Coherence Lifetime Research Articles

Correlated Protein Environments Drive Quantum Coherence Lifetimes in Photosynthetic Pigment-Protein Complexes

Summary Early reports of long-lived quantum beating signals in photosynthetic pigment-protein complexes were interpreted to suggest that electronic coherence benefits from protection by the protein, but many subsequent studies have suggested instead that vibrational or vibronic contributions are responsible for the observed signals. Here, we devised two 2D-spectroscopy methods to observe how each exciton is perturbed by its nuclear environment in a photosynthetic complex. The first approach simultaneously monitors each exciton's energy fluctuations over time to obtain its time-dependent electronic-nuclear interactions. The second method isolates evidence of coupled interexcitonic environmental motions. The techniques are validated with Nile Blue A and subsequently used on the Fenna-Matthews-Olson (FMO) complex. The FMO data reveal that each exciton experiences nearly identical spectral motion after excitation and that spectral motion of one excited exciton induces similar motion on unpopulated neighboring excitonic states. These synchronized and correlated spectral dynamics prolong coherences in the FMO complex after femtosecond excitation.

Controlled size reduction of rare earth doped nanoparticles for optical quantum technologies.

Rare earth doped nanoparticles with sub-wavelength size can be coupled to optical micro- or nano-cavities to enable efficient single ion readout and control, a key requirement for quantum processors and high-fidelity single-ion quantum memories. However, producing small nanoparticles with good dispersion and exploitable optical coherence properties, another key aspect for these applications, is highly challenging by most synthesis and nano-fabrication methods. We report here on the wet chemical etching of Eu3+:Y2O3 nanoparticles and demonstrate that a controlled size reduction down to 150 nm, well below the wavelength of interest, 580 nm, can be achieved. The etching mechanism is found to proceed by reaction with grain boundaries and isolated grains, based on obtained particles size, morphology and polycrystalline structure. Furthermore, this method allows maintaining long optical coherence lifetimes (T2): the 12.5 μs and 9.3 μs values obtained for 430 nm initial particles and 150 nm etched particles respectively, revealing a broadening of only 10 kHz after etching. These values are the longest T2 values reported for any nanoparticles, opening the way to new rare-earth based nanoscale quantum technologies.

Topological protection of coherence in a dissipative environment

One dimensional topological insulators are characterized by edge states with exponentially small energies. According to one generalization of topological phases to non-Hermitian systems, a finite system in a non-trivial topological phase displays surface states with exponentially long life times. In this work we explore the possibility of exploiting such non-Hermitian topological phases to enhance the quantum coherence of a fiducial qubit embedded in a dissipative environment. We first show that a network of qubits interacting with lossy cavities can be represented, in a suitable super-one-particle sector, by a non-Hermitian "Hamiltonian" of the desired form. We then study, both analytically and numerically, one-dimensional geometries with up to three sites per unit cell, and up to a topological winding number $W=2$. For finite-size systems the number of edge modes is a complicated function of $W$ and the system size $N$. However we find that there are precisely $W$ modes localized at one end of the chain. In such topological phases the quibt's coherence lifetime is exponentially large in the system size. We verify that, for $W>1$, at large times, the Lindbladian evolution is approximately a non-trivial unitary. For $W=2$ this results in Rabi-like oscillations of the qubit's coherence measure.

Dependence of high density nitrogen-vacancy center ensemble coherence on electron irradiation doses and annealing time

Negatively charged nitrogen-vacancy (NV−) center ensembles in diamond have proved to have great potential for use in highly sensitive, small-package solid-state quantum sensors. One way to improve sensitivity is to produce a high-density NV− center ensemble on a large scale with a long coherence lifetime. In this work, the NV− center ensemble is prepared in type-Ib diamond using high energy electron irradiation and annealing, and the transverse relaxation time of the ensemble—T2—was systematically investigated as a function of the irradiation electron dose and annealing time. Dynamical decoupling sequences were used to characterize T2. To overcome the problem of low signal-to-noise ratio in T2 measurement, a coupled strip lines waveguide was used to synchronously manipulate NV− centers along three directions to improve fluorescence signal contrast. Finally, NV− center ensembles with a high concentration of roughly 1015 mm−3 were manipulated within a ~10 µs coherence time. By applying a multi-coupled strip-lines waveguide to improve the effective volume of the diamond, a sub-femtotesla sensitivity for AC field magnetometry can be achieved. The long-coherence high-density large-scale NV− center ensemble in diamond means that types of room-temperature micro-sized solid-state quantum sensors with ultra-high sensitivity can be further developed in the near future.

