Quantum Control of Diatomic Molecular Vibration States Using Space-Time Discretization

Alicia B Magann, Matthew Grace, HERSCHEL A. RABITZ +1 more

Ryan R. Zaari, Alex Brown

The importance of the ro-vibrational state energies on the ability to produce high fidelity binary shaped laser pulses for quantum logic gates is investigated. The single frequency 2-qubit ACNOT(1) and double frequency 2-qubit NOT(2) quantum gates are used as test cases to examine this behaviour. A range of diatomics is sampled. The laser pulses are optimized using a genetic algorithm for binary (two amplitude and two phase parameter) variation on a discretized frequency spectrum. The resulting trends in the fidelities were attributed to the intrinsic molecular properties and not the choice of method: a discretized frequency spectrum with genetic algorithm optimization. This is verified by using other common laser pulse optimization methods (including iterative optimal control theory), which result in the same qualitative trends in fidelity. The results differ from other studies that used vibrational state energies only. Moreover, appropriate choice of diatomic (relative ro-vibrational state arrangement) is critical for producing high fidelity optimized quantum logic gates. It is also suggested that global phase alignment imposes a significant restriction on obtaining high fidelity regions within the parameter search space. Overall, this indicates a complexity in the ability to provide appropriate binary laser pulse control of diatomics for molecular quantum computing.

Bern Kohler, Jeffrey L. Krause, Robert M. Whitnell +3 more

Benhui Yang, Robert C. Forrey, Phillip C. Stancil +2 more

Highly efficient and specific energy transfer mechanisms that involve rotation-rotation, vibration-vibration, and vibration-rotation exchange in diatomic molecules are examined theoretically in ultracold H(2), D(2), and HD self-collisions as a function of initial vibrational level v. The three quasiresonant mechanisms are found to operate for all vibrational levels and yield complex scattering lengths which vary smoothly with v. Exceptions to this trend occur at select high values of v where the scattering lengths are modulated by orders of magnitude corresponding to the location of an s-wave zero-energy resonance in "vibration space." The quasiresonant mechanisms, which are not very sensitive to the details of the interaction potential, generally control the final distribution of molecular states for any given initial distribution. The zero-energy resonances are more sensitive to the potential and may be used to vibrationally "tune" the interaction strength, similar to methods which vary applied external fields.

Hisaki Oka

Abstract We theoretically investigate molecular vibrational dynamics driven by intense and broadband terahertz (THz) pulses. The light–nuclei interaction is derived from a fully quantum-mechanical minimal coupling Hamiltonian to analyse the direct excitation of vibrational modes by the THz pulses. We show that for diatomic molecules a ‘vibrational moment’ can be defined in a manner similar to the dipole moment and that it has non-zero values only for heteronuclear diatomic molecules. Taking ultra-cold heteronuclear diatomic molecules as examples, we show that vibrational-mode excitation by THz pulse irradiation occurs in an almost stepwise manner, especially for low vibrational modes. A coherent control method using a frequency-controlled THz pulse sequence is also proposed.

Ciann‐Dong Yang, Hung-Jen Weng