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The unsteady flow in the front side window region of the vehicle can generate hydrodynamic and acoustic pressure on the front side window, which can influence the interior sound field. The hydrodynamic pressure on the front side window was achieved by the incompressible wall-modeled large-scale eddy simulation (WMLES) or improved delayed detached eddy simulation (IDDES), and the hybrid computational aeroacoustics (CAA) method based on acoustic perturbation equations (APE) was employed to achieve the acoustic pressure on the front side window. The numerical results of both hydrodynamic and acoustic pressure ware validated by the wind tunnel experiment, especially the corrected force analysis technique (CFAT) is employed to validate the acoustic pressure. The comparison of hydrodynamic and acoustic pressure on the front side window was performed by the Dynamic Mode Decomposition (DMD). Results show that the hydrodynamic pressure regionally distributes on the front side window and most energy concentrates on area interacted with the side mirror wake, while the acoustic pressure distributes uniformly on the front side window acting as a diffusion field and the energy disperses in frequency region.

The polarization switching behaviors of ultrathin films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) capacitors with different thicknesses of top PEDOT-PSSH layer have been studied. The polarization switching results are related to an effective internal electric field, and a large, permanent, internal electric field can be ensured by introducing a symmetric PEDOT-PSSH layer into the capacitors. The switching speed is potentially influenced by the PEDOT-PSSH layer and the asymmetric PEDOT-PSSH layer leads to a different density of surface charge, resulting in a build-in field. As a result, these enhanced switching dynamics in spin-coating 25 nm P(VDF-TrFE) copolymer films are achieved by introduced a symmetric electroactive layer to optimize morphology and the internal electric field. At high temperature (60 °C), we also show that the enhanced switching speed is two times as fast as what is measured at room temperature (25 °C) because the temperature depended active field is related to the switching kinetic energy. Switching time of P(VDF-TrFE) films with an electroactive layer as a function of the number of switching cycles show the excellent switching characteristics of ferroelectric capacitors.

Magnetic sensing based on nitrogen-vacancy (NV) center ensembles provides unique signal-to-noise ratio and sensitivity for physical and biological measurements at room temperature. However, inhomogeneous microwave (MW) coupling with NV ensembles and different field projections onto the NV axes lead to nonsynchronous spin dephasing and pose severe challenges to magnetic sensing. In this paper, we experimentally investigated the impact of the MW field uniformity on the DC magnetic sensing based on NV ensembles. We obtained different MW field uniformity between 42% and 93% in the same direction using a square split ring resonator. The uniformity and direction of the MW field was mapped using a wide-field magnetic imaging system. The magnetic sensing with the MW frequency modulation technology confirmed the consistence between the theoretical analysis and experimental results. The sensitivity was enhanced by nearly seven times with the best MW field uniformity. Our study can promote the further application and development of magnetic sensing devices based on solid-state spin ensembles.

The pressure excitation on automotive front side window acts as an indicator of the unsteady flow and wind noise in the front side window region. The complex unsteady flow in this area generates a wider range of vortex structures resulting in the nonhomogeneous and complex pressure excitation on side glass. The description of the pressure field, which can consider the nonhomogeneous in the exact space, is needed to better solve the vibration and noise problems. The turbulent pressure excitation on side window was achieved by the incompressible improved delayed detached-eddy simulation, which is validated by the wind tunnel experiment. The reduced-order modeling methods, including the proper orthogonal decomposition and the dynamic mode decomposition, were employed to describe the pressure excitation on side glass. The dynamic mode decomposition modes separate the pressure excitation into three parts just corresponding to three main flow structures in the front side window region: the vortex shedding of the side mirror (lower frequency range), pedestal vortex (middle frequency range), and A-pillar vortex (higher frequency range). The turbulent pressure excitation generated by the vortex shedding of the side mirror contributes most of the vibration of the side glass and then the wind noise in the cabin in the low-frequency range. (The characteristic frequency is around 60 Hz, which is close to both the measured coincidence frequency and the theoretical derivation value.) The dynamic mode decomposition analysis with the unique and exact frequency for each mode, considering the nonhomogeneous of the pressure excitation, has potential to understand and solve the vibration and wind noise problems.

