Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W38: Quantum Metrology and Sensing VRecordings Available

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Sponsoring Units: DQI Chair: Leah Weiss, Oak Ridge National Laboratory Room: McCormick Place W195 
Thursday, March 17, 2022 3:00PM  3:12PM 
W38.00001: Atomic scale quantum sensing from ultrafast coherence measurement of single H_{2} molecules in the STM junction Likun Wang, Yunpeng Xia, Wilson Ho Quantum sensing takes advantage of the quantum properties of a qubit system to realize measurements with high sensitivity surpassing its classical counterpart. Scanning tunneling microscopy combined with femtosecond THz spectroscopy has shown the capability of coherence measurement of single molecules. We report here, by performing time domain THz rectification spectroscopy, the discovery of a molecular hydrogen quantum sensor in the STM junction with unprecedented atomicscale spatial and femtosecond temporal resolution and sensitivity. The hydrogen molecule in the STM junction behaves as a twolevel system with its coherent oscillation period and decoherence time highly sensitive to the underlying surface potential of Cu_{2}N grown on Cu(100) surface. We explore the sensitivity of single hydrogen molecules to the surrounding environment in a controllable way in all three spatial dimensions. Time domain THz rectification imaging was then acquired with various pumpprobe delays to reveal the atomic scale surface potential distribution of Cu_{2}N. Our results demonstrate a new class of quantum sensor in the STM junction with ultrahigh resolution and sensitivity outperforming existing sensing protocols. 
Thursday, March 17, 2022 3:12PM  3:24PM 
W38.00002: Single molecule coherence for sensing point defects with spacetime resolution within the STM Cavity Dan Bai, Likun Wang, Wilson Ho The quantum coherence of a single molecule can have extreme sensitively on its environment and thus provide unmatched precision in measurements. Here, we demonstrate that the coherent oscillations of a hydrogen molecule adsorbed on a surface arise from the superposition of its twolevel quantum states that sensitively depend on its environment. Combining a femtosecond terahertz laser with a low temperature scanning tunneling microscope, we present a comprehensive study of the coherence of a hydrogen molecule as it probes point defects in Cu_{2}N islands grown on Cu(100) with simultaneous spatial and temporal resolutions . Single hydrogen molecules serve as quantum sensors of different types of point defects and surface heterogeneity as determined from variations in the frequency, phase and decoherence time of the coherent oscillations. Time resolved imaging of the molecular coherence provides a movie of the changes in space and time. The extreme position sensitivity of the phase and frequency reveals a spatially varying local potential surrounding the defects. These results validate the supremacy of quantum sensing based on the coherent properties of single molecules that should be widely applicable to other nanoscale systems. 
Thursday, March 17, 2022 3:24PM  3:36PM 
W38.00003: Accelerated Bayesian experiment design for highspeed quantum sensing Sean M Blakley, Robert D McMichael In a sweptfrequency magnetic resonance measurement, most of the acquisition time is often spent on system settings with little bearing on the results. Optimal Bayesian experiment design methods use Bayesian statistics to adaptively predict which system settings yield useful data; our previous implementation of these methods has provided a 40fold reduction in the number of data points needed to produce accurate magnetic field measurements with a nitrogenvacancy (NV) diamond sensor. [Phys. Rev. Appl. 14, 054036 (2020)] While the acquisition speed in this example was limited by the emission rate of the NV centers, the computation time of the adaptive algorithm will ultimately become the limiting factor for stronger signals. In this presentation, a simplified adaptive Bayesian algorithm for high speed quantum sensing with NV centers is described, yielding a 2.5 fold speed improvement in settings computation over previous versions. Fixedpoint arithmetic and parallel execution of the Bayesian inference, optimization, and resampling subroutines make this algorithm suitable for implementation in a fieldprogrammable gate array (FPGA) platform, which we project will result in a 1000 fold reduction in processing time compared to the software version. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W38.00004: Adaptive measurement protocols for Ramsey sequence quantum sensing Sean M Blakley, Robert D McMichael The Ramsey sequence is a canonical example quantum interferometry and a mainstay of quantum sensing. When readout fidelity is high, the phase can be determined with Heisenberg scaling where precision Δφ is proportional to 1/N after N measurements. Here, we address Ramsey measurements with lowfidelity readout in a comparison of protocols for choosing the phase accumulation time setting, τ. In simulations of Ramsey measurements using NV^{} center spin qbits for magnetic field sensing, we compare a new optimal Bayesian experiment design protocol to an adaptive heuristic protocol, a quantum phase estimation algorithm, and random setting choices. When precession frequency is the lone parameter to estimate, the Bayesian design is faster by factors of roughly 2, 4, and 5 relative to the adaptive heuristic, random τ choices, and the quantum phase estimation algorithm, respectively. When four parameters are to be determined, Bayesian experiment design and random τ choices can converge to roughly equivalent sensitivity, but the Bayesian method converges four times faster. 
