Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X51: Challenging Conventional Quantum Limits in Measurements and MetrologyFocus

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Sponsoring Units: GQI Chair: Gabriel Durkin, Berkeley Center for Quantum Computing and Computation Room: 398 
Friday, March 17, 2017 8:00AM  8:36AM 
X51.00001: Quantum Theory of Superresolution for Incoherent Optical Imaging Invited Speaker: Mankei Tsang Rayleigh's criterion for resolving two incoherent point sources has been the most influential measure of optical imaging resolution for over a century. In the context of statistical image processing, violation of the criterion is especially detrimental to the estimation of the separation between the sources, and modern farfield superresolution techniques rely on suppressing the emission of close sources to enhance the localization precision. Using quantum optics, quantum metrology, and statistical analysis, here we show that, even if two close incoherent sources emit simultaneously, measurements with linear optics and photon counting can estimate their separation from the far field almost as precisely as conventional methods do for isolated sources, rendering Rayleigh's criterion irrelevant to the problem. Our results demonstrate that superresolution can be achieved not only for fluorophores but also for stars. Recent progress in generalizing our theory for multiple sources and spectroscopy will also be discussed. [Preview Abstract] 
Friday, March 17, 2017 8:36AM  9:12AM 
X51.00002: Quantum back action free measurement of motion in a negative mass reference frame Invited Speaker: Eugene Polzik It has been proposed [13] that a measurement of motion with precision beyond the vacuum state uncertainty in \underline {both} position and momentum is possible if it is carried out in a quantum reference frame with an effective negative mass. In such a measurement, the quantum back action is evaded due to the destructive interference of the back action on the object and on the reference frame. The reference frame can be implemented with an oscillator which has its first excited state energy below the ground state energy, such for an atomic spin oscillator oriented along the magnetic field [4]. We report on the experiment where the motion of the oscillator is tracked in the reference frame of the spin oscillator by probing this hybrid quantum system with light. The mechanical oscillator is a macroscopic millimeter size membrane [6]. The atomic oscillator is a long lived collective spin of an atomic ensemble [4]. We demonstrate the evasion of the quantum back action of the measurement in the hybrid system and study an intricate interplay between quantum back action and the optomechanical cooling force. The negative mass reference frame physics opens the way towards generation of entanglement between the mechanical oscillator and an atomic spin, leading to applications in fundamental physics of entangled macroscopic objects, and force, gravitation and acceleration measurements beyond standard quantum limits. [1] K. Hammerer, M. Aspelmeyer, E.S. Polzik, P. Zoller. \textit{Phys. Rev. Lett. }102, 020501 (2009). [2] E.S. Polzik and K.Hammerer. \textit{Annalen der Physyk}. 527, No. 12, A15A20 (2015). [3] M. Tsang and C. Caves, \textit{Phys. Rev. Lett. }105(12), (2010). [4] G. Vasilakis et al. Nature Physics, doi:10.1038/nphys3280 (2015). [5] C. M{\o}ller, R. Thomas, G. Vasilakis, E. Zeuthen, Y. Tsaturyan, K. Jensen, A. Schliesser, K. Hammerer and E.S. Polzik. Manuscript in preparation. [6] Y. Tsaturyan et al. Optics Express, Vol. 22, Issue 6, pp. 68106821 (2014). [Preview Abstract] 
Friday, March 17, 2017 9:12AM  9:24AM 
X51.00003: Quantumlimited evanescent single molecule sensing. Warwick Bowen, Nicolas Mauranyapin, Lars Madsen, Michael Taylor, Muhammad Waleed Sensors that are able to detect and track single unlabeled biomolecules are an important tool both to understand biomolecular dynamics and interactions, and for medical diagnostics operating at their ultimate detection limits. Recently, exceptional sensitivity has been achieved using the strongly enhanced evanescent fields provided by optical microcavities and plasmonic resonators[1]. However, at high field intensities photodamage to the biological specimen becomes increasingly problematic[2]. Here, we introduce a new approach that combines dark field illumination and heterodyne detection in an optical nanofibre[3]. This allows operation at the fundamental precision limit introduced by quantisation of light. We achieve stateoftheart sensitivity with a four orderofmagnitude reduction in optical intensity. This enables quantum noise limited tracking of single biomolecules as small as 3.5 nm and surfacemolecule interactions to be montored over extended periods. By achieving quantum noise limited precision, our approach provides a pathway towards quantumenhanced singlemolecule biosensors. [1] Baaske et al, Nat. Nano. \textbf{9} 933 (2014); Pang and Gordon, Nano Letters \textbf{12} 402 (2012). [2] E.g. Mirsaidov et al., Phys. Rev. E. \textbf{78 }021910 (2008). [3] Mauranyapin et al. arxiv:1609.05979 (2016). [Preview Abstract] 
