Nonlinear spectroscopy has revealed long-lasting oscillations in the optical response of a variety of photosynthetic complexes. Different theoretical models that involve the coherent coupling of electronic (excitonic) or electronic-vibrational (vibronic) degrees of freedom have been put forward to explain these observations. The ensuing debate concerning the relevance of either mechanism may have obscured their complementarity. To illustrate this balance, we quantify how the excitonic delocalization in the LH2 unit of Rhodopseudomonas acidophila purple bacterium leads to correlations of excitonic energy fluctuations, relevant coherent vibronic coupling, and importantly, a decrease in the excitonic dephasing rates. Combining these effects, we identify a feasible origin for the long-lasting oscillations observed in fluorescent traces from time-delayed two-pulse single-molecule experiments performed on this photosynthetic complex and use this approach to discuss the role of this complementarity in other photosynthetic systems.

Non-Markovian effects in the evolution of open quantum systems have recently attracted widespread interest, particularly in the context of assessing the efficiency of energy and charge transfer in nanoscale biomolecular networks and quantum technologies. With the aid of many-body simulation methods, we uncover and analyse an ultrafast environmental process that causes energy relaxation in the reduced system to depend explicitly on the phase relation of the initial state preparation. Remarkably, for particular phases and system parameters, the net energy flow is uphill, transiently violating the principle of detailed balance, and implying that energy is spontaneously taken up from the environment. A theoretical analysis reveals that non-secular contributions, significant only within the environmental correlation time, underlie this effect. This suggests that environmental energy harvesting will be observable across a wide range of coupled quantum systems.

Diffusion broadening of spectral lines is the main limitation to frequency resolution in non-polarized liquid state nano-NMR. This problem arises from the limited amount of information that can be extracted from the signal before losing coherence. For liquid state NMR as with most generic sensing experiments, the signal is thought to decay exponentially, severely limiting resolution. However, there is theoretical evidence that predicts a power law decay of the signal’s correlations due to diffusion noise in the non-polarized nano-NMR scenario. In this work we show that in the NV based nano-NMR setup such diffusion noise results in high spectral resolution.

Charge separation is a critical process for achieving high efficiencies in organic photovoltaic cells. The initial tightly bound excitonic electron-hole pair has to dissociate fast enough in order to avoid photocurrent generation and thus power conversion efficiency loss via geminate recombination. Such process takes place assisted by transitional states that lie between the initial exciton and the free charge state. Due to spin conservation rules these intermediate charge transfer states typically have singlet character. Here we propose a donor-acceptor model for a generic organic photovoltaic cell in which the process of charge separation is modulated by a magnetic field which tunes the energy levels. The impact of a magnetic field is to intensify the generation of charge transfer states with triplet character via inter-system crossing. As the ground state of the system has singlet character, triplet states are recombination-protected, thus leading to a higher probability of successful charge separation. Using the open quantum systems formalism we demonstrate that the population of triplet charge transfer states grows in the presence of a magnetic field, and discuss the impact on carrier population and hence photocurrent, highlighting its potential as a tool for research on charge transfer kinetics in this complex systems.

The limits of frequency resolution in nano NMR experiments have been discussed extensively in recent years. It is believed that there is a crucial difference between the ability to resolve a few frequencies and the precision of estimating a single one. Whereas the efficiency of single frequency estimation gradually increases with the square root of the number of measurements, the ability to resolve two frequencies is limited by the specific time scale of the signal and cannot be compensated for by extra measurements. Here we show theoretically and demonstrate experimentally that the relationship between these quantities is more subtle and both are only limited by the Cramér-Rao bound of a single frequency estimation.We consider the problem of spectral resolution; i.e., differentiating between two close frequency components of a signal. In the nano NMR setting this can be formulated as follows. A time dependent signal is coupled to a two-level system by a term such as H = f (t)σ z , where σ z is a Pauli matrix, with the aim to assess the spectral content of f (t). This problem has been extensively examined in the past few years via NV centers in diamond [1][2][3][4][5][6][7][8][9][10][11]. The limit of resolution of the frequency spectrum of signals is believed to be set by the linewidth of the power spectrum [1][2][3] where the liquid state is dominated by diffusion [9,10,12,13] . The main problem is illustrated in Fig. 1a, where two signals that are close enough manifest a power spectrum which is similar to that of a single broad frequency. This intuition that resolution is limited by the line-width is based on the Rayleigh criterion from optics [17,18] where an analogy is drawn between the wavelength and the line-width. This notion is one of the main pillars of spectroscopy. Here, we challenge this concept.The traditional method of spectroscopy with quantum sensors uses dynamical decoupling pulses for a certain duration, since the fluorescence as a function of the dynamical decoupling frequency reflects the spectrum of the signal [19,20]. Thus when implementing this method two frequencies are only resolvable if the difference between them is larger than, roughly, T −1 2 , where T 2 is the coherence time of the probe. This was believed to impose a fundamental limit on frequency resolution. However, it was realized that by transferring the quantum phase of the sensor to the state population that survives up to longer T 1 relaxation times [13,21], resolution could be improved. Moreover, by using a hybrid quantum system where an additional long-lived qubit acts as a more stable clock [4,5,10,[22][23][24] the limit could be extended to the coherence time of the ancilla qubit. Recently it was realized that the quantum memory could be replaced by a classical one [1][2][3]25]. In these contributions it was shown that although the efficiency of estimating a single frequency improves with the number of measurements, the resolution limit is set by the specific time scale of the scheme.