The wake of a circular cylinder coated with porous media was experimentally investigated by hot-wire anemometry, particle image velocimetry (PIV) and smoke-wire visualization. Mean velocity and Reynolds stresses in the streamwise and lateral are eliminated obviously for porous layer coating, especially in the centerline. The vortex formation length is elongated, and the width of the wake region is expanded by coating porous media. The Strouhal number of vortex shedding is 0.162 for the porous cylinder, which is smaller than 0.182 for the bare one due to the widened wake. Additionally, porous layer significantly regulates the flow field, which can be detected distinctly by smoke-wire visualization and transient PIV data. The proper orthogonal decomposition (POD) was further used to analyze PIV data with a view to extract dominant coherent structures in the wake. Moreover, a phase-averaged low-order model was applied to provide a comprehensive description of the dynamics of the first four modes. The first two POD modes, showing the signature of asymmetrical vortex shedding and predominating in the wake of the bare cylinder, are weakened dramatically for porous layer coating and present abrupt degradation in the midline. Meanwhile, mode 3 and mode 4, representing the symmetrical vortex shedding, occupy higher energy for porous cylinder. The study reveals that the porous layer coating delays the roll-up of shear layer and weakens the interaction between them, and thus, the vortex region is shifted further downstream.

The Heisenberg scaling, which scales as N^{-1} in terms of the number of particles or T^{-1} in terms of the evolution time, serves as a fundamental limit in quantum metrology. Better scalings, dubbed as "super-Heisenberg scaling," however, can also arise when the generator of the parameter involves many-body interactions or when it is time dependent. All these different scalings can actually be seen as manifestations of the Heisenberg uncertainty relations. While there is only one best scaling in the single-parameter quantum metrology, different scalings can coexist for the estimation of multiple parameters, which can be characterized by multiple Heisenberg uncertainty relations. We demonstrate the coexistence of two different scalings via the simultaneous estimation of the magnitude and frequency of a field where the best precisions, characterized by two Heisenberg uncertainty relations, scale as T^{-1} and T^{-2}, respectively (in terms of the standard deviation). We show that the simultaneous saturation of two Heisenberg uncertainty relations can be achieved by the optimal protocol, which prepares the optimal probe state, implements the optimal control, and performs the optimal measurement. The optimal protocol is experimentally implemented on an optical platform that demonstrates the saturation of the two Heisenberg uncertainty relations simultaneously, with up to five controls. As the first demonstration of simultaneously achieving two different Heisenberg scalings, our study deepens the understanding on the connection between the precision limit and the uncertainty relations, which has wide implications in practical applications of multiparameter quantum estimation.

Quantum control can be employed in quantum metrology to improve the precision limit for the estimation of unknown parameters. The optimal control, however, typically depends on the actual values of the parameters and thus needs to be designed adaptively with the updated estimations of those parameters. Traditional methods, such as gradient ascent pulse engineering (GRAPE), need to be rerun for each new set of parameters encountered, making the optimization costly, especially when many parameters are involved. Here we study the generalizability of optimal control, namely, optimal controls that can be systematically updated across a range of parameters with minimal cost. In cases where control channels can completely reverse the shift in the Hamiltonian due to a change in parameters, we provide an analytical method which efficiently generates optimal controls for any parameter starting from an initial optimal control found by either GRAPE or reinforcement learning. When the control channels are restricted, the analytical scheme is invalid, but reinforcement learning still retains a level of generalizability, albeit in a narrower range. In cases where the shift in the Hamiltonian is impossible to decompose to available control channels, no generalizability is found for either the reinforcement learning or the analytical scheme. We argue that the generalization of reinforcement learning is through a mechanism similar to the analytical scheme. Our results provide insights into when and how the optimal control in multiparameter quantum metrology can be generalized, thereby facilitating efficient implementation of optimal quantum estimation of multiple parameters, particularly for an ensemble of systems with ranges of parameters., 13 pages, 12 figures

In this paper, we consider the system where a superconducting qubit is coupled to a resonator acting as a harmonic oscillator. Unlike the existing work regarding estimation of the coupling strength or oscillator frequency, we aim to sense the amplitude of a microwave drive by utilizing the qubit as a quantum probe. Furthermore, in order to improve the estimation precision, we employ a periodic control protocol that is easy to be implemented in practice. Such improvement does not require any manipulation of the oscillator. The proposed strategy sheds light on a systematic study on estimation of the driving field in superconducting circuits by means of adding control.

