastic - invented by Peyghambarian and unveiled today in the journal Nature. "The new material combines the three essential properties we need to make this technology - to write the image data very rapidly, to hold it for a long time and to have the erasability function," he says. "These properties combine perfectly, like a recipe of spices."

The holographic screen is made up of tiny units of refractive material - 1mm by 1mm. Each can be loaded with the image of a particular point on the skull, photographed from many different perspectives. When a laser passes through the unit, it scatters the light in such a way that the image is projected in realistic 3-D.

"To picture the object, we photograph it simultaneously from different angles, capturing all the 2-D perspectives. These are reassembled in the hologram, so the viewer sees a 3-D picture, just like a human eye would," says Peyghambarian.

"But at present, the picture is effectively static, because it takes two minutes to write each new image. To get into the area of 3-D movies and videos, we would need to write the information much faster - dozens of images per second. That's very much possible, but we have to make the material respond much faster."

He wants the palm-size screen to become larger in size - metres rather than centimetres - and to project full-colour images instead of monochrome. "We're talking about the next two to five years to work on these," he says. "I think we've already cleared the big hurdle - the quantum leap."

For film fans and video-gamers itching to get inside the action, the wait might not be too long.