Because of the fixed and relatively small inter-chromophore separation in TIPS-BP1' dimers, and because dilution inhibits triplet diffusion, we assume that J is large, that excitons are immobile, and that rare fluctuations in J drive transitions between specific spin sublevels. Unlike the spectrum from most SF systems in the literature, triplet ( S = 1) photoproducts are undetectable-the observed spectrum is entirely from the quintet state. Because the Rabi oscillations self-modulate the trEPR signal, they allow for a new frequency-selective detection scheme for the spectrum that we report here. These coherences, which form the basis of quantum gates, persist for microseconds, orders of magnitude longer than the gate switching time for electrons. ![]() Electron spin coherence between the 5TT M sublevels produces observable Rabi oscillations in the trEPR spectrum at 75 K. We report on the time-resolved electron paramagnetic resonance (trEPR) spectrum from a dilute glass of these dimers. In this article, we take a bottom-up approach and initialize specific quantum spin states of biexcitons at high temperatures using SF in rigid molecular dimers called TIPS-BP1'. If the triplets are independent, there is no quantum advantage to preparing them using SF-they might as well have been prepared through conventional intersystem crossing. Geometrical fluctuations can play a similar role to hopping. In fact, most molecular dimers are conformationally flexible 13, 14, 15, 16, which allows the relative geometry of the two chromophores to fluctuate. There have been no reports of an aligned system of molecular SF dimers, where chromophores are covalently coupled through synthetic design. Triplet pairs may form at sites where J is large, but in crystals the excitons are mobile and hopping unpairs the biexciton state, which can then decohere 8, 9, 10, 11, 12. When they are not satisfied, both static and dynamic sources of decoherence emerge.įor example, in solid-state SF systems, individual chromophores are aligned through crystallization. In practice, these conditions are very difficult to fulfill simultaneously. The final two conditions require that the system is spatially ordered and dilute. Second, the inter-chromophore exchange interaction, J, that splits the spin states, must be large. First, the chromophores should share common molecular axes. They showed that relaxation from 1TT populates a pure quintet sublevels when four conditions are met. In an earlier theoretical publication, Smyser and Eaves 4 suggested that the state initialization problem might be solved in singlet fission (SF)-a photophysical process that generates a maximally entangled two-triplet state with singlet multiplicity 1TT-by directing the relaxation from the 1TT state into a specific M-sublevel of the quintet state, 5TT M. Like many other quantum materials, however, those molecules only exhibit quantum function near liquid helium temperatures. Recent molecular analogs to the color centers suggest that a bottom-up approach from synthetic chemistry might ultimately lead to more scalable architectures 6, 7. But controlling the placement of such defects in crystals is challenging, which makes scaling the number of qubits in these materials a formidable hurdle. For example, in color centers, like nitrogen-vacancy centers in diamond, a weak-field optical excitation initializes the system into a non-equilibrium state-a magnetic sublevel-where strong-field magnetic resonance pulses perform gate operations 5. Removing the uncertainty in the initial condition of the wavefunction solves the so-called “state-initialization problem,” a requirement for quantum computation that DiVincenzo articulated more than 20 years ago 1. This “tyranny of temperature” makes quantum circuits classical for temperatures above a few kelvin 4. Because the resonant frequencies are much smaller than the thermal energy at room temperature, without extreme cooling or other means of control, a significant population in the excited state generates thermal uncertainty in the initial state of the wavefunction 3. In strong-field experiments, microwave or radio frequencies manipulate the qubits to perform operations 2. Quantum logic uses qubits that are built upon fragile non-equilibrium quantum states that irreversibly decay to Boltzmann equilibrium. But unlike classical computing, where the solid-state transistor has become ubiquitous, we remain in the discovery phase for quantum materials. Quantum information promises advances in science and computing not seen since the revolutions in classical computing that have unfolded over the last 80 years 1.
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