March 23, 2011

Diamond Quantum Memory - Chip-Scale Nanofabrication of Single Spins and Spin Arrays in Diamond

Nanoletters - Chip-Scale Nanofabrication of Single Spins and Spin Arrays in Diamond

Scientists have developed a new way to manipulate atoms inside diamond crystals so that they store information long enough to function as quantum memory, which encodes information not as the zeros and ones crunched by conventional computers but in states that are both zero and one at the same time. Physicists use such quantum data to send information securely, and hope to eventually build quantum computers capable of solving problems beyond the reach of today's technology.

* Awschalom has now figured out how to link the spin of a electron to the spin of the nearby nitrogen’s nucleus. This transfer, triggered by magnetic fields, is fast — about 100 nanoseconds, comparable to how long it takes to store information on a stick of RAM. The fidelity is 90-99%.

* This diamond memory works at room temperature.

* The spins inside the diamond can be both changed and measured by shining laser light into the diamond. This could make diamond an attractive material for scientists developing nanophotonic systems designed to move and store information in packets of light.

* It lasts for a very long time by quantum standards. The nuclear spin remains coherent for more than a millisecond, with the potential to improve to seconds.

* it can be scaled up to larger sizes.

* They have developed a technique for creating customizable patterns of nitrogen atoms inside a diamond, using lasers to implant thousands of atoms in a grid.

* Awschalom’s diamond quantum memory could also be useful for building large quantum networks many kilometers in size and with quantum repeaters over larger scales.

We demonstrate a technique to nanofabricate nitrogen vacancy (NV) centers in diamond based on broad-beam nitrogen implantation through apertures in electron beam lithography resist. This method enables high-throughput nanofabrication of single NV centers on sub-100-nm length scales. Secondary ion mass spectroscopy measurements facilitate depth profiling of the implanted nitrogen to provide three-dimensional characterization of the NV center spatial distribution. Measurements of NV center coherence with on-chip coplanar waveguides suggest a pathway for incorporating this scalable nanofabrication technique in future quantum applications.

Figure 1. (a) SEM micrograph of an 30 nm diameter resist aperture used to mask nitrogen ion implantation to spatially control NV center formation. (b) SEM micrograph of an 60 nm diameter resist aperture. After fabrication, the NV centers resulting from this aperture diameter were analyzed in photoluminescence and ESR experiments. (c) SEM micrograph of an array of 60 nm diameter apertures. (d) Spatial photoluminescence image of an array of NV centers resulting from resist masking, implantation, and annealing. The white dots in the image result from NV center photoluminescence. A lithographic, short-terminated CPW is also visible. Two single NV centers used in further measurements are circled. (e) Photon antibunching curve from the NV center circled in yellow in Figure 1d. g(2)(0) < 0.5, indicating it is a single quantum emitter. The data are uncorrected for background luminescence and dark counts of the photon detectors. (f) A histogram of resist aperture sites yielding zero, one, two, and three NV centers per aperture site (blue dots) determined by photon antibunching measurements on 32 apertures sites. The results are compared with a Poisson distribution with mean 1.45 (red squares).

Solid-state spins are attractive for quantum information processing in part because mature nanofabrication techniques developed for the semiconductor industry can be used for their production. Among single-spin systems the negatively charged nitrogen vacancy (NV) center in diamond stands out because of its individual addressability,(1) optical spin polarization, and millisecond room-temperature spin coherence.(2) These properties have generated intense interest in the use of NV centers for quantum device applications in dipolar-coupled quantum registers(3) and hybrid quantum computing architectures.(4-8) Many of these architectures present exacting device requirements that necessitate the development of methods to place NV centers in diamond on 10 nm length scales. Nitrogen implantation into high-quality chemical vapor deposition (CVD) diamond is a promising method for engineering NV centers and has been used to fabricate single-qubit and two-qubit NV center devices. Proposals for nanometer-scale, spatially controlled NV center fabrication for quantum device applications have commonly relied on serial nitrogen implantation with scanning probe microscope apertures and ultracold ion sources. We demonstrate an alternative approach for spatially controlling NV center formation by using electron beam lithography resist masks to pattern ion implantation. This approach offers high spatial resolution and high throughput: resist aperture arrays are scalable to 10 nm aperture diameters and 30 nm aperture pitches and are patterned at a rate of 10^3 per second. This nanofabrication technique makes possible the production of dense, large-scale NV center arrays in diamond for spin-based quantum computing architectures.
The nanofabrication technique described in this Letter relies on nitrogen implantation into masked, high-purity CVD diamond substrates. Secondary ion mass spectroscopy (SIMS) measurements showed the intrinsic nitrogen content of the substrates to be 7 × 10^15 cm−3. Before masking, a 10 nm SiO2 layer was deposited on the diamond surfaces to mitigate ion channeling during implantation. Positive electron beam lithography resist, approximately 300 nm thick, was applied to the samples, and resist apertures were patterned using a 100 kV electron beam lithography system. The minimum aperture diameter achieved after resist development, as measured with a scanning electron microscope (SEM), was 30 nm. Aperture diameters smaller than 30 nm are possible but require thinner resist masks. This spot size, in combination with ion straggling during implantation, determines the lateral NV center placement accuracy of this technique. After development the apertures were implanted with 20 keV 15N+ ions, which enabled isotopic tagging of the implanted nitrogen to distinguish it from 14N in the diamond substrate. 15N has a low natural abundance (0.4%) and a distinct nuclear spin (I = 1/2) from 14N (I = 1). Monte Carlo simulations using the Stopping Range of Ions in Matter (SRIM) program for these implantation conditions predict a lateral straggle of 9 nm. Following implantation, the samples were annealed in Ar at 850 °C to induce vacancy diffusion to form NV centers and then annealed in O2 at 420 °C to reduce photochromism. Diffusion of nitrogen atoms, a potential source of NV center placement uncertainty, is expected to be negligible under these annealing conditions. Further process details are provided in Materials and Methods.

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