Session Q39: Quantum Metrology and Sensing II

Towards spin-squeezing in two-dimensional ensembles of solid-state defects

Generating spin-squeezed states in quantum simulators with power-law interactions is a key experimental challenge with limited theoretical guidance. While numerical evidence suggests it should be possible to achieve spin squeezing with sufficiently long-range (but still energetically extensive) XXZ Hamiltonians, the precise requirements remain unclear. We conjecture an explanation for the "squeezing phase diagram" of long-range XXZ models. While squeezing in such models is dynamically generated by time evolution from simple product states, our explanation is intimately connected to the presence of finite-temperature equilibrium order in the Hamiltonian. Using a variety of numerical methods, we test our conjecture in one-dimensional models and find necessary and sufficient conditions for spin squeezing. We discuss the implications of these conditions for realizing spin-squeezing in a two-dimensional ensemble of solid-state defects coupled via magnetic dipole-dipole interactions.   

Preparation of Metrological States in Dipolar Interacting Spins Systems

The creation and control of highly entangled states lies at the heart of quantum metrology and promises sensing beyond the Standard Quantum Limit. Dipolar interacting spins in atomic and solid-state systems have recently emerged as an attractive candidate for engineering such states. This talk will discuss a novel variational method that efficiently generates metrologically relevant entangled states in small dipolar interacting spin ensembles using only limited qubit control and no knowledge of the actual spin configuration. Our results show that the generated entangled states provide sensitivity approaching the Heisenberg Limit. Depending on the depth of the variational ansatz the resulting metrological states resemble Greenberger–Horne–Zeilinger (GHZ) or Squeezed Spin states. We further show that these results hold in the presence of experimental imperfections, such as finite initialization/readout fidelity and coherence. The developed black-box variational optimization techniques provide a deeper understanding of the connections between spin arrangement (random vs regular arrays), entanglement, and obtainable sensitivity. Our results are directly applicable to systems in which the number of spins is limited, such as diamond-based nanoscale sensing.   

Transverse spin scanning relaxometry of a high temperature superconductor

The NV center in diamond has emerged as a versatile nanoscale sensor due to its long spin coherence times and spin-dependent fluorescence response over a wide temperature range. Increasing the coherence lifetime of shallow NVs without loss of sensitivity has been a challenge due to concomitant paramagnetic impurities near the surface. Here, we monitor and spatially map the coherence lifetime of a single NV spin in an all-diamond scanning probe brought in close proximity with a thin film of TBCCO, a high temperature superconductor. We find a heterogeneous response, with some sections of the sample exhibiting accelerated transverse spin relaxation. Remarkably, we also identify regions where the NV coherence lifetime grows, which we attribute to a Meissner-effect-induced cancelation of the environmental spin noise. Through the use of multi-pulse spin control sequences with variable timing, we also extract the spectral response of the superconductor. These observations open intriguing opportunities to engineer shallow NVs with improved spin coherence.   

Magnetic sensing of iron in biomolecules using diamond nitrogen vacancy centers

Iron is an essential yet toxic redox active element that is found in many cells, including neurons and glial cells. Several techniques have been used to quantify iron in neurons and cells; however, most are incapable of high-resolution imaging inside a single cell. Magnetic field sensors based on diamond nitrogen vacancy (NV) centers have emerged as a powerful tool for detecting magnetic signal in iron-containing biological samples with a good combination of spatial resolution and sensitivity [1-3]. In this study we use NV based T₁ relaxometry technique [2] to map iron in cytochrome C (Cyt C) proteins. Cyt C plays an important role in the electron transport chain of mitochondria and it is in the Fe(III) paramagnetic state under ambient conditions. We measure Cyt C under different concentrations and locations of the 10-nm NV doped diamond chip. We show a reduction of the NV T₁ from few milliseconds to hundreds of microseconds, explained by the spin noise from the intracellular iron spins. We discuss plans of imaging Cyt C and other iron-containing proteins on nanostructured diamond chip integrated with gratings. [1] A. Ermakova, et al., Nan. Lett. 13(7), 3305 (2013). [2] P. Wang, et al., Sci. Adv. 5(4), eaau8038 (2019). [3] I. Fescenko, et al., Phy. Rev. App. 11(3), 034029 (2019).   

