Defects – are they merely an inconvenience? Not in the quantum universe! Scientists have recently found a promising new class of atomic-scale defects in hexagonal boron nitride [1], a two-dimensional material. The information storage capabilities of these impurities could enable powerful quantum technologies.
Take, for example, quantum communication. As more of our personal data is shared online, the need for an ultra-secure network has become vitally important. Quantum communication protocols leverage the laws of quantum physics to protect our data. Particles – typically photons (or ‘packets of light’) transmitted along optical fibres – can exist in a state of superposition (some combination of 1 and 0). These particles, termed qubits or quantum bits, are remarkably sensitive. Why is this useful you may ask? Imagine an eavesdropper wants to read the state of this qubit during transmission. As soon as an attempt is made, the qubit will immediately “collapse” into either 1 or 0, leaving behind unmistakable evidence of their snooping. In theory, these quantum networks will offer unparalleled security. But finding a qubit fit for the job is no easy task.

Most current material platforms for quantum emitters and qubits are difficult to make and only function well at extremely low temperatures. Hexagonal boron nitride (hBN), on the other hand, is a cheap and scalable two-dimensional material synthesised by chemical vapour deposition [3]. Under the right growth conditions, we can deliberately create atomic-scale defects in the hBN lattice. Some of these defects can emit single photons at room temperature. Further investigations have also revealed that the light emitted from these isolated impurities gives information about a quantum property, called spin, that can be used to store quantum information [1]. Therefore, data can be encoded and retrieved using light, and at room temperature. Exciting as this is, we still need to better understand the physics of these spin-active defects and explore various operation regimes for possible applications such as quantum sensing, communication and computation.

Charge noise describes the rapid switching between different defect charge states as charged particles in the host material wander around unpredictably. Radically different spin and optical properties emerge when the charge state varies. Therefore, it is crucial to be able to initialise the charge state on demand and ensure it remains stable during qubit operation [5]. That is to say, we need to demonstrate coherent spin and optical transitions for the same quantum emitter, in order to develop a reliable spinphoton interface or qubit for quantum technologies. This is rather challenging given the significant charge noise observed in hBN.
In light of this, my research aims to control or eliminate this charge noise. To do this, we have designed a device in which the defect-containing sample is sandwiched between two gold electrodes. This device can therefore be used to apply an electric field of varying strength to the hBN material. By firing a laser at the sample at room temperature, we can observe how this applied field affects the optical and spin properties of a single defect. We will use a technique called photoluminescence (PL) spectroscopy, where we measure the energy and intensity of the light emitted by the defect when we shoot a green laser at it. On the spin side of things, we can carry out optically detected magnetic resonance (ODMR) experiments. Here, we use a tiny gold antenna to deliver microwaves to the sample. We then carefully analyse the optical response to infer information about the defect’s spin, or intrinsic angular momentum. We believe this research will bring us closer to our ultimate goal of entangling spins with photons in this unique 2D system – and hopefully pave the way for scalable quantum technologies set to change the world as we know it.
References:
- Stern, H. L. et al. Room-temperature optically detected magnetic resonance of single defects in hexagonal boron nitride. Nat. Commun. 13, 618 (2022).
- https://www.volkswagenag.com/en/news/stories/2019/11/where-is-theelectron-and-how-many-of-them.html
- Liu, H. et al. Synthesis of hexagonal boron nitrides by chemical vapor deposition and their use as single photon emitters. Nano Mater. Sci. 3, 291–312 (2021).
- https://www.advancedsciencenews.com/tag/advanced-quantum-technologies/
- Wolfowicz, G., Heremans, F.J., Anderson, C.P. et al. Quantum guidelines for solid-state spin defects. Nat. Rev. Mater. 6, 906–925 (2021).
NanoDTC PhD Student, c2022