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arXiv:cond-mat/0412521

Quantum electromechanics: Qubits from buckling nanobars

Sergey E. Savel'ev, Xuedong Hu, Franco Nori

Published Dec 20, 2004• 1 citation
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Abstract

We propose a mechanical qubit based on buckling nanobars--a NEMS so small as to be quantum coherent.To establish buckling nanobars as legitimate candidates for qubits, we calculate the effective buckling potential that produces the two-level system and identify the tunnel coupling between the two local states. We propose different designs of nanomechanical qubits and describe how they can be manipulated. Also we outline possible decoherence channels and detection schemes. A comparison between the well studied charge and flux superconducting qubits suggests several experimental setups which could be realized using available technology.

DOI
10.48550/arxiv.cond-mat/0412521

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We propose a mechanical qubit based on buckling nanobars--a NEMS so small as to be quantum coherent.To establish buckling nanobars as legitimate candidates for qubits, we calculate the effective buckling potential that produces the two-level system and identify the tunnel coupling between the two local states. We propose different designs of nanomechanical qubits and describe how they can be manipulated. Also we outline possible decoherence channels and detection schemes. A comparison between the well studied charge and flux superconducting qubits suggests several experimental setups which could be realized using available technology.
We propose a mechanical qubit based on buckling nanobars—a nano-electromechanical system so small as to be quantum coherent. To establish buckling nanobars as legitimate candidates for qubits, we calculate the effective buckling potential that produces the two-level system and identify the tunnel coupling between the two local states. We propose different designs of nanomechanical qubits and describe how they can be manipulated. Also we outline possible decoherence channels and detection schemes. A comparison between the well-studied charge and flux superconducting qubits suggests several experimental setups which could be realized using available technology.
Analyzing recent experimental results [Reulet et al., Phys. Rev. Lett. 85, 2829 (2000); Izmalkov et al., Europhys. Lett. 65, 844 (2004)], we find strikingly similar behaviors between two very different systems: three-junction superconducting qubits and suspended carbon nanotubes. When these different systems are ac-driven near their resonances, the resonance single-peak, observed at weak driving amplitudes, splits into two subpeaks for strong driving amplitudes. We describe this unusual behavior by considering quantum tunneling in a double well potential. Inspired by these experiments, we propose a mechanical qubit based on buckling nanoscale bars (nanobars)---a nanoelectromechanical system so small as to be quantum coherent. We consider how this nanomechanical qubit can be manipulated. A comparison between nanobars and superconducting qubits suggests several future experiments on quantum electromechanics.
We describe a possible implementation of the nanomechanical quantum superposition generation and detection scheme described in the preceding, companion paper (Armour A D and Blencowe M P 2008 New. J. Phys. 10 095004). The implementation is based on the circuit quantum electrodynamics (QED) set-up, with the addition of a mechanical degree of freedom formed out of a suspended, doubly-clamped segment of the superconducting loop of a dc SQUID located directly opposite the centre conductor of a coplanar waveguide (CPW). The relative merits of two SQUID based qubit realizations are addressed, in particular a capacitively coupled charge qubit and inductively coupled flux qubit. It is found that both realizations are equally promising, with comparable qubit–mechanical resonator mode as well as qubit–microwave resonator mode coupling strengths.
Quantum nanomechanics represents a frontier where macroscopic mechanical systems are engineered and manipulated to exhibit quantum behaviors such as superposition, entanglement, and quantized energy states. By integrating nanoelectromechanical systems (NEMS) with quantum control protocols, researchers have realized novel platforms that probe the quantum-classical boundary. This interdisciplinary field bridges quantum optics, materials science, and mechanical engineering, enabling ultra-sensitive force detection, quantum information transduction, and tests of fundamental physics. The current work reviews key developments, identifies dominant experimental platforms, and outlines challenges toward scalable quantum mechanical systems. Emphasis is placed on system coherence, coupling strategies, and the potential for hybrid architectures in quantum technologies.
We propose a scheme in which the quantum coherence of a nanomechanical resonator can be probed using a superconducting qubit. We consider a mechanical resonator coupled capacitively to a Cooper-pair box and assume that the superconducting qubit is tuned to the degeneracy point so that its coherence time is maximised and the electro-mechanical coupling can be approximated by a dispersive Hamiltonian. When the qubit is prepared in a superposition of states this drives the mechanical resonator progressively into a superposition which in turn leads to apparent decoherence of the qubit. Applying a suitable control pulse to the qubit allows its population to be inverted resulting in a reversal of the resonator dynamics. However, the resonator's interactions with its environment mean that the dynamics is not completely reversible. We show that this irreversibility is largely due to the decoherence of the mechanical resonator and can be inferred from appropriate measurements on the qubit alone. Using estimates for the parameters involved based on a specific realization of the system we show that it should be possible to carry out this scheme with existing device technology.
We suggest a nanoelectromechanical setup for designing a quantum-nanomechanical motion of a charge qubit in two-dimensional space. The quantum character of such motion comes through the fact that states of the qubit are entangled with nanomechanical cat states. The setup is based on mesoscopic terminal utilizing the ac Josephson effect between voltage-biased superconducting electrodes and mesoscopic superconducting grain—the charge qubit, vibrating in plane fixed on a nanomechanical cantilever. Required functionality is achieved by operating two external parameters: bias voltage between superconducting electrodes and controlling voltage applied over the gate electrodes. While the resonant tunneling of Cooper pairs between the superconductors and charge qubit builds nanomechanical coherent states comprising the cat state, gate voltage has a specific role of tuning their two-dimensional character in the form of the in-plane linear motion under the voltage-controlled direction. Controlling the spatial motion of a charge qubit opens a possibility of quantum communication between different terminals located in space. Directional control of nanomechanical motion entangled with charge qubit states we call the “quantum switch”.