A modular design of molecular qubits to implement universal quantum gates

Abstract

The physical implementation of quantum information processing relies on individual modules—qubits—and operations that modify such modules either individually or in groups—quantum gates. Two examples of gates that entangle pairs of qubits are the controlled NOT-gate (CNOT) gate, which flips the state of one qubit depending on the state of another, and the gate that brings a two-qubit product state into a superposition involving partially swapping the qubit states. Here we show that through supramolecular chemistry a single simple module, molecular {Cr7Ni} rings, which act as the qubits, can be assembled into structures suitable for either the CNOT or gate by choice of linker, and we characterize these structures by electron spin resonance spectroscopy. We introduce two schemes for implementing such gates with these supramolecular assemblies and perform detailed simulations, based on the measured parameters including decoherence, to demonstrate how the gates would operate.

Molecular Spins in the Context of Quantum Technologies

“…Proposals for the implementation of two-qubit gates with specific molecular spins systems have appeared and preliminary experiments have been reported using binuclear lanthanides [31], radicals [32], and antiferromagnetic rings [33]. Moreover, an intense search of a suitable bi-nuclear molecular system is currently under way and many potential candidates are ready to be tested.…”

Supramolecular Complexes for Quantum Simulation

“…The elementary unit of the scalable array consists of an AB pair of qubits linked by an interposed switch, in which A and B transitions are spectroscopically distinguishable. As a physical implementation of this architecture, we consider a couple of perpendicularly arranged Cr 7 Ni rings, connected through a Co 2+ complex, as reported in [20]. This two-qubit unit can be extended to form the ABAB one-dimensional register schematically shown in Figure 1.…”

“…To this aim, we have recently introduced a scalable quantum computation scheme which allows one to control the qubits on a linear register by using uniform magnetic pulses as the only manipulation tool [18][19][20]. It is based on two classes of magnetic units that play two distinct roles: effective S = 1/2 spins are used to encode the qubits, whereas interposed molecular complexes (or ions) are used as switches of the effective qubit-qubit interaction.…”

“…We employ an extended version of the supra-molecular assembly introduced in [20], in which Cr 7 Ni rings are employed to encode the qubits, whereas Co ions act as switches of the qubit-qubit interaction. The perpendicular arrangement of the Cr 7 Ni qubits makes the A and B transitions spectroscopically distinguishable, thanks to the anisotropy of the g tensors of the rings.…”

Measuring Spin⋅⋅⋅Spin Interactions between Heterospins in a Hybrid [2]Rotaxane

“…This underpins the g ‐engineering idea proposed by Takui and co‐workers for organic radicals5 and work employing heterometallic lanthanide dimers 6. Large differences in g ‐values could also be used as a means to implement entangling two qubit gates 7. Quantifying weak interactions between very different spins is challenging.…”