Coherence Lifetime Research Articles

Seeing excitations in a new light

By combining the benefits of multidimensional spectroscopy with photoemission electron microscopy, scientists in Germany have successfully mapped the coherence lifetimes of plasmons in silver with nanoscale spatial resolution and femtosecond temporal resolution.

Physical Basis for Long-Lived Electronic Coherence in Photosynthetic Light-Harvesting Systems

The physical basis for observed long-lived electronic coherence in photosynthetic light-harvesting systems is identified using an analytically soluble model. Three physical features are found to be responsible for their long coherence lifetimes, (i) the small energy gap between excitonic states, (ii) the small ratio of the energy gap to the coupling between excitonic states, and (iii) the fact that the molecular characteristics place the system in an effective low-temperature regime, even at ambient conditions. Using this approach, we obtain decoherence times for a dimer model with FMO parameters of ∼160 fs at 77 K and ∼80 fs at 277 K. As such, significant oscillations are found to persist for 600 and 300 fs, respectively, in accord with the experiment and with previous computations. Similar good agreement is found for PC645 at room temperature, with oscillations persisting for 400 fs. The analytic expressions obtained provide direct insight into the parameter dependence of the decoherence time scales.

Rotating frame spin dynamics of a nitrogen-vacancy center in a diamond nanocrystal

We investigate the spin dynamics of a Nitrogen-Vacancy (NV) center contained in an individual diamond nanocrystal in the presence of continuous microwave excitation. Upon periodic reversal of the microwave phase, we observe a train of 'Solomon echoes' that effectively extends the system coherence lifetime to reach several tens of microseconds, depending on the microwave power and phase inversion rate. Starting from a model where the NV center interacts with a bath of paramagnetic defects on the nanocrystal surface, we use average Hamiltonian theory to compute the signal envelope from its amplitude at the echo maxima. Comparison between the effective Rabi and Solomon propagators shows that the observed response can be understood as a form of higher-order decoupling from the spin bath.

Coherence of an electron spin in quantum dots generated by a resonant optical pulse with elliptic polarization

We have experimentally observed the spin polarization process of single electrons in InP/InGaP quantum dots by time-resolved Kerr rotation measurements. It is found that the inversion of the spin polarization direction occurs with the variation of the intensity of the optical pulse. The spin coherence lifetime abruptly changes on the occurrence of the inversion. We have reproduced the inversion in numerical simulations using the density operator of the electron–trion four-level system, assuming a small deviation of the optical pulse from circular polarization. The change of the spin lifetime is attributed to the qualitative change of the four-level system in the electric polarization state.

Extending Timescales and Narrowing Linewidths in NMR

Among the different fields of research in nuclear magnetic resonance (NMR) which are currently investigated in the Laboratory of Biomolecular Magnetic Resonance (LRMB), two subjects that are closely related to each other are presented in this article. On the one hand, we show how to populate long-lived states (LLS) that have long lifetimes T(LLS) which allow one to go beyond the usual limits imposed by the longitudinal relaxation time T1. This makes it possible to extend NMR experiments to longer time-scales. As an application, we demonstrate the extension of the timescale of diffusion measurements by NMR spectroscopy. On the other hand, we review our work on long-lived coherences (LLC), a particular type of coherence between two spin states that oscillates with the frequency of the scalar coupling constant J(IS) and decays with a time constant T(LLC). Again, this time constant T(LLC) can be much longer than the transverse relaxation time T2. By extending the coherence lifetimes, we can narrow the linewidths to an unprecedented extent. J-couplings and residual dipolar couplings (RDCs) in weakly-oriented phases can be measured with the highest precision.

Hyperfine characterization and spin coherence lifetime extension in Pr3+:La2(WO4)3

Rare-earth ions in dielectric crystals are interesting candidates for storing quantum states of photons. A limiting factor on the optical density and thus the conversion efficiency is the distortion introduced in the crystal by doping elements of one type into a crystal matrix of another type. Here, we investigate the system Pr3+:La2(WO4)3, where the similarity of the ionic radii of Pr and La minimizes distortions due to doping. We characterize the praseodymium hyperfine interaction of the ground state (3H4) and one excited state (1D2) and determine the spin Hamiltonian parameters by numerical analysis of Raman-heterodyne spectra, which were collected for a range of static external magnetic field strengths and orientations. On the basis of a crystal field analysis, we discuss the physical origin of the experimentally determined quadrupole and Zeeman tensor characteristics. We show the potential for quantum memory applications by measuring the spin coherence lifetime in a magnetic field that is chosen such that additional magnetic fields do not shift the transition frequency in first order. Experimental results demonstrate a spin coherence lifetime of 158 ms - almost three orders of magnitude longer than in zero field.

