We present a numerical investigation of the implementation of quantum logic gates through time–frequency resolved coherent anti-Stokes Raman scattering (TFRCARS) in iodine vapour. A specific scheme is given whereby two qubits are encoded in the tensor product space of vibrational and rotational molecular eigenstates. Single-qubit and controlled-logic gates are applied to test the viability of this encoding scheme. Possible experimental constraints are investigated, including the necessary precision in the timing of successive CARS pulses and the minimum resolution of spectral components within a given pulse. It is found that these requirements can be satisfied by current technology. The two-qubit Grover search is performed to demonstrate the implementation of a sequence of quantum operations. The results of our simulations suggest that TFRCARS is a promising experimental system for simple quantum information processing applications.
We present a numerical investigation of the implementation of quantum logic gates through time–frequency resolved coherent anti-Stokes Raman scattering (TFRCARS) in iodine vapour. A specific scheme is given whereby two qubits are encoded in the tensor product space of vibrational and rotational molecular eigenstates. Single-qubit and controlled-logic gates are applied to test the viability of this encoding scheme. Possible experimental constraints are investigated, including the necessary precision in the timing of successive CARS pulses and the minimum resolution of spectral components within a given pulse. It is found that these requirements can be satisfied by current technology. The two-qubit Grover search is performed to demonstrate the implementation of a sequence of quantum operations. The results of our simulations suggest that TFRCARS is a promising experimental system for simple quantum information processing applications.
While formally quantum computation has become a rather well defined theoretical science, physical demonstrations are few, limited to several qubits, and to several steps in processing. At present, there is not a recognized, scalable physical method of implementation that meets the promise of true computation. We have proposed nonlinear spectroscopic methods of coherence manipulation, and in particular, Time-Frequency Resolved Coherent Anti-Stokes Raman Scattering (TFRCARS) in the molecular re-vibronic Hilbert space as a novel scheme for implementing quantum logic. The method has advantages with its demonstrable massive parallelism, therefore well suited for information transfer, while its scalability remains unresolved, both theoretically and with regard to physical implementations. We are actively following the latter, with the concept of an inhomogeneously indexed array of single quantum centers as the physical realization.
Time-Frequency Resolved Coherent Anti-Stokes Raman Scattering (TFRCARS) was recently proposed as a means to implement quantum logic using the molecular ro-vibrational manifold as a quantum register [R. Zadoyan et al., Chem. Phys. 266, (2001) 323]. We give a concrete example of how this can be accomplished through an illustrative algorithm that solves the Deutsch-Jozsa problem. We use realistic molecular parameters to recognize that, as the problem size expands, shaped pulses must be tailored to maintain fidelity of the algorithm. Key words: Four-wave mixing, coherent spectroscopy, quantum computing
Time-resolved coherent anti-Stokes Raman scattering is applied to prepare and interrogate vibrational coherences on the ground electronic surface of molecular iodine isolated in Ar matrices. The coherence decay time shows a linear dependence on vibrational quantum numbers for v=3–15. The temperature dependence of decoherence rates is negligible for v<7 in the experimental range T=18–32 K. For a v=13, 14 superposition, the temperature dependence indicates dephasing by a 66 cm−1 pseudo-local phonon, just outside the Debye edge of the solid. The accuracy of the data is limited due to two-photon induced dissociation of the molecule, a process which is characterized using polarized fields. The T→0 limit of dephasing is discussed.
Time-resolved, electronically resonant, coherent anti-Stokes Raman scattering is used to prepare and interrogate vibronic coherences of molecular iodine in matrix Ar. Coherences that involve evolution on the excited B(3Π0u) state, first- and third-order coherences, decay in less than one vibrational period (τ<300 fs). In contrast, as many as 200 vibrational periods of motion can be observed for Raman-prepared wave packets consisting of zero-phonon vibrational superpositions on the ground electronic state (second-order coherence). Packets consisting of v=4, 5 and v=3, 4, 5 on the X(1Σg) state decay with a half-life of 10±1 ps at 31 K, allowing a more accurate measure of vibrational level spacings and decoherence time than has been possible in frequency domain. The harmonic frequency of the molecule is reduced by 1.5 cm−1 (0.7%) in the matrix. The lack of recurrence in the excited electronic state ensures that the resonant anti-Stokes scattering arises only from the negative momentum component of the Raman packet. This momentum filter, which should be ubiquitous in condensed media, leads to a signal with deeper modulation than in the gas phase.
Time-resolved coherent anti-Stokes Raman-scattering (CARS) measurements are carried out for iodine (I2) in solid krypton matrices. The dependence of vibrational dephasing time on temperature and vibrational quantum number v is studied. The v dependence is approximately quadratic, while the temperature dependence of both vibrational dephasing and spectral shift, although weak, fits the exponential form characteristic of dephasing by pseudolocal phonons. The analysis of the data indicates that the frequency of the pseudolocal phonons is approximately 30 cm(-1). The longest dephasing times are observed for v = 2 being approximately 300 ps and limited by inhomogeneous broadening. An increase in the dephasing rate of v = 2 as the temperature is lowered to T = 2.6 K is taken as a clear indication of lattice-strain-induced inhomogeneity of the ensemble coherence.
A semiclassical theory that describes the generation of a coherent anti-Stokes Raman scattering (CARS) signal is presented that maximizes vibrational coherence in a mode predetermined by the pump, the Stokes, and the probe chirped pulse trains and takes into account the field propagation effects in a cloud of molecules. The buildup of the anti-Stokes signal, which may be used as a molecular signature in the backward CARS signal, is demonstrated numerically. The theory is based on the solution of the coupled Maxwell's and Liouville--von Neumann equations and focuses on the quantum effects induced in the target molecules by the control pulse trains. A deep convolutional neural network technique is implemented to evaluate time-dependent phase characteristics of the control fields. The effect of decoherence induced by spontaneous decay and collisional dephasing is examined.