Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B32: Quantum Metrology and Sensing IIFocus Live

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Sponsoring Units: DQI Chair: Martin Koppenhoefer 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B32.00001: RLD Fisher Information Bound for Multiparameter Estimation of Quantum Channels Vishal Katariya, Mark Wilde One of the fundamental tasks in quantum metrology is to estimate multiple parameters embedded in a noisy process, i.e., a quantum channel. In this work, we study fundamental limits to quantum channel estimation via the concept of amortization and the right logarithmic derivative (RLD) Fisher information value. Our key technical result is the proof of a chainrule inequality for the RLD Fisher information value, which implies that amortization, i.e., access to a catalyst state family, does not increase the RLD Fisher information value of quantum channels. This technical result leads to a fundamental and efficiently computable limitation for multiparameter channel estimation in the sequential setting, in terms of the RLD Fisher information value. As a consequence, we conclude that if the RLD Fisher information value is finite, then Heisenberg scaling is unattainable in the multiparameter setting. 
Monday, March 15, 2021 11:42AM  11:54AM Live 
B32.00002: Quantum metrology in noncollective, spatiotemporally correlated quantum noise environments Francisco Riberi, Leigh M Norris, Felix Beaudoin, Lorenza Viola We study the impact of noncollectivity on frequency estimation by Ramsey interferometry in the presence of spatiotemporally correlated Gaussian quantum noise. In previous work [1], it was found that the uncontrolled buildup of spatiotemporal correlations between the probes mediated by a quantum environment leads to subSQL scaling in entanglementassisted parameter estimation for the paradigmatic case of qubit probes subject to collective noise from a bosonic environment. With that in mind, we consider two types of noncollective couplings between probes and environment: the simplest digital departure from a collective model, and a limit where the position of the probes are Gaussian random variables. We find that, in both cases, noncollectivity can be regarded as a metrological resource reducing the frequency uncertainty, leading to a constant factor advantage with respect to the collective result in the first case, and to superclassical scaling in the second. 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B32.00003: Activating hidden metrological usefulness Geza Toth, Tamas Vertesi, Pawel Horodecki, Ryszard Horodecki We consider bipartite entangled states that cannot outperform separable states in any linear interferometer. Then, we show that these states can still be more useful metrologically than separable states if several copies of the state are provided or an ancilla is added to the quantum system. We present a general method to find the local Hamiltonian for which a given quantum state performs the best compared to separable states. We obtain analytically the optimal Hamiltonian for some quantum states with a high symmetry. We show that all bipartite entangled pure states outperform separable states in metrology. Some potential applications of the results are also suggested. 
Monday, March 15, 2021 12:06PM  12:18PM Live 
B32.00004: Surprises in reservoirengineering approaches to spinsqueezing Peter Groszkowski, HoiKwan Lau, Martin Koppenhoefer, Aashish Clerk Spinsqueezed states can be useful in metrological applications as they allow for sensing beyond the standard quantum limit. In recent years, a variety of mechanisms for generating such states have been proposed theoretically, as well as in some cases, realized experimentally. In this talk, we revisit the theory of dissipative protocols, where appropriately engineered noise processes can be utilized for spinsqueezing generation [1][2]. In the Markovian limit, we analyze the surprising consequences of singlespin dephasing and relaxation, and outline optimized mitigation strategies. We also analyze a nonMarkovian regime directly relevant to realizations where an NV center spin ensemble is coupled to a mechanical mode. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B32.00005: SuperSensitive Quantum Metrology with ManyParticle Separable State Mayukh Lahiri, Manuel Erhard

Monday, March 15, 2021 12:30PM  12:42PM Live 
B32.00006: EntanglementAssisted Absorption Spectroscopy Haowei Shi, Zheshen Zhang, Stefano Pirandola, Quntao Zhuang Spectroscopy is an important tool for probing the properties of materials, chemicals and biological samples. We design a practical transmitterreceiver system that exploits entanglement to achieve a provable quantum advantage over all spectroscopic schemes based on classical sources. To probe the absorption spectra, modelled as pattern of transmissivities among different frequency modes, we employ broadband signalidler pairs in twomode squeezed vacuum states. At the receiver side, we apply photodetection after optical parametric amplification. Finally, we perform a maximallikehihood decision test on the measurement results, achieving ordersofmagnitudelower error probability than the optimum classical systems in various examples, including `winetasting' and `drugtesting' where real molecules are considered. In detecting the presence of an absorption line, our quantum scheme achieves the optimum performance allowed by quantum mechanics. The quantum advantage in our system is robust against noise and loss, which makes nearterm experimental demonstration possible. 
Monday, March 15, 2021 12:42PM  12:54PM Live 
B32.00007: Theory of entanglementassisted metrology for quantum channels Sisi Zhou, Liang Jiang The quantum Fisher information (QFI) measures the amount of information that a quantum state carries about an unknown parameter. The (entanglementassisted) QFI of a quantum channel is defined by the maximum QFI of the output state assuming an entangled input state over a single probe and an ancilla. In quantum metrology, people are interested in computing the QFI of N identical copies of a quantum channel when N→∞, which we call the asymptotic QFI. It was known that the asymptotic QFI grows either linearly or quadratically with N. Here we obtain a simple criterion that determines whether the scaling is linear or quadratic. In both cases, the asymptotic QFI and a quantum error correction protocol to achieve it are solvable via a semidefinite program. When the scaling is quadratic, the Heisenberg limit is recovered. When the scaling is linear, the asymptotic QFI is still in general larger than N times the singlechannel QFI and furthermore, sequential estimation strategies provide no advantage over parallel ones. For details, see arXiv: 2003.10559. 
