Session K07: Quantum Metrology and Sensing II

Sensing multiple NV centers for nanoscale covariance magnetometry

Nitrogen vacancy (NV) defect centers in diamond have been used as a versatile platform for nanoscale sensing of many condensed matter systems, but experiments have typically only taken advantage of a single NV center at a time or else performed widefield imaging without nanoscale resolution. Here we discuss what can be learned by simultaneous addressing of multiple NV centers; measuring correlated dynamics in NV centers would provide simultaneous information at two different NV center locations (~0.1 to 100 micron length scales), and optionally at two different sensing times limited only by the experimental clock (~1 ns resolution). Understanding dynamics at these length and time scales is important for many microscopic systems, and measurements of spatiotemporal correlations may provide useful information about the electron mean free path, signatures of hydrodynamic flow, or the nature of local NV center noise sources like surface spins. Assuming two individual NV centers experience a shared 'global' magnetic field as well as noisy 'local' fields, we analytically derive the expected measurable correlation between the sensed signals from the NV centers. We then consider the experimental approach, deriving the sensitivity of such a measurement with a focus on the roles of readout noise and fidelity.   

Digital noise spectroscopy with a quantum sensor

Characterizing the environmental noise in quantum systems is an essential task in both fundamental physics and quantum applications. Typical protocols for quantum noise spectroscopy rely on periodic dynamical decoupling sequences that yield narrow frequency filters, and sample the noise spectrum in frequency space using the $\delta$-filter approximation. Here we propose a digital noise spectroscopy protocol based on Walsh-function dynamical decoupling. By measuring the decoherence of a qubit under a set of Walsh modulation sequences, the auto-correlation of a stationary Gaussian noise is directly reconstructed,  while its discrete Fourier transform gives the corresponding noise spectrum. In comparison to previous noise spectroscopy methods, the accuracy of our method is only limited by the sampling in time-space. We then perform a proof-of-principle demonstration by using a single nitrogen-vacancy center in diamond to characterize its environmental noise dominated by the surrounding $^{13}$C nuclear spins, and discuss practical limitations of the reconstruction accuracy and avenues for its improvement.   

Ramsey Envelope Modulation in NV Diamond Magnetometry

Nitrogen-vacancy (NV) ensembles in diamond offer promising applications in magnetic sensing and imaging, including studies of biological, geological and solid-state systems. The recent development of high-purity ¹⁵N-doped CVD diamond (Ι = 1/2) offers advantages over the naturally occurring ¹⁴N isotope (I = 1) for magnetometry. However, the lack of a quadruple moment in ¹⁵N-doped diamond leads to pronounced envelope modulation effects in sensing modalities that require a misaligned bias magnetic field. While such effects in spin echo experiments have been well studied, discussion of analogous effects in Ramsey measurements and the implications for magnetometry remain under-explored. We derive the modulated Ramsey response and compare to experimental results, revealing significant beating and sensitivity loss if left unaddressed. Double-quantum coherences that utilize the full spin-1 ground state system can suppress these modulations, while additionally proving robust to local strain and temperature shifts.   

Spatially Resolved Charge Dynamics in Diamond and Super-Resolution Microscopy with Airy Disks

Quantum technologies based on color centers in diamond are promising for numerous applications but face many outstanding challenges and open questions. For example, controlling the charge state of nitrogen vacancy (NV) centers is critical to utilizing them in applications ranging from sensing to quantum repeaters. However, little is known about charge transport and the dynamics of charged defect in diamond. We introduce a novel experimental technique to probe charge diffusion in diamond using single-shot charge state readout of an isolated NV center. By spatially mapping the change in the NV charge state, we study charge release, transport, and capture processes of surrounding defects and identify the dark state of silicon vacancy (SiV) centers under laser illumination as SiV^2-. Furthermore, we demonstrate that spatial resolutions far below the diffraction limit can be achieved by exploiting the Airy pattern formed by diffraction through the objective lens aperture. This new super-resolution technique can be used to localize and manipulate color centers individually, even when they are spatially separated by less than the diffraction limit. The primary advantage of this technique is that it can be employed in a standard confocal microscope without requiring any specialized optics or physical modifications. Part of this work was performed under the auspices of US DOE by LLNL under Contract DE-AC52-07NA27344   

Towards single atom state readout of Rb atoms trapped in solid neon.

In prior work, we studied the optical and spin coherence properties of Rb atoms trapped in solid neon. Working with ensembles, we have demonstrated their ability to sense nearby Ne-21 spins inside the matrix and successfully performed NMR spectroscopy of Ne-21. By scaling down to single-atom measurements, we aim to use a single Rb sensor atom to perform NMR spectroscopy of single nuclear spins co-trapped inside the neon matrix. Progress towards this goal will be discussed.    

Towards a cw superradiant laser: Continuous strong coupling and transport of 88Sr atoms in a ring cavity

Superradiant lasers are a promising path towards realizing a narrow-linewidth, high-precision and high-bandwidth active frequency reference [1]. They shift the phase memory from the optical cavity, which is subject to technical and thermal vibration noise, to an ultra-narrow optical atomic transition of an ensemble of cold atoms trapped inside the cavity. Our previous demonstration of pulsed superradiance on the mHz transition in ⁸⁷Sr [2,3] achieved a fractional Allan deviation of 6.7·10^-16 at 1s of averaging. Moving towards continuous-wave superradiance promises to further improve the short-term frequency stability by orders of magnitude. A key challenge in realizing a cw superradiant laser is the continuous supply of cold atoms into a cavity, while staying in the collective strong coupling regime. 

