1,720,990 research outputs found
A pedagogical introduction to continuously monitored quantum systems and measurement-based feedback
Full text linkIn this manuscript we present a pedagogical introduction to continuously monitored quantum systems. We
start by giving a simplified derivation of the Markovian master equation in Lindblad form, in the spirit
of collision models and input-output theory, which describes the unconditional dynamics of a continuously
monitored system. The same formalism is then exploited to derive stochastic master equations that describe the
conditional dynamics. We focus on the two most paradigmatic examples of continuous monitoring: continuous
photodetection, leading to a discontinuous dynamics with “quantum jumps”, and continuous homodyne
measurements, leading to a diffusive dynamics. We then present a derivation of feedback master equations that
describe the dynamics (either conditional or unconditional) when the continuous measurement photocurrents
are fed back to the system as a linear driving Hamiltonian, a paradigm known as linear Markovian feedback.
In the second part of the manuscript we focus on continuous-variable Gaussian systems: we first present the
equations for first and second moments describing the dynamics under continuous general-dyne measurements,
and we then discuss in more detail the conditional and unconditional dynamics under Markovian and state-based
feedback
Tensor network approach to sensing quantum light-matter interactions
No full textWe present the fundamental limits to the precision of estimating parameters of a quantum matter system probed by light, even when some of the light is lost. This practically inevitable scenario leads to a tripartite quantum system of matter, and light-detected and lost. Evaluating fundamental information theoretic quantities such as the quantum Fisher information of only the detected light was heretofore impossible. We succeed by expressing the final quantum state of the detected light as a matrix product operator. We apply our method to resonance fluorescence and pulsed spectroscopy. For both, we quantify the sub-optimality of continuous homodyning and photo-counting measurements in parameter estimation. For the latter, we find that single-photon Fock state pulses allow higher precision per photon than pulses of coherent states. Our method should be valuable in studies of quantum light-matter interactions, quantum light spectroscopy, quantum stochastic thermodynamics, and quantum clocks
Simultaneous optical phase and loss estimation revisited : measurement and probe incompatibility
Full text linkQuantum multiparameter metrology is hindered by incompatibility issues, such as finding a single probe state (probe incompatibility) and a single measurement (measurement incompatibility) optimal for all parameters. The simultaneous estimation of phase shift and loss in a single optical mode is a paradigmatic multiparameter metrological problem in which such tradeoffs are present. We consider two settings: single-mode or two-mode probes (with a reference lossless mode), and for each setting we consider either Gaussian states or arbitrary quantum states of light restricted only by a maximal number of photons allowed. We find numerically that, as the number of photons increases, there are quantum states of light for which probe incompatibility disappears both in the single- and two-mode scenarios. On the other hand, for Gaussian states, probe incompatibility is present in the single-mode case and may be removed only in the two-mode setting thanks to the entanglement with the reference mode. Finally, we provide strong arguments that the fundamental incompatibility aspect of the model is measurement incompatibility, which persists for all the scenarios considered, and unlike probe-incompatibility cannot be overcome even in the large photon number limit
Semiparametric estimation of the Hong-Ou-Mandel profile
Full text linkWe apply the theory of semiparametric estimation to a Hong-Ou-Mandel interference experiment with a spectrally entangled two-photon state generated by spontaneous parametric down-conversion. Thanks to the semiparametric approach, we can evaluate the Cramér-Rao bound and find an optimal estimator for a particular parameter of interest without assuming perfect knowledge of the two-photon wave function, formally treated as an infinity of nuisance parameters. In particular, we focus on the estimation of the Hermite-Gauss components of the marginal symmetrized wave function, whose Fourier transform governs the shape of the temporal coincidence profile. We show that negativity of these components is an entanglement witness of the two-photon state
Enhanced estimation of loss in the presence of Kerr nonlinearity
Full text linkWe address the characterization of dissipative bosonic channels and show that estimation of the loss rate by Gaussian probes (coherent or squeezed) is improved in the presence of Kerr nonlinearity. In particular, enhancement of precision may be substantial for short interaction time, i.e., for media of moderate size, e.g., biological samples. We analyze in detail the behavior of the quantum Fisher information (QFI), and determine the values of nonlinearity maximizing the QFI as a function of the interaction time and of the parameters of the input signal. We also discuss the precision achievable by photon counting and quadrature measurement and present additional results for truncated, few-photon, probe signals. Finally, we discuss the origin of the precision enhancement, showing that it cannot be linked quantitatively to the non-Gaussianity or the nonclassicality of the interacting probe signal
Pseudomode treatment of strong-coupling quantum thermodynamics