Continuous generation of delayed light

We use a four-wave mixing process to read-out light from atomic coherence which is continuously written. The light is continuously generated after an effective delay, allowing the atomic coherence to evolve during the process. Contrary to slow-light delay, which depends on the medium optical depth, here the generation delay is determined solely by the intensive properties of the system, approaching the atomic coherence lifetime at the weak driving limit. The atomic evolution during the generation delay is further manifested in the spatial profile of the generated light due to atomic diffusion. Continuous generation of light with a long intrinsic delay can replace discrete write–read procedures when the atomic evolution is the subject of interest.

Disruption of Magnetic Compass Orientation in Migratory Birds by Radiofrequency Electromagnetic Fields

The radical-pair mechanism has been put forward as the basis of the magnetic compass sense of migratory birds. Some of the strongest supporting evidence has come from behavioral experiments in which birds exposed to weak time-dependent magnetic fields lose their ability to orient in the geomagnetic field. However, conflicting results and skepticism about the requirement for abnormally long quantum coherence lifetimes have cast a shroud of uncertainty over these potentially pivotal studies. Using a recently developed computational approach, we explore the effects of various radiofrequency magnetic fields on biologically plausible radicals within the theoretical framework of radical-pair magnetoreception. We conclude that the current model of radical-pair magnetoreception is unable to explain the findings of the reported behavioral experiments. Assuming that an unknown mechanism amplifies the predicted effects, we suggest experimental conditions that have the potential to distinguish convincingly between the two distinct families of radical pairs currently postulated as magnetic compass sensors. We end by making recommendations for experimental protocols that we hope will increase the chance that future experiments can be independently replicated.

Scandium doped Tm:YAG ceramics and single crystals: Coherent and high resolution spectroscopy

We report a detailed study about the effect of Scandium (Sc3+) doping on the 3H6-3H4 transition in Tm3+:Y3Al5O12 ceramics at cryogenic temperature. Inhomogeneous, homogeneous linewidths and Zeeman lifetime as a function of the Sc3+ codoping are presented and compared to values obtained in single crystals. We demonstrate that Sc3+ can be used to increase the inhomogeneous broadening without being detrimental to coherence properties and spectral hole persistence. This controlled disorder therefore provides larger bandwidth for signal processing applications such as spectral analyzers and quantum information processing. Moreover, we show that opaque ceramics prepared by simple solid-state reaction exhibit a long coherence lifetime and even narrower inhomogeneous linewidths than single crystals. Opaque ceramics are very attractive for material development because they offer ease and low cost of fabrication and enable the study of a larger range of composition compared to single crystals. However we show that the manufacturing and the initial stoichiometry of the ceramics have a significant impact on the inhomogeneous broadening.

Filling the dead-time gap in zero echo time MRI: Principles compared.

MRI of tissues with short coherence lifetimes T2 or T2* can be performed efficiently using zero echo time (ZTE) techniques such as algebraic ZTE, pointwise encoding time reduction with radial acquisition (PETRA), and water- and fat-suppressed proton projection MRI (WASPI). They share the principal challenge of recovering data in central k-space missed due to an initial radiofrequency dead time. The purpose of this study was to compare the three techniques directly, with a particular focus on their behavior in the presence of ultra-short-lived spins. The most direct comparison was enabled by aligning acquisition and reconstruction strategies of the three techniques. Image quality and short- T2* performance were investigated using point spread functions, 3D simulations, and imaging of phantom and bone samples with short (<1 ms) and ultra-short (<100 μs) T2*. Algebraic ZTE offers favorable properties but is limited to k-space gaps up to approximately three Nyquist dwells. At larger gaps, PETRA enables robust imaging with little compromise in image quality, whereas WASPI may be prone to artifacts from ultra-short T2* species. For small k-space gaps (<4 dwells) and T2* much larger than the dead time, all techniques enable artifact-free short- T2* MRI. However, if these requirements are not fulfilled careful consideration is needed and PETRA will generally achieve better image quality. Magn Reson Med 79:2036-2045, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E′ and E″ phonon modes.

Is protein deuteration beneficial for proton detected solid-state NMR at and above 100 kHz magic-angle spinning?

1H-detection in solid-state NMR of proteins has been traditionally combined with deuteration for both resolution and sensitivity reasons, with the optimal level of proton dilution being dependent on MAS rate. Here we present 1H-detected 15N and 13C CP-HSQC spectra on two microcrystalline samples acquired at 60 and 111 kHz MAS and at ultra-high field. We critically compare the benefits of three labeling schemes yielding different levels of proton content in terms of resolution, coherence lifetimes and feasibility of scalar-based 2D correlations under these experimental conditions. We observe unexpectedly high resolution and sensitivity of aromatic resonances in 2D 13C-1H correlation spectra of protonated samples. Ultrafast MAS reduces or even removes the necessity of 1H dilution for high-resolution 1H-detection in biomolecular solid-state NMR. It yields 15N,1H and 13C,1H fingerprint spectra of exceptional resolution for fully protonated samples, with notably superior 1H and 13C lineshapes for side-chain resonances.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E’ and E” phonon modes.