Friday, March 17, 2017 9:24AM  9:36AM 
X51.00004: Quantum Metrology and ManyBody Decoherence. Mathieu Beau, Aurelia Chenu, Jianshu Cao, Adolfo del Campo We introduce a scheme for the quantum simulation of manybody decoherence that relies on the unitary evolution generated by a stochastic Hamiltonian including $k$body interactions \footnote{A. Chenu, M. Beau, J. Cao, and A. del Campo, Quantum Simulation of ManyBody Decoherence: Noise as a Resource, arxiv/1608.01317 (2016).}. We propose to modulate the strength of the interactions with a stochastic process, and show that the dynamics of the noiseaveraged density matrix is effectively open and governed by $k$body Lindblad operators. Our proposal can be readily implemented on a variety of quantum platforms such as optical lattices, superconducting circuits, and trapped ions. It also has interesting applications in quantum metrology. After deriving the Quantum Cram\'{e}rRao bound for quantum open systems, we provide the conditions for robustness of the Heisenberg limit in the presence of manybody decoherence. [Preview Abstract] 
Friday, March 17, 2017 9:36AM  9:48AM 
X51.00005: Quantum detectors of vector potential and their modeling Armen Gulian, Gurgen Melkonyan, Ellen Gulian Proportionality of current to vector potential is a feature not allowed in classical physics, but is one of the pillars in quantum theory. For superconductors, in particular, it allows us to describe the Meissner effect. Since the phase of the quantum wave function couples with the vectorpotential, the related expressions are gaugeinvariant. Is it possible to measure this gaugeinvariant quantity locally? The answer is definitely ``yes'', as soon as the current is involved. Indeed, the electric current generates a magnetic field which can be measured straightforwardly. However, one can consider situations like the AharonovBohm effect where the classical magnetic field is locally absent in the area occupied by the quantum object (i.e., superconductor in our case). Despite the local absence of the magnetic field, current is, nevertheless, building up. From what source is it acquiring its energy? Locally, only a vector potential is present. Is the current formation a result of a truly nonlocal quantum action, or does the local action of the vector potential have experimental consequences on the quantum system, which then can be considered as a detector of the vector potential? We discuss possible experimental schemes on the level of COMSOL modeling. [Preview Abstract] 
Friday, March 17, 2017 9:48AM  10:00AM 
X51.00006: Scalable Heisenberg limited metrology using mixed states Geng Chen, yaron kedem Improving the precision of measurements is a prime challenge of the scientific community. Quantum metrology provides methods to overcome the standard quantum limit (SQL) of $1/\sqrt{N}$ and to reach the fundamental Heisenberg limit (HL), $1/N$ . While a lot of theoretical and experimental work has been dedicated to this task, most of the attempts focused on utilizing NOON and squeezed states, which exhibit unique quantum correlations. However, it was not yet experimentally demonstrated that schemes using these states are scalable. Here we present, and experimentally implement, a new scheme for precision measurements that enables reaching the HL. Our scheme is based on a probe with a large uncertainty, combined with a postselection, such that the Fisher information is maximized, and the CarmerRao bound is saturated. We performed a Heisenberg limited measurement of the Kerr nonlinearity at the single photon level, and report on an unprecedented precision $ \simeq 10^{9}$ of a Kerr phase. [Preview Abstract] 
Friday, March 17, 2017 10:00AM  10:12AM 
X51.00007: Optimal and nearoptimal probe states for quantum metrology of number conserving twomode bosonic Hamiltonians Tyler Volkoff We derive families of optimal and nearoptimal probe states for quantum estimation of the coupling constants of a general twomode numberconserving bosonic Hamiltonian describing onebody and twobody dynamics. For $\mathfrak{su}(2)$ dynamics and for interactions diagonal in the basis of Dicke states, families of superpositions of antipodal $SU(2)$ coherent states maximize the quantum Fisher information appearing in the quantum CramerRao bound. For nonlinear tunneling processes such as pair tunneling and densitydependent single particle tunneling, respectively, we present new classes of variational superposition probe states that provide near perfect saturation of the corresponding quantum CramerRao bounds. We show that the ground state of a pair tunneling Hamiltonian exhibits high fidelity with an optimal state for estimation of a single particle tunneling amplitude, and thereby conclude that a highperformance probe state for tunneling amplitude estimation may be produced by tuning the twomode system through a quantum phase transition. [Preview Abstract] 