Experiments demonstrate very high precisions achieved simultaneously for multiple parameters with noncommuting generators., Quantum estimation of a single parameter has been studied extensively. Practical applications, however, typically involve multiple parameters, for which the ultimate precision is much less understood. Here, by relating the precision limit directly to the Heisenberg uncertainty relation, we show that to achieve the highest precisions for multiple parameters at the same time requires the saturation of multiple Heisenberg uncertainty relations simultaneously. Guided by this insight, we experimentally demonstrate an optimally controlled multipass scheme, which saturates three Heisenberg uncertainty relations simultaneously and achieves the highest precisions for the estimation of all three parameters in SU(2) operators. With eight controls, we achieve a 13.27-dB improvement in terms of the variance (6.63 dB for the SD) over the classical scheme with the same loss. As an experiment demonstrating the simultaneous achievement of the ultimate precisions for multiple parameters, our work marks an important step in multiparameter quantum metrology with wide implications.

Two-mode interferometers lay the foundations for quantum metrology. Instead of exploring quantum entanglement in the two-mode interferometers, a single bosonic mode also promises a measurement precision beyond the shot-noise limit (SNL) by taking advantage of the infinite-dimensional Hilbert space of Fock states. Here, we demonstrate a single-mode phase estimation that approaches the Heisenberg limit (HL) unconditionally. Due to the strong dispersive nonlinearity and long coherence time of a microwave cavity, quantum states of the form \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\left| 0 \right\rangle + \left| N \right\rangle } \right)/\sqrt 2$$\end{document}0+N∕2 can be generated, manipulated and detected with high fidelities, leading to an experimental phase estimation precision scaling as ∼N−0.94. A 9.1 dB enhancement of the precision over the SNL at N = 12 is achieved, which is only 1.7 dB away from the HL. Our experimental architecture is hardware efficient and can be combined with quantum error correction techniques to fight against decoherence, and thus promises quantum-enhanced sensing in practical applications., Reaching a quantum advantage in metrology usually requires hard-to-prepare two-mode entangled states such as NOON states. Here, instead, the authors demonstrate single-mode phase estimation using Fock states superpositions in a superconducting qubit-oscillator system.

Under the unitary evolutions it is always possible to distinguish different Hamiltonians with zero-error probability within a finite time. At the presence of noises, however, the error probability does not always go to zero within a finite time. In this article, we give a necessary and sufficient condition for Markovian dynamics to achieve zero-error quantum hypothesis testing in a finite time. We show that when the condition fails, the maximal fidelity of the output states under two different Hamiltonians, aided with arbitrary control operations, is always lower bounded by an exponential function, which remains positive at any finite time; zero-error quantum hypothesis testing thus cannot be achieved within any finite time. However, when the condition holds, quantum error corrections can be used to correct the noises and partially maintain the coherent evolutions, zero-error quantum hypothesis testing can then be achieved within a finite time.

Dissipations in the waveguide-cavity systems destroy quantum interference between the incident signal photons and the cavity photons linearly and, thus, usually limit the manipulation of the photon transport. Here, we adopt a four-mode microring cavity including both second-order (${\ensuremath{\chi}}^{(2)}$) and third-order (${\ensuremath{\chi}}^{(3)}$) nonlinearities side coupled to two different waveguides to manipulate photon transport against dissipative effects. The results show that the incident signal photons can be amplified due to the optical parametric amplification (OPA) originated from ${\ensuremath{\chi}}^{(3)}$ nonlinearities. Owing to the coherent interference between cavity photons with different frequencies, the transmission spectra of the signal field are split into two amplified peaks. The transmission intensities are not always linearly reduced by increasing the dissipations but determined by the competition between the OPA and the dissipations. It means that we can utilize the OPA process to overcome the dissipative effects to manipulate photon transport and, thus, control the transmission intensities of the incident signal photon from ${T}_{c}=0$ to ${T}_{c}g100%$. This competition can also be used to convert the signal field to the idler field with amplified efficiencies.

