Trafalgar Square, home of Nelson’s Column and a snarling tangle of black cabs and bus routes, is not the sort of place you look for enchantment. Until now. For three nights this week, should you scurry across this London landmark, you might find yourself in the midst of a phantom ballet, surrounded by ghostly dancers and accompanied by faint but familiar music.

Flock is an interactive installation from the digital artists KMA and the dancer and choreographer Tom Sapsford, commissioned by the ICA. The piece is based on Tchaikovsky’s Swan Lake and behind its simple beauty is some very high-tech wizardry.

An area of pavement will be marked out and, as you walk into it, a thermal imaging camera registers your heat signal (yes, like the Terminator). This triggers a spotlight to fall on you and track you around the space until you step out of the marked out area. The space is constantly filled with projections of dancing figures, but these can be seen only when you cross their paths and your spotlight illuminates them for a moment, before they disappear again into the surrounding darkness. (One dancer was filmed making the movements, then the images were built up layer upon layer.)

Following these figures, which move with the music that your presence also sets off, means that you mirror their movements; in your own way, in high heels or work boots, you enter the dance with them. The more people who come into the space, the more dancers appear and the louder the music gets.

If there is nobody in the space, nothing will be heard or seen, says Kit Monkman, KMA’s creative director. “It’s like there’s a kind of ghostly ballet going on but it’s brought to life by the presence of people. If there are lots of people in the space it becomes a big, brassy, camp experience, but if there’s just one person you just hear the very faintest tune.”

Vivienne Gaskin, until recently the ICA’s director of performing arts and digital media, was inspired to stage the project after seeing KMA’s collaboration with Darshan Singh Bhuller for Phoenix Dance Theatre, entitled Eng-er-land, which flooded the stage with projections of a virtual city that shifted in response to the movements of dancers.

Sapsford has made his mark in choreography with projects such as Hypnos, which used film footage of the dancers under hypnosis. To Gaskin, working with KMA seemed an obvious idea. “One of the things I’ve been looking at for the past four or five years is the relationship between technology and liveness — whether that’s interaction with the public, immersive environments or a way of expanding the stage space to create a link between audience and performer,” she says. There was no brief — the team was effectively told to get on with creating some art — and the result of this £10,000 gamble is Flock.

High-tech though the work may be, it has its roots firmly in the very traditional art of dance. Sapsford, who has danced with Michael Clark, Siobhan Davies and the Royal Ballet, says: “The combination of the Tchaikovsky music and the classical pointe work will trigger associations for anybody that will walk across the square.” Flock presented a completely different challenge, he says, to choreographing a complete work like Hypnos. “It doesn’t exist in the physical world in any sense, it’s all imagination. So I’ve tried to keep the patterns simple, yet interesting.”

What makes Flock particularly engaging is the immersiveness of the experience. It’s almost like a life-sized, 3-D computer game in which the players (pedestrians) are in control but working together to enhance the experience.

Sapsford thinks that technology will inevitably become incorporated into performance. “This is an exciting period. In a sense it’s like when people had discovered film but not really realised what a film could be,” he says. “I’ve worked a lot with digital films but this is the first time that I’ve worked on something with such a level of interaction. I love the idea of making the theatrical experience somehow deeper.”

Flock, Trafalgar Square, London WC2 (www.ica.org.uk 020-7930 3647), Thur-Sat, 5-10pm

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