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arXiv:quant-ph/0605201

Polar molecules near superconducting resonators: a coherent, all-electrical, molecule-mesoscopic interface

Axel André, David P. DeMille, John M. Doyle, Mikhail D. Lukin, Stephen Maxwell, Peter Rabl, R. J. Schoelkopf, P. Zoller

Published May 23, 2006• 5 citations
arXivHTMLPDF

Abstract

The challenge of building a scalable quantum processor requires consolidation of the conflicting requirements of achieving coherent control and preservation of quantum coherence in a large scale quantum system. Moreover, the system should be compatible with miniaturization and integration of quantum circuits. Mesoscopic solid state systems such as superconducting islands and quantum dots feature robust control techniques using local electrical signals and self-evident scaling based on advances in fabrication; however, in general the quantum states of solid state devices tend to decohere rapidly. In contrast, quantum optical systems based on trapped ions and neutral atoms exhibit dramatically better coherence properties, while miniaturization of atomic and molecular systems, and their integration with mesoscopic electrical circuits, remains an important challenge. Below we describe methods for the integration of a single particle system -- an isolated polar molecule -- with mesoscopic solid state devices in a way that produces robust, coherent, quantum-level control. The methods described include the trapping, cooling, detection, coherent manipulation and quantum coupling of isolated polar molecules at sub-micron dimensions near cryogenic stripline microwave resonators. We show that electrostatically trapped polar molecules can exhibit strong confinement and fast, purely electrical gate control. Furthermore, the effect of electrical noise sources, a key issue in quantum information processing, can be suppressed to very low levels via appropriate preparation and manipulation of the polar molecules. Our setup provides a scalable cavity QED-type quantum computer architecture, where entanglement of distant qubits stored in long-lived rotational molecular states is achieved via exchange of microwave photons.

