arXiv subjects

Harry L. Anderson

Publications and source records attributed to Harry L. Anderson.

14 recordsLinked to original sources

arXiv2026

Exploring pathways towards quantum advantage in quantum chemistry: the case of a molecule with half-M\"obius topology

We report quantum chemistry calculations performed on superconducting quantum processors for a molecule exhibiting the half-M\"obius electronic topology originally introduced by Ron\v{c}evi\'c et al. Using SqDRIFT, a randomized sample-based Krylov quantum diagonalization algorithm, we achieve reliable quantum simulations on active spaces corresponding to 36 orbitals (72 qubits) and extend previous studies up to 50 orbitals (100 qubits). We demonstrate that a systematic increase of active space sizes, which has a concrete impact on the accuracy of the electronic structure description, is achievable with state-of-the-art quantum processors, thus offering a promising path towards practically relevant quantum-assisted electronic-structure calculations.

quant-ph↗

arXiv2026

On-surface synthesis and aromaticity of large cyclocarbons

Molecular rings of N carbon atoms, that is, cyclo[N]carbons, or C$_N$, can be formed by tip-induced chemistry [1-7]. Because of their monocyclic geometry, cyclocarbons are fundamentally important for testing theoretical models of aromaticity [8-11]. Here, we synthesized large cyclo[N]carbons, with N up to 88, by tip-induced chemistry on a NaCl surface and studied their aromaticity by measuring their transport gaps by scanning tunnelling spectroscopy. We first generated C$_{20}$ and C$_{22}$, and then fused multiple cyclocarbons [5-7] by means of atom manipulation, obtaining C$_{42}$, C$_{44}$, C$_{46}$, C$_{66}$ and C$_{88}$. In agreement with predictions obtained using a finely tuned density functional [12-15] and large active space approximate configuration interaction calculations executed on quantum hardware [16, 17], we observe a substantially smaller transport gap for C$_{20}$ (N = 4n) compared to C$_{22}$ (4n+2), and for C$_{44}$ (4n) relative to C$_{42}$ (4n+2). In larger cyclocarbons, the oscillation of the transport gap between anti-aromatic N = 4n and aromatic N = 4n+2 cyclocarbons becomes smaller, and is expected to eventually vanish with increasing N, indicating non-aromaticity. Our experimental results show that aromaticity persists at N = 42, and theory predicts ring currents comparable in magnitude to that of benzene in cyclocarbons of this size. In the future, such large cyclocarbons could be used to study conductance, quantum interference, and the effects of aromaticity in single atomic carbon wires and circuits.

cond-mat.mes-hall↗

arXiv2025

A molecule with half-M\"obius topology

Stereoisomers of C$_{13}$Cl$_2$ exhibiting helical orbitals around a ring of carbon atoms were synthesized by atom manipulation on NaCl surfaces. We resolved the enantiomeric geometries of the singlet states by atomic force microscopy and mapped their helical orbital densities by scanning tunnelling microscopy. A ${\pi}$-orbital basis of the helical, non-planar singlets that twists by 90{\deg} in one circulation is consistent with a half-M\"obius topology. In such a topology, the ${\pi}$-orbital basis changes sign with respect to two circumnavigations and is periodic with respect to four circumnavigations. A quasiparticle on a ring with this boundary condition could be interpreted as carrying a Berry phase of ${\pi}$/2. We demonstrate reversible switching of the topology, between the two singlets of oppositely threaded half-M\"obius topology, and the planar, topologically trivial, triplet state. Multireference calculations, including large-scale sample-based ab initio calculations executed on quantum hardware, reveal that the switching is associated with a helical pseudo Jahn-Teller effect.

cond-mat.mes-hall↗

arXiv2024

Electron Phase Detection in Single Molecules by Interferometry

Interferometry has underpinned a century of discoveries, ranging from the disproval of the ether theory to the detection of gravitational waves, offering insights into wave dynamics with unrivalled precision through the measurement of phase relationships. In electronics, phase-sensitive measurements can probe the nature of transmissive topological and quantum states, but are only possible using complex device structures in magnetic fields. Here we demonstrate electronic interferometry in a single-molecule device through the study of non-equilibrium Fano resonances. We show the phase difference between an electronic orbital and a coupled Fabry-Perot resonance are tuneable through electric fields, and consequently it is possible to read out quantum information in the smallest devices, offering new avenues for the coherent manipulation down to single molecules.

