The bars are cut, they open them slowly as the camera goes through and put them back before the camera comes back. If you look closely as the camera goes through the one on the right moves a little faster then the one on the right.

From wikipedia . . .

The long final shot:

There were a number of reasons why the shot proved so difficult to accomplish and is so studied by film students. The shot needed to be taken in the evening towards dusk to minimize the light difference between interior and exterior. Since the shot was continuous, it was not possible to adjust the lens aperture at the moment when the camera passed from the room to the square. As such, the scene could only be shot between 5:00 and 7:30 in the evening.

Difficulties were further compounded by atmospheric conditions. The weather in Spain was windy and dusty. For the shot to work, the atmosphere needed to be still to ensure that the movement of the camera would be smooth. Antonioni tried to encase the camera in a sphere to lessen the impact of the wind, but then it couldn't get through the window.

Then there were further technical problems. The camera ran on a ceiling track in the hotel room, and when it emerged outside the window it was picked up by a hook suspended on a giant crane that was nearly thirty metres high. A system of gyroscopes had to be fitted to the camera to mask the change from a smooth track to the less smooth and more mobile crane. The bars on the outside of the window were fitted on hinges. As the camera came up to the bars they were swung away at the same time as the hook of the crane attached itself to the camera as it left the tracks. The whole operation was co-ordinated by Antonioni from a van by means of monitors and microphones to assistants who, in turn, communicated his instructions to the actors and the operators.

In the DVD commentary, Nicholson states that Antonioni constructed the entire hotel entirely so that the final shot could be accomplished.