Michigan State University Team Builds Nickel-Vacancy Qubit for Millisecond Quantum Memory

A new diamond-based platform offers key advantages for building scalable quantum networks. I. M. Morris at Michigan State University, and colleagues, in collaboration with University of Leipzig, Element Six Global Innovation Centre; Didcot, Coatings and Diamond Technologies, and Felix Bloch Institute, demonstrate that transition-metal defects in diamond enable efficient optical access, coherent control, and long-lived quantum memory. Their findings show nickel-vacancy (NiV^-) defects provide spin-orbit protected coherence, all-optical control, and near-infrared emission, resulting in a coherence time exceeding one millisecond at 1.65 K and establishing NiV^- as a promising candidate for a deployable diamond spin-photon interface.

Millisecond coherence achieved in nickel-vacancy centres via all-optical dynamical decoupling

Scientists at Michigan State University and collaborating institutions have extended the coherence of a nickel-vacancy (NiV−) defect in diamond to 1.27 milliseconds from 371 nanoseconds, utilising all-optical dynamical decoupling. This breakthrough is a key threshold for building deployable diamond spin-photon interfaces, previously hindered by short coherence times. Operating at 1.65 K, this all-optically controlled diamond spin qubit requires a temperature readily achievable with standard cryogenics, avoiding complex cooling systems.

Raman Rabi oscillations and Ramsey interferometry confirmed the NiV− defect’s potential as a stable platform for quantum information storage and processing, opening a new design space for diamond-based quantum technologies beyond the limitations of existing defects. Utilising all-optical dynamical decoupling, a coherence time of 1.27 milliseconds was achieved in a nickel-vacancy (NiV−) defect within diamond by researchers at Michigan State University and their colleagues. These experiments involved manipulating the spin of the nickel-vacancy using precisely timed laser pulses, verifying the NiV− defect’s capability to reliably store and process quantum information. Temperature-dependent measurements revealed a Hahn-echo coherence time of 125 microseconds at 1.65 K, decreasing to 3.2 microseconds at 3.3 K, indicating a transition from spin-bath limited to phonon-limited coherence. Despite these promising results, the team acknowledges that achieving coherence times approaching the theoretically predicted 30 milliseconds at 1.65 K still requires overcoming challenges related to material purity and isotopic enrichment.

All-optical coherence extension via pulsed dynamical decoupling

Dynamical decoupling was central to extending the coherence of the nickel-vacancy (NiV−) defect, a key property defining how long a quantum bit can reliably store information. This is similar to a spinning top maintaining its spin before it topples. Environmental noise, such as vibrations and electromagnetic fluctuations, constantly threatens to disrupt the delicate quantum state of the qubit. A carefully timed series of optical pulses was therefore employed, analogous to noise-cancelling headphones blocking out unwanted sounds.

At 1.65 Kelvin, a low temperature achievable with standard cryogenics, a nickel-vacancy (NiV−) defect in diamond had its coherence extended to over one millisecond. This single NiV− defect served as the qubit, controlled entirely using optical pulses, avoiding the need for microwave signals used in many other qubit systems. Dynamical decoupling, employing timed optical pulses to shield the qubit from environmental noise, increased coherence from 371 nanoseconds to 1.27 milliseconds.

Millisecond coherence in diamond qubits achieved using accessible cryogenic temperatures

Building a quantum internet demands qubits capable of reliably storing and processing information for extended durations, and scientists are actively pursuing this goal. Work with nickel-vacancy defects in diamond offers a potential solution, avoiding limitations found in more established qubit technologies like nitrogen-vacancy centres. Maintaining a frigid 1.65 K, achieved using cryogenics, is currently required for the demonstration of millisecond-level coherence, introducing practical hurdles for widespread deployment and scalability.

Nevertheless, achieving millisecond-level coherence at such low temperatures presents a vital obstacle to practical application, as current cryogenic systems are bulky and expensive to operate. The team acknowledges this limitation, but highlights that 1.65 K is readily attainable with compact, closed-cycle cryogenics, unlike the liquid helium temperatures often required by other qubit platforms. This represents a key step towards miniaturisation and wider accessibility of quantum technologies.

Nickel-vacancy defects in diamond offer a promising platform for quantum information storage and processing. Currently, achieving millisecond-level coherence demands cryogenic cooling to 1.65 K. The demonstration of millisecond-level coherence using a nickel-vacancy defect represents a major advance in identifying viable qubit platforms. This all-optical control, combined with near-infrared emission, distinguishes this defect from other diamond colour centres and circumvents limitations previously encountered with nitrogen-vacancy and silicon-vacancy centres. Accessible at 1.65 K with standard cryogenics, this extended coherence suggests a pathway towards more practical quantum technologies. The team’s findings now prompt investigation into consistently creating and controlling multiple nickel-vacancy defects, essential for building scalable quantum devices and realising a fully functional quantum internet.

The research demonstrated millisecond-level coherence in a nickel-vacancy defect within diamond, achieved through all-optical control at 1.65 K. This is significant because it establishes this defect as a viable candidate for a deployable diamond spin-photon interface, offering a potential alternative to existing qubit technologies. Maintaining this coherence currently requires cryogenic cooling, but 1.65 K is attainable with compact cryogenics, unlike some other platforms. The authors intend to focus on creating and controlling multiple nickel-vacancy defects to progress towards scalable quantum devices.