arXiv subjects

Johannes Flick

Publications and source records attributed to Johannes Flick.

At least 37 records · Page 2Linked to original sources

arXiv2023

Simulating polaritonic ground states on noisy quantum devices

The recent advent of quantum algorithms for noisy quantum devices offers a new route toward simulating strong light-matter interactions of molecules in optical cavities for polaritonic chemistry. In this work, we introduce a general framework for simulating electron-photon coupled systems on small, noisy quantum devices. This method is based on the variational quantum eigensolver (VQE) with the polaritonic unitary coupled cluster (PUCC) ansatz. To achieve chemical accuracy, we exploit various symmetries in qubit reduction methods, such as electron-photon parity, and use recently developed error mitigation schemes, such as the reference zero-noise extrapolation method. We explore the robustness of the VQE-PUCC approach across a diverse set of regimes for the bond length, cavity frequency, and coupling strength of the H$_2$ molecule in an optical cavity. To quantify the performance, we measure two properties: ground-state energy, fundamentally relevant to chemical reactivity, and photon number, an experimentally accessible general indicator of electron-photon correlation.

quant-ph↗

arXiv2023

Detection and modeling of hole capture by single point defects under variable electric fields

Understanding carrier trapping in solids has proven key to semiconductor technologies but observations thus far have relied on ensembles of point defects, where the impact of neighboring traps or carrier screening is often important. Here, we investigate the capture of photo-generated holes by an individual negatively-charged nitrogen-vacancy (NV) center in diamond at room temperature. Using an externally gated potential to minimize space-charge effects, we find the capture probability under electric fields of variable sign and amplitude shows an asymmetric-bell-shaped response with maximum at zero voltage. To interpret these observations, we run semi-classical Monte Carlo simulations modeling carrier trapping through a cascade process of phonon emission, and obtain electric-field-dependent capture probabilities in good agreement with experiment. Since the mechanisms at play are insensitive to the trap characteristics, the capture cross sections we observe - largely exceeding those derived from ensemble measurements - should also be present in materials platforms other than diamond.

cond-mat.mes-hall↗

arXiv2022

Chemical reactivity under collective vibrational strong coupling

Recent experiments of chemical reactions in optical cavities have shown great promise to alter and steer chemical reactions but still remain poorly understood theoretically. In particular the origin of resonant effects between the cavity and certain vibrational modes in the collective limit is still subject to active research. In this paper, we study unimolecular dissociation reactions of many molecules collectively interacting with an infrared cavity mode through their vibrational dipole moment. We find that the reaction rate can slow down by increasing the number of aligned molecules if the cavity mode is resonant with a vibrational frequency of the molecules. We also discover a simple scaling relation that scales with the collective Rabi splitting to estimate the onset of reaction rate modification by collective vibrational strong coupling and numerically demonstrate these effects for up to 10,000 molecules.

physics.chem-ph↗

arXiv2022

Shining Light on the Microscopic Resonant Mechanism Responsible for Cavity-Mediated Chemical Reactivity

Strong light-matter interaction in cavity environments is emerging as a promising approach to control chemical reactions in a non-intrusive and efficient manner. The underlying mechanism that distinguishes between steering, accelerating, or decelerating a chemical reaction has, however, remained unclear, hampering progress in this frontier area of research. We leverage quantum-electrodynamical density-functional theory to unveil the microscopic mechanism behind the experimentally observed reduced reaction rate under cavity induced resonant vibrational strong light-matter coupling. We observe multiple resonances and obtain the thus far theoretically elusive but experimentally critical resonant feature for a single strongly-coupled molecule undergoing the reaction. While we do not explicitly account for collective coupling or intermolecular interactions, the qualitative agreement with experimental measurements suggests that our conclusions can be largely abstracted towards the experimental realization. Specifically, we find that the cavity mode acts as mediator between different vibrational modes. In effect, vibrational energy localized in single bonds that are critical for the reaction is redistributed differently which ultimately inhibits the reaction.

quant-ph↗

arXiv2022

Error rate reduction of single-qubit gates via noise-aware decomposition into native gates

In the current era of Noisy Intermediate-Scale Quantum (NISQ) technology, the practical use of quantum computers remains inhibited by our inability to aptly decouple qubits from their environment to mitigate computational errors. In this work, we introduce an approach by which knowledge of a qubit's initial quantum state and the standard parameters describing its decoherence can be leveraged to mitigate the noise present during the execution of a single-qubit gate. We benchmark our protocol using cloud-based access to IBM quantum processors. On ibmq_rome, we demonstrate a reduction of the single-qubit error rate by $38\%$, from $1.6 \times 10 ^{-3}$ to $1.0 \times 10 ^{-3}$, provided the initial state of the input qubit is known. On ibmq_bogota, we prove that our protocol will never decrease gate fidelity, provided the system's $T_1$ and $T_2$ times have not drifted above $100$ times their assumed values. The protocol can be used to reduce quantum state preparation errors, as well as to improve the fidelity of quantum circuits for which some knowledge of the qubits' intermediate states can be inferred. This work presents a pathway to using information about noise levels and quantum state distributions to significantly reduce error rates associated with quantum gates via optimized decomposition into native gates.

