An atom–molecule platform for quantum computing

Abstract

We propose a combined atom-molecule system for quantum information processing in individual traps, such as provided by optical lattices. In this platform, different species of atoms-one atom carrying a qubit and the other enabling the interaction-are used to store and process quantum information via intermediate molecular states. We show how gates, initialization, and readout operations could be implemented using this approach. In particular, we describe in some detail the implementation of a two-qubit phase gate in which a pair of atoms is transferred into the ground rovibrational state of a polar molecule with a large dipole moment, thus allowing atoms transferred into molecules to interact via their dipole-dipole interaction. We also discuss how the reverse process could be used as a non-destructive readout tool of molecular qubit states. Finally, we generalize these ideas to use a decoherence-free subspace for qubit encoding to minimize the decoherence due to magnetic field fluctuations. In this case, qubits will be encoded into field-insensitive states of two identical atoms, while a third atom of a different species will be used to realize a phase gate.

Manipulation of molecules with electromagnetic fields

“…Schemes for robust quantum computation with polar molecules and experimental feasibility thereof have been analyzed by Yelin, Kuznetsova, and coauthors [811,812]. Kuznetsova et al proposed a platform based on polar molecules and neutral atoms for quantum information processing [813,814]. They also proposed a scheme for cluster state generation using van der Waals and dipole-dipole interactions between atoms and molecules in an optical lattice [815].…”

Encoding a qubit in a molecule

“…The rigid rotor Hamiltonian describing molecular rotational motion is inherently anharmonic; because the energy levels are unevenly spaced, transitions between levels can be individually addressed using microwave fields. Hence, proposals for storing quantum information in molecules [12][13][14][15][16][17][18][19][20][21][22][23] (see also [24,25]) typically pick out two low-lying long-lived energy eigenstates as basis states for a qubit. One can also introduce an external electric field, and encode a qubit using the resulting "pendular" eigenstates [26][27][28].…”

Two-qubit entangling gates between distant atomic qubits in a lattice

“…However, in a large array of qubits, it is highly desirable to be able to perform entangling gates between arbitrarily far apart atoms in the lattice. A few proposals addressing this problem have been made [34][35][36][37][38][39]. One idea put forward is to use a spin chain as a quantum bus to perform quantum gates between distant qubits [34].…”

“…It is based on the adiabatic following of the ground state of the spin chain across the paramagnet to crystal phase transition. Another proposal is to use moving carrier atoms of a different species while mediating the quantum gate with molecular states [35][36][37]. Alternatively, it has been suggested to transport the state of the control qubit near the target qubit via optical lattice modulations [38,39].…”