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The green laser excites charge carriers in the NV centers, which are then captured by surface states. The scanning tip moves over the surface and measures a potential difference around an NV center. The spin states of the NV centers can be manipulated using microwaves. Credit: Martin Künsting / HZB
A research team at HZB has developed a clever technique to read quantum spin states in diamonds using electrical signals instead of light. This breakthrough could dramatically simplify quantum sensors and computing hardware.
Diamonds that contain specific optically active defects, known as color centers, can serve as highly sensitive sensors or as qubits for quantum computers, with quantum information stored in their electron spin states. Traditionally, reading these spin states requires optical methods, which are often complex and difficult to implement. Now, researchers at HZB have developed a more streamlined approach: using photovoltage to detect the spin states of individual defects. This method could pave the way for much smaller and more compact quantum sensors.
Harnessing Defects for Spin States
While defects in solids are often seen as problematic, they can also offer surprising advantages, especially in diamonds. By introducing nitrogen vacancy (NV) centers into the crystal, researchers can create defects whose electron spin states can be controlled using microwaves. These spin states can carry information, making NV-doped diamonds useful not only as ultra-sensitive sensors but also as qubits for quantum computing.
Optical Spin Detection Challenges
Until now, reading the spin state of an NV center required detecting photons emitted from the defect. Since only a single photon is typically released when a spin flips, the signal is extremely weak. This makes the setup for detecting these spins both technically challenging and complex.
A Voltage-Based Breakthrough
A team at HZB has developed a new way to address this challenge. “The idea was that such defect centres not only possess a spin state, but also electrical charge,” explains Dr. Boris Naydenov.
To measure these charges, the team adapted a technique called Kelvin probe force microscopy (KPFM), a variation of atomic force microscopy. In their method, a laser excites the NV centers, generating free charge carriers. These charges interact with surface states and create a measurable voltage around the NV center, allowing the spin state to be detected electrically.
Reading Spins Through Charge
“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.
Toward Compact Quantum 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.
Reference: “Voltage detected single spin dynamics in diamond at ambient conditions” by Sergei Trofimov, Klaus Lips and Boris Naydenov, 14 April 2025, Nature Communications.
DOI: 10.1038/s41467-025-58635-3
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