Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K28: Quantum Measurement and Sensing IFocus

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Sponsoring Units: DQI Chair: Anja Metelmann, Princeton University Room: BCEC 161 
Wednesday, March 6, 2019 8:00AM  8:36AM 
K28.00001: Quantum metrology in the presence of dissipation with superconducting qubits. Invited Speaker: Kater Murch While quantum systems can offer significant advantages over classical systems in metrology, often the quantum states that confer such advantages are degraded by decoherence. I will discuss experiments that explore strategies to mitigate this degradation through continuous quantum measurement and feedback. Furthermore, dissipation also yields potential for further metrological advantages that are associated with exceptional point degeneracies in nonHermitian systems. I will discuss how such degeneracies can be realized in single dissipative qubits and present investigations of enhanced measurement sensitivity in single quantum systems. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K28.00002: NonHermitian quantum sensing: exceptional point and nonreciprocal approaches HoiKwan Lau, Aashish Clerk Unconventional properties of nonHermitian systems, such as the existence of exceptional points, have recently been suggested as a resource for sensing [1,2]. The impact of noise and utility in quantum regimes however remains unclear. We describe here a full analysis of quantum parametric sensing using coupled mode systems described by effective nonHermitian Hamiltonians; our approach rigorously accounts for quantum noise effects [3]. Focusing on twomode devices, we derive fundamental bounds on the signal power and signaltonoise ratio for any such sensor. We use these to demonstrate that enhanced signal power requires gain, but not necessarily any proximity to an exceptional point. Further, when noise is included, we show that nonreciprocity is a powerful resource for quantum sensing: it allows one to exceed the fundamental bounds constraining any conventional, reciprocal sensor. Nonreciprocal quantum sensors could be implemented in a variety of systems, including superconducting quantum circuits and quantum optomechanical systems. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K28.00003: Saturating the quantum CramérRao bound using LOCC Sisi Zhou, ChangLing Zou, Liang Jiang The quantum CramérRao bound (QCRB) provides an ultimate precision limit allowed by quantum mechanics in parameter estimation. Given any quantum state dependent on a single parameter, there is always a positiveoperator valued measurement (POVM) saturating the QCRB. However, the QCRBsaturating POVM cannot always be implemented efficiently, especially in multipartite systems. In this talk, we show that the POVM based on local operations and classical communication (LOCC) is QCRBsaturating for arbitrary pure states or ranktwo mixed states with varying probability distributions over fixed eigenbasis. We also analyze the robustness of our LOCC protocol against noise and show how it can be made noiseresilient. Finally, a fourqubit system is studied as an example of a nontrivial LOCC protocol saturating the QCRB. For details, see ArXiv: 1809.06017. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K28.00004: Ramsey interferometry in correlated quantum noise environments Leigh Norris, Felix Beaudoin, Lorenza Viola We quantify the impact of spatiotemporally correlated Gaussian quantum noise on frequency estimation by Ramsey interferometry. While correlations in a classical noise environment can be exploited to reduce uncertainty relative to the uncorrelated case, we show that quantum noise environments with frequency asymmetric spectra generally introduce additional sources of uncertainty due to uncontrolled entanglement of the sensing system mediated by the bath. For the representative case of collective noise from bosonic sources, and experimentally relevant collective spin observables, we find that the uncertainty can increase exponentially with the number of probes. As a concrete application, we show that correlated quantum noise due to a lattice vibrational mode can preclude superclassical precision scaling in current amplitude sensing experiments with trapped ions. This work was recently reported in PRA (Rapid Communications) 98, 020102 (2018). 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K28.00005: Hamiltonian engineering for quantum sensing YiXiang Liu, Ashok Ajoy, Jordan Hines, Paola Cappellaro Quantum sensing utilizes the interaction between the quantum sensor and the object to be detected. However, limited by the intrinsic system Hamiltonian or experimental capabilities, the implementable Hamiltonian evolution may not be optimal to reveal the information of the target object. Inspired by digital quantum simulation, here we present a general framework in which we use Trotterlike combinations of unitaries to engineer better Hamiltonians for sensing and provide an efficient protocol to reduce the approximation error. We show the application of Hamiltonian engineering to nanoscale magnetic resonance imaging. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K28.00006: Provably optimal controls for magnetometry in cluttered environments Virginia Frey, Leigh Norris, Lorenza Viola, Michael Jordan Biercuk The extreme fragility of quantum systems makes them ideally suited for sensing applications such as magnetometry, biological imaging and noise characterization for quantum computing. However, interpreting a qubitbased sensor's output is generally complicated by background clutter arising from both outofband spectral leakage, and ambiguity in signal origin when the implemented qubit drive is imperfect. Here we present a novel sensing protocol based on the optimal bandlimited Slepian functions that overcomes both of these challenges. We construct Slepianbased controls using a finitedifference method that preserves the relevant spectral concentration while removing nonlinearities in the sensor response which arise when targeting ambient noise signatures that couple to the sensor's signal through an additive dephasing Hamiltonian term, such as magnetic field fluctuations. We experimentally implement a tomographic measurement framework which separates multiaxes contributions using projective measurements on a single trapped ion magnetometer. Experiments validate the spectral concentration of the new finitedifference controls and allow for simultaneous, narrowband spectrum reconstruction of both environmental dephasing and control noise fields. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K28.00007: Quantum state tomography of a mechanical oscillator Robert Delaney, Adam P Reed, Reed Andrews, Konrad Lehnert Quantum mechanics places strict limits on how precisely the two motional quadratures of an object can be simultaneously measured. To noiselessly reconstruct an unknown quantum state of motion, a single quadrature measurement can be repeated over all of phase space, and the density matrix describing the quantum state can be reconstructed via quantum state tomography. Here we demonstrate a pulsed measurement of the motion of a micromechanical oscillator embedded in a superconducting electromechanical circuit. The measurement can be tuned from a nearly quantumlimited simultaneous measurement of both quadratures, to a single quadrature measurement with added noise of 10 dB relative to vacuum fluctuations in one quadrature, resulting in a total measurement efficiency of 92% . The high efficiency measurement is used to accurately reconstruct the density matrix of a dissipatively squeezed mechanical oscillator (3.3 dB relative to vacuum fluctuations), demonstrating that the measurement is suitable for tomography of arbitrary quantum states of the mechanical oscillator. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K28.00008: Quantum sensing of magnons with a superconducting qubit Dany LachanceQuirion, Samuel Piotr Wolski, Yutaka Tabuchi, Shingo Kono, Koji Usami, Yasunobu Nakamura Opportunities for quantum sensing of quanta of collective spin excitations in a ferromagnet, called magnons, are now possible thanks to the demonstrations of both strong resonant and dispersive couplings between a superconducting qubit and the uniform precession mode, or Kittel mode, of a ferromagnetic sphere [1,2]. Based on operations on the qubit conditional on the state of the Kittel mode, singleshot detection of a single magnon with an efficiency reaching about 50% is demonstrated using the protocol of Ref. [3]. The detection efficiency is mainly limited by the qubit readout fidelity. Furthermore, a magnon detection sensitivity of about 10^{3} magnons/√Hz is demonstrated using a standard Ramsey interferometry technique. These two complementary quantum sensing methods could find applications in quantum technologies based on magnonics and the detection of axions in dark matter searches. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K28.00009: Spinorbit semiconductors as dark matter detector targets using firstprinciples calculations. Katherine Inzani, Sinead Griffin The direct detection of light dark matter (DM) relies on harnessing lowthreshold events in target materials. One such event is the scattering of an electron across a small gap, where our target material’s threshold will determine the range of masses of DM particles we are sensitive to. In this work we investigate spinorbit semiconductors as DM detection targets. We report the results of a highthroughput search for new smallgapped semiconductors, and estimate their reach as DM targets using firstprinciples methods. We also discuss how the calculated material properties influence the detector size and geometry. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K28.00010: Axion Dark Matter Detection with Superconducting Qubits Akash Dixit, David Schuster, Aaron Chou, Ankur Agrawal, Srivatsan Chakram, Ravi Naik The axion is a potential solution to the strong CP problem in QCD and could account for the abundance of dark matter observed in the universe. In the presence of an applied magnetic field, the axion field will source a current used to drive a resonant cavity to single photon occupation. A transmon qubit operating as a microwave photon sensor is a viable readout system at frequencies where the added noise of quantum limited amplifiers overwhelms the signal rate. The use of a direct dispersive quantum nondemolition measurement of the photon number decouples the measurement back action from the experimental uncertainties. In this regime background and dark counts become the dominant sources of detector error. For a transmon qubit operating as a photon counter, the dark rate is typically 110% and is orders of magnitude greater than the anticipated signal rate. In order to mitigate the effect of individual bit flip errors of the detector we operate multiple detectors in the same cavity volume. The error rate of the joint Nqubit detector could potentially have exponentially suppressed error rates as compared to the single detector. We will report theoretical performance and progress towards characterizing fidelity and correlations of multiqubit detectors. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K28.00011: Quantum noise limits for a class of nonlinear amplifiers Jeffrey Epstein, Joshua Combes We introduce a class of nonlinear amplifier inputoutput relations that allow the measurement of any normal operator using linear measurements with only vacuum fluctuations added at the output. In the limit of large gain, such devices would effectively implement perfect measurements of nonlinear operators in the same way that linear amplifiers permit effectively perfect linear measurements even when paired with realistic, noisy measurement devices. We analyze the application of these amplifiers to photon number measurements and state estimation. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K28.00012: One from many: Scalar estimation in a multiparameter context Jonathan Gross, Carlton Morris Caves Achievable sensitivity bounds are difficult to formulate for quantum multiparameter estimation. We consider a specialized case: many parameters of a Hamiltonian are unknown and one seeks an estimate for a scalar function of the Hamiltonian. This problem exhibits genuine multiparameter behavior, though it is superficially similar to singleparameter estimation. By uniting saturable singleparameter quantum bounds with geometric reasoning we prove the conditions, necessary and sufficient, for saturating the fundamental and attainable bound in this context. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K28.00013: Optimal measurements in simultaneous multiparameter estimation of quantum systems Jing Yang, Shengshi Pang, Yiyu Zhou, Andrew N Jordan Simultaneous estimation of multiple parameters is a wellknown challenge in quantum metrology. The underlying difficulty is to identify the common optimal measurements for all the parameters, which typically do not coincide or commute. For a general probe state and a projective measurement of arbitrary rank, we find the necessary and sufficient conditions under which the measurement gives rise to the multiparameter quantum CramerRao matrix bound. We also give an application of these conditions to the specific problem of estimating threedimensional separation of two point incoherent sources of equal intensities from single photon measurements. By considering the hardaperture and paraxial approximated pupil function, we find a local optimal measurement for simultaneous estimation of the small three dimensional separation. Furthermore, regardless of the magnitude of longitudinal separation, a local optimal measurement for simultaneously estimating the small transverse separation is also found. The saturation conditions of the multiparameter quantum CramerRao bound may be further explored in quantum sensing and imaging. 
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