Photonic entanglement enhanced multidimensional spectroscopy for probing exciton correlations: theory and applications to photosynthetic aggregates

Abstract

Nonlinear spectroscopic techniques using entangled photon pairs provide an opportunity to exploit non-classical correlations encoded in two-photon wavefunctions to manipulate two-exciton wavefunctions. We propose an entangled photon pair-enhanced multidimensional spectroscopic technique which is sensitive to exciton-exciton interactions and correlations at the ultrafast timescale. Simulations for a dissipative photosynthetic aggregate reveal the superior ability of entangled photon pairs, compared to both transform-limited and frequency-chirped laser pulses, to manipulate excited-state absorption pathways. The corresponding spectral features in the two-dimensional spectrogram are interpreted in terms of one- and two-exciton resonances. The signal scales linearly with the incoming intensity of the photon sources. It is argued that classifying these resonances using entangled photon source at the perturbative limit allow for probing exciton correlations at the natural energy scale. These insights can be used to explore multi-exciton dynamics using multiphoton entanglement.

Figures (6)

• Figure 1: (A) Simplified schematic description of two interacting exciton levels (denoted $|j\rangle$, where $j\in \{a,b,c\})$ at sites $m,n$) highlighting exciton nonlinearities (denoted $\Delta_m, \Delta_n$). The simulation contains $14$ such sites. In the delocalized basis, the two-exciton states are affected by both overtone and combination nonlinearities. (B) The exciton pathways that contribute to the signal are presented using Albrecht notation. The process, involving $\omega_1$ and $\omega_2$, is common to both pathways. The process, involving $\omega_3$ and $\omega_4$, is discriminative depending on the differential nature of the two pathways. (C) The corresponding Keldysh-Schwinger diagram is presented.
• Figure 2: Simulated DQC spectra using entangled photon pairs. The sum frequencies of the photon pairs are tuned to the two-exciton states $f_{39}$ (top panel) and $f_{81}$ (bottom panel). The left column provides a reference simulation with an entanglement time $\tilde{T}_{\text{ent}} = 10$ fs and an SPDC pump temporal width $\tau_0 = 50$ fs. The middle column shows the effects of varying the SPDC pump temporal width, while the right column shows the effects of varying the entanglement time relative to the references. For a detailed discussion, see Section \ref{['subsec:resultQ']}.
• Figure 3: Simulated DQC spectra using classical laser pulses. The central frequencies match the parameters employed for the entangled photon pairs in Fig. \ref{['fig:dqc3981']}. The left column presents reference simulations with a temporal width of $\tau_{j,0} = 10$ fs. The middle column shows the effect of an increased temporal width ($\tau_{j,0} = 20$ fs), which decreases the spectral bandwidth and suppresses several resonances. The right column displays spectra obtained using linearly chirped pulses; notably, the negatively chirped pulse was unable to recover the suppressed resonances. For a detailed discussion, see Section \ref{['subsec:resultC']}.
• Figure 4: Simulated DQC spectra using a variation of excitonic nonlinearity parameters which influences both the two-exciton wavefunction and the transition dipole moments. The lower (upper) panel presents the case for classical laser sources (entangled photon pairs). In the upper-left and upper-middle panel, using only higher overtone excitonic nonlinearities and two different two-exciton states are targeted using the same photonic parameter as in the reference simulation in Fig. \ref{['fig:dqc3981']}. The upper-right uses degenerate entangled photon parameters to track the down-shifted resonances. The lower-left and lower-middle panel uses higher and lower overtone excitonic nonlinearities(compared to the results in Fig. \ref{['fig:dqc3981c']}) respectively. The lower-right employs degenerate pulse parameters to explore the down-shifted resonances. For detailed discussions, see Section \ref{['subsec:resultU']}.
• Figure 5: The absolute exciton nonlinearity values are listed here in the order in which they appear in the local two-exciton basis. For a detailed discussion, see section \ref{['subsec:ham']} and section \ref{['subsec:resultU']}.