Tuning the spin coherence time of Cu(II)-(bis)oxamato and Cu(II)-(bis)oxamidato complexes by advanced ESR pulse protocols.

We have investigated with the pulsed ESR technique at X- and Q-band frequencies the coherence and relaxation of Cu spins S = 1/2 in single crystals of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opba)] (1%) in the host lattice of [n-Bu4N]2[Ni(opba)] (99%, opba = o-phenylenebis(oxamato)) and of diamagnetically diluted mononuclear [n-Bu4N]2[Cu(opbon-Pr2)] (1%) in the host lattice of [n-Bu4N]2[Ni(opbon-Pr2)] (99%, opbon-Pr2 = o-phenylenebis(N(propyl)oxamidato)). For that we have measured the electron spin dephasing time Tm at different temperatures with the two-pulse primary echo and with the special Carr–Purcell–Meiboom–Gill (CPMG) multiple microwave pulse sequence. Application of the CPMG protocol has led to a substantial increase of the spin coherence lifetime in both complexes as compared to the primary echo results. It shows the efficiency of the suppression of the electron spin decoherence channel in the studied complexes arising due to spectral diffusion induced by a random modulation of the hyperfine interaction with the nuclear spins. We argue that this method can be used as a test for the relevance of the spectral diffusion for the electron spin decoherence. Our results have revealed a prominent role of the opba4– and opbon-Pr24– ligands for the dephasing of the Cu spins. The presence of additional 14N nuclei and protons in [Cu(opbon-Pr2)]2– as compared to [Cu(opba)]2– yields significantly shorter Tm times. Such a detrimental effect of the opbon-Pr24− ligands has to be considered when discussing a potential application of the Cu(II)−(bis)oxamato and Cu(II)−(bis)oxamidato complexes as building blocks of more complex molecular structures in prototype spintronic devices. Furthermore, in our work we propose an improved CPMG pulse protocol that enables elimination of unwanted echoes that inevitably appear in the case of inhomogeneously broadened ESR spectra due to the selective excitation of electron spins.

Effects of mechanical processing and annealing on optical coherence properties of [formula omitted]:LiNbO3 powders

Optical coherence lifetimes and decoherence processes in erbium-doped lithium niobate (Er3+:LiNbO3) crystalline powders are investigated for materials that underwent different mechanical and thermal treatments. Several complimentary methods are used to assess the coherence lifetimes for these highly scattering media. Direct intensity or heterodyne detection of two-pulse photon echo techniques was employed for samples with longer coherence lifetimes and larger signal strengths, while time-delayed optical free induction decays were found to work well for shorter coherence lifetimes and weaker signal strengths. Spectral hole burning techniques were also used to characterize samples with very rapid dephasing processes. The results on powders are compared to the properties of a bulk crystal, with observed differences explained by the random orientation of the particles in the powders combined with new decoherence mechanisms introduced by the powder fabrication. Modeling of the coherence decay shows that paramagnetic materials such as Er3+:LiNbO3 that have highly anisotropic interactions with an applied magnetic field can still exhibit long coherence lifetimes and relatively simple decay shapes even for a powder of randomly oriented particles. We find that coherence properties degrade rapidly from mechanical treatment when grinding powders from bulk samples, leading to the appearance of amorphous-like behavior and a broadening of up to three orders of magnitude for the homogeneous linewidth even when low-energy grinding methods are employed. Annealing at high temperatures can improve the properties in some samples, with homogeneous linewidths reduced to less than 10kHz, approaching the bulk crystal linewidth of 3kHz under the same experimental conditions.

Properties of a Rare-Earth-Ion-Doped Waveguide at Sub-Kelvin Temperatures for Quantum Signal Processing

We characterize the 795nm ^{3}H_{6} to ^{3}H_{4} transition of Tm^{3+} in a Ti^{4+}:LiNbO_{3} waveguide at temperatures as low as 800mK. Coherence and hyperfine population lifetimes-up to 117 μs and 2.5h, respectively-exceed those at 3K at least tenfold, and are equivalent to those observed in a bulk Tm^{3+}:LiNbO_{3} crystal under similar conditions. We also find a transition dipole moment that is equivalent to that of the bulk. Finally, we prepare a 0.5GHz-bandwidth atomic frequency comb of finesse >2 on a vanishing background. These results demonstrate the suitability of rare-earth-ion-doped waveguides created using industry-standard Ti indiffusion in LiNbO_{3} for on-chip quantum applications.

Atomistic Analysis of Room Temperature Quantum Coherence in Two-Dimensional CdSe Nanostructures.