Full text linkThe treatment of quantum thermodynamic systems beyond weak coupling is of
increasing relevance, yet extremely challenging. The evaluation of
thermodynamic quantities in strong-coupling regimes requires a nonperturbative
knowledge of the bath dynamics, which in turn relies on heavy numerical
simulations. To tame these difficulties, considering thermal bosonic baths
linearly coupled to the open system, we derive expressions for heat, work, and
average system-bath interaction energy that only involve the autocorrelation
function of the bath and two-time expectation values of system operators. We
then exploit the pseudomode approach, which replaces the physical continuous
bosonic bath with a small finite number of damped, possibly interacting, modes,
to numerically evaluate these relevant thermodynamic quantities. We show in
particular that this method allows for an efficient numerical evaluation of
thermodynamic quantities in terms of one-time expectation values of the open
system and the pseudomodes. We apply this framework to the investigation of two
paradigmatic situations. In the first instance, we study the entropy production
for a two-level system coupled to an ohmic bath, simulated via interacting
pseudomodes, allowing for the presence of time-dependent driving. Secondly, we
consider a quantum thermal machine composed of a two-level system interacting
with two thermal baths at different temperatures, showing that an appropriate
sinusoidal modulation of the coupling with the cold bath only is enough to
obtain work extraction.Comment: 23 pages, 5 figure
Role of chirping in pulsed single-photon spectroscopy
Full text linkWe investigate the precision of estimating the interaction strength between a two-level system (TLS) and a single-photon pulse when the latter is subject to chirping. We consider linear, quadratic, and sinusoidal temporal phases applied to Gaussian and exponential temporal profiles. At the asymptotic time, when the TLS has fully decayed to its ground state, the fundamental precision depends solely on the magnitude of its spectral amplitude. For quadratically phase-modulated Gaussian pulses, this is entirely determined by the spectral bandwidth. We provide expressions for evaluating the fundamental precision for general temporal profiles and phase modulations. Finally, we show that experimentally feasible mode-resolved measurements are optimal, or close to it, for chirped, pulsed single-photon spectroscopy
Does entanglement enhance single-molecule pulsed biphoton spectroscopy?
Full text linkIt depends. For a single molecule interacting with one mode of a biphoton
probe, we show that the spectroscopic information has three contributions, only
one of which is a genuine two-photon contribution. When all the scattered light
can be measured, solely this contribution exists and can be fully extracted
using unentangled measurements. Furthermore, this two-photon contribution can,
in principle, be matched by an optimised but unentangled single-photon probe.
When the matter system spontaneously emits into inaccessible modes, an
advantage due to entanglement can not be ruled out. In practice, time-frequency
entanglement does enhance spectroscopic performance of the oft-studied
weakly-pumped spontaneous parametric down conversion (PDC) probes. For
two-level systems and coupled dimers, more entangled PDC probes yield more
spectroscopic information, even in the presence of emission into inaccessible
modes. Moreover, simple, unentangled measurements can capture between 60% - 90%
of the spectroscopic information. We thus establish that biphoton spectroscopy
using source-engineered PDC probes and unentangled measurements can provide
tangible quantum enhancement. Our work underscores the intricate role of
entanglement in single-molecule spectroscopy using quantum light.Comment: 18+14 pages, 12+14 figures, 1+0 table
Fisher-information susceptibility for multiparameter quantum estimation
Full text linkNoise affects the performance of quantum technologies, hence the importance of elaborating operative figures of merit that can capture its impact in exact terms. In quantum metrology, the introduction of the Fisher-information measurement noise susceptibility now allows one to quantify the robustness of measurement for single-parameter estimation. Here we extend this notion to the multiparameter quantum estimation scenario. We provide its mathematical definition in the form of a semidefinite program. Although a closed formula could not be found, we further derive an upper and a lower bound to the susceptibility. We then apply these techniques to two paradigmatic examples of multiparameter estimation: the joint estimation of phase and phase diffusion and the estimation of the different parameters describing the incoherent mixture of optical point sources. Our figure of merit provides clear indications on conditions allowing or hampering robustness of multiparameter measurements
Kramers–Kronig relations and precision limits in quantum phase estimation
Full text linkPhase measurements are of paramount importance in quantum optical sensing. However, the promise of a quantum advantage, the celebrated Heisenberg scaling, is severely curtailed in the presence of noise and loss. Here we investigate systems in which phase and absorption profiles are linked by Kramers–Kronig relations and show that, in the limit of a large photon number, their use connects the uncertainties on the profiles attainable by optimal probes for loss and phase. This underlines a physical motivation for which the Heisenberg scaling for the phase is lost. Our results bear practical implications, revealing the metrological capabilities of absorption measurements in determining phase profiles
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