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arXiv:quant-ph/0412081 · Europhysics Letters (EPL)

Single-spin detection by qubit SWAP to a molecular nanomagnet

M Feng, Jason Twamley

Published Jan 25, 2005• 4 citations
arXivHTMLPDF

Abstract

Spin state detection is a key but very challenging step for any spin-based solid-state quantum computing technology. In fullerene-based quantum computer technologies, we here propose to detect the single spin inside a fullerene by transferring the quantum information from the endohedral spin to the ground states of a molecular nanomagnet Fe 8 , with large spin S = 10. We show how to perform the required SWAP operation and how to read out the information through state-of-the-art techniques such as micro-SQUID.

DOI
10.1209/epl/i2004-10406-0

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Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences•Jul 15, 2003•62 citations
Spin state detection is a key but very challenging step for any spin-based solid-state quantum computing technology. In fullerene-based quantum computer technologies, we here propose to detect the single spin inside a fullerene by transferring the quantum information from the endohedral spin to the ground states of a molecular nanomagnet Fe 8 , with large spin S = 10. We show how to perform the required SWAP operation and how to read out the information through state-of-the-art techniques such as micro-SQUID.
Spin state detection is a key but very challenging step for any spin-based solid-state quantum computing technology. In fullerene based quantum computer technologies, we here propose to detect the single spin inside a fullerene by transferring the quantum information from the endohedral spin to the ground states of a molecular nanomagnet Fe8_{8}8​, with large spin S=10. We show how to perform the required SWAP operation and how to read out the information through state-of-the-art techniques such as micro-SQUID.
We propose an alternative concept for a scalable spin quantum computer that combines aspects of other proposals with the advantageous features of endohedral fullerenes. The key advantages are that electron spins instead of nuclear spins are used and that the manipulation of fullerene molecules is fairly easy. Qubits are set and read out via pulsed electron-spin resonance. Addressing is provided by local magnetic fields or field gradients (A(A(A gate). The qubit-qubit interaction is mediated by magnetic dipolar coupling and can be controlled via the direction of the magnetic field with respect to the distance vector of the qubits (J(J(J gate). Molecular as well as solid-state architectures are discussed.
We propose a scheme to implement the two-qubit gates between the nuclear spins of the encapsulated atoms in endohedral fullerenes 15N@C60^{15}\mathrm{N}@{\mathrm{C}}_{60}15N@C60​ or 31P@C60^{31}\mathrm{P}@{\mathrm{C}}_{60}31P@C60​, within today's magnetic resonance techniques. Since there is no interaction between the nuclear spins, this scheme employs the electronic spins as medium and two swap operations are proposed to transfer the information between the nuclear spin 12\frac{1}{2}21​ and the electronic spin 32\frac{3}{2}23​, and between two electronic spins 32\frac{3}{2}23​. These two-qubit gates, along with the single-qubit rotation gates, compose a universal set of quantum gates in fullerene-based quantum computation.
We propose to carry out two-qubit quantum gates with only one-step operations on endohedral fullerenes N@C60N@C_{60}N@C60​ or P@C60P@C_{60}P@C60​. The qubits are stored in electronic spin degrees of freedom of the doped atom NNN or PPP. By means of the magnetic dipolar coupling between two neighboring fullerenes, the two-qubit controlled-NOT gate is performed by a selective microwave pulse, and the two-qubit conditional phase gate is done by simultaneous radiation by two detuned microwaves. We will show that our proposal is useful for both quantum gating and the readout of quantum information from the spin-based qubit state.
Molecular nanomagnets (MNMs), molecules containing interacting spins, have been a playground for quantum mechanics. They are characterized by many accessible low-energy levels that can be exploited to store and process quantum information. This naturally opens the possibility of using them as qudits, thus enlarging the tools of quantum logic with respect to qubit-based architectures. These additional degrees of freedom recently prompted the proposal for encoding qubits with embedded quantum error correction (QEC) in single molecules. QEC is the holy grail of quantum computing and this qudit approach could circumvent the large overhead of physical qubits typical of standard multi-qubit codes. Another important strength of the molecular approach is the extremely high degree of control achieved in preparing complex supramolecular structures where individual qudits are linked preserving their individual properties and coherence. This is particularly relevant for building quantum simulators, controllable systems able to mimic the dynamics of other quantum objects. The use of MNMs for quantum information processing is a rapidly evolving field which still requires to be fully experimentally explored. The key issues to be settled are related to scaling up the number of qudits/qubits and their individual addressing. Several promising possibilities are being intensively explored, ranging from the use of single-molecule transistors or superconducting devices to optical readout techniques. Moreover, new tools from chemistry could be also at hand, like the chiral-induced spin selectivity. In this paper, we will review the present status of this interdisciplinary research field, discuss the open challenges and envisioned solution paths which could finally unleash the very large potential of molecular spins for quantum technologies.
Most experts agree that it is too early to say how quantum computers will eventually be built, and several nanoscale solid-state schemes are being implemented in a range of materials. Nanofabricated quantum dots can be made in designer configurations, with established technology for controlling interactions and for reading out results. Epitaxial quantum dots can be grown in vertical arrays in semiconductors, and ultrafast optical techniques are available for controlling and measuring their excitations. Single-walled carbon nanotubes can be used for molecular self-assembly of endohedral fullerenes, which can embody quantum information in the electron spin. The challenges of individual addressing in such tiny structures could rapidly become intractable with increasing numbers of qubits, but these schemes are amenable to global addressing methods for computation.