Recent experiments on CdSe nanoplatelets synthesized with precisely controlled thickness that eliminates ensemble disorder have allowed accurate measurement of quantum coherence at room temperature. Matching exactly the CdSe cores of the experimentally studied particles and considering several defects, we establish the atomistic origins of the loss of coherence between heavy and light hole excitations in two-dimensional CdSe and CdSe/CdZnS core/shell structures. The coherence times obtained using molecular dynamics based on tight-binding density functional theory are in excellent agreement with the measured values. We show that a long coherence time is a consequence of both small fluctuations in the energy gap between the excited state pair, which is much less than thermal energy, and a slow decay of correlation between the energies of the two states. Anionic defects at the core/shell interface have little effect on the coherence lifetime, while cationic defects strongly perturb the electronic structure, destroying the experimentally observed coherence. By coupling to the same phonon modes, the heavy and light holes synchronize their energy fluctuations, facilitating long-lived coherence. We further demonstrate that the electronic excitations are localized close to the surface of these narrow nanoscale systems, and therefore, they couple most strongly to surface acoustic phonons. The established features of electron-phonon coupling and the influence of defects, surfaces, and core/shell interfaces provide important insights into quantum coherence in nanoscale materials in general.

Controlling quantum-beating signals in 2D electronic spectra by packing synthetic heterodimers on single-walled carbon nanotubes.

In multidimensional spectroscopy, dynamics of coherences between excited states report on the interactions between electronic states and their environment. The prolonged coherence lifetimes revealed through beating signals in the spectra of some systems may result from vibronic coupling between nearly degenerate excited states, and recent observations confirm the existence of such coupling in both model systems and photosynthetic complexes. Understanding the origin of beating signals in the spectra of photosynthetic complexes has been given considerable attention; however, strategies to generate them in artificial systems that would allow us to test the hypotheses in detail are still lacking. Here we demonstrate control over the presence of quantum-beating signals by packing structurally flexible synthetic heterodimers on single-walled carbon nanotubes, and thereby restrict the motions of chromophores. Using two-dimensional electronic spectroscopy, we find that both limiting the relative rotation of chromophores and tuning the energy difference between the two electronic transitions in the dimer to match a vibrational mode of the lower-energy monomer are necessary to enhance the observed quantum-beating signals.

Reliable nanometre-range distance distributions from 5-pulse double electron electron resonance.

Double electron electron resonance (DEER) enables determination of distance distributions in the nanometre range. A recently introduced 5-pulse version of this experiment prolongs the electron spin coherence lifetime and thus provides improved sensitivity or an extended distance range, but suffers from artefacts due to partial excitation and excitation band overlap. In particular, the partial excitation artefact is hard to eliminate experimentally at frequencies where DEER is most sensitive or on spectrometers that provide only monochromatic pulses. Here, a data post-processing method is introduced that removes the partial excitation artefact without relying on previous knowledge of its amplitude and without sensitivity loss. The method is based on acquisition of two traces with shifted positions of the artefact and computation of the artefact shape from the difference of the two traces. Artefact removal was successfully tested both on simulated and experimental data. It was found to be stable for a variety of distance distributions and down to low signal-to-noise ratios in the presence of moderate background decay. The artefact correction method also performs well in the regime of rather strong partial excitation artefacts that is usually encountered with rectangular monochromatic pump pulses on widely available commercial spectrometers.

Phonon-based scalable platform for chip-scale quantum computing

We present a scalable phonon-based quantum computer on a phononic crystal platform. Practical schemes involve selective placement of a single acceptor atom in the peak of the strain field in a high-Q phononic crystal cavity that enables coupling of the phonon modes to the energy levels of the atom. We show theoretical optimization of the cavity design and coupling waveguide, along with estimated performance figures of the coupled system. A qubit can be created by entangling a phonon at the resonance frequency of the cavity with the atom states. Qubits based on this half-sound, half-matter quasi-particle, called a phoniton, may outcompete other quantum architectures in terms of combined emission rate, coherence lifetime, and fabrication demands.

Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides.

Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.

Initiating and imaging the coherent surface dynamics of charge carriers in real space.

The tip of a scanning tunnelling microscope is an atomic-scale source of electrons and holes. As the injected charge spreads out, it can induce adsorbed molecules to react. By comparing large-scale ‘before' and ‘after' images of an adsorbate covered surface, the spatial extent of the nonlocal manipulation is revealed. Here, we measure the nonlocal manipulation of toluene molecules on the Si(111)-7 × 7 surface at room temperature. Both the range and probability of nonlocal manipulation have a voltage dependence. A region within 5–15 nm of the injection site shows a marked reduction in manipulation. We propose that this region marks the extent of the initial coherent (that is, ballistic) time-dependent evolution of the injected charge carrier. Using scanning tunnelling spectroscopy, we develop a model of this time-dependent expansion of the initially localized hole wavepacket within a particular surface state and deduce a quantum coherence (ballistic) lifetime of ∼10 fs.