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Dynamic nuclear polarization (DNP) is the process of achieving a strong nuclear polarization by polarizing electron spins under cryogenic conditions. Common applications are in nuclear magnetic resonance (NMR). Recent advancements have employed optical centres using the nitrogen-vacancy (NV) centres in diamond to achieve DNP under ambient conditions. However, nitrogen vacancy centres are known to have a distribution of values. A team of international researchers has addressed this issue by designing microwave fields with time-dependent frequencies to improve the exchange couplings between the NV centres and nuclear spins.

Nitrogen-vacancy (NV) centres in diamond provide appealing advantages for many applications including nuclear magnetic resonance (NMR) due to not only providing nanoscale spatial resolution, but also room temperature operating conditions. NV centres have been tested as quantum sensors and can even detect individual nuclear spins.

NV quantum sensing has found promise through two main routes- pulsed schemes and continuous driving schemes. Both schemes are dependent upon the resonance condition between the induced effective frequency of the NV spin and the Larmor frequency of nuclear spins.

A principle known as the Hartmann-Hahn condition is present in these sensors, and is especially prevalent in continuous driving schemes. The Hartmann-Hahn resonance condition arises from tuning the intensity of the microwaves, where the resonant exchange interactions of the NV centre spin matches the Larmor frequency.

In real scenarios, both bulk diamond and diamond powder contain a large number of NV centres which generally leads to a distribution of the transition frequencies in the NV. This has been the biggest challenge facing DNP applications. When a single microwave is used in these scenarios, only a small portion of the NV centres satisfy the Hartmann-Hahn condition, and the inhomogenous broadening of the NV centres limits the effectiveness of the DNP protocols.

The researchers have recently addressed this major challenge by designing microwave fields with multiple time-dependent frequencies to improve the exchange couplings between the large number of NV centres and nuclear spins. Unlike other approaches, the employed frequency-modulated microwave driving field allows the Hartmann-Hahn resonance to be achieved for a large number of NV centres producing a major improvement in the DNP.

The principles of the modulated microwave driving field were employed on an ensemble of NV centres with inhomogeneous linebroadening (Lorentzian shape) and random orientations. The researchers have generalized the DNP process in diamond (powder) by using one NV centre to sweep the Rabi frequency of the microwave field (an oscillation for a given atomic transition in a given light field).

The sweeping process was found to enhance the coupling between the NV centres which enabled a large amount of the NV centres to make a contribution to the dynamical nuclear polarization. The frequency sweeping speed was also found to have an impact on the DNP protocols, and was dependent upon the type of NV centre. In Lorentzian NV centres, a longer interaction time is required to reach the resonant point, whereas, in randomly orientated NV centres, lower sweeping frequency speeds can be consistently employed to increase the DNP efficiency. In scenarios where possible, a lower frequency speed is preferable for Lorentzian NV centres as well, as it increase the DNP efficiency, but it is not always a possible option.

The advancement of such DNP protocols is an extension of the standard DNP procedures that employ a Hartmann-Hahn resonance and is expected to be easily implemented in experimental processes. The main application in which this research will find itself in is single molecule magnetic resonance spectroscopy using a single NV spin sensor.

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This information has been sourced, reviewed and adapted from materials provided by SpingerOpen.