Quantum estimation of parameters defining open-system dynamics may be enhanced by using ancillas that are entangled with the probe but are not submitted to the dynamics. Here we consider the important problem of estimation of transmission of light by a sample, with losses due to absorption and scattering. We show, through the determination of the quantum Fisher information, that the ancilla strategy leads to the best possible precision in single-mode estimation, the one obtained for a Fock state input, through joint photon-counting of probe and ancilla, which are modes of a bimodal squeezed state produced by an optical parametric amplifier. This proposal overcomes the challenge of producing and detecting high photon-number Fock states, and it is quite robust against additional noise: we show that it is immune to phase noise and the precision does not change if the incoming state gets disentangled. Furthermore, the quantum gain is still present under moderate photon losses of the input beams. We also discuss an alternative to joint photon counting, which is readily implementable with present technology, and approaches the quantum Fisher information result for weak absorption, even with moderate photons losses of the input beams before the sample is probed: a time-reversal procedure, placing the sample between two optical parametric amplifiers, with the second undoing the squeezing produced by the first one. The precision of estimation of the loss parameter is obtained from the average outgoing total photon number and its variance. In both procedures, the state of the probe and the detection procedure are independent of the value of the parameter.

Quantum sensors allow the estimation of parameters with precision higher than that obtained with classical strategies. Devices based on quantum physics have allowed the precise estimation of the gravitational field, the detailed imaging of the brain, the detection of gravitational-wave sources more than 400 million light years away, and an ever-increasing precision in the measurement of time. Quantum metrology, which is the conceptual framework that encompasses all these devices, is reviewed here, emphasizing recent results regarding noisy systems.

Establishing the limits of precision in the estimation of parameters for noisy quantum channels probed by qubits is important for many areas of quantum information, such as quantum sensing, computation, and communication. Here we consider the estimation of parameters characterizing a general class of noisy Pauli channels. We show that two entangled qubits, such that only one of them probes the channel, may lead, under an entangling measurement, to strong enhancement of the precision in the estimation, as compared to the precision corresponding to sending the pair, entangled or not, through the channel. We prove that entanglement plays an essential role, as does the entangling detection procedure, consisting in projecting the final state onto a Bell-state basis. We also prove that quantum advantage is obtained only when the output state, after interaction with the sample, is not entangled anymore. This behavior has striking similarities with quantum illumination, where initial entanglement of probe and ancilla beams, followed by an entangling measurement, lead to enhancement of the sensitivity of photodetection, even after the output beams are disentangled. Similarities and differences with ghost imaging are also discussed.

We show that a bimodal squeezed state leads to optimal precision in the estimation of loss, overcoming challenges in producing and detecting high photon number Fock states. Using a time-reversal procedure provides a precise estimation that approaches the bound for weak absorption.

We show that the strategy of ghost imaging can be successfully adapted to the estimation of parameters of open systems. Two entangled qubits, so that only one of them interacts with a depolarizing sample, may lead to precision in the estimation of the depolarizing parameter better than the one corresponding to sending the pair, entangled or not, through the sample. As opposed to ghost imaging, entanglement plays an essential role in this case, as does the entangling detection procedure, consisting in projecting the final state on a Bell-state basis. For isotropic depolarization, quantum advantage is obtained only when the final state, after interaction with the sample, is not entangled anymore.

O texto destaca os desafios enfrentados pelo Brasil na busca pelo desenvolvimento sustentável, ressaltando a necessidade de investimentos em ciência, tecnologia e educação para enfrentar questões como desigualdade social, degradação ambiental e dependência de commodities. Apesar do potencial do país em biodiversidade e recursos naturais, cortes no orçamento de pesquisa e desenvolvimento, aumento do desmatamento e déficits na educação e inovação representam obstáculos significativos. Uma revisão da agenda nacional, com foco em políticas de longo prazo e maior igualdade de oportunidades, é considerada crucial para o avanço socioeconômico e ambiental do Brasil.

We present a unified framework, based on quantum metrology concepts, for defining and quantifying deter-ministic noiseless quantum amplification of parameter-dependent processes, which plays an important role in increasing the precision of quantum sensing. Recent experiments [Burd et al., Science 364, 1163 (2019)] can be encompassed by this concept, which also leads to new suggestions of experimental work. The unified view presented here allows the identification of the basic steps for quantum amplification and of the measurements that lead to the best possible precision, beyond the quantum standard limit, in the estimation of parameters involved in the process. This is applied to the estimation of displacements of trapped ions and of the phase in SU(1,1) optical interferometers and atomic interferometry.

We determine quantum precision limits for estimation of damping constants and temperature of lossy bosonic channels. A direct application would be the use of light for estimation of the absorption and the temperature of a transparent slab. Analytic lower bounds are obtained for the uncertainty in the estimation, through a purification procedure that replaces the master equation description by a unitary evolution involving the system and ad hoc environments. For zero temperature, Fock states are shown to lead to the minimal uncertainty in the estimation of damping, with boson-counting being the best measurement procedure. In both damping and temperature estimates, sequential prethermalization measurements, through a stream of single bosons, may lead to huge gain in precision.