AC Sensing of NV Centers under Hydrostatic Pressure

Nitrogen-vacancy color centers in diamond have attracted broad attention as quantum sensors for both static and dynamic magnetic, electrical, strain and thermal fields, and are particularly attractive for quantum sensing under pressure in diamond anvil cells. Optically-based nuclear magnetic resonance may be possible at pressures greater than a few GPa, and offers an attractive alternative to conventional Faraday-induction based detection. In this talk, we will discuss AC sensing of NV Centers under hydrostatic pressure inside of a diamond anvil cell, with a focus on technical challenges of performing these measurements. In this talk, we will discuss AC sensing of NV Centers under hydrostatic pressure inside of a diamond anvil cell, with a focus on technical challenges of performing these measurements.   

Nuclear spin sensing under hydrostatic pressure using nitrogen vacancy centers

Nitrogen vacancy centers have attracted broad attention as quantum sensors particularly for quantum sensing under geochemical hydrostatic pressure conditions(hundreds of kbars) . Here we present AC sensing results up to 6 GPa with sensitivity to detect nuclear spin inside the diamond anvil cell. These experiments enable the possibility for all-optical high resolution magnetic resonance of nanoliter sample volumes to study geochemical processes.   

Electric-field sensing using Quantum spin defect/multiferroic hybrids

Quantum spin defects (QSD) are promising candidates for probing various condensed matter phenomena owing to their nanometer-scale resolution, wide temperature range, and noninvasive mode of operation. While the spin energy states of QSD are naturally susceptible to external magnetic noise, sensing electric field has so far relied on exploiting the weak Stark effect and/or high-density ensembles [Nature Phys., 7, 459 (2011), Phys. Rev. Applied 16, 024024 (2021)]. In this work, we propose an alternate route towards probing the DC electric field by utilizing the voltage control of magnon-QSD coupling in a ferroelectric/ferromagnetic/Nitrogen vacancy center hybrid. Our results demonstrate nearly an order of magnitude enhancement in sensitivity compared to the Stark effect, thus paving the way towards devising a nanoscale, room temperature hybrid E-field sensor.   

Optimization of shallow surface Nitrogen Vacancy yield with Hot Implant of Nitrogen-Implanted Diamond

Nitrogen vacancies (NVs) are diamond defects with long coherence times with excellent magnetic sensitivity.  For AC sensing and sensing of small length scales, a shallow surface NV layer is desired.  However, the standard process of nitrogen implant followed by anneal has low yield at the surface, and increasing the implantation density results in greater P1 impurities in the diamond and can lead to graphitization.  Improved NV yield has previously been demonstrated by introducing new vacancies through carbon co-implantation at high temperatures, which introduces no new impurities and allows the lattice to heal in situ.  With techniques such as double electron electron resonance (DEER), we explore the dynamics of carbon implants at high temperatures ("hot implant") on nitrogen-implanted diamonds towards optimizing creation efficiency of shallow surface NV layers.   

Quantum sensing of photonic spin density

Nitrogen-vacancy (NV) centers in diamond have emerged as promising room-temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids and magnons to hydrodynamic flow of current. Here we demonstrate that the NV center can be used as a quantum sensor for detecting the photonic spin density (PSD). PSD is the spatial distribution of light's spin angular momentum in the nanoscale. The PSD is associated with spin-momentum locking for light and topological effects such as photonic skyrmion. The spinning field of light induces an effective static magnetic field in the spin qubit of the NV centers. We experimentally detect this effective field (10s of nanotesla) with a single NV center and an ensemble of NV centers at room temperature. We use ac magnetometry techniques to reach the required sensitivity using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-sub-wavelength volumes. The direct detection of the photonic spin density at the nanoscale opens interesting quantum metrological avenues for studying exotic phases of photons, nanoscale properties of structured light as well as future on-chip applications in and all-optical control of spin qubits.   