quant-ph↗

arXiv2022

Ab-initio investigation of Er3+ defects in tungsten disulfide

We use density functional theory (DFT) to explore the physical properties of an $Er_{ W}$ point defect in monolayer $WS_{ 2}$. Our calculations indicate that electrons localize at the dangling bonds associated with a tungsten vacancy ($V_{W}$) and at the $Er^{ 3+}$ ion site, even in the presence of a net negative charge in the supercell. The system features a set of intra-gap defect states, some of which are reminiscent of those present in isolated $Er^{ 3+}$ ions. In both instances, the level of hybridization is low, i.e., orbitals show either strong Er or W character. Through the calculation of the absorption spectrum as a function of wavelength, we identify a broad set of transitions, including one possibly consistent with the $Er^{ 3+}$ $4I_{ 15/2} \rightarrow 4I_{ 13/2}$ observed in other hosts. Combined with the low native concentration of spin-active nuclei as well as the two-dimensional nature of the host, these properties reveal $Er:WS_{ 2}$ as a potential platform for realizing spin qubits that can be subsequently integrated with other nanoscale optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Frequency-dependent Sternheimer linear-response formalism for strongly coupled light-matter systems

The rapid progress in quantum-optical experiments especially in the field of cavity quantum electrodynamics and nanoplasmonics, allows to substantially modify and control chemical and physical properties of atoms, molecules and solids by strongly coupling to the quantized field. Alongside such experimental advances has been the recent development of ab-initio approaches such as quantum electrodynamical density-functional theory (QEDFT) that is capable of describing these strongly coupled systems from first-principles. To investigate response properties of relatively large systems coupled to a wide range of photon modes, ab-initio methods that scale well with system size become relevant. In light of this, we extend the linear-response Sternheimer approach within the framework of QEDFT to efficiently compute excited-state properties of strongly coupled light-matter systems. Using this method, we capture features of strong light-matter coupling both in the dispersion and absorption properties of a molecular system strongly coupled to the modes of a cavity. We exemplify the efficiency of the Sternheimer approach by coupling the matter system to the continuum of an electromagnetic field. We observe changes in the spectral features of the coupled system as Lorentzian line shapes turn into Fano resonances when the molecule interacts strongly with the continuum of modes. This work provides an alternative approach for computing efficiently excited-state properties of large molecular systems interacting with the quantized electromagnetic field.

physics.chem-ph↗

arXiv2021

Cavity-Modulated Proton Transfer Reactions

Proton transfer is ubiquitous in many fundamental chemical and biological processes, and the ability to modulate and control the proton transfer rate would have a major impact on numerous quantum technological advances. One possibility to modulate the reaction rate of proton transfer processes is given by exploiting the strong light-matter coupling of chemical systems inside optical or nanoplasmonic cavities. In this work, we investigate the proton transfer reactions in the prototype malonaldehyde and Z-3-amino-propenal (aminopropenal) molecules using different quantum electrodynamics methods, in particular quantum electrodynamics coupled cluster theory (QED-CC) and quantum electrodynamical density functional theory (QEDFT). Depending on the cavity mode polarization direction, we show that the optical cavity can increase the reaction energy barrier by 10--20$\%$ or decrease the reaction barrier by $\sim$5$\%$. By using first principles methods, this work establishes strong light-matter coupling as a viable and practical route to alter and catalyze proton transfer reactions.

physics.chem-ph↗

arXiv2021

Cavity-modified unimolecular dissociation reactions via intramolecular vibrational energy redistribution

While the emerging field of vibrational polariton chemistry has the potential to overcome traditional limitations of synthetic chemistry, the underlying mechanism is not yet well understood. Here, we explore how the dynamics of unimolecular dissociation reactions that are rate-limited by intramolecular vibrational energy redistribution (IVR) can be modified inside an infrared optical cavity. We study a classical model of a bent triatomic molecule, where the two outer atoms are bound by anharmonic Morse potentials to the center atom coupled to a harmonic bending mode. We show that an optical cavity resonantly coupled to particular anharmonic vibrational modes can interfere with IVR and alter unimolecular dissociation reaction rates when the cavity mode acts as a reservoir for vibrational energy. We find a strong dependence on the initial state of the cavity and molecule. In particular, when the cavity is initially empty, the dissociation rate decreases, while when the cavity is initially hotter than the molecule, the cavity can instead accelerate the reaction rate. These results lay the foundation for further theoretical work toward understanding the intriguing experimental results of vibrational polaritonic chemistry within the context of IVR.

physics.chem-ph↗

arXiv2021

Optical activation and detection of charge transport between individual color centers in room-temperature diamond