Thursday, March 17, 2022 3:48PM  4:00PM Withdrawn 
W38.00005: Optimal Control for Quantum Metrology via Pontryagin's principle Chungwei Lin, Dries Sels Quantum metrology comprises a set of techniques and protocols that utilize quantum features for parameter estimation which can in principle outperform any procedure based on classical physics. We formulate the quantum metrology in terms of an optimal control problem and apply Pontryagin's Maximum Principle to determine the optimal protocol that maximizes the quantum Fisher information for a given evolution time. As the quantum Fisher information involves a derivative with respect to the parameter which one wants to estimate, we devise an augmented dynamical system that explicitly includes gradients of the quantum Fisher information. The necessary conditions derived from Pontryagin's Maximum Principle are used to quantify the quality of the numerical solution. The proposed formalism is generalized to problems with control constraints, and can also be used to maximize the classical Fisher information for a chosen measurement. 
Thursday, March 17, 2022 4:00PM  4:12PM 
W38.00006: Virus quantum sensor based on nitrogenvacancy centers in diamond Changhao Li, Rouhollah Soleyman, Mohammad Kohandel, Paola Cappellaro The development of highly sensitive and rapid biosensing tools targeted to virus such as SARSCoV2 is critical to tackling the COVID19 pandemic and other highly contagious viral disease. Quantum sensors can play an important role, thanks to their superior sensitivity and fast improvements in recent years. Here we propose a molecular transducer designed for nitrogenvacancy (NV) centers in nanodiamonds, translating the presence of virus RNA into an unambiguous magnetic noise signal that can be optically read out. With SARSCoV2 as an example, we evaluate the performance of the hybrid sensor, including its sensitivity and false negative rate, and compare it to widespread diagnostic methods. The proposed method is fast and promises to reach a sensitivity down to a few hundreds of RNA copies with false negative rate less than 1%. The proposed hybrid sensor can be further implemented with different solidstate defects and substrates and integrated with CRISPR technology. We will also show preliminary experimental results including characterization of the hybrid sensor and its response in the presence of virus RNA. 
Thursday, March 17, 2022 4:12PM  4:24PM 
W38.00007: Sensing diffusion with spincenters Denis R Candido, Michael E Flatté In this work we study the electric noise in spin defects due to both fluctuation of the surface charge density and the electrostatic potential at the surface of a crystal. Surprisingly, we show that the depth dependence of the electric noise spectral density is strongly influenced by the twopoint correlation function of both the charged particles' positions and the surface electrostatic potential, rather than solely by the character of the charge fluctuators, e.g., pointlike or dipole. Furthermore, the theory developed here identifies the fingerprints and signatures of diffusion of charged particles in the spin defect's T1 and T2. Similarly to some manybody sensing techniques, here both the defect spin decay and dephasing contain a crossover as a function of time around the characteristic correlation time of the fluctuators, determined by the diffusion coefficients. Hence, spin defects can also be used for sensing diffusion phenomena and extracting the corresponding correlation time and diffusion constant. 
Thursday, March 17, 2022 4:24PM  4:36PM 
W38.00008: Minimum Entanglement Protocols for Function Estimation Adam Ehrenberg, Jacob A Bringewatt, Alexey V Gorshkov We derive a family of optimal protocols, in the sense of saturating the quantum CramérRao bound, for measuring a linear combination of d field amplitudes with quantum sensor networks, a key subprotocol of general quantum sensor networks applications. We demonstrate how to select different protocols from this family under various constraints via linear programming. Focusing on entanglementbased constraints, we prove the surprising result that highly entangled states are not necessary to achieve optimality in many cases. Specifically, we prove necessary and sufficient conditions for the existence of optimal protocols using at most kpartite entangled catlike states. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W38.00009: Efficient tomography of multiple bosonic modes with bounded total excitations Yat Wong, Alireza Seif, Ming Yuan, Kevin He, Srivatsan Chakram, David Schuster, Liang Jiang With recent advances in highfidelity control and quantum error correction over multiple bosonic modes, it becomes an outstanding challenge to efficiently characterize quantum systems of multiple cavities. The standard Wigner tomography suffers exponential overhead with the number of bosonic modes (e.g., limiting us to characterizing only up to two or three bosonic modes). However, we may use the prior knowledge about the quantum state to enable efficient tomography. When we limit the total number of excitations, the size of the space is reduced to a polynomial of the number of bosonic modes. Based on this observation, we propose an efficient tomography protocol to characterize the projected state with polynomial measurements of some product operators. