The manipulation of massive ro-vibronic superpositions using time-frequency-resolved coherent anti-Stokes Raman scattering (TFRCARS): from quantum control to quantum computing

Molecular ro-vibronic coherences, joint energy-time distributions of quantu m amplitudes, are selectively prepared, manipulated, and imaged in time-fre quency-resolved coherent anti-Stokes Raman scattering (TFRCARS) measurement s using femtosecond laser pulses. The studies are implemented in iodine vap or, with its thermally occupied statistical re-vibrational density serving as initial state. The evolution of the massive ro-vibronic superpositions, consisting of 10(3) eigenstates, is followed through two-dimensional images . The first- and second-order coherences are captured using time-integrated frequency-resolved CARS, while the third-order coherence is captured using time-gated frequency-resolved CARS. The Fourier filtering provided by time -integrated detection projects out single ro-vibronic transitions, while ti me-gated detection allows the projection of arbitrary ro-vibronic superposi tions from the coherent third-order polarization. A detailed analysis of th e data is provided to highlight the salient features of this four-wave mixi ng process. The richly patterned images of the re-vibrational coherences ca n be understood in terms of phase evolution in rotation-vibration-electroni c Hilbert space, using time-circuit diagrams. Beside the control and imagin g of chemistry, the controlled manipulation of massive quantum coherences s uggests the possibility of quantum computing. We argue that the universal l ogic gates necessary for arbitrary quantum computing - all single qubit ope rations and the two-qubit controlled-NOT (CNOT) gate - are available in tim e-resolved four-wave mixing in a molecule. The molecular rotational manifol d is naturally "wired" for carrying out all single qubit operations efficie ntly, and in parallel. We identify vibronic coherences as one example of a naturally available two-qubit CNOT gate, wherein the vibrational qubit cont rols the switching of the targeted electronic qubit. (C) 2001 Elsevier Scie nce B.V. All rights reserved.