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arXiv:quant-ph/0608182 · Physical Review A

Quantum phase gate and controlled entanglement with polar molecules

Éric Charron, P. Milman, Arne Keller, Osman Atabek

Published Mar 26, 2007• 60 citations
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Abstract

We propose an alternative scenario for the generation of entanglement between rotational quantum states of two polar molecules. This entanglement arises from dipole-dipole interaction, and is controlled by a sequence of laser pulses simultaneously exciting both molecules. We study the efficiency of the process, and discuss possible experimental implementations with cold molecules trapped in optical lattices or in solid matrices. Finally, various entanglement detection procedures are presented, and their suitability for these two physical situations is analyzed.

DOI
10.1103/physreva.75.033414

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We propose an alternative scenario for the generation of entanglement between rotational quantum states of two polar molecules. This entanglement arises from dipole-dipole interaction, and is controlled by a sequence of laser pulses simultaneously exciting both molecules. We study the efficiency of the process, and discuss possible experimental implementations with cold molecules trapped in optical lattices or in solid matrices. Finally, various entanglement detection procedures are presented, and their suitability for these two physical situations is analyzed.
We analyze a recently proposed physical implementation of a quantum computer based on polar molecules with ``switchable'' dipoles, i.e., dipole moments that can be switched ``on'' and ``off''. Conditional dipole-dipole interaction is an efficient tool for realizing two-qubit quantum gates necessary to construct universal gates. A set of general requirements for a molecular system is presented, which would provide an optimal combination of quantum gate times, coherence times, number of operations, high gate accuracy, and experimental feasibility. We proceed with an analysis of a two-qubit phase gate realization based on switchable dipole-dipole interactions between polar molecules in an optical lattice architecture. We consider one of the schemes proposed in our previous work [Phys. Rev. A 74, 050301(R) (2006)], using specific molecules, such as CO and NF. We suggest suitable electronic states and transitions, and investigate requirements for the laser pulses driving them. Finally, we analyze possible sources of decoherence and list practical difficulties of the scheme.
Quantum entanglement and coherence are both essential physical resources in quantum theory. Cold polar molecules have long coherence time and strong dipole-dipole interaction and thus have been suggested as a platform for quantum information processing. In this paper, we employ the pendular states of the polar molecules trapped in static electric fields as the qubits, and put forward several theoretical schemes to generate the entanglement and coherence for two coupled dipoles by using optimal control theory. Through the designs of appropriate laser pulses, the transitions from the ground state to the Bell state and maximally coherent state can be realized with high fidelities 0.9906 and 0.9943 in the two-dipole system, respectively. Meanwhile, we show that the degrees of entanglement and coherence between the two pendular qubits are effectively enhanced with the help of optimized control fields. Furthermore, our schemes are generalized to the preparation of the Hardy state and even to the creation of arbitrary two-qubit states. Our findings can shed some light on the implementation of quantum information tasks with the molecular pendular states.
While many-particle entanglement can be found in natural solids and strongly interacting atomic and molecular gases, generating highly entangled states between weakly interacting particles in a controlled and scalable way presents a significant challenge. We describe here a one-step method to generate entanglement in a dilute gas of cold polar molecules. For molecules in optical traps separated by a few micrometers, we show that maximally entangled states can be created using the strong off-resonant pulses that are routinely used in molecular alignment experiments. We show that the resulting alignment-mediated entanglement can be detected by measuring laser-induced fluorescence with single-site resolution and that signatures of this molecular entanglement also appear in the microwave absorption spectra of the molecular ensemble. We analyze the robustness of these entangled molecular states with respect to intensity fluctuations of the trapping laser and discuss possible applications of the system for quantum information processing.
Optically trapped ultracold polar molecules can have multiple long-lived states for coding quantum information, and can exhibit electric dipole-dipole interactions~(DDI) which enables entanglement generation. The general understanding on the quantized motion~(QM) of molecules in the traps is that it causes fluctuation of DDI. Here, we find that the molecular QM can realize an asymmetric quantum Rabi model, which is of specific importance in the study of fundamental physics. The molecular QM can also lead to an exotic trap-dipole resonance, resulting in excess population loss to uncoupled motional states, and, hence, should be avoided in a general quantum control over polar molecules. To examine the impact of QM on quantum computing based on polar molecules, we introduce two gate protocols, a fast iSWAP gate which can be realized by a global microwave pulse of pulse area smaller than 2π2π2π, and a controlled-phase gate with an arbitrary controlled phase, and find that both gates can attain a high fidelity.
"We analyze recently proposed physical implementations of a quantum computer based on polar molecules with switchable dipoles. Switch- able dipole-dipole interaction is an efficient tool for realization of two-qubit quantum gates, necessary to construct a universal set of gates. We specifically study a phase gate realization based on switchable dipole-dipole interactions between polar molecules in an optical lattice architecture. We consider one possible realization of the phase gate using a specific molecule, such as 13 CO."
We analyse recently proposed physical implementations of a quantum computer based on polar molecules. A set of general requirements for a molecular system is presented, which would provide an optimal combination of quantum gate times, coherence times, number of operations, high gate accuracy and experimental feasibility. We proceed with a detailed analysis of a scheme utilizing switchable dipole-dipole interactions between polar molecules. Switchable dipole-dipole interaction is an efficient tool for realization of two-qubit quantum gates, necessary to construct a universal set of gates. We consider three possible realizations of a phase gate using specific molecules, such as CO, NF, alkali dimers and alkaline-earth monohalides. We suggest suitable electronic states and ransitions and investigate requirements for the pulses driving them. Finally, we analyse possible sources of decoherence.