Sub-millihertz magnetic spectroscopy with a nanoscale quantum sensor (1706.02103v1)

Abstract: Precise timekeeping is critical to metrology, forming the basis by which standards of time, length and fundamental constants are determined. Stable clocks are particularly valuable in spectroscopy as they define the ultimate frequency precision that can be reached. In quantum metrology, where the phase of a qubit is used to detect external fields, the clock stability is defined by the qubit coherence time, which determines the spectral linewidth and frequency precision. Here we demonstrate a quantum sensing protocol where the spectral precision goes beyond the sensor coherence time and is limited by the stability of a classical clock. Using this technique, we observe a precision in frequency estimation scaling in time $T$, as $T^{-3/2}$ for classical oscillating fields. The narrow linewidth magnetometer based on single spins in diamond is used to sense nanoscale magnetic fields with an intrinsic frequency resolution of 607 $\mu$Hz, 8 orders of magnitude narrower than the qubit coherence time.

• The paper introduces Qdyne, a quantum heterodyne detection method that decouples spectral resolution from the NV center's coherence time.
• It employs NV centers with a classical oscillator to achieve a frequency resolution as fine as 607 µHz and precision scaling with T^(-3/2).
• Experimental results show significant improvements in nanoscale magnetic sensing and NMR, paving the way for advanced quantum metrology applications.

Sub-millihertz Magnetic Spectroscopy with a Nanoscale Quantum Sensor

The paper presented by Schmitt et al. details a novel quantum spectroscopy technique that allows for sub-millihertz frequency resolution using nitrogen vacancy (NV) centers in diamond as the sensing element. This work demonstrates significant advancements in quantum metrology, especially in overcoming coherence time limitations inherent in quantum sensors.

Quantum Sensing and Spectroscopy

The authors address a fundamental limitation in quantum sensing, where the coherence time of a qubit traditionally sets a bound on spectral resolution. In typical quantum spectroscopy, the probe coherence time limits the frequency resolution due to the need for sequential sampling of frequency components (dynamical decoupling), leading to a compromise between resolution and interaction time with the target field.

Qdyne: Quantum Heterodyne Detection

The authors propose a new quantum detection methodology, Qdyne, analogous to classical heterodyne detection but with a novel implementation using a quantum probe. This hybrid quantum-classical approach utilizes NV centers in diamond as the quantum sensor, mixed with a classical oscillator, enabling a frequency resolution potentially defined by the stability of the local oscillator rather than the coherence time of the quantum sensor.

Qdyne's unique ability allows for frequency resolutions such as 607 µHz, eight orders of magnitude below the qubit coherence time, and improves precision scaling with a temporal dependency of $T^{-3/2}$. This marks a significant development, providing a practical pathway to frequency estimation precision not achievable through conventional techniques timed to the coherence duration of quantum probes.

Experimental Validation

The experimental configuration involves NV centers just nanometers beneath a diamond surface, used to detect an oscillating magnetic field. Through careful timing and photon-based measurement correlated with the local oscillator, the researchers demonstrated a significant enhancement in both frequency resolution and sensitivity compared to dynamical decoupling methods. Notably, Qdyne outperformed traditional methods in practical experiments by orders of magnitude in terms of frequency precision and resolution.

Implications for Nanoscale NMR

The extended capabilities of this technique were further applied to nanoscale nuclear magnetic resonance (NMR), showcasing Qdyne's utility in capturing weak interactions such as those from statistically polarized molecules near diamond surfaces. This offers significant implications for molecular structural determination at scales unreachable by conventional NMR.

Significance and Future Prospects

The introduction of Qdyne as a high-precision, low-overhead spectroscopic technique opens new frontiers in quantum metrology. Its capability to decouple spectral resolution from quantum coherence time and enhance frequency estimation precision paves the way for greater advancements in nanoscale magnetic sensing and quantum technologies. Future developments could further exploit this approach's potential, improving the precision and scalability of quantum sensing applications, contributing deeply to both theoretical foundations and practical advancements in quantum science and technology.

The authors' findings represent a substantial contribution to quantum metrology, illustrating how integration with classical timing devices can overcome inherent quantum limitations, enabling unprecedented resolution and precision for practical, high-impact scientific investigations. This refined control over frequency estimation has the potential to significantly impact numerous fields, including quantum computing, materials paper, and biochemical analysis.