DOI
10.48550/arxiv.quant-ph/0605201

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The challenge of building a scalable quantum processor requires consolidation of the conflicting requirements of achieving coherent control and preservation of quantum coherence in a large scale quantum system. Moreover, the system should be compatible with miniaturization and integration of quantum circuits. Mesoscopic solid state systems such as superconducting islands and quantum dots feature robust control techniques using local electrical signals and self-evident scaling based on advances in fabrication; however, in general the quantum states of solid state devices tend to decohere rapidly. In contrast, quantum optical systems based on trapped ions and neutral atoms exhibit dramatically better coherence properties, while miniaturization of atomic and molecular systems, and their integration with mesoscopic electrical circuits, remains an important challenge. Below we describe methods for the integration of a single particle system -- an isolated polar molecule -- with mesoscopic solid state devices in a way that produces robust, coherent, quantum-level control. The methods described include the trapping, cooling, detection, coherent manipulation and quantum coupling of isolated polar molecules at sub-micron dimensions near cryogenic stripline microwave resonators. We show that electrostatically trapped polar molecules can exhibit strong confinement and fast, purely electrical gate control. Furthermore, the effect of electrical noise sources, a key issue in quantum information processing, can be suppressed to very low levels via appropriate preparation and manipulation of the polar molecules. Our setup provides a scalable cavity QED-type quantum computer architecture, where entanglement of distant qubits stored in long-lived rotational molecular states is achieved via exchange of microwave photons.
Superconducting qubits are fabricated "loss-free" electrical circuits on a chip with size features of tens of nanometers. If cooled to cryogenic temperatures below -273 °C they behave as quantum elements, similar to atoms and molecules. Such a qubit can be manipulated by fast-oscillating magnetic fields and its resulting quantum state can be measured using a proper magnetic detector. These qubits could be the basic elements for a quantum processor. To that order, multiple qubits have to controllably interact, and to that purpose very high quality resonators are fabricated on the same chip. Effectively the interaction results from the phase-coherent exchange of a single photon between qubit and resonator. In this way it is highly analogous to the so-called cavity QED in atomic physics where single atoms couple to a single trapped photon, all inside a cavity resonator. In this thesis, a superconducting flux qubit has been coupled to an LC resonator and coherent exchange of a single photon between qubit and resonator has been achieved. A striking option in these superconducting circuits, not attainable in "common" atomic cavity-QED, is that the interaction between qubit and photon can be made very large, up to the limit that one no longer can talk about the qubit and resonator as such. This limit requires a fully different description and it leads to novel effects, e.g. in the energy spectrum of the qubit-resonator composite. The Bloch-Siegert spectral shift is experimentally demonstrated in this thesis. As a small but critical technical aside a new type of control line filter has been developed, which allows strong suppression of excess electrical noise that forms a threat for qubit coherence. Having two modes of operation, it shorts all low-frequency noise if control is idle while being transparent if control is active, where short or open is determined by the control line signal level itself.
Arrays of trapped neutral atoms and polar molecules have separately emerged as powerful and complementary platforms for quantum science. Neutral atoms enable fast, programmable interactions through excitation to Rydberg states, whereas polar molecules possess long-lived rotational states that are attractive for quantum memories and qudits. Combining these platforms would create new possibilities, but requires a coherent interface between individual atoms and molecules, which so far has not been realised. Here, we establish such an interface between a single neutral atom and a single polar molecule. By tuning atomic and molecular transitions into resonance, we realise strong state-dependent dipolar interactions at micrometre separations in an optical tweezer platform with single-particle control. We exploit this interface to perform quantum operations between the two particles, demonstrating atom-mediated readout of a molecular qubit, coherent spin exchange, and entanglement using a blockade-based controlled-NOT operation. These results establish a new hybrid quantum platform at the intersection of atomic and molecular physics, in which long-lived molecular quantum information can be rapidly mapped onto an atom for readout or onward coherent transfer. The platform provides a route to hybrid quantum processors utilising atom-mediated readout and entanglement of molecular qubits, mixed-species quantum simulators of dipolar systems, and macroscopic entangled molecular states for quantum-enhanced metrology and precision sensing.
Polar molecules are an emerging platform for quantum technologies based on their long-range electric dipole--dipole interactions, which open new possibilities for quantum information processing and the quantum simulation of strongly correlated systems. Here, we use magnetic and microwave fields to design a fast entangling gate with >0.999>0.999>0.999 fidelity and which is robust with respect to fluctuations in the trapping and control fields and to small thermal excitations. These results establish the feasibility to build a scalable quantum processor with a broad range of molecular species in optical-lattice and optical-tweezers setups.
Polar molecules, with their rich internal structure, offer immense potential for fundamental physics, quantum technology, and controlled chemistry. However, their utilization is currently limited because of slow and imperfect state detection and weak dipolar interaction, limiting fast and large-scale entanglement generation. We propose and analyze a scheme for quantum logic control and measurement-based state preparation in a hybrid platform of polar molecules and neutral atoms. The method leverages fast, high-fidelity atom-molecule gates and high-fidelity atomic ancilla measurements to overcome the common challenges in molecule-only platforms, while preserving their diverse structural advantages. The proposed atom-molecule controlled-phase gate is based on resonant dipole-dipole exchange between a molecular rotational transition and an atomic Rydberg transition, rendering it three orders of magnitude faster than any direct molecule-molecule entangling gate. We further study several applications of our scheme including the preparation of molecular GHZ states for quantum enhanced precision measurements, the preparation of exotic molecular qudit states with topological order, and measurement-altered criticality. Our scheme is applicable to any polar molecule. It expands the paradigm of quantum logic control and paves the way to large-scale molecular entangled states. More generally, it highlights a concrete hybrid quantum system in which each qubit is utilized in an optimal way and where the measurement-based approach can yield a significant advantage in near-term devices.
We show that an array of polar molecules interacting with Rydberg atoms is a promising hybrid system for scalable quantum computation.Quantum information is stored in long-lived hyperfine or rotational states of molecules, which interact indirectly through resonant dipole-dipole interactions with Rydberg atoms.A two-qubit gate based on this interaction has a duration of 1 μs and an achievable fidelity of 99.9%.The gate has little sensitivity to the motional states of the particles-the molecules can be in thermal states, the atoms do not need to be trapped during Rydberg excitation, the gate does not heat the molecules, and heating of the atoms has a negligible effect.Within a large, static array, the gate can be applied to arbitrary pairs of molecules separated by tens of micrometres, making the scheme highly scalable.The molecule-atom interaction can also be used for rapid qubit initialization and efficient, nondestructive qubit readout, without driving any molecular transitions.Single-qubit gates are driven using microwave pulses alone, exploiting the strong electric dipole transitions between rotational states.Thus, all operations required for large-scale quantum computation can be done without moving the molecules or exciting them out of their ground electronic states.
We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as long-lived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman processes. We show that, for convenient trap-surface distances of a few microm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.