New research bridges quantum devices and optical networks

As early as 1935, both Albert Einstein and Erwin Schroedinger saw that entanglement lies at the heart of the quantum mechanics of elementary and atomic particles. However, the ability to observe and control entanglement in macroscopic objects has been a long-sought goal. Achieving it reliably at room temperature could have far-reaching implications for quantum technologies.

New research published in Science, shows how entangling macroscopic matter with travelling light could help connect future quantum technologies. Physicists have demonstrated quantum entanglement between the motion of a 0.1µm wide levitating glass sphere and a beam of light at room temperature. This advance could help bridge the gap between stationary quantum devices and optical quantum communications.

Macroscopic Entanglement

Quantum entanglement is the coupling between objects whereby, no matter how far apart the objects sit, measuring a property of one tells reveals information about a property of the other. The coupling is not mediated by a signal traveling between the two objects. Instead, it is a collective characteristic of both, created the instant the entanglement coupling forms. However, the larger an object is, the harder (much harder) it becomes to form and maintain that coupling.

Light moves quickly and can carry information over long distances, whereas a solid object can hold information in place, allowing it to act as a memory or storage. This underpins the importance of bringing together three normally difficult ingredients: a macroscopic mechanical object, travelling light, and quantum entanglement. And, all operating at room temperature.

The research described by researchers in Science points towards achieving this goal, enabling new interfaces between stationary quantum systems and light that can carry quantum correlations elsewhere. Their experiment used a silica nanosphere roughly 0.1µm wide, held without physical contact by an optical tweezer inside an optical cavity at room temperature.  

Although entanglement is routinely demonstrated with microscopic particles such as photons, creating persistent entanglement involving the motion of a larger mechanical object is much harder because interaction with its environment tends to destroy fragile quantum states. Previous optomechanical demonstrations have therefore often relied upon ultra-cryogenic conditions.

The new research demonstrates a different approach. Two optical fields with opposite detunings interacted with the levitated particle. A red-detuned field cooled and stabilised its mechanical motion, while a blue-detuned field produced a two-mode squeezing interaction capable of entangling that motion with the optical field. The researchers observed the tell-tale signs distinguishing entangled states.

Quantum Communication

Crucially, the researchers detected entanglement not simply between systems confined inside the cavity, but between the nanosphere's centre-of-mass motion and a propagating optical mode leaving the interaction region.

That distinction could be technologically important. A mechanical oscillator can provide a stationary quantum system, whereas light provides a natural means of transmitting quantum information through space. Entangling the two therefore provides an interface between stationary and travelling, or “flying”, quantum degrees of freedom.

The researchers consequently identify levitated optomechanical systems as promising platforms for quantum communication. The result potentially points the way to a room-temperature quantum memory or network, and demonstrates an important ingredient: quantum correlations between a comparatively macroscopic mechanical system and light that can propagate beyond it.