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Quantum metrology with entangled resources aims to achieve sensitivity beyond the standard quantum limit by harnessing quantum effects even in the presence of environmental noise. So far, sensitivity has been mainly discussed from the viewpoint of reducing statistical errors under the assumption of perfect knowledge of a noise model. However, we cannot always obtain complete information about a noise model due to coherence time fluctuations, which are frequently observed in experiments. Such unknown fluctuating noise leads to systematic errors and nullifies the quantum advantages. Here, we propose an error-mitigated quantum metrology that can filter out unknown fluctuating noise with the aid of purification-based quantum error mitigation. We demonstrate that our protocol mitigates systematic errors and recovers superclassical scaling in a practical situation with time-inhomogeneous bias-inducing noise. Our results reveal the usefulness of purification-based error mitigation for unknown fluctuating noise, thus paving the way not only for practical quantum metrology but also for quantum computation affected by such noise., Comment: 6+11 pages, 3+4 figures, close to the published version

The authors find that short-range charge correlations in prototypical electronic ferroelectrics show ultrafast enhancement upon photoexcitation, in contrast to the photoinduced melting of the long-range charge order.

This work presents materials informatics methodology which combines the first-principles transport calculations and the Bayesian optimization to design the optimal structure of disordered MgAl${}_{2}$O${}_{4}$ in Fe/ MgAl${}_{2}$O${}_{4}$/Fe (001) with the highest tunnel magnetoresistance ratio. The authors show that machine learning do provide useful guidelines for future experimental investigations on the tunnel magnetoresistance effects of MgAl${}_{2}$O${}_{4}$-based magnetic tunnel junctions

The strong light-field effect of (TMTTF)2AsF6 was investigated utilizing 1.5-cycle, 7-fs infrared pulses. The ultarfast (20 fs) and large (40%) response of the plasma-like reflectivity edge (0.7 eV) was analyzed by the changes in omega_p=sqrt(ne2/(epsilon_0*epsilon(infty)*m)} (n: number of charges in the 1/4 filled-band, m: mass of charge, epsilon(infty): dielectric constants for high-frequency and vacuum, e: elementary charge). The 3% reduction in omega_p is attributed to the 6% increase in m. Furthermore, 20 fs oscillation of omega_p in the time domain indicates that the plasma-like edge is affected by the charge gap (0.2 eV) nature. Theoretical calculations suggest that the Coulomb repulsion plays an important role in the increase in m.

A dip-shaped anomaly appearing in the infrared spectrum of charge-transfer organic complexes has been investigated. The anomaly appears at approximately the same frequency (\ensuremath{\sim}2700 cm${}^{\ensuremath{-}1}$), irrespective of light polarization as well as a composition of the complex, when the compounds undergo charge ordering. Isotope-shift measurements for \ensuremath{\theta}-(BEDT-TTF)${}_{2}$RbZn(SCN)${}_{4}$ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] indicates a relationship between the overtone of a C=C stretching mode of the BEDT-TTF molecule and this anomalous signal. Calculations of electron-molecular vibration coupling based on a diatomic molecular dimer model reveals that the overtone is activated by an anharmonicity developed in the adiabatic potential in a charge-separated system. It is presented that numerical calculation based on the simple cluster model reproduces essential features of the experimentally obtained conductivity spectrum.

Ultrafast dynamics of the light-matter interaction in a charge-ordered molecular insulator α-(BEDT-TTF) 2 I 3 were studied by pump-probe spectroscopy using few-optical-cycle infrared pulses (pulse width 12 fs). Coherent oscillation of the correlated electrons and subsequent Fano destructive interference with intramolecular vibration were observed in time domain; the results indicated a crucial role for electron-electron interplay in the light-matter interaction leading to the photoinduced insulator-to-metal transition. The qualitative features of this correlated electron motion were reproduced by calculations based on exact many-electron-phonon wave functions.

