NV center associated projects

Quantum Bit Resources – The Nitrogen Vacancy Center in Diamond

The probabilistic nature of today’s single photon sources makes it difficult to generate multiphoton interference. The probability of creating single photons simultaneously decreases dramatically with the number of photons which makes such sources less attractive as sources of travelling quantum bits (qubits).

In our single photon projects we pursue an efficient On-demand single photon source. By ‘On-demand’ is meant that the probability of creating a single photon which can be captured in a single mode waveguide should be almost 100% per single laser excitation pulse.

NV center in diamond

Our focus is on the negatively charged nitrogen vacancy (NV) defect center in diamond depicted above. In a bandgap diagram, illustrated below, the NV center has two optical active levels within the diamond bandgap and by laser excitation single photons are emitted. Our research involves both 30 nm sized nanodiamonds and bulk diamond substrates. The NV center is a robust, stable single photon emitter at room temperature. Furthermore, the NV center has an electronic spin of 1 and the state of this spin can be detected by measuring the level of emission from the NV center, also at room temperature. The coherence time of the spin state is on the order of 100 μs and can be manipulated by applying microwave pulses. These features make the NV center very attractive in the scope of single photon emission and spin-qubit storage.

NV energy levels

Photon harvesting

Our challenge is to tailor the environment of the single photon emitter to gain, firstly, near unity the collection of the emitted photons, and secondly, a high emission rate. Both objectives we pursue by manipulating the spontaneous emission by designing specific material and geometry selected waveguides of various kinds. The different approaches are found under different projects:

Extreme confinement of light in metallic nanowires

We experiment with the coupling between silver nanowires and the NV center where we couple photons emitted by the NV center to radiative modes and to special surface modes propagating along the wire/air interface. The situation is sketched below.

NV NW geometry coupling

The surface mode excitations are denoted ‘surface plasmon polaritons’ ( or ‘plasmons’).  These modes have the ability to support extreme confinement of light scaled below the diffraction limit of light. By coupling to these modes, we are able to strongly change the lifetime of the NV center. This we do using different wires arrangements yielding a variety of single and common modes which we can couple our NV center to.

NV NW coupling galvo

 Emission stemming from plasmon generation we can observe in the far field as shown above. The change of lifetime for an NV center due to coupling is depicted below.

NV decay rate change

Capturing the single photons in a fiber cavity

Another way to control and enhance the emission from a dipole emitter is to enclose it in a cavity constructed of two mirrors whose separation matches the wavelength of the emitted light. In this project one of the mirrors is a partially transmitting coated fiber into which emission can be harvested, aiming at an on-demand single photon source for quantum communication experiments. An artistic view of the project goal is depicted below.

NV in fiber cavity

Fabricating structures of bulk diamond

Instead of using nanodiamonds containing NV centers one can use bulk diamond. It introduces the opportunity of shaping the emission pattern of the NV center by tailoring the surrounding diamond structure. By carefully designing the structures an optimal spatial coupling to a dielectric or a plasmonic waveguide is possible.

Due to the hardness of diamond, fabrication is not an easy task and it is not a standard cleanroom processing material. But fabrication can be performed by careful Reactive Ion Etching (RIE) methods which we are working on. A preliminary result is seen below which show formation of diamond pillars due to an RIE etch.

Diamond pillars

Manipulating a single spin in an atomic lattice

A key feature of the NV center is the spin dependent fluorescence level. Here we manipulate the spin properties by external magnetic fields and microwaves which make it possible to measure Rabi-oscillations between initial and excited spin-states through the collected emission from the NV center. Such Rabi-oscillations is seen below. Since a single spin can be addressed, very sensitive magnetic field measurements of the surroundings of the NV center can be performed and is one of the main features to explore.