Friday, March 17, 2017 10:12AM  10:24AM 
X51.00008: CramerRao bound for timecontinuous measurements in linear Gaussian quantum systems Marco G. Genoni We describe a compact and reliable method to calculate the Fisher information for the estimation of a dynamical parameter in a continuously measured linear Gaussian quantum system. Unlike previous methods in the literature, that involve the numerical integration of a stochastic master equation for the corresponding density operator in a Hilbert space of infinite dimension, the formulas here derived depends only on the evolution of first and second moments of the quantum states, and thus can be easily evaluated without the need of any approximation. We also present some basic but physically meaningful examples where this result is exploited, calculating analytical and numerical bounds on the estimation of the squeezing parameter for a quantum parametric amplifier, and of a constant force acting on a mechanical oscillator in a standard optomechanical scenario. [Preview Abstract] 
Friday, March 17, 2017 10:24AM  10:36AM 
X51.00009: Quantum metrology with LandauZener transitions Jing Yang, Shengshi Pang, Andrew Jordan In this talk, we present the fundamental precision limits in estimating the parameters with LandauZener transitions. For the case of a single LandauZener transition, rather than using the celebrated LandauZener transition probabilities, where the precision is quantified by the classical Fisher information, we show that using the acquired phase, higher precision may be obtained. The measurement precision is quantified by the quantum Fisher information for this scheme, which scales asymptotically as $T^{4}$ for estimating the sweeping velocity and $lnT$ for estimating the tunneling amplitude, where $T$ is the elapsed time, which can be further improved with controls. We also consider the case of multiple transitions before measurement, "LandauZenerStueckelberg interferometry", and show that with proper quantum controls quantum Fisher information for estimating the transition frequency can still achieve $T^{4}$ scaling, although the Hamiltonian is bounded. [Preview Abstract] 
Friday, March 17, 2017 10:36AM  10:48AM 
X51.00010: Continuous wave noise spectroscopy beyond weak coupling and Markov approximations Kyungdeock Park, Kyle Willick, Jonathan Baugh The optimization of dynamical decoupling and error correction for a particular qubit realization relies on accurate noise characterization. Recently probing the spectral density $S(w)$ of semiclassical phase noise by using a spin interacting with continuouswave (CW) onresonance field has gained attention. Standard CW noise spectroscopy is designed based on the generalized Bloch equations (GBE) or the filter function formalism assuming weak coupling to the Markovian bath. Under such simplifications, the qubit coherence decays exponentially at a rate proportional to S($\Omega$) where $\Omega$ is the CW field's Rabi frequency. However, naive application of the standard CW protocol can substantially underestimate $S(w)$ at low frequency. We derive the coherence decay function beyond the analysis of the standard CW protocol by extending it to higher orders in the noise strength and discarding the Markov approximation. Simulations show qualitatively that our result is a much improved description of the spin dynamics compared to the simple exponential decay. Exploiting more accurate picture of the spin dynamics, we devise a protocol that extends the range over which $S(w)$ can be reliably reconstructed to beyond the weak coupling and the Markovian regimes. [Preview Abstract] 
Friday, March 17, 2017 10:48AM  11:00AM 
X51.00011: Electron paramagnetic resonance spectroscopy using a superconducting flux qubit directly coupled to an electron spin ensemble Hiraku Toida, Yuichiro Matsuzaki, Kosuke Kakuyanagi, Xiaobo Zhu, William Munro, Kae Nemoto, Hiroshi Yamaguchi, Shiro Saito Electron paramagnetic resonance (EPR) is a powerful spectroscopic tool to investigate unpaired electrons in materials. Conventional EPR spectrometers rely on a cavity to detect the microwave signal from electron spins. On the other hand, in our spectrometer, polarization of an electron spin ensemble is detected by a magnetometer, which is directly bonded to the spin ensemble. Here, we report EPR spectroscopy with a superconducting flux qubit, which is used as a magnetometer. The electron spin ensemble is excited by applying a continuous microwave signal with an onchip microstrip. EPR is detected as a change in resonance frequency of the flux qubit. We estimate the sensing volume and the sensitivity to be $\sim5\times10^{11}$ cm$^{3}$ ($\sim50$ fL) and $\sim500$ spins/$\sqrt{\mathrm{Hz}}$, respectively. This result paves the way towards realizing onchip EPR spectroscopy of a single spin, or highly sensitive nuclear spin detection. [Preview Abstract] 
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