We demonstrate continuous loading of cold ⁸⁸Sr atoms into a ring cavity, after several stages of laser cooling and slowing. We characterize controlled transport of the atoms within the ring cavity using an intracavity travelling wave optical lattice, and demonstrate continuous strong collective atom-cavity coupling through continuous measurement of the vacuum Rabi splitting.

[1] D. Meiser et al., Phys. Rev. Lett. 102, 163601 (2009).

[2] M. A. Norcia et al., Science Advances 2, e1601231 (2016)

[3] M. A. Norcia et al., PRX 8, 021036 (2018)   

Cavity-mediated rapid state detection of single atoms in tweezer arrays

The ability to measure a subset of a quantum system without perturbing the rest paves the way for quantum error correction, quantum teleportation, and other real-time feedback processes. Here, we demonstrate rapid, localized, and lossless state measurement of single ⁸⁷Rb atoms in an optical tweezer array adjacent to a high-finesse optical cavity. This extends the capabilities of atomic array experiments by offering an alternative to global detection relying on state-selective atom loss and fluorescence imaging. In our work, single atoms are moved into the cavity for fast fluorescence- or transmission-based readout that differentiates among the ground state hyperfine levels and empty cavity. We achieve measurement fidelities exceeding 99% in timescales of tens of microseconds. To establish the local nature of this measurement, we initialize a two-atom array and perform a microwave Ramsey experiment, with a cavity measurement of the first atom between Ramsey pulses on the second atom. The second atom’s coherence is unperturbed by the first atom measurement.   

A Microfabricated Chip-scale Atomic Beam System with Self-sustained Vacuum

Atomic beams are a key technology for realizing navigation grade, rack-mounted Cs atomic clocks, and offer a promising approach to realize gyroscopes based on atom interferometry. Miniaturization of atom beam technology will enable new quantum sensor architectures benefitting from foundry production and microfabrication approaches. This work presents the design, fabrication, and characterization of a centimeter-size planar atomic beam platform with a self-sustained vacuum given by graphite and non-evaporable getters. Monte Carlo simulations guide our design on atomic beam formation and vacuum maintenance. Deep reactive ion etching of silicon defines three functional areas: an atom reservoir, a microchannel array for atomic beam collimation, and a drift region for its ballistic propagation. Finally, anodic bonding hermetically seals the silicon functional and glass capping layers together. Our measured absorption and fluorescence spectra possessing narrow Doppler features indicate the ballistic transport of atomic beams in the drift region. A detailed comparison of these with Monte Carlo simulations is still in process. In addition, we have benchmarked two-zone Ramsey interferometry and transverse laser cooling to boost atomic beam brightness on a free-space apparatus for future implementation of atomic sensing protocols on this chip-scale platform.   

A quantum network of entangled optical atomic clocks

Optical atomic clocks are our most precise tools to measure time and frequency. They enable precision frequency comparisons between atoms in separate locations to probe the space-time variation of fundamental constants, the properties of dark matter, and for geodesy. Measurements on independent systems are limited by the standard quantum limit (SQL); measurements on entangled systems, in contrast, can surpass the SQL to reach the ultimate precision allowed by quantum theory - the so-called Heisenberg limit. While local entangling operations have been used to demonstrate this enhancement at microscopic distances, frequency comparisons between remote atomic clocks require rapid high-fidelity entanglement between separate systems that have no intrinsic interactions. We demonstrate the first quantum network of entangled optical clocks [1] using two 88Sr+ ions separated by a macroscopic distance (2 m), that are entangled using a photonic link. We characterise the entanglement enhancement for frequency comparisons between the ions. We find that entanglement reduces the measurement uncertainty by a factor close to √2, as predicted for the Heisenberg limit, thus halving the number of measurements required to reach a given precision. Practically, today's optical clocks are typically limited by laser dephasing; in this regime, we find that using entangled clocks confers an even greater benefit, yielding a factor 4 reduction in the number of measurements compared to conventional correlation spectroscopy techniques. As a proof of principle, we demonstrate this enhancement for measuring a frequency shift applied to one of the clocks. Our results show that quantum networks have now attained sufficient maturity for enhanced metrology. This two-node network could be extended to additional nodes, to other species of trapped particles, or to larger entangled systems via local operations.

 

[1] arXiv:2111.10336   

Application-focused R&D of Rydberg atom electric field sensors

Rydberg atom electric field sensors are projected to enable novel capabilities for communications and sensing due to their small size, high sensitivity, and broad tunability. While these atomic electric field sensors will not replace traditional receivers in commodity applications for RF signal reception, these sensors could be an enabling technology in niche application spaces. We will quantitatively compare the performance of Rydberg atom electric field sensors to traditional devices and discuss their advantages and limitations for select applications with a particular focus on resilient communications and spectrum situational awareness.  We will also discuss design considerations with creating a deployable sensor.