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W38.00010: Achieving Heisenberg Scaling on Measurement of A ThreeQubit System via Quantum Error Correction Andrew N Jordan, Le Hu In manybody quantum systems, the quantum Fisher information an observer can obtain is susceptible to decoherence. Consequently, quantum enhanced metrology, such as Heisenberg scaling, cannot usually be achieved. We show that by applying periodic quantum error corrections, we can achieve the Heisenberg scaling for an extended period of time on a threequbit TavisCumming Model, where three twolevel atoms interact with a single cavity mode, under certain approximations. The generalization to arbitrary number of atoms case is also discussed. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W38.00011: Adaptive RealTime Protocols for Improved Sensing with NV Centers Yonatan Cohen, Yoav Romach, Niv Drucker, Nir Halay, Nissim Ofek, Inbar Zohar, Amit Finkler, Nabeel Aslam The Quantum Machines' Quantum Orchestration Platform (QOP) has advanced realtime capabilities that can be used for various applications. Here we will present several results from groups working with the QOP that utilize its unique capabilities. We will show how realtime feedback and adaptive thresholding can improve the singleshot readout (SSRO) of a threelevel nuclear spin next to an NV center. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W38.00012: Robust quantum sensing via manybody scars Shane Dooley In most quantum sensing schemes, interactions between the constituent particles of the sensor are expected to lead to thermalization and degraded sensitivity. Recently, however, a new mechanism to slow down, or even avoid thermalisation in isolated quantum systems was discovered theoretically and observed experimentally, and was given the name "quantum manybody scarring". It is natural to ask: can this new mechanism be exploited to protect quantum sensing against thermalisation? I show that manybody scars can indeed be exploited for sensing that is robust against certain strong interactions. In the ideal case, the sensor is completely shielded from thermalisation and the optimal sensing time diverges. I demonstrate the idea with two examples: a spin1 model with DzyaloshinskiiMoriya interaction and a spin1/2 mixedfield Ising model. 
Thursday, March 17, 2022 5:24PM  5:36PM 
W38.00013: Errormitigated quantum metrology Kaoru Yamamoto, Suguru Endo, Hideaki Hakoshima, Yuichiro Matsuzaki, Yuuki Tokunaga Quantum metrology with entangled resources has the potential to achieve the sensitivity that is called the Heisenberg limit. Since the sensitivity is reduced by environmental noise, many theoretical efforts have been made to recover the sensitivity under the effect of decoherence. Conventionally, most of the theoretical protocols have focused only on statistical errors under the assumption that the noise model can be fully characterized. However, contrary to theoretical interests, noise fluctuates in time, for example, a fluctuation of the coherence time has been observed; accordingly, noise characterization is intractable, and then leads to systematic errors. Systematic errors usually come from a difference between a theoretical model and the actual one, and remain almost unexplored theoretically for the quantum metrology. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W38.00014: Spiderweb nanomechanicalresonator with a novel torsionalsoft clampingmotionfound by Bayesian optimization Dongil Shin, Andrea Cupertino, Matthijs H de Jong, Peter G Steeneken, Miguel A Bessa, Richard A Norte Nanomechanical resonators are key enablers of nextgeneration technologies, from ultrasensitive detectors of fundamental forces to quantumlimited commercial sensors and quantum networks operating at room temperature. Yet, the rational design of nanomechanical resonators is far from trivial. Apart from basic principles derived from onedimensional analytical models and the broad use of silicon nitride as a highly tensile base material, human intuition remains the driving force behind the design process. Here, inspired by nature and guided by machine learning, a spiderweblike resonator concept is presented that exhibits a novel vibration mode that reduces radiation losses without using phononic shields. This vibration mode was discovered by the datadriven exploration and was found to be essential for obtaining an unprecedented quality factor (1.8 billion) in a compact design (3 mm characteristic length) at low frequencies (around 130 kHz). This work demonstrates that machine learning and Bayesian optimization can play a key role in uncovering practical new directions in nanotechnology. 
Thursday, March 17, 2022 5:48PM  6:00PM 
W38.00015: RareEarth ions spin detected with a microwave photon counter Eric Billaud We report the spin resonance spectroscopy of rareearth ions using the newly developed method of spin microwave fluorescence detection at millikelvin temperature [1]. The rareearth ion is Er3+ in a CaWO4 crystal, which behaves as an effective electron spin½ with high gyromagnetic ratio and long coherence time [2]. The spins are magnetically coupled to a micronsize superconducting microwave resonator deposited on top of the crystal, which enhances their radiative relaxation rate via the Purcell effect [3]. The ions are excited by a resonant microwave pulse, and their radiative relaxation is detected at 10mK using a Single Microwave Photon Counter based on a transmon qubit [4]. 
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