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The Journal of Physical Chemistry Letters

Optically Driven Both Classical and Quantum Unary, Binary, and Ternary Logic Gates on Co-Decorated Graphene Nanoflakes

Yiming Zhang, Shuai Xu, Jing Liu, Wei Jin, Georgios Lefkidis, Wolfgang Hübner, Chun Li

Published Sep 1, 2023• 11 citations
Publisher

Abstract

many-body theory, we suggest a nanoscale framework for building quantum computation elements, based on individual magnetic atoms deposited on graphene nanoflakes. We show the great possibilities of this proposal by exemplarily presenting four quantum gates, namely, the unary Pauli-X, Pauli-Y, Pauli-Z, and Hadamard gates, as well as the universal classical ternary Toffoli gate, which preserves information entropy and is therefore fully reversible and minimally energy consuming. All our gates operate within the subpicosecond time scale and reach fidelities well above 90%. We demonstrate the ability to control the spin direction and localization, as well as to create superposition states and to control the quantum phase of states, which are indispensable ingredients of quantum computers. Additionally, being optically driven, their predicted operating speed by far beats that of modern quantum computers.

DOI
10.1021/acs.jpclett.3c01917

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many-body theory, we suggest a nanoscale framework for building quantum computation elements, based on individual magnetic atoms deposited on graphene nanoflakes. We show the great possibilities of this proposal by exemplarily presenting four quantum gates, namely, the unary Pauli-X, Pauli-Y, Pauli-Z, and Hadamard gates, as well as the universal classical ternary Toffoli gate, which preserves information entropy and is therefore fully reversible and minimally energy consuming. All our gates operate within the subpicosecond time scale and reach fidelities well above 90%. We demonstrate the ability to control the spin direction and localization, as well as to create superposition states and to control the quantum phase of states, which are indispensable ingredients of quantum computers. Additionally, being optically driven, their predicted operating speed by far beats that of modern quantum computers.
Quantum computing, based on different principles than classical computing, has raised high expectations regarding the increase of computational speed in nano-sized quantum systems. Therefore, the search for implementations of quantum logic gates in photons, spin states, atom/ion traps or superconducting materials, for example, is a very active research area. Graphene has demonstrated already the possibility of implementing reversible logic gates, therefore becoming a compelling candidate for quantum computing applications. The paper presents several proposals of quantum logic gates implementation in graphene, which could work at room temperature and require only current measurements as readout procedures; examples of such quantum gates are Hadamard, C-NOT, C-phase and Toffoli gates. Besides these gates, it is shown that quantum algorithms, such as the modified Deutsch-Jozsa algorithm, can be implemented also in graphene.
Quantum computing is expected to increase significantly the computational speed since it operates on completely different principles in comparison to classical computers. Quantum computing can be implemented in small, nano-sized systems, such as trapped atom or ions, spin states, superconductors or conductors in the ballistic transport regime. In particular, graphene is a suitable material for implementing quantum logic gates due to its large room-temperature mean free path. Several graphene-based configurations of quantum gates such as Hadamard, C-phase, C-NOT, and universal Toffoli gates, as well as of a modified Deutsch-Jozsa quantum algorithm are presented in this paper. In all cases, the quantum logic circuits operate at room temperature and the readout procedure implies current measurements only.
Rising energy demand from data-center and AI applications has renewed interest in reversible computation, where logic need not dissipate heat at every step if information is uncomputed. Implementations have so far been classical: adiabatic CMOS recovers part of the switching energy but still moves thousands of kBTk_BTkB​T per logic node at room temperature. Here we propose classical reversible logic implemented by coherent spin dynamics in a spin quantum-dot array, with inputs and outputs in classical basis states and no algorithmic use of superposition. The same spin stores, transports, and computes, with unitary rotation replacing irreversible switching. The universal building block is an iToffoli gate driven by DC voltage pulses and exchange between hole spins in Ge/SiGe quantum dots. Simulations with realistic model parameters reproduce the Toffoli truth table and yield a testable error landscape. Because shuttling transports the bit without measurement, logic and data movement remain unitary until readout. Millivolt pulses on femtofarad gates with superconducting lines dissipate only dielectric loss and have no thermodynamic floor. With the loss parameters assumed for a demonstrated device the gate energy is ∼102 kBTln⁡2\sim 10^{2}\,k_BT\ln 2∼102kB​Tln2 at 4 K; at a design point within reach of existing devices it can fall below the Landauer scale kBTln⁡2k_BT\ln 2kB​Tln2, five to seven orders of magnitude less than a CMOS Toffoli. The same semiconductor hardware therefore serves both purposes, supporting quantum algorithms when superposition is used and classical reversible logic otherwise.
Atomic-scale logic and the minimization of heating (dissipation) are both very high on the agenda for future computation hardware. An approach to achieve these would be to replace networks of transistors directly by classical reversible logic gates built from the coherent dynamics of a few interacting atoms. As superpositions are unnecessary before and after each such gate (inputs and outputs are bits), the dephasing time only needs to exceed a single gate operation time, while fault tolerance should be achieved with low overhead, by classical coding. Such gates could thus be a spin-off of quantum technology much before full-scale quantum computation. Thus motivated, we propose methods to realize the 3-bit Toffoli and Fredkin gates universal for classical reversible logic using a single time-independent 3-qubit Hamiltonian with realistic nearest neighbour two-body interactions. We also exemplify how these gates can be composed to make a larger circuit. We show that trapped ions may soon be scalable simulators for such architectures, and investigate the prospects with dopants in silicon.
The works that have been devoted to the topic of quantum computer design have been examined. The main issues surrounding the development of a quantum computer are explored. The “up to bottom” concept has been presented and supported as a fundamentally novel way to solve the problem of building a really quantum computer. The technique can be accomplished by using nanotriggers made of two-dimensional materials, such as graphene, to visualise the quantum states of qubits in advance. This refers to the visualisation (materialisation) of all states, including entangled states, which primarily determine the quantum computer's theoretically large mathematical resource. The proposal is for an electronic device block diagram based on “a priory” quantum states of q-bits. It is demonstrated that each realised (visualised) Shor's cell should correspond to an element of the electronic system for the execution of quantum computation procedures. A block containing at least 1010 nanotriggers that act as q-bits in quantum computation and are generated using graphene nanoribbons and controlled by a specific element is included in the device. The latter is a self-organizing quantum dot with two essentially different magnetic states. This quantum dot is made from a compound whose molecules are distinguished by intramolular rearrangement. Nanotriggers are used to create reversible logic gates or blocks. Three triggers are included in each gate to perform logical operations. The offered device is an additional electronic unit implanted in a digital computer that allows the computational process to be implemented in accordance with the requirements of quantum physics.