We study the joint estimation of multiple Rabi frequencies in a multilevel system. By analytically identifying the optimal probe state and the optimal measurement, we obtain the highest precision limit for the joint estimation of multiple Rabi frequencies. We show that the joint estimation does not always outperform the separate estimation. In different regimes the joint estimation can either outperform or underperform the separate estimation. We further show that if additional adaptive quantum controls are allowed then the advantage of the joint estimation over the separate estimation can be reestablished.

The advantage of quantum metrology has been experimentally demonstrated for phase estimations where the dynamics are commuting. General noncommuting dynamics, however, can have distinct features. For example, the direct sequential scheme, which can achieve the Heisenberg scaling for the phase estimation under commuting dynamics, can have even worse performances than the classical scheme when the dynamics are noncommuting. Here we realize a scalable optimally controlled sequential scheme, which can achieve the Heisenberg precision under general noncommuting dynamics. We also present an intuitive geometrical framework for the controlled scheme and identify sweet spots in time at which the optimal controls used in the scheme can be prefixed without adaptation, which simplifies the experimental protocols significantly. We successfully implement the scheme up to eight controls in an optical platform and demonstrate a precision near the Heisenberg limit. Our work opens the avenue for harvesting the power of quantum control in quantum metrology, and provides a control-enhanced recipe to achieve the Heisenberg precision under general noncommuting dynamics.

The ultra-wide-gap β-Ga2O3 has been regarded as a promising material for next-generation power electronic and deep-ultraviolet (UV) photodetectors. Exploring a suitable electrode is vital for realizing high performance β-Ga2O3 based nanodevices. Herein, the structural and contact properties of graphene/Ga2O3 interfaces are tuned and investigated by using the first-principles calculations. Results show that the small n-type Schottky barrier of about 0.07 eV for the graphene/Ga2O3 interface with weak interlayer interaction is irrespective of the interface stacking arrangement. Moreover, the intrinsic electronic property of Ga2O3 is well preserved in the interface. More interestingly, the n-type Schottky barrier to Ohmic contact transition can be obtained by shorting the interlayer distance, or increasing the graphene layers or applying a negative external electric field for the interface. Moreover, applying a large positive external electric field can realize the p-type Schottky barrier to Ohmic contact transition for graphene/Ga2O3 interface. These results are uncovered by analyzing the interfacial dipole and potential step of graphene/Ga2O3 interface, and expected to enhance the application potential of graphene electrode in the β-Ga2O3 based electronic and optoelectronic devices.

Both the hydrodynamic and acoustic pressure on automotive front side window have the potential contribution to the interior sound pressure of the vehicle. The corrected force analysis technique (CFAT) is employed to extract the acoustic pressure on front side window. The sensitivity of all pressure fluctuation inside and outside the vehicle to the variation of wind speed, yaw angle and the geometry of side mirror is analyzed. The scaling law of both hydrodynamic and acoustic pressure is derived, compared and used to confirm the validation of CFAT. The coherence function and contribution coefficient are used to analyze the relative contribution of hydrodynamic and acoustic pressure on front side window to the interior sound pressure. Results show that in the middle and high frequency range, although the magnitude of acoustic pressure on front side window is much smaller than that of hydrodynamic pressure, the acoustic pressure on front side window contributes much more than the hydrodynamic pressure into the interior sound pressure.

