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Transport properties

Diamond bull’s-eye collects polarized photons at a rapid rate

18 Mar 2015 Isabelle Dumé
On target: bull's-eye-shaped grating helps extract photons

A new optical grating shaped like a “bull’s-eye” that is extremely efficient at collecting photons from diamond nitrogen vacancy (NV) centres has been built by physicists in the US. The device can collect nearly three million photons per second from a single NV, which is the highest value reported to date. The grating could find use in a number of emerging technologies including nanoscale sensors, single-photon sources and quantum memories.

Atomic impurities, or defects, in natural diamond lead to the pink, blue and yellow colours seen in some 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.

Entangled with photons

For anyone trying to build a quantum computer, NVs are useful because they have an electronic spin that is extremely well isolated from the surrounding lattice – so if an NV is placed in a certain spin state, then it will remain in that state for a long time, even at room temperature. 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 of the future.

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

Efficiently collecting NV light

When illuminated with green laser light, an NV centre emits red light by fluorescence. The intensity of this light 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 Dirk Englund of the Massachusetts Institute of Technology (MIT). Collecting this light has proved to be 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 researchers at Columbia University in New York and Element Six in California, say that by etching a circular, “bull’s-eye”-shaped grating in the diamond membrane containing the NV, they can collect nearly three million photons per second from the structure. This is the highest value reported to date from a single NV.

The bull’s-eye grating consists of concentric slits etched into a diamond membrane containing the NV. The diamond membrane has a thickness that is about half a wavelength of visible light. The grating is centred on the NV and its period satisfies the so-called second-order Bragg condition. This helps to 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,” explain the researchers.

Enduring spins

Moreover, the researchers say that they have also measured a spin-coherence time for the NV (the time that it maintains its spin state) of around 1.7 ms. This value not only compares well with the highest reported spin-coherence times measured in previous NVs at room temperature, but also proves that the bull’s-eye fabrication process does not degrade the spin properties of an NV.

“The efficiency with which we can collect photons from an NV determines how fast we can measure the NV’s spin state,” Englund explains. “The more fluorescence we detect, the higher the signal. 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.”

“Nice advance”

Ronald Walsworth of Harvard University, who was not involved in the 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”.

Englund and colleagues believe 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 bull’s-eye device is described in Nano Letters.

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