physics.chem-ph↗

arXiv2023

Quantum Interference Enhances the Performance of Single-Molecule Transistors

An unresolved challenge facing electronics at a few-nm scale is that resistive channels start leaking due to quantum tunneling. This affects the performance of nanoscale transistors, with single-molecule devices displaying particularly low switching ratios and operating frequencies, combined with large subthreshold swings.1 The usual strategy to mitigate quantum effects has been to increase device complexity, but theory shows that if quantum effects are exploited correctly, they can simultaneously lower energy consumption and boost device performance.2-6 Here, we demonstrate experimentally how the performance of molecular transistors can be improved when the resistive channel contains two destructively-interfering waves. We use a zinc-porphyrin coupled to graphene electrodes in a three-terminal transistor device to demonstrate a >104 conductance-switching ratio, a subthreshold swing at the thermionic limit, a > 7 kHz operating frequency, and stability over >105 cycles. This performance is competitive with the best nanoelectronic transistors. We fully map the antiresonance interference features in conductance, reproduce the behaviour by density functional theory calculations, and trace back this high performance to the coupling between molecular orbitals and graphene edge states. These results demonstrate how the quantum nature of electron transmission at the nanoscale can enhance, rather than degrade, device performance, and highlight directions for future development of miniaturised electronics.

physics.chem-ph↗

arXiv2022

Phase-Coherent Charge Transport through a Porphyrin Nanoribbon

Quantum interference in nano-electronic devices could lead to reduced-energy computing and efficient thermoelectric energy harvesting. When devices are shrunk down to the molecular level it is still unclear to what extent electron transmission is phase coherent, as molecules usually act as scattering centres, without the possibility of showing particle-wave duality. Here we show electron transmission remains phase coherent in molecular porphyrin nanoribbons, synthesized with perfectly defined geometry, connected to graphene electrodes. The device acts as a graphene Fabry-Pérot interferometer, allowing direct probing of the transport mechanisms throughout several regimes, including the Kondo one. Electrostatic gating allows measurement of the molecular conductance in multiple molecular oxidation states, demonstrating a thousand-fold increase of the current by interference, and unravelling molecular and graphene transport pathways. These results demonstrate a platform for the use of interferometric effects in single-molecule junctions, opening up new avenues for studying quantum coherence in molecular electronic and spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Charge-state dependent vibrational relaxation in a single-molecule junction

The interplay between nuclear and electronic degrees of freedom strongly influences molecular charge transport. Herein, we report on transport through a porphyrin dimer molecule, weakly coupled to graphene electrodes, that displays sequential tunneling within the Coulomb-blockade regime. The sequential transport is initiated by current-induced phonon absorption and proceeds by rapid sequential transport via a non-equilibrium vibrational distribution. We demonstrate this is possible only when the vibrational dissipation is slow relative to sequential tunneling rates, and obtain a lower bound for the vibrational relaxation time of 8 ns, a value that is dependent on the molecular charge state.

cond-mat.mes-hall↗

arXiv2021

Single-electron transport in a molecular Hubbard dimer

Many-body electron interactions are at the heart of chemistry and solid-state physics. Understanding these interactions is crucial for the development of molecular-scale quantum and nanoelectronic devices. Here, we investigate single-electron tunneling through an edge-fused porphyrin oligomer and demonstrate that its transport behavior is well described by the Hubbard dimer model. This allows us to study the role of electron-electron interactions in the transport setting. In particular, we empirically determine the molecule's on-site and inter-site electron-electron repulsion energies, which are in good agreement with density functional calculations, and establish the molecular electronic structure within various charge states. The gate-dependent rectification behavior is used to further confirm the selection rules and state degeneracies resulting from the Hubbard model. We therefore demonstrate that current flow through the molecule is governed by a non-trivial set of vibrationally coupled electronic transitions between various many-body states, and experimentally confirm the importance of electron-electron interactions in single-molecule devices.

cond-mat.mes-hall↗

arXiv2020

A cyanine dye rotaxane porphyrin nanoring complex as a model light harvesting system