Fabricating low-dimensional ultra-wide-bandgap β-Ga2O3 has been regarded as a promising approach to optimize the performance of β-GaO Ga2O3–based optoelectronics. Controlling the surface-dangling bonds of low-dimensional Ga2O3 is crucial for the performance modulation. Here, the structural stability and optoelectronic properties of two-dimensional (2D) Ga2O3 tuned by halogen passivation are comparatively investigated by density functional theory. No matter how full passivation and semipassivation, both chlorine and fluorine passivation can change the indirect bandgap into direct bandgap for 2D Ga2O3 because of the higher energy of bonding orbital for Ga-halogen bond than that of Ga–O bond. Moreover, surface chlorine passivation promotes the formation of p-type 2D Ga2O3 with reduced bandgap because chlorine passivation supplies electron to Ga2O3, while fluorine passivation introduces some subconduction bands into the enlarged bandgap because of the π∗ antibonding between the F-py and O-py orbitals. In addition, just chlorine passivation can simultaneously improve the electron and hole mobility of 2D Ga2O3, while opposite characters are observed for fluorine-passivated 2D Ga2O3. The hole mobility of chlorine semipassivated p-type monolayer Ga2O3 along b direction increases to 3913.52 cm2V−1s−1. Moreover, surface chlorine passivation not only enhances the absorption in the ultravisible region but also extends the absorption range of 2D Ga2O3 from the ultravisible to visible region. Note that the fluorine-passivated 2D Ga2O3 has more stability than the chlorine-passivated one. This work deeply reveals the effect of surface halogen passivation and provides useful guidance for realizing high-performance p-type Ga2O3 optoelectronics.

Quantum Fisher information matrix (QFIM) is a core concept in theoretical quantum metrology due to the significant importance of quantum Cram\'{e}r-Rao bound in quantum parameter estimation. However, studies in recent years have revealed wide connections between QFIM and other aspects of quantum mechanics, including quantum thermodynamics, quantum phase transition, entanglement witness, quantum speed limit and non-Markovianity. These connections indicate that QFIM is more than a concept in quantum metrology, but rather a fundamental quantity in quantum mechanics. In this paper, we summarize the properties and existing calculation techniques of QFIM for various cases, and review the development of QFIM in some aspects of quantum mechanics apart from quantum metrology. On the other hand, as the main application of QFIM, the second part of this paper reviews the quantum multiparameter Cram\'{e}r-Rao bound, its attainability condition and the associated optimal measurements. Moreover, recent developments in a few typical scenarios of quantum multiparameter estimation and the quantum advantages are also thoroughly discussed in this part., Comment: ~70 pages, close to published version, open access

The photonic router is a key device in optical quantum networks. Conventional routers, however, can only transfer a photon from the input port to the desired port probabilistically. Here, we propose to use chiral photon-atom interactions for targeted routing that can transfer a single photon from the input port to an arbitrarily selected output port deterministically, i.e., with $100%$ probability. The configuration of the proposed router consists of a driven three-level atom which is chirally coupled to two photonic waveguides simultaneously. It is shown that, by properly adjusting the driving field on the atom, a single photon inputting from one port of the first waveguide can be deterministically transferred to one of the selected output ports of the second waveguide. Both resonant and nonresonant photons can be deterministically routed to the desired ports. We also examine how the routing fidelity is influenced by dissipations and imperfections, and investigate the transmission characters of two resonant photons.

The characteristics of ferroelectric capacitors with poly(vinylidene fluoride-trifluoroethlene) (P(VDF-TrFE)) films have been studied at different structures of cell electrodes. It is suggested that the effect of electrode structures could induce changes of performance. Remarkably, cells with line electrodes display a better polarization and fatigue resistance than those with flat electrodes. For P(VDF-TrFE) ultrathin films with different electrode structures, the models of charge compensation mechanism for depolarization field and domain fatigue decomposition are used to explain the effect of electrode structure. Furthermore, the driving voltage based on normal speed-functionality is designed, and the testing results show that the line electrode structure could induce a robust switching, which is determined by the free charges concentration in active layer. These findings provide an effective route to design the optimum structure for a ferroelectric capacitor based on P(VDF-TrFE) copolymer ultrathin film.