Enhanced Resolution and Coherence Lifetimes in the Solid-State NMR Spectroscopy of Perdeuterated Proteins under Ultrafast Magic-Angle Spinning

We investigate the combined effect of perdeuteration and fast magic-angle spinning on the resolution and sensitivity of proton-detected protein NMR spectra and on coherence lifetimes. With 60 kHz spinning of a microcrystalline α-spectrin SH3 sample at a field strength of 23 T, a regime is attained where there is no substantial difference in resolution between perdeuterated samples with 10 or 100% protons at the exchangeable sites.1H resolution is then limited by inhomogeneous contributions. Upon fast spinning, the most dramatic line narrowing effects are observed for residues in the loop or bend regions of the protein, probably due to the removal of destructive dynamics effects. This investigation paves the way for using samples with 100% protons at the exchangeable sites in structure determination protocols, since all backbone amide sites can now contribute to the signal.

Multiple resonance heteronuclear decoupling under MAS: Dramatic increase of spectral resolution at moderate magnetic field and MAS frequencies

The effects of multiple-resonance heteronuclear decoupling under magic angle spinning (MAS) on the resolution of one-dimensional 19F and 31P and various two-dimensional MAS NMR spectra and on the residual non-refocusable coherence lifetimes in fluorinated aluminophosphate AlPO 4-CJ2, i.e. a compound that contains numerous highly abundant nuclei but no homonuclear spin bath, has been investigated. The design of the four-channel ( 1H, 19F, 27Al, 31P) MAS probe used for this study is first described. 1H and 1H– 27Al double-resonance decouplings allows lengthening the optimized transverse relaxation T 2 opt and increasing the resolution in the 19F and 31P dimensions. Under the application of multi-nuclear decoupling, a two-dimensional 19F– 31P CP-HETCOR correlation spectrum for AlPO 4-CJ2 is recorded with unprecedented high-resolution in the two dimensions. Moreover, because 1H-decoupling increases the 19F T 2 opt , it has been applied during the entire duration of the 2D NMR experiments, allowing the direct use of residual small interactions to generate 19F– 19F and 19F– 27Al 2D NMR correlation spectra in AlPO 4-CJ2.

Concentration dependence of absorption and optical and hyperfine transition dynamics in Pr3 +:La2(WO4)3

Optical and hyperfine parameters of the 3H4–1D2 transition are investigated in Pr3 +:La2(WO4)3 for doping concentrations ranging from 0.02 to 3 at.%, at liquid helium temperatures. This includes optical inhomogeneous and homogeneous linewidths, peak absorption, excited-state population decays, spectral hole lifetime and inhomogeneous and homogeneous hyperfine linewidths. Optical inhomogeneous broadening with increasing Pr3 + concentration is limited due to close atomic radii between Pr3 + and La3 +. The peak absorption coefficient increases with increasing Pr3 + doping, reaching up to 28 cm−1. At the highest Pr3 + concentration, optical coherence lifetime is 4.6 μs and hole lifetime 2 s, which should allow one to transfer coherences and tailor the optical inhomogeneous linewidth, as required for rare earth-based optical quantum memories. Raman echo experiments show that hyperfine inhomogeneous linewidth and coherence lifetime (50 kHz and 250 μs respectively) are independent of Pr3 + concentration. These results suggest that highly doped crystals are useful for efficient and long-storage time quantum memories.

