Quantum Computation with Vibrationally Excited Molecules

A new physical implementation for quantum computation is proposed. The vibrational modes of molecules are used to encode qubit systems. Global quantum logic gates are realized using shaped femtosecond laser pulses which are calculated applying optimal control theory. The scaling of the system is favorable; sources for decoherence can be eliminated. A complete set of one- and two-quantum gates is presented for a specific molecule. Detailed analysis regarding experimental realization shows that the structural resolution of today’s pulse shapers is easily sufficient for pulse formation.

Figure 1

Controlled NOT gate for a two-qubit system in vibrationally excited acetylene. Depicted are the strength of the electric field, a fast Fourier transformation, and frequency versus time. The efficiency of the gate is $\ge 90\%$.

Figure 2

Preparation of a Bell state: $\frac{1}{\sqrt{2}}[|00⟩+|10⟩]\to \frac{1}{\sqrt{2}}[|00⟩+|11⟩]$, efficiency $\ge 98\%$. $R$ is the coordinate of the cis-bending mode, $d$ describes the asymmetric CH-stretching mode. Displayed are the laser pulse and the time evolution of the real part of the wave function during the pulse and afterwards. The * indicates the initial and target state; the box labels stay with the same $|\psi {|}^2$.

Figure 3

Mask function for the CNOT gate. For explanation see text.