Efficient Photon Collection from a Nitrogen Vacancy Center in a Circular Bullseye Grating in Diamond
Authors: Luozhou Li, Edward Chen, Dirk Englund
Affiliation: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, USA.
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The nitrogen-vacancy center (NV)  behaves much like an atom trapped in the diamond lattice. Because of the high band gap and the mostly spin-free composition of the diamond host, the NV is well isolated from the environment, so it shows well-behaved atom-like properties. Most importantly, it’s possible to optically prepare and measure the long-lived states of the associated electron and nuclear spins. NVs are potentially promising building blocks for a large-scale quantum network where the optical addressability of the NV allows flying qubits, or photons, to connect nodes of this network together. One of the fundamental bottlenecks for this to be made into a reality is the flux of photons collected from an NV, which determines how quickly the NV’s spin state can be measured and compared: the more fluorescent photons that are collected, the faster new connections can be made. The same photon collection limitation is also true for using the NV as a highly sensitive quantum sensor, where the sensitivity to electric, magnetic and temperature fields increase with increased photon collection. Thus, higher photon detection of the NV’s photoluminescence is of central importance to many NV quantum technologies, such as communication, computing, and even sensing.
In our recent work , we introduce a circular “bullseye” grating in diamond (Figure 1), which enables record-high photon collection from the nitrogen-vacancy (NV) color center. The bullseye grating consists of concentric slits etched into a diamond membrane , which itself is about half of a wavelength in thickness. The grating period satisfies the second-order Bragg condition, giving rise to the scattering of light out of the membrane. The scattered light from each grating interferes constructively out of the plane and into the far field, thereby enabling significantly higher collection efficiency. With this circular grating, we have shown that it’s possible to collect about an order of magnitude more fluorescence than is possible from an NV in un-patterned diamond.
Figure 1: (a) Illustration of an array of diamond bullseye gratings adjacent to a microwave strip line. (b) Schematic of the circular grating. ‘a’ denotes the lattice constant and ‘gap’ the air spacing between circular gratings. (c) Simulated electric field intensity (log scale) in the x = 0 plane with air above and glass below the diamond. A dipole emitter was placed in the center of the bullseye grating, and was oriented along the horizontal direction.
Achieving higher collection efficiency from the NV impacts several applications such as improved sensing of static or dynamic electromagnetic fields just outside the diamond, higher luminosity room-temperature single photon sources, and better quantum memories for quantum computing and networking. For example, NV researchers  have recently shown that the NV is even sensitive to changes of single proton spins, paving the way for magnetic resonance imaging of individual molecules in liquid — and this application would be improved by better fluorescence collection from the NV.
The efficient photon collection should allow for a range of new measurements, such as non-demolition measurements of NV spins — i.e., you could make a measurement and then act back on the NV spin state. We’re also using the efficient collection for medium-scale quantum registers, which would contain on the order of tens of qubits each, and for quantum sensing.
 Marcus W. Doherty, Neil B. Manson, Paul Delaney, Fedor Jelezko, Jörg Wrachtrup, Lloyd CL Hollenberg, "The nitrogen-vacancy colour centre in diamond." Physics Reports, 528, 1-45 (2013). Abstract.
 Luozhou Li, Edward H. Chen, Jiabao Zheng, Sara L. Mouradian, Florian Dolde, Tim Schröder, Sinan Karaveli, Matthew L. Markham, Daniel J. Twitchen, and Dirk Englund, "Efficient photon collection from a nitrogen vacancy center in a circular bullseye grating." Nano letters, 15, 1493 (2015). Abstract.
 Luozhou Li, Igal Bayn, Ming Lu, Chang-Yong Nam, Tim Schröder, Aaron Stein, Nicholas C. Harris, Dirk Englund. "Nanofabrication on unconventional substrates using transferred hard masks." Scientific reports, 5, Article number 7802 (2015). Article.
 A. O. Sushkov, I. Lovchinsky, N. Chisholm, R. L. Walsworth, H. Park, M. D. Lukin, "Magnetic resonance detection of individual proton spins using quantum reporters." Physical Review Letters, 113, 197601 (2014). Abstract.