Atomic impurities, or defects, in natural diamond lead to the colour seen in pink, blue and yellow diamonds. One such defect, the nitrogen-vacancy (NV) centre, occurs when two neighbouring carbon atoms in diamond are replaced by a nitrogen atom and an empty lattice site.

The NV defect behaves very much like an ion trapped in the diamond crystal, explains team leader Dirk Englund of the Massachusetts Institute of Technology (MIT). This is because the electrons on the NV are well decoupled from the diamond material. This also means that researchers can orient the spins of these electrons and measure them using ordinary optical spectroscopy techniques. What is more, an NV’s electron spin can be entangled with the polarization state of a photon and such spin-photon entanglement might help in the development of quantum networks and distributed quantum computers in the future.

NVs in nanodiamond could also be used as biological probes and sensors because they are non-toxic, photostable and can easily be inserted into living cells. They are capable of detecting the very weak magnetic fields that come from surrounding electronic or nuclear spins too, and so can be used as highly sensitive magnetic resonance probes capable of monitoring local spin changes in a target material over distances of just tens of nanometres.

Efficiently collecting NV light

When illuminated with green laser light, an NV centre emits red fluorescence, whose intensity depends on the orientation of the NV’s electron spin. A major challenge here is to efficiently detect this light, and the more light that can be detected, the better the NV application, says Englund. Collecting this light has proved difficult until now because of the high refractive index of diamond, which traps light by total internal reflection. Previous attempts to overcome this problem have included coupling NVs to optical cavities to enhance the light emitted from the defects, and building solid immersion lenses around NVs.

Now, Englund and colleagues at MIT together with co-workers at Columbia University in New York and Element Six in Santa Clara, California, say that by etching a circular, bull's-eye-shaped grating in the diamond membrane containing the NV, they have succeeded in collecting nearly three million photons per second from the structure. This is the highest ever value reported to date from a single NV.

Moreover, the researchers say that they have also measured a spin coherence time (the time the defect conserves its spin state before “decohering”, or collapsing) for their NV of around 1.7 milliseconds. This value not only compares well to the highest reported spin coherence times measured in previously made NVs at room temperature, but also proves that the fabrication process to make the bull's-eye does not degrade the spin properties of an NV.

An order of magnitude more fluorescence

“The efficiency with which we can collect photons from an NV determines how fast we can measure the NV’s spin state,” Englund tells nanotechweb.org. “The more fluorescence we detect, the higher the signal is. Detecting more photons is crucially important for many NV technologies, such as sensing, communication and computing, and with our circular grating, we are able to collect about an order of magnitude more fluorescence than is possible from an NV in unpatterned diamond.”

The bull's-eye grating itself consists of concentric slits etched into the diamond membrane containing the NV. The diamond membrane is about half a wavelength in thickness. The grating period satisfies the so-called second-order Brag condition, which helps scatter light out of the membrane, say MIT team members Luozhou Li and Ed Chen. “The scattered light from each grating interferes constructively out of the plane of the membrane and into the far field – and it is this phenomenon that allows us to collect significantly more photons.”

"Nice advance"

Ronald Walsworth of Harvard University, who was not involved in this work, says that "using a bull's-eye diamond grating to enhance the photon collection efficiency of single NV centres in diamond, while maintaining good NV spin coherence time, is a nice advance that may aid diamond-based sensing and metrology."

The MIT researchers reckon that such efficient photon collection should allow for a whole new range of, hitherto impossible, experiments, such as “non-demolition” measurements of NV spins. “Here, you could measure an NV spin and then ‘act back’ on this spin state,” explains Englund. “We are also using our technique to make medium-scale quantum registers that would contain tens of quantum bits (or qubits) made from NVs – for quantum sensing applications.”

The present work is detailed in Nano Letters DOI: 10.1021/nl503451j.