An elegant method for the detection of single spins using photovoltage

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.

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

Diamonds with certain optically active defects can be used as highly sensitive sensors or qubits for quantum computers, where the quantum information is stored in the electron spin state of these colour centres. However, the spin states have to be read out optically, which is often experimentally complex. Now, a team at HZB has developed an elegant method using a photo voltage to detect the individual and local spin states of these defects. This could lead to a much more compact design of quantum sensors.

Defects in solids are often undesirable, but they can also provide wonderful new opportunities, for example in diamonds: Here, nitrogen vacancy centres (NV centres) can be introduced whose spin states can be manipulated with microwaves. The information from a single spin can then be read out using light. This makes NV-doped diamonds suitable not only as highly sensitive sensors, but also as qubits for quantum computers.

Up to now: optical detection

However, to determine the state of each individual spin, the photons emitted from the colour centre (carrying the spin) must be measured. Since only single photons are emitted when spins flip, this signal is very weak, leading to a complex experimental set-up for the detection.

Now: photovoltage measured by KPFM

A team at HZB has now presented a novel method to solve this problem. ‘The idea was that such defect centres not only possess a spin state, but also electrical charge,’ says Dr Boris Naydenov. To probe these charges, they modified a variant of atomic force microscopy known as Kelvin probe force microscopy (KPFM): in this process, a laser excites the NV centres, generating free charge carriers that are captured by surface states and thus produce a measurable voltage around an NV centre.

Capturing spin dynamics

‘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.

Outlook: tiny diamond based devices

‘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,’ Lips estimates.

arö

  • Copy link

You might also be interested in

  • Sodium-ion batteries: New storage mechanism for cathode materials
    Science Highlight
    18.07.2025
    Sodium-ion batteries: New storage mechanism for cathode materials
    Li-ion and Na-ion batteries operate through a process called intercalation, where ions are stored and exchanged between two chemically different electrodes. In contrast, co-intercalation, a process in which both ions and solvent molecules are stored simultaneously, has traditionally been considered undesirable due to its tendency to cause rapid battery failure. Against this traditional view, an international research team led by Philipp Adelhelm has now demonstrated that co-intercalation can be a reversible and fast process for cathode materials in Na-ion batteries. The approach of jointly storing ions and solvents in cathode materials provides a new handle for the designing batteries with high efficiency and fast charging capabilities. The results are published in Nature Materials.
  • 10 million euros in funding for UNITE – Startup Factory Berlin-Brandenburg
    News
    16.07.2025
    10 million euros in funding for UNITE – Startup Factory Berlin-Brandenburg
    UNITE – Startup Factory Berlin-Brandenburg has been recognised by the Federal Ministry for Economic Affairs and Energy as one of ten nationwide flagship projects for science-based start-ups. UNITE is to be established as a central transfer platform for technology-driven spin-offs from science and industry in the capital region. The Helmholtz Centre Berlin will also benefit from this.

  • Hydrogen storage in MXene: It all depends on diffusion processes
    Science Highlight
    23.06.2025
    Hydrogen storage in MXene: It all depends on diffusion processes
    Two-dimensional (2D) materials such as MXene are of great interest for hydrogen storage. An expert from HZB has investigated the diffusion of hydrogen in MXene using density functional theory. This modelling provides valuable insights into the key diffusion mechanisms and hydrogen's interaction with Ti₃C₂ MXene, offering a solid foundation for further experimental research.