Most studies in multiparameter estimation assume the dynamics is fixed and focus on identifying the optimal probe state and the optimal measurements. In practice, however, controls are usually available to alter the dynamics, which provides another degree of freedom. In this paper we employ optimal control methods, particularly the gradient ascent pulse engineering (GRAPE), to design optimal controls for the improvement of the precision limit in multiparameter estimation. We show that the controlled schemes not only capable to provide a higher precision limit, but also have a higher stability to the inaccuracy of the time point performing the measurements. This high time stability will benefit the practical metrology where it is hard to perform the measurement at a very accurate time point due to the response time of the measurement apparatus., 11 pages, 2 figures, PRA published version

Various strategies exist in quantum metrology, such as with or without ancillary system, with a fixed or optimized measurement, with or without monitoring the environment, etc. Different set of tools are usually needed for different strategies. In this article, we provide a unified framework for these different settings, in particular we introduce a quantum metrology matrix and show that the precision limits of different settings can all be obtained from the trace or the trace norm of the quantum metrology matrix. Furthermore, the probe state enters into the quantum metrology matrix linearly, which makes the identification of the optimal probe states, one of the main quests in quantum metrology, much more efficient than conventional methods.

We study the existence of the maximal quantum Fisher information matrix in multi-parameter quantum estimation, which bounds the ultimate precision limit. We show that when the maximal quantum Fisher information matrix exists, it can be directly obtained from the underlying dynamics. Examples are then provided to demonstrate the usefulness of the maximal quantum Fisher information matrix by deriving various tradeoff relations in multi-parameter quantum estimation and obtaining the bounds for the scalings of the precision limit., Comment: 6 pages

We propose a hierarchy of Bell-type inequalities for arbitrary $n$-partite systems that identify the different degrees of nonlocality ranging from standard to genuine multipartite nonlocality. After introducing the definition of nonsignaling $m$-locality, we show that the observed joint probabilities in any nonsignaling $m$-local realistic models should satisfy the $(m-1)$-th Bell-type inequality. When $m=2$ the corresponding inequality reduces to the one shown in [Phys. Rev. Lett. 112, 140404 (2014)] whose violation indicates genuine multipartite nonlocality, and when $m=n$ the corresponding inequality is just Hardy's inequality whose violation indicates standard multipartite nonlocality. Furthermore, several examples are provided to demonstrate their hierarchy of multipartite nonlocality., 6 pages

A pivotal task in quantum metrology, and quantum parameter estimation in general, is to de- sign schemes that achieve the highest precision with given resources. Standard models of quantum metrology usually assume the dynamics is fixed, the highest precision is achieved by preparing the optimal probe states and performing optimal measurements. However, in many practical experimental settings, additional controls are usually available to alter the dynamics. Here we propose to use optimal control methods for further improvement on the precision limit of quantum parameter estimation. We show that by exploring the additional degree of freedom offered by the controls higher precision limit can be achieved. In particular we show that the precision limit under the controlled schemes can go beyond the constraints put by the coherent time, which is in contrast to the standard scheme where the precision limit is always bounded by the coherent time., 15 pages, 9 figures, PRA published version

Measurement and estimation of parameters are essential for science and engineering, where the main quest is to find out the highest achievable precision with given resources and design schemes to attain it. Two schemes, the sequential feedback scheme and the parallel scheme, are usually studied in quantum parameter estimation. While the sequential feedback scheme represents the most general scheme, it remains unknown whether it can outperform the parallel scheme for any quantum estimation tasks. In this Letter we show that the sequential feedback scheme has a 3-fold improvement over the parallel scheme for Hamiltonian parameter estimations on 2-dimensional systems, and an order of $O(d+1)$ improvement for Hamiltonian parameter estimation on $d-$dimensional systems. We also show that, contrary to the conventional belief, it is possible to simultaneously achieve the highest precision for estimating all three components of a magnetic field, which sets a benchmark on the local precision limit for the estimation of a magnetic field., 11 pages. Accepted by Phys. Rev. Lett

International audience; Here we study the optimum efficiency of the excitation of maximum quantum (MaxQ) coherence using analytical and numerical methods based on optimal control theory. The theoretical limit of the achievable MaxQ amplitude and the minimum time to achieve this limit are explored for a set of model systems consisting of up to five coupled spins. In addition to arbitrary pulse shapes, two simple pulse sequence families of practical interest are considered in the optimizations. Compared to conventional approaches, substantial gains were found both in terms of the achieved MaxQ amplitude and in pulse sequence durations. For a model system, theoretically predicted gains of a factor of three compared to the conventional pulse sequence were experimentally demonstrated. Motivated by the numerical results, also two novel analytical transfer schemes were found: Compared to conventional approaches based on non-selective pulses and delays, double-quantum coherence in two-spin systems can be created twice as fast using isotropic mixing and hard spin-selective pulses. Also it is proved that in a chain of three weakly coupled spins with the same coupling constants, triple-quantum coherence can be created in a time-optimal fashion using so-called geodesic pulses.