Enhanced Ramsey-based quantum metrology using dissipative spin amplification

Quantum metrology protocols exploiting ensembles of two-level systems and Ramsey-style measurements are ubiquitous. While spin squeezing can allow the sensitivity of these measurements to surpass the standard quantum limit (SQL), this is only a useful strategy if excess detection noise is sufficiently small compared to spin-projection noise. This is not the case in many settings, especially sensing protocols based on ensembles of solid-state defect spins. In this work, we present a phase-insensitive "spin amplification" protocol that allows one to dramatically improve the sensitivity of schemes limited by excess detection noise. Our method is based on exploiting collective spin decay, an effect that is usually seen as a nuisance because it limits spin-squeezing protocols. We show that this approach can allow a system with extremely imperfect detection to approach the SQL within a factor of 2. Our ideas are compatible with several state-of-the-art experimental platforms where an ensemble of solid-state spins (NV centers, SiV centers) is coupled to a common microwave or mechanical mode.   

Magnetic sensing employing long-lived coherences in quantum spin ensembles

Ensembles of interacting quantum spins, driven by imperfect control pulses, often demonstrate coherence that survives far beyond the "usual" decay time, i.e. beyond the Hahn echo decay time [1,2]. This effect occurs in various spin systems of different dimensionalities. The long-lived coherences arise due to accumulation of the control imperfections; they are very robust, and can extend the coherence time by up to five orders of magnitude.

We present theoretical design and analysis of control protocols which employ the long-lived coherences for quantum-assisted sensing of ac magnetic fields. In these protocols the direction of the control pulses periodically changes, inducing the long-lived oscillations with the frequency which depends on the magnitude of the ac magnetic field; the baseline of the oscillations also depends on the ac field, enabling two modalities of magnetic sensing. Theory shows that this approach is a promising alternative to the sensing methods based on more traditional dynamical decoupling techniques. Experimental results utilizing ensembles of dipolar interacting nitrogen vacancy centers in diamond are presented.   

Temperature dependence of metrological sensitivities of nitrogen-vacancy centers in diamond

Fluorescence count rate, spin state fluorescence contrast and spin coherence times are key parameters determining the metrological sensitivity of NV centres in diamond, as well as their applicability in quantum information processing applications [1-2]. These parameters are well studied at room and low temperatures (~ 4 K ). However, understanding how they vary between 100 - 4 K has important implications on utilising NVs to study material systems which display key phase transitions in this intermediate temperature regime [3]. We present a temperature dependent study of how these parameters vary in near surface ensembles and single NVs.

[1] L. Rondin et al. Magnetometry with nitrogen-vacancy defects in diamond Reports on progress in physics, 77(5):056503, (2014) 

[2] J. O’Gorman. et al. A silicon-based surface code quantum computer. npj Quantum Inf 2, 15019 (2016).

[3] Nathan J. McLaughlin et al ‘Strong Correlation Between Superconductivity and Ferromagnetism in an Fe-Chalcogenide Superconductor’ Nano Letters 21 (17), 7277-7283  (2021)   

Imaging spin dynamics in ferromagnets using a single spin scanning magnetometer

Probing spin dynamics in ferromagnets at nanometer scale has drawn increasing attention due to the potential usage of spin waves as information carriers in future spintronic devices. Moreover, coupled with solid-state spin qubits, the spin waves may serve as information transducer between distant qubits. Here, we study spin dynamics in ferromagnets using a nitrogen-vacancy (NV) spin qubit in diamond. With the help of a single spin scanning magnetometer that hosts a NV center at the probe tip, we are able to image DC and AC magnetic field generated from the magnetic samples. For instance, we study dynamic magnetic properties of a thin Py square by mapping out Rabi oscillation frequency. From the comparison with static magnetic image, we find very distinct spin dynamics particularly around the vortex. We will further discuss on-going efforts of imaging spin waves in various ferromagnetic materials.