We investigate interferometric techniques to estimate the deflection angle of an optical beam and compare them to the direct detection of the beam deflection. We show that quantum metrology methods lead to a unifying treatment for both single photons and classical fields. Using the Fisher information to assess the precision limits of the interferometric schemes, we show that the precision can be increased by exploiting the initial transverse displacement of the beam. This gain, which is present for both Sagnac and Mach-Zehnder-like configurations, can be considerable when compared to non-interferometric methods. In addition to the fundamental increase in precision, the interferometric schemes have the technical advantage that (i) the precision limits can be saturated by a sole polarization measurement on the field, and that (ii) the detection system can be placed at any longitudinal position along the beam. We also consider position-dependent polarization measurements, and show that in this case the precision increases with the propagation distance, as well as the initial transverse displacement.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text L. Davidovich, "Recent Strategies in Quantum Metrology: The Quest for the Ultimate Precision Limits," in Rochester Conference on Coherence and Quantum Optics (CQO-11), OSA Technical Digest (Optica Publishing Group, 2019), paper Tu3A.1. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

These lectures will focus on quantum metrology, which aims to determine the ultimate precision limits in the estimation of parameters. It also helps to understand some basic problems in quantum physics, like for instance the energy-time uncertainty and its extension to noisy systems. Applications to several systems will be discussed, including phase estimation in optical interferometers, frequency estimation in atomic spectroscopy, force estimation, weak-value amplification, and the quantum s...

Hyperentanglement --- simultaneous entanglement between multiple degrees of freedom of two or more systems --- has been used to enhance quantum information tasks such as quantum communication and photonic quantum computing. Here we show that hyperentanglement can lead to increased quantum advantage in metrology, with contributions from the entanglement in each degree of freedom, allowing for Heisenberg scaling in the precision of parameter estimation. Our experiment employs photon pairs entangled in polarization and spatial degrees of freedom to estimate a small tilt angle of a mirror. Precision limits beyond shot noise are saturated through a simple binary measurement of the polarization state. The broad validity of the dynamics considered here implies that similar strategies based on hyperentanglement can offer improvement in a wide variety of metrological tasks.

We investigate the generation of entanglement between two noninteracting qubits coupled to a common reservoir. An experimental setup was conceived to encode one qubit on the polarization of an optical beam and another qubit on its transverse mode. The action of the reservoir is implemented as conditional operations on these two qubits, controlled by the longitudinal path as an ancillary degree of freedom. An entanglement witness and the two-qubit concurrence are easily evaluated from direct intensity measurements showing an excellent agreement with the theoretical prediction.

M. H. M. Passos, W. F. Balthazar, A. Z. Khoury, M. Hor–Meyll, L. Davidovich, J. A. O. Huguenin 1Instituto Ciências Exatas Universidade Federal Fluminense Volta Redonda RJ Brasil 2Instituto Federal do Rio de Janeiro Volta Redonda RJ Brasil 3Institutode F́ısica Universidade Federal Fluminense Niterói RJ Brasil 4Instituto F́ısica Universidade Federal do Rio de Janeiro Rio de Janeiro RJ Brasil

We propose a hitherto-unexplored concept in quantum thermodynamics: catalysis of heat-to-work conversion by quantum nonlinear pumping of the piston mode which extracts work from the machine. This concept is analogous to chemical reaction catalysis: Small energy investment by the catalyst (pump) may yield a large increase in heat-to-work conversion. Since it is powered by thermal baths, the catalyzed machine adheres to the Carnot bound, but may strongly enhance its efficiency and power compared with its noncatalyzed counterparts. This enhancement stems from the increased ability of the squeezed piston to store work. Remarkably, the fraction of piston energy that is convertible into work may then approach unity. The present machine and its counterparts powered by squeezed baths share a common feature: Neither is a genuine heat engine. However, a squeezed pump that catalyzes heat-to-work conversion by small investment of work is much more advantageous than a squeezed bath that simply transduces part of the work invested in its squeezing into work performed by the machine.

Post-selection strategies have been proposed with the aim of amplifying weak signals, which may help to overcome detection thresholds associated with technical noise in high-precision measurements. Here we use an optical setup to experimentally explore two different post-selection protocols for the estimation of a small parameter: a weak-value amplification procedure and an alternative method, that does not provide amplification, but nonetheless is shown to be more robust for the sake of parameter estimation. Each technique leads approximately to the saturation of quantum limits for the estimation precision, expressed by the Cramér-Rao bound. For both situations, we show that information on the parameter is obtained jointly from the measuring device and the post-selection statistics.

Since the beginning of quantum physics, the relation between the properties of the microscopic quantum and the macroscopic classical world has been an important source for the development of the theory, and has led to new insights on the role of the environment in the transition from quantum to classical physics. Decoherence affects both coherence and entanglement of open systems. Quantum optics and cavity quantum electrodynamics have allowed detailed investigations of this phenomenon, within the framework of microwaves and light waves. In this paper, I present a personal account of theoretical and experimental developments that have led to the probing of the subtle frontier between quantum and classical phenomena.

We report a quantum measurement beyond the standard quantum limit (SQL) for the amplitude of a small displacement acting on a cavity field. This measurement uses as a resource an entangled mesoscopic state, prepared by the resonant interaction of a circular Rydberg atom with a field stored in a superconducting cavity. We analyze the measurement process in terms of Fisher information and prove that it is, in principle, optimal. The experimental precision achieved, 2.4 dB below the SQL, is well understood in terms of experimental imperfections. This method could be transposed to other systems, particularly to circuit QED, for the precise measurement of weak forces acting on oscillators.

Luiz Davidovich, Instituto de Fisica, Universidade Federal do Rio de Janeiro These lectures will focus on quantum metrology, which aims to determine the ultimate precision limits in the estimation of parameters. It also helps to understand some basic problems in quantum physics, like for instance the energy-time uncertainty and its extension to noisy systems. Applications to several systems will be discussed, including phase estimation in optical interferometers, frequency estimation in atom...