Quantum Sensing

Technical Overview

The ability to harness the quantum nature of systems provides revolutionary capability when applied to sensing. Indeed, when it is possible to base measurements on quantum phenomena, such as the energy difference between two well-defined quantum states, sensors have the ability to reach unprecedented precision and accuracy that doesn't drift over time. For decades, quantum measurements have been used in metrology to define fundamental constants such as time. More recently, this approach is being applied to sensing. High sensitivity and stability in combination with a small form factor provide transformational capabilities with applications spanning from GPS-free global navigation and cryogen-free high-precision magnetometry to sensing within mesoscale structure and measurements of individual nuclear or electron spins.

Program Goals

Solid state diamond Figure 1. Diamonds occur in various colors, including various shades of pink, blue, yellow, green, orange, brown, gray, and black. Predominantly, the color of the diamond results from imperfections in the crystal lattice. The most common color center in synthetically grown diamond is formed when a nitrogen atom replaces a carbon atom in the lattice. When paired with a missing carbon atom or vacancy, an atom-like quantum system is formed. The unique properties of the nitrogen vacancy allow it to be addressed optically with a green laser and read out with the generated red fluorescence, which is known to give these diamonds a red/orange hue.

Quantum sensing at Lincoln Laboratory focuses on devices that exceed the capability of their classical counterparts. In this newly initiated effort, our first sensors take advantage of the nitrogen vacancy (NV), a quantum system embedded in a solid-state diamond (Figure 1). These nitrogen-vacancy pairs, or color centers as they are often called, have atom-like quantized energy levels that can be manipulated to sense electromagnetic fields, rotation, temperature, pressure, stress, and even to measure time (frequency). While precision quantum mechanical measurements typically sprawl across several optical tables or require cryogen dewars, devices based on color centers have the promise of a room-temperature, all-solid-state solution to quantum sensing.

To understand the operating principle of the sensor, we can use magnetic field detection as an example. Because the energy levels of this atom-like system split in the presence of a magnetic field (Figure 2), the shift can be read out to determine the magnitude of the externally applied field. Compared to other color centers, the NV's uniqueness stems from its ability to be addressed and read out optically; this is known as optically detected magnetic resonance (ODMR). For the nitrogen vacancy in diamond, a green laser initiates the atomic-like system into a well-known state, RF fields near 3 GHz control the state coherence, and red fluorescence or absorption is read out to make the measurement. Current goals focus on improving the diamond magnetometer and gradiometer sensitivity, enhancing its coherence, improving its spatial resolution, and devising strategies to achieve sensitivity beyond the Heisenberg limit. From quantum non-demolition measurement to dynamical decoupling and spin squeezing, techniques from quantum information science are critical in achieving these goals.

Data showing magnetic field aligned to a single nitrogen vacancy axis Figure 2. Data showing magnetic field aligned to a single nitrogen-vacancy axis. The magnetic field is tied to fundamental constants by measuring the splitting of the lines. Increased accuracy and noise immunity is obtained by using a lock-in amplifier.
(Click on image to view larger version.)


Research Highlights

Recent work focuses on ensemble NV-based magnetometers employing N unentangled color centers to realize a factor of up to √N improvement in sensitivity. In order to realize fully this signal enhancement, we are developing new techniques to excite efficiently and to collect fluorescence from large NV ensembles. In a collaboration with MIT professor Dirk Englund, we are using a novel light-trapping diamond waveguide geometry that enables both high fluorescence collection (~ 20%) and efficient pump absorption. This new geometry enables in excess of 2% conversion efficiency of pump photons into optically detected magnetic resonance fluorescence, an improvement of three orders of magnitude over previous single-pass geometries. The apparatus is depicted in Figure 3. By applying these techniques to low-frequency magnetometry, vector magnetic field sensitivity has exceeded nanoTesla precision.

diamond nitrogen vacancy apparatusFigure 3. (a) Photograph of a bulk diamond nitrogen-vacancy apparatus depicting Helmholtz coils for field control and diamond mounted on temperature-controlled pedestal; (b) top view of 3 mm x 3 mm x 0.3 mm diamond showing fluorescence from nitrogen vacancies as a green excitation laser enters through the prism facet and undergoes total internal reflection; (c) side view of diamond on top of the pedestal.



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