Physicists in the US have developed a new platform for trapping and rapidly manipulating the positions of nanoscale quantum objects. Justus Ndukaife and colleagues at Vanderbilt University and Oak Ridge National Laboratory used a combination of gold nanopillar arrays and a specialized optical tweezer to transport individual nanodiamonds to specific locations within just a few seconds. Their techniques could pave the way for a diverse range of advanced quantum technologies.
Suspended colloidal nanodiamonds are highly effective tools for enhancing interactions between light and matter. Measuring less than 100 nm in diameter, each nanodiamond contains a point defect known as a nitrogen-vacancy centre that can emit single photons under room-temperature conditions – a key building block for quantum photonics.
To exploit these point defects in practical applications, the nanodiamonds’ emission properties must first be enhanced by trapping groups of them and then creating entanglement between the spin states of their nitrogen-vacancy centres. Previously, this has been done via a combination of optical tweezers and arrays of nanopillars that act like tiny antennas. When illuminated by their resonant wavelength, these structures create highly localized and enhanced electromagnetic fields within volumes that are much smaller than the smallest possible spot sizes of optical tweezer laser beams, thereby trapping nanoparticles within deep, narrow wells.
Despite these advanced capabilities, however, researchers have so far only been able to rapidly confine nanodiamonds at specific positions defined by the locations of the nanoantennas. It remains extremely difficult to transport individual particles into positions outside these “hotspots”, meaning that it can take hours to assemble groups of nanodiamonds with entangled nitrogen vacancy centres.
Field distortion
In their study, which is published in Nano Letters, Ndukaife’s team overcame this issue by developing a new manipulation tool known as a low-frequency electrothermoplasmonic tweezer (LFET). As well as a laser beam, this device incorporates an alternating current (AC) electric field that induces thermal gradients in the nanopillar array, distorting the electric field it experiences. This combination allowed the researchers to establish a robust electrohydrodynamic potential capable of stably trapping and dynamically manipulating individual nanoparticles.
Tiny optical tweezer traps nanoscale objects
As a proof of concept, the researchers used the LFET to trap a single nanodiamond on top of an array of gold nanopillars and manipulate it by moving a near-infrared laser beam over the array. This nanoparticle transport method proved so rapid that Ndukaife and colleagues ultimately cut the time required to assemble a group of nanodiamonds from several hours to just a few seconds.
The researchers hope that their techniques will pave the way for scalable assemblies of ultra-bright sources of single photons. Ultimately, the LFET could become a reliable tool for fabricating large, stable systems of quantum bits (qubits), thereby opening up new capabilities for technologies such as on-chip quantum information processing and low-noise, high-resolution quantum sensors.
