New photovoltage technique promises simpler quantum sensor design

While defects in solid-state materials are typically considered problematic, they can also unlock valuable functionalities. In diamonds, for example, nitrogen vacancy (NV) centres – optically active defects where a nitrogen atom replaces a carbon atom adjacent to a vacant site in the lattice – can be exploited for advanced applications. The electron spin states of these NV centres can be manipulated using microwaves, with information read out optically. This capability makes NV-doped diamonds attractive candidates both as highly sensitive sensors and as qubits within quantum computers.

Traditionally, determining the spin state of each NV centre has relied on optical methods. Photons emitted by the colour centres carry information about the spin, but as only single photons are produced when spins flip, the resulting signals are exceptionally weak. Consequently, highly complex experimental set-ups have been required to accurately detect these signals, limiting the practicality and scalability of such systems.

A research team at Helmholtz-Zentrum Berlin (HZB) has now introduced a more straightforward method, using photovoltage measurements to detect individual spin states. The team recognised that NV centres are not solely defined by their spin state, but also by their electrical charge.

To access this property, the researchers adapted Kelvin probe force microscopy (KPFM), a variant of atomic force microscopy. By using a laser to excite the NV centres, the system generates free charge carriers. These carriers become trapped by surface states near the defect, producing a measurable photovoltage in the immediate vicinity of each NV centre.

Dr Boris Naydenov explained: “The idea was that such defect centres not only possess a spin state, but also electrical charge.”

This technique allows for the detection of local spin states via electrical signals rather than weak photon emissions, which could simplify quantum sensor architectures. It offers the prospect of significantly reducing the complexity of readout systems for quantum sensors and, potentially, future quantum computing platforms based on diamond NV centres.

“The photovoltage depends on the electron spin state of the NV centre, and so we can actually read out the individual spin,” says Sergei Trofimov, who carried out the measurements as part of his PhD project. Moreover, with the new method, it is even possible to capture the spin dynamics by coherently manipulating the spin states using microwave excitation.

“This would open the way to the development of really tiny and compact diamond-based devices, since all that is needed are suitable contacts instead of complex microscopic optics and single-photon detectors,” says Prof. Klaus Lips, Head of the Spins in Energy Conversion and Quantum Information Science department. “The newly developed readout method could also be used in other solid-state physics systems where electron spin resonance of spin defects has been observed.”

Above image: The green laser excites charge carriers in the NV centres, which are then captured by surface states. The scanning tip moves over the surface and measures a potential difference around a NV centre. The spin states of the NV centres can be manipulated using microwaves. © Martin Künsting/HZB