Quantum devices with solid-state defects

Coherent, controllable quantum systems underlie the best clocks and the most precise sensors, and may someday form building blocks for information processing devices.  Our research uses techniques developed in quantum optics and atomic physics to understand and control the quantum states of defect centers in crystalline hosts, while exploring their potential applications in quantum information science and metrology.

The NV center in diamond

The nitrogen-vacancy (NV) center is a defect in diamond that behaves remarkably like an isolated atom or ion trapped in the crystal lattice.  With narrow optical transitions and extraordinarily long-lived spin states, it has been proposed as a platform for both quantum information and precision sensing applications.   Our group uses the optical transitions of these defects to access and control individual spins in the diamond lattice; one long-term goal is to identify and study interactions that could be extended to large-scale networks, opening the door to meaningful applications in quantum information science.  Intriguingly, these efforts may also be relevant for fundamental condensed matter physics and practical sensor devices: the same superposition states that underlie quantum information processing are also exquisitely sensitive to their surroundings, creating opportunities to study the mesoscopic environment or monitor (sense) external fields.

Fabry-perot microcavities for diamond photonics

Combining high fidelity control over NV-based spins with high finesse Fabry-Perot cavities can provide a coherent optical connection over macroscopic distances. Owing to the tight spatial and long temporal confinement of photons in the cavity, interactions between the defect and light are strongly enhanced.
A sketch of a fiber cavity enclosing
a diamond membrane (blue)

We work with low-mode volume, high quality factor microcavities formed by a mirror deposited on the tip of specially laser-ablated optical
fiber in close proximity to a flat mirror. The cavity encloses a few-micron-thick membrane of diamond, and maintains a quality factor in excess of one million. At the same time, the fairly thick diamond membrane permits us to work with defects that are not strongly affected by proximal surfaces.

Erika JanitzMaximilian RufMark DimockAlexandre BourassaJack Sankey and Lilian Childress, Fabry-Perot microcavity for diamond-based photonics, Physical Review A 92, 043884 (2015).

Charging states of the NV center 

Fluorescence traces showing jumps in
the charge state of the NV
Understanding the charge state dynamics of the NV center is critical for applications that use the spin states of the negative NV as well as for readout based on spin-to-charge conversion. The charge state can be measured using wavelengths of light that only excite the negative charge state; observing the resulting fluorescence allows preparation and detection of the NV charge with high fidelity (see e.g. Aslam et al NJP 15013064). 

In addition to studying charge-state readout fundamentally, we are exploring the mechanisms for optically induced ionization and recombination in the context of optical nanoscopy. 

D'Anjou, L. Kuret, L. Childress and W. A. Coish, Maximal adaptive-decision speedups in quantum-state readout, Physical Review X 6011017 (2016).

Spin-torque-driven magnetic circuits

Ferromagnetic resonance spectrum from a permalloy
nanowire, observed via anisotropic magnetoresistance.
The frequency range matches the typical spin splittings
for the NV center.

The Zeeman effect makes individual spins an efficient sensor of their local magnetic field. We are interested in using NV spins to sense stray magnetic fields from nanoscale ferromagnetic devices driven by spin currents. In the long run, the information gleaned from spin sensors could provide spatially resolved data not easily obtained in transport experiments.

This work is a collaboration with Prof. Sankey's lab at McGill.