A nanoring-rotaxane supramolecular assembly, with a Cy7 cyanine dye (hexamethylindotricarbocyanine) threaded along the axis of the nanoring, has been synthesized as a model for the energy transfer between the light harvesting complex LH1 and the reaction center in purple bacteria photosynthesis. The complex displays efficient energy transfer from the central cyanine dye to the surrounding zinc porphyrin nanoring. We present a theoretical model that reproduces the absorption spectrum of the nanoring and quantifies the excitonic coupling between the nanoring and the central dye, explaining the efficient energy transfer and elucidating the similarity with structurally related natural light harvesting systems.

physics.chem-ph↗

arXiv2019

Understanding resonant charge transport through weakly coupled single-molecule junctions

Off-resonant charge transport through molecular junctions has been extensively studied since the advent of single-molecule electronics and it is now well understood within the framework of the non-interacting Landauer approach. Conversely, gaining a qualitative and quantitative understanding of the resonant transport regime has proven more elusive. Here, we study resonant charge transport through graphene-based zinc-porphyrin junctions. We experimentally demonstrate an inadequacy of the non-interacting Landauer theory as well as the conventional single-mode Franck-Condon model. Instead, we model the overall charge transport as a sequence of non-adiabatic electron transfers, the rates of which depend on both outer and inner-sphere vibrational interactions. We show that the transport properties of our molecular junctions are determined by a combination of electron-electron and electron-vibrational coupling, and are sensitive to the interactions with the wider local environment. Furthermore, we assess the importance of nuclear tunnelling and examine the suitability of semi-classical Marcus theory as a description of charge transport in molecular devices.

cond-mat.mes-hall↗

arXiv2019

An sp-hybridized molecular carbon allotrope, cyclo[18]carbon

Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized, due to their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu(111) at 5 Kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

physics.chem-ph↗

arXiv2015

Diversity of coherences and origin of electronic transitions of supermolecular nanoring

Quantum coherence is highly involved in photochemical functioning of complex molecular systems. Co-existence and intermixing of electronic and/or vibrational coherences, while never unambiguously identified experimentally, has been proposed to be responsible for this phenomenon. Analysis of multidimensional spectra of a synthetic belt-shaped molecular six-porphyrin nanoring with an inner template clearly shows a great diversity of separable electronic, vibrational and mixed coherences and their cooperation shaping the optical response. The results yield clear assignment of electronic and vibronic states, estimation of excitation transfer rates, and decoherence times. Theoretical considerations prove that the complexity of excitation dynamics and spectral features of the nanoring excitation spectrum is due to combined effect of cyclic symmetry, small geometrical deformations, and vibronic coupling.

physics.chem-ph↗

arXiv2012

Ultrafast entangling gates between nuclear spins using photo-excited triplet states

The representation of information within the spins of electrons and nuclei has been powerful in the ongoing development of quantum computers. Although nuclear spins are advantageous as quantum bits (qubits) due to their long coherence lifetimes (exceeding seconds), they exhibit very slow spin interactions and have weak polarisation. A coupled electron spin can be used to polarise the nuclear spin and create fast single-qubit gates, however, the permanent presence of electron spins is a source of nuclear decoherence. Here we show how a transient electron spin, arising from the optically excited triplet state of C60, can be used to hyperpolarise, manipulate and measure two nearby nuclear spins. Implementing a scheme which uses the spinor nature of the electron, we performed an entangling gate in hundreds of nanoseconds: five orders of magnitude faster than the liquid-state J coupling. This approach can be widely applied to systems comprising an electron spin coupled to multiple nuclear spins, such as NV centres, while the successful use of a transient electron spin motivates the design of new molecules able to exploit photo-excited triplet states.

quant-ph↗

arXiv2010

Entangling remote nuclear spins linked by a chromophore

Molecular nanostructures may constitute the fabric of future quantum technologies, if their degrees of freedom can be fully harnessed. Ideally one might use nuclear spins as low-decoherence qubits and optical excitations for fast controllable interactions. Here, we present a method for entangling two nuclear spins through their mutual coupling to a transient optically-excited electron spin, and investigate its feasibility through density functional theory and experiments on a test molecule. From our calculations we identify the specific molecular properties that permit high entangling power gates under simple optical and microwave pulses; synthesis of such molecules is possible with established techniques.

quant-ph↗