The fidelity function of quantum states have been widely used in quantum information science and frequently arises in the quantification of optimal performances for the estimation and distinguish of quantum states. A fidelity function on quantum channel is expected to have same wide applications in quantum information science. In this paper we propose a fidelity function on quantum channels and show that various distance measures on quantum channels can be obtained from this fidelity function, for example the Bures angle and the Bures distance can be extended to quantum channels via this fidelity function. We then show that the distances between quantum channels lead naturally to a new Fisher information which quantifies the ultimate precision limit in quantum metrology, the ultimate precision limit can thus be seen as a manifestation of the distances between quantum channels. We also show that the fidelity on quantum channels provides a unified framework for perfect quantum channel discrimination and quantum metrology, in particular we show that the minimum number of uses needed for perfect channel discrimination is exactly the counterpart of the precision limit in quantum metrology, and various useful lower bounds for the minimum number of uses needed for perfect channel discrimination can be obtained via this connection., 15 pages. Comments are particularly welcome

We present observable lower bounds for several bipartite entanglement measures including entanglement of formation, geometric measure of entanglement, concurrence, convex-roof extended negativity, and G-concurrence. The lower bounds facilitate estimates of these entanglement measures for arbitrary finite-dimensional bipartite states. Moreover, these lower bounds can be calculated analytically from the expectation value of a single observable. Based on our results, we use several real experimental measurement data to get lower bounds of entanglement measures for these experimentally realized states. In addition, we also study the relations between entanglement measures., Comment: 10 pages, 4 figures

One of the main quests in quantum metrology, and quantum parameter estimation in general, is to find out the highest achievable precision with given resources and design schemes that attain that precision. In this article we present a general framework for quantum parameter estimation which relates the ultimate precision limit directly to the underlying dynamics. With this framework we present systematical methods for computing the ultimate precision limit and optimal probe states. We further demonstrate the power of the framework by deriving a sufficient condition on when ancillary systems are not useful for improving the precision limit., Comment: 14 pages. Comments are particularly welcome

The widely used quantum Cramer-Rao bound (QCRB) sets a lower bound for the mean square error of unbiased estimators in quantum parameter estimation, however, in general QCRB is only tight in the asymptotical limit. With a limited number of measurements biased estimators can have a far better performance for which QCRB cannot calibrate. Here we introduce a valid lower bound for all estimators, either biased or unbiased, which can serve as standard of merit for all quantum parameter estimations., Comment: 9 pages, 5 figures

Optimal control theory is a versatile tool that presents a route to significantly improving figures of merit for quantum information tasks. We combine it here with the geometric theory for local equivalence classes of two-qubit operations to derive an optimization algorithm that determines the best entangling two-qubit gate for a given physical setting. We demonstrate the power of this approach for trapped polar molecules and neutral atoms., Comment: extended version; Phys. Rev. A (2011)

In this paper, we study time-optimal control problems related to system of two coupled qubits where the time scales involved in performing unitary transformations on each qubit are significantly different. In particular, we address the case where unitary transformations produced by evolutions of the coupling take much longer time as compared to the time required to produce unitary transformations on the first qubit but much shorter time as compared to the time to produce unitary transformations on the second qubit. We present a canonical decomposition of SU(4) in terms of the subgroup SU(2)xSU(2)xU(1), which is natural in understanding the time-optimal control problem of such a coupled qubit system with significantly different time scales. A typical setting involves dynamics of a coupled electron-nuclear spin system in pulsed electron paramagnetic resonance experiments at high fields. Using the proposed canonical decomposition, we give time-optimal control algorithms to synthesize various unitary transformations of interest in coherent spectroscopy and quantum information processing., 8 pages, 3 figures