The charge order (CO) state can be observed by microprobe measurements such as nuclear magnetic resonance (NMR) and infrared spectroscopy. We used the infrared spectroscopy to examine ${(\text{TMTTF})}_{2}{\text{AsF}}_{6}$ and confirmed the CO transition seen in previous $^{13}\text{C}$-NMR studies. However we found the degree of charge disproportionation, $\ensuremath{\Delta}\ensuremath{\rho}=0.16$, to be smaller than that obtained from the analysis of ${T}_{1}$ in $^{13}\text{C}$-NMR and the redistribution of charge density below spin Peierls transition predicted by $^{13}\text{C}$-NMR was not observed at low temperature. We estimated the degree of spin disproportionation to be 0.11 from the analysis of the NMR shift in previous NMR studies. This small disproportionation does not require the redistribution of charge density at low temperature. The different behaviors of ${T}_{1}$ at charge-rich and charge-poor molecular sites observed by $^{13}\text{C}$-NMR can be examined by the contribution of the commensurate antiferromagnetic fluctuation with $4{k}_{F}$.

Photoinduced melting of charge-order (CO) in [bis(ethylenedithiolo)]-tetrathiafulvalene (BEDT-TTF) salts was investigated by femotosecond spectroscopy. Comparative studies on two polytypes exhibiting large [theta-(BEDT-TTF)2RbZn(SCN)_{4}] and small [alpha-(BEDT-TTF)2I3] molecular rearrangements through the CO transition were performed. Ultrafast melting of CO for both compounds demonstrates the major contribution of the electronic instability which is due to Coulomb interaction. The roles of the molecular rearrangements on the formation of the CO and the metallic domain are discussed on the basis of low-frequency lattice dynamics.

The charge distribution and molecular arrangement of a quarter-filled quasi-one-dimensional system${(\mathrm{DI}\text{\ensuremath{-}}\mathrm{DCNQI})}_{2}\mathrm{Ag}$ $(\mathrm{DI}\text{\ensuremath{-}}\mathrm{DCNQI}=2,5\text{\ensuremath{-}}\text{diiodo}\text{\ensuremath{-}}\text{dicyanoquinediimine})$ has been studied by IR and Raman spectroscopy. The charge localization of this material was believed to be a one-dimensional generalization of a Wigner crystal driven by intersite Coulomb repulsion. While charge disproportionation is confirmed via the splitting of ${b}_{u}$ modes in the infrared spectrum, the appearance of intense IR vibronic bands of ${a}_{g}$ modes strongly suggests the presence of the dimerization that is not expected by the proposed 1010 charge ordering model. In addition, the selection rules for the IR and Raman signals cannot be explained without a further symmetry reduction of the unit cell. To explain the vibrational behavior observed for a single crystal, we show that a more appropriate model for the charge ordering is 0110 ($2{k}_{F}$ charge-density wave and $4{k}_{F}$ bond-order wave).

We present the electronic and vibrational spectra of quasi-one-dimensional $\frac{3}{4}$-filled ${(\mathrm{EDO}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ ($\mathrm{EDO}\text{\ensuremath{-}}\mathrm{TTF}=\text{ethylenedioxy-tetrathiafulvalene}$, $X={\mathrm{PF}}_{6}$ and ${\mathrm{AsF}}_{6}$) above and below the metal-insulator phase transition (${T}_{\mathrm{MI}}=280\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the ${\mathrm{PF}}_{6}$ salt and $268\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the ${\mathrm{AsF}}_{6}$ salt). For the low-temperature insulating phase, the pattern of both bond and charge order was identified. Almost all charge density is localized on the strongly bound central pair of 0110 tetramer giving rise to a characteristic spectrum of electronic excitations. Infrared spectra along the stacking axis show evidence of strong electron-molecular vibration coupling between the charge transfer band within the pair and some specific intramolecular vibrations. This charge order is assisted by a molecular deformation.