A theoretical investigation of super‐resolution CARS imaging via coherent and incoherent saturation of transitions

AbstractWe review two approaches to achieving sub‐diffraction‐limited resolution coherent anti‐Stokes Raman scattering (CARS) microscopy (Beeker et al., Opt. Express, 2009, 17, 22632 and Beeker et al., J. Herek, Phys. Rev. A, 2010, 81, 012507). We performed a numerical investigation, based on the density matrix model, of the CARS emission process and identified two modified CARS experiments that lead to sub‐diffraction‐limited resolution images. At the heart of both processes is the spatial manipulation of the coherence between the ground state and the vibrational state being probed by the CARS process via a control state and a control laser that is resonant with the ground state to control state transition. We find two possible regimes of operation: in the first regime, the control and vibrational states are coupled via incoherent processes so that the populations of the two states reach equilibrium very quickly compared to the relevant coherence times. Under these conditions, pre‐populating the control state provides a saturable suppression of the coherence between the ground state and the vibrational state, suppressing CARS emission. By using a donut mode to pre‐populate the control state, CARS is suppressed everywhere but the central node, allowing sub‐diffraction‐limited resolution imaging. In the second regime, the control state has a rather long coherence lifetime, and the resonant laser drives Rabi oscillations that periodically deplete the ground state. As a result, the CARS emission process is amplitude‐modulated, which appear as sidebands on the CARS spectrum. By a process of spectral resolution and trilateration, sub‐diffraction‐limited resolution images can be obtained. Copyright © 2011 John Wiley & Sons, Ltd.

Coherent State Transfer between an Electron and Nuclear Spin inN15@C60

Electron spin qubits in molecular systems offer high reproducibility and the ability to self-assemble into larger architectures. However, interactions between neighboring qubits are "always on," and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both effectively turn on or off interqubit coupling mediated by dipolar interactions and benefit from the long nuclear spin decoherence times (T(2n)). We transfer qubit states between the electron and (15)N nuclear spin in (15)N@C(60) with a two-way process fidelity of 88%, using a series of tuned microwave and radio frequency pulses and measure a nuclear spin coherence lifetime of over 100 ms.

Efficient solid state memories for quantum cryptography

Abstract Long distance quantum cryptography requires quantum repeaters which use quantum memories. The latter are designed to store and retrieve photon quantum states on demand. Although quantum memories have been demonstrated in atomic vapors and ultra cold gases, a solid state alternative may better fulfill quantum memories requirements. Rare earth based crystals, which exhibit long coherence lifetimes, are actively studied for this purpose. Memory efficiency, i.e. the probability to retrieve a photon after storage, should be close to unity for practical applications. This can be achieved in highly doped crystals. Although Pr–Pr interactions could be detrimental in this case, we show that in a 3% Pr3+ doped La2(WO4)3 crystal ground state hyperfine coherence lifetime is still close to that measured at low Pr concentration. Since the latter determines the memory storage time, this result suggests that highly doped crystals may be useful for efficient quantum memories.

Direct measurement of rotationally resolved H2 Q-branch Raman coherence lifetimes using time-resolved picosecond coherent anti-Stokes Raman scattering

We report direct measurement of H2 Q-branch Raman coherence lifetimes using time-resolved picosecond coherent anti-Stokes Raman scattering (ps-CARS). A custom-built, high-peak-power, nearly transform-limited ps laser system offers an ideal combination of frequency and temporal resolution for such measurements. The coherence lifetimes measured for pure H2 at room temperature are in good agreement with decay rates that were derived from previous high-resolution studies. Measurements were also performed in binary mixtures of H2–X (X=Ar, N2, CH4, and C2H4). These measurements can be useful for accurate H2 ps-CARS thermometry as well as for studying various H2 collisional energy-transfer processes.

Long-lived quantum coherence in photosynthetic complexes at physiological temperature.

Photosynthetic antenna complexes capture and concentrate solar radiation by transferring the excitation to the reaction center that stores energy from the photon in chemical bonds. This process occurs with near-perfect quantum efficiency. Recent experiments at cryogenic temperatures have revealed that coherent energy transfer--a wave-like transfer mechanism--occurs in many photosynthetic pigment-protein complexes. Using the Fenna-Matthews-Olson antenna complex (FMO) as a model system, theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature; these studies also show that this mechanism simultaneously improves the robustness of the energy transfer process. This theory requires long-lived quantum coherence at room temperature, which never has been observed in FMO. Here we present evidence that quantum coherence survives in FMO at physiological temperature for at least 300 fs, long enough to impact biological energy transport. These data prove that the wave-like energy transfer process discovered at 77 K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77 K and 277 K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations.

