Small glass ring elevates quantum mechanics to a larger scale
Quantum connection between light and movement
Physicists have demonstrated a system in which light can be converted into a mechanical vibration and vice versa. The interaction is so strong that the movement of an object visible to the naked eye can be controlled at the level where quantum mechanics determines the behaviour of the object. Such quantum behaviour is normally only visible in far smaller systems the size of a few atoms or molecules. Former FOM PhD Dr. Ewold Verhagen is part of the team of researchers from the Swiss EPFL that published the results this week in the journal Nature.
Since the last century it has been clear that the movement of objects must satisfy the laws of quantum mechanics. These predicted a number of amazing possibilities: an object could, in principle, be found at two locations at once, and it will always move a bit, even at the temperature of absolute zero – it will then be in the quantum ‘ground state'. Yet we never see such amazing behaviour in everyday objects because quantum effects can only be observed in systems that are extremely well isolated from their surroundings. In general, large objects are poorly isolated and so the influence of the surroundings quickly erase the quantum mechanical properties of the object in a process called 'decoherence'.
Until recently, scientists could therefore only observe quantum effects in miniscule objects the size of several atoms and molecules. Now the physicists from EPFL under the leadership of Tobias Kippenberg have demonstrated that the movement of an object containing a 100 million million atoms can be controlled at the level where quantum mechanics dominates. The trick is to illuminate the object with laser light.
A racing circuit for light
The researchers made a carefully shaped glass doughnut on a chip, with a diameter of 30 micrometres (about half the thickness of a hair). Just like a wine glass, the glass doughnut can vibrate at the right frequency. Yet at the same time the shape acts as a racing circuit for light, which can circle around in the glass doughnut. As the light goes round the bend it exerts a force on the surface of the glass. Despite its small size, this force has a significant effect in these structures because the light can circle 1 million times around the glass ring. The light force can set the ring in motion and let it vibrate, just like a finger rubbing over the edge of the wine glass. However, it can also attenuate the vibration and in so doing cool down the doughnut.
Cold, colder, ...
Cooling is vital to achieve the state in which quantum mechanical movements can be observed as such movement is normally overshadowed by random thermal movement. The researchers therefore placed the glass doughnut in a cryostat –a piece of equipment that ensures a constant low temperature –which brought it down to a temperature less than one degree above absolute zero. The light force of the laser light shone upon the doughnut then cools the movement of the doughnut 100 times further still until it is nearly at the quantum ground state. An even more important effect, however, is that the interaction between light and movement is so strong that they form an unusual connection: a small light pulse can be completely converted into a small vibration and vice versa. For the first time, this conversion from light to movement happened so quickly that the quantum properties of the light pulse were not erased by decoherence during the transformation process and were therefore transferred to the movement of the structure.
By overcoming decoherence, this result provides a powerful method for manipulating the quantum characteristics of mechanical movement and for observing the amazing predictions of quantum mechanics at work in relatively large man-made objects. This makes it possible to explore the boundary between quantum mechanical and classic mechanical behaviour and to test the laws of quantum mechanics at a much larger scale than has so far been possible. However, there are more possibilities: mechanical oscillators can be coupled not only to light but also to a range of other quantum systems (for example, electric currents). They could therefore be used to 'translate' quantum information from these systems into light signals, which have the advantage that they can be easily transmitted over large distances through optical fibres.
Rubicon
With his thesis, Ewold Verhagen won the FOM Physics Thesis Award 2010. After that, he left for a 24-month period at the École Polytechnique Fédérale de Lausanne, where he carries out his research with a Rucibon grant from NWO. Rubicon is aimed at talented young scientists who have recently gained their doctorates. With the Rubicon grant they gain the opportunity to acquire up to two years of experience at a foreign university or research institute.