We present a comparative study of the charge-ordered states in orthorhombic and monoclinic forms of $\ensuremath{\theta}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}{\mathrm{TlZn}(\mathrm{SCN})}_{4}$ $[\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}=\mathrm{bis}(\mathrm{ethylenedithiolo})$ tetrathiafulvalene] by means of vibrational spectroscopy with the aid of electrical resistivity and x-ray diffraction experiments. Theoretical consideration is performed for the splitting of the vibronic modes of the Raman-active $\mathrm{C}=\mathrm{C}$ stretching of BEDT-TTF. Based on the assignment and factor group analysis of the vibronic modes, we examined the charge-ordering pattern. The orthorhombic form provides a horizontal stripe of the localized holes, which is the same result as in the isostructural $\ensuremath{\theta}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}{\mathrm{RbZn}(\mathrm{SCN})}_{4}.$ The insulator-to-insulator phase transition of the monoclinic form has been shown to be accompanied by charge ordering. In contrast to the orthorhombic form, the charge-ordering pattern is a diagonal stripe. An unusual nonmetallic state in the high-temperature phase is discussed based on the line shape of the Raman spectrum.

Charge distributions in the donor layer for the paramagnetic semimetal-like and nonmagnetic insulating phases of the $\ensuremath{\alpha}\ensuremath{-}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F}{)}_{2}{\mathrm{I}}_{3}$ single crystal [BEDT-TTF: bis(ethylenedithiolo)tetrathiafulvalene] were investigated under quasihydrostatic pressures up to 3.6 GPa using Raman spectroscopy. The results confirmed the appearance of charge disproportionation below the metal-insulator phase transition temperature ${(T}_{\mathrm{MI}}).$ The average charge-disproportionation ratio, 0.2--0.8, at ambient pressure was quite pressure sensitive and decreased by ca. $0.1e/\mathrm{GPa}$ with increasing pressure. This result is ascribed to the faster increase of the transfer integrals than the intersite Coulomb repulsions under pressure. The loss of the inversion centers in the nonmagnetic insulating phase, concluded by the selection rule, suggests that the charge is ordered such that it forms a so-called horizontal stripe. Nonuniform charge distributions were also found at temperatures above ${T}_{\mathrm{MI}}$ for the paramagnetic semimetal-like phase. The experimentally determined small charge difference among BEDT-TTF at crystallographically nonequivalent sites is consistent with the band calculations.

We present the polarized Raman spectra and magnetic susceptibility of $\ensuremath{\theta}\ensuremath{-}(\mathrm{B}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{P}{)}_{2}\mathrm{Cu}(\mathrm{NCS}{)}_{2}$ which undergoes a charge-ordering phase transition at ${T}_{\mathrm{CO}}\ensuremath{\sim}250\mathrm{K}.$ From the splitting of the ${\ensuremath{\nu}}_{2}$ band, the charge-disproportionation ratio \ensuremath{\delta} is estimated as 0.4 at \ensuremath{\sim}10 K. The linewidth of ${\ensuremath{\nu}}_{2}$ suggests the fluctuation in charge distribution above ${T}_{\mathrm{CO}}.$ The magnetic susceptibility of $30\mathrm{K}lTl300\mathrm{K}$ follows the Curie--Weiss Law with the Curie constant of 0.154 emuK/mol and Weiss temperature of -29 K. The single-crystal ESR experiment shows a magnetic phase transition to a spin-singlet state at 5 K. The polarized Raman spectra and magnetic property suggest the view that the positive charge is vertically ordered along the b-axis.

Charge distribution in the insulating phase of $\ensuremath{\theta}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{RbZn}(\mathrm{SCN}{)}_{4}$ [BEDT-TTF denotes bis(ethylenedithiolo)tetrathiafulvalene] is investigated using the polarized Raman and infrared (IR) spectroscopy. Both of the Raman and IR spectra showed a multiple-peaks pattern below metal-insulator transition temperature. We assigned the carbon-double-bond $(\mathrm{C}=\mathrm{C})$ stretching-modes based on the isotope shift in the ${}^{13}\mathrm{C}$-substituted sample, and performed a factor-group analysis for the polarized IR and Raman spectra. These results proved that charge disproportionation (CD) takes place in the insulating phase, and that the spatial pattern of the charge ordering agrees with the symmetry determined by the x-ray diffraction. The CD ratio was estimated from the frequency of a ring $\mathrm{C}=\mathrm{C}$ stretching mode that is weakly coupled with electronic excited states. A systematic way to investigate the CD phenomenon based on vibrational spectroscopy is discussed.