Nanophotonics for quantum optics using nitrogen-vacancy centers in diamond

Optical microcavities and waveguides coupled to diamond are needed to enable efficientcommunication between quantum systems such as nitrogen-vacancy centers which areknown already to have long electron spin coherence lifetimes. This paper describes recentprogress in realizing microcavities with low loss and small mode volume in twohybrid systems: silica microdisks coupled to diamond nanoparticles, and galliumphosphide microdisks coupled to single-crystal diamond. A theoretical proposal for agallium phosphide nanowire photonic crystal cavity coupled to diamond is alsodiscussed. Comparing the two material systems, silica microdisks are easier tofabricate and test. However, at low temperature, nitrogen-vacancy centers in bulkdiamond are spectrally more stable, and we expect that in the long term the bulkdiamond approach will be better suited for on-chip integration of a photonicnetwork.

Optical induced vortices and persistent currents in polariton condensates

Observation of quantized vortices in non-equilibrium polariton condensates, suggesting parallelisms with conventional superfluids, have been reported either by spontaneous formation and pinning in the presence of disorder [1,2] or by imprinting it onto the signal or idler of an optical parametric oscillator (OPO) [2]. Here we report the first observation of a polariton condensate receiving a quantized angular momentum by means of a short optical pulse and maintaining its rotation for a time much longer than the pulse duration, the polariton and coherence lifetimes. This observation shows a peculiar character of polariton condensates and reveals analogies with supercurrents in superconductors or persistent flow in condensates [3].

Efficient and accurate simulations of two-dimensional electronic photon-echo signals: Illustration for a simple model of the Fenna–Matthews–Olson complex

Two-dimensional (2D) photon-echo spectra of a single subunit of the Fenna-Matthews-Olson (FMO) bacteriochlorophyll trimer of Chlorobium tepidum are simulated, employing the equation-of-motion phase-matching approach (EOM-PMA). We consider a slightly extended version of the previously proposed Frenkel exciton model, which explicitly accounts for exciton coherences in the secular approximation. The study is motivated by a recent experiment reporting long-lived coherent oscillations in 2D transients [Engel et al., Nature 446, 782 (2007)] and aims primarily at accurate simulations of the spectroscopic signals, with the focus on oscillations of 2D peak intensities with population time. The EOM-PMA accurately accounts for finite pulse durations as well as pulse-overlap effects and does not invoke approximations apart from the weak-field limit for a given material system. The population relaxation parameters of the exciton model are taken from the literature. The effects of various dephasing mechanisms on coherence lifetimes are thoroughly studied. It is found that the experimentally detected multiple frequencies in peak oscillations cannot be reproduced by the employed FMO model, which calls for the development of a more sophisticated exciton model of the FMO complex.

Direct Observation of the Two Lowest Exciton Zero-Phonon Lines in SingleCdSe/ZnSNanocrystals

We report a spectroscopic study of highly photostable individual CdSe/ZnS colloidal nanocrystals. At low temperature, photoluminescence spectra display two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest levels of the band-edge exciton fine structure. For the first time, resonant photoluminescence excitation spectra of these lines is performed, and spectral diffusion broadening of 10 microeV is measured over integration times of 100 ms, corresponding to an optical coherence lifetime longer than 100 ps.

Thulium doped crystals for quantum information storage

Optically driving nuclear spin waves in a Tm:YAG crystal, we experimentally demonstrate the feasibility of a three-level Λ system in this material, which is a foundation step in the prospect of quantum memory investigations. Varying the spin state splitting with an external magnetic field, we show that the nuclear spin coherence lifetime remains close to 350 μ s over a wide range of variation of this splitting. Finally, we demonstrate fast coherent population transfer between the spin states.

Long coherence lifetime and electromagnetically induced transparency in a highly-spin-concentrated solid

We have studied a rare-earth-doped crystal, Pr3+:La-2(WO4)(3), which exhibits a high magnetic-moment density. Although the latter favors rare-earth dephasing, a nuclear-spin coherence lifetime of 250 mu s has been observed, as well as electromagnetically induced transparency (EIT). This suggests that a broad range of materials could be considered for quantum information applications such as quantum memories for light, where solids are especially attractive. Absorption and dispersion curves are independently in very good agreement with EIT theory. Fano-like profiles have also been observed.