Monday, March 15, 2021 12:54PM  1:06PM Live 
B32.00008: Observing a Topological Transition in WeakMeasurementInduced Geometric Phases Zoe Wang, Kyrylo Snizhko, Alessandro Romito, Yuval Gefen, Kater Murch Quantum measurements induce backaction on quantum states resulting in measurement induced dynamics. A sequence of weak measurements can consequently realize a cyclic motion for the qubit state in Hilbert space and thus induce a geometric phase. As the measurement strength is varied between weak and strong regimes, we expect a topological transition corresponding to a change in the Chern number of the surface tracked by the qubit’s cyclic motion. To experimentally measure this transition, we employ quantum nondemolition measurement of a superconducting transmon circuit in the strong dispersive regime. This transition is revealed as a quantized jump in the averaged geometric phase when tuning the strength for the measurement sequence, giving new insights into how weak measurements are a powerful tool for quantum control. 
Monday, March 15, 2021 1:06PM  1:42PM Live 
B32.00009: Quantum Metrology in the Era of Quantum Information Invited Speaker: Rafal DemkowiczDobrzanski A comprehensive overview of the most recent advances in theoretical methods of quantum metrology will be presented, that in particular benefit from the quantum information related concepts such as quantum errorcorrection or matrix product states formalism. The theory developed allows to determine whether the Heisenberg scaling of precision is possible for a quantum sensor subject to a general Markovian noise. The theory takes into account all the possible quantum strategies, including entangling the sensor with ancillary systems, adaptive strategies such as e.g. quantum error correction protocols. Moreover, effective algorithms, based on the matrix product states/matrix product operator formalism, are developed that allow to identify the optimal metrological protocols in presence of noise (also correlated noise) in the limit of large number of probes, inaccessible by the stateoftheart methods. Finally, by reversing the line of reasoning why may also use the fundamental metrological precission bounds to establish limits on efficiency of quantum errorcorrecting protocols. 
Monday, March 15, 2021 1:42PM  1:54PM Live 
B32.00010: A geometric pathway to scalable quantum sensing Gavin Brennen, Mattias Johnsson, Nabomita Roy Mukty, Daniel Burgarth, Thomas Volz Entangled resources enable quantum sensing that achieves Heisenberg scaling, a quadratic improvement on the standard quantum limit, but preparing large N spin entangled states is challenging in the presence of decoherence. We present a quantum control strategy using highly nonlinear geometric phase gates which can be used for generic state or unitary synthesis on the Dicke subspace with O(N) or O(N^2) gates, respectively. The method uses a dispersive coupling of the spins to a common bosonic mode and does not require addressability, special geometric layouts or detunings, or interactions between the spins. By using amplitude amplification our control sequence for preparing states ideal for metrology can be significantly simplified to O(N^5/4) geometric phase gates with size O(1/N) action angles that are more robust to mode decay. The geometrically closed path of the control operations ensures the gates are insensitive to the initial state of the mode and the sequence has builtin dynamical decoupling providing resilience to dephasing errors. We describe implementations using trapped ions or Rydberg atoms and show how to prepare quantum error correction code words in the Dicke space. 
Monday, March 15, 2021 1:54PM  2:06PM Live 
B32.00011: Signatures of the quantum nature of gravity: Principle and practice Animesh Datta, Haixing Miao We study the quantum nature of the gravitational interaction between two identical masses in the Newtonian limit. Since gravity only depends on the relative position of the two masses, the differential motion of their centers of mass is a simple and elegant way of exploring its quantum signatures. If the gravitational interaction between the two masses is quantum in the Newtonian limit, the differential mode exhibits quantum squeezing. If the gravitational interaction is semiclassical of the SchrodingerNewton type, both the differential and common modes exhibit identical quantum squeezing. We thus show the origins of quantum entanglement between two masses in the former case, and their absence in the latter. We close with prospects of experimentally detecting of these signatures. 
Monday, March 15, 2021 2:06PM  2:18PM Live 
B32.00012: Enhancing qubit noise spectroscopy using a quantum quench Yuxin Wang, Aashish Clerk Understanding in detail the dissipative environment of a qubit or quantum processor is crucial for the design of optimal dynamical decoupling protocols and highfidelity operations. Qubitbased quantum noise spectroscopy seeks to use a probe qubit and tailored control pulses as a tool for reconstructing environmental noise spectra [1]. While standard approaches treat the environment as a fixed noise source, true quantum environments have a dynamical nature that can play an important role in dissipative effects. Here, we discuss modifications of standard Ramseybased noise spectroscopy protocols that incorporate an effective quantum quench of the bath. These techniques allow one to extract bath properties that are not accessible via standard methods (e.g., parameterfree measurements of bath temperature, or evidence that the bath is not in thermal equilibrium). We discuss how our approaches are readily compatible with standard diamond NVcenter based quantum sensing platforms. 
Monday, March 15, 2021 2:18PM  2:30PM Live 
B32.00013: Quantumlimited Estimation of Coherence Under Thermal Noise in Photonstarved States Zi Chua, Jonathan Habif, Federico Maximiliano Spedalieri To most efficiently estimate a parameter of a quantum system, one needs to choose the optimal measurement that creates the conditions for the most efficient estimator to act. Specifically, one needs to implement the measurement that achieves the quantum Fisher information (qFi) for that particular estimation task. In this work, we search for an optimal measurement for estimating the coherence, parameterised as η, of a mixed state composed of coherent light and thermal (incoherent) light. Our search is limited to photonstarved signals, with signal strength of n_bar << 1. We found that the classical choice of measurement, direct detection, was suboptimal, whereas homodyne detection approached the qFi for estimating coherence for the low coherence (η ~ 0) range. 
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