Charge control of color centers in semiconductors promises opportunities for novel forms of sensing and quantum information processing. Here, we articulate confocal fluorescence microscopy and magnetic resonance protocols to induce and probe charge transport between discrete sets of engineered nitrogen-vacancy (NV) centers in diamond, down to the level of individual defects. In our experiments, a "source" NV undergoes optically-driven cycles of ionization and recombination to produce a stream of photo-generated carriers, one of which we subsequently capture via a "target" NV several micrometers away. We use a spin-to-charge conversion scheme to encode the spin state of the source color center into the charge state of the target, in the process allowing us to set an upper bound to carrier injection from other background defects. We attribute our observations to the action of unscreened Coulomb potentials producing giant carrier capture cross-sections, orders of magnitude greater than those typically attained in ensemble measurements. Besides their fundamental interest, these results open intriguing prospects in the use of free carriers as a quantum bus to mediate effective interactions between paramagnetic defects in a solid-state chip.

cond-mat.mes-hall↗

arXiv2021

Investigation of photon emitters in Ce-implanted hexagonal boron nitride

Color centers in hexagonal boron nitride (hBN) are presently attracting broad interest as a novel platform for nanoscale sensing and quantum information processing. Unfortunately, their atomic structures remain largely elusive and only a small percentage of the emitters studied thus far has the properties required to serve as optically addressable spin qubits. Here, we use confocal fluorescence microscopy at variable temperature to study a new class of point defects produced via cerium ion implantation in thin hBN flakes. We find that, to a significant fraction, emitters show bright room-temperature emission, and good optical stability suggesting the formation of Ce-based point defects. Using density functional theory (DFT) we calculate the emission properties of candidate emitters, and single out the CeVB center - formed by an interlayer Ce atom adjacent to a boron vacancy - as one possible microscopic model. Our results suggest an intriguing route to defect engineering that simultaneously exploits the singular properties of rare-earth ions and the versatility of two-dimensional material hosts.

cond-mat.mes-hall↗

arXiv2021

Ab initio linear-response approach to vibro-polaritons in the cavity Born-Oppenheimer approximation

Recent years have seen significant developments in the study of strong light-matter coupling including the control of chemical reactions by altering the vibrational normal modes of molecules. In the vibrational strong coupling regime the normal modes of the system become hybrid modes which mix nuclear, electronic, and photonic degrees of freedom. First principles methods capable of treating light and matter degrees of freedom on the same level of theory are an important tool in understanding such systems. In this work, we develop and apply a generalized force constant matrix approach to the study of mixed vibration-photon (vibro-polariton) states of molecules based on the cavity Born-Oppenheimer approximation and quantum-electrodynamical density-functional theory. With this method vibro-polariton modes and infrared spectra can be computed via linear response techniques analogous to those widely used for conventional vibrations and phonons. We also develop an accurate model that highlights the consistent treatment of cavity coupled electrons in the vibrational strong coupling regime. These electronic effects appear as new terms previously disregarded by simpler models. This effective model also allows for an accurate extrapolation of single and two molecule calculations to the collective strong coupling limit of hundreds of molecules. We benchmark these approaches for single and many CO$_2$ molecules coupled to a single photon mode and the iron-pentacarbonyl Fe(CO)$_5$ molecule coupled to a few photon modes. Our results are the first ab-initio results for collective vibrational strong coupling effects. This framework for efficient computations of vibro-polaritons paves the way to a systematic description and improved understanding of the behavior of chemical systems in vibrational strong coupling.

quant-ph↗

arXiv2021

Machine learning band gaps from the electron density

A remarkable consequence of the Hohenberg-Kohn theorem of density functional theory is the existence of an injective map between the electronic density and any observable of the many electron problem in an external potential. In this work, we study the problem of predicting a particular observable, the band gap of semiconductors and band insulators, from the knowledge of the local electronic density. Using state-of-the-art machine learning techniques, we predict the experimental band gaps from computationally inexpensive density functional theory calculations. We propose a modified Behler-Parrinello (BP) architecture that greatly improves the model capacity while maintaining the symmetry properties of the BP architecture. Using this scheme, we obtain band gaps at a level of accuracy comparable to those obtained with state of the art and computationally intensive hybrid functionals, thus significantly reducing the computational cost of the task.

cond-mat.dis-nn↗

arXiv2021

Polaritonic Unitary Coupled Cluster for Quantum Computations

In the field of polaritonic chemistry, strong light-matter interactions are used to alter a chemical reaction inside an optical cavity. To explain and understand these processes, the development of reliable theoretical models is essential. While traditional methods have to balance accuracy and system size, new developments in quantum computing, in particular the Variational Quantum Eigensolver (VQE), offer a path for an accurate solution of the electronic Schrödinger equation with the promise of polynomial scaling and eventual quasi-exact solutions on currently available quantum devices. In this work, we combine these two fields. In particular, we introduce the quantum electrodynamics unitary coupled cluster (QED-UCC) method combined with the VQE algorithm, as well as the quantum electrodynamics equation-of-motion (QED-EOM) method formulated in the qubit basis that allows an accurate calculation of the ground-state and the excited-state properties of strongly coupled light-matter systems on a quantum computer. The accuracy and performance of the developed methods is tested for a H$_4$ molecule inside an optical cavity in a regime where strong electronic correlations become significant. For the first time, we explicitly include two photon effects from first principles. We show that the developed methods are in excellent agreement with the exact reference results and can outperform their traditional counterparts. The work presented here sets the stage for future developments of polaritonic quantum chemistry methods suitable for both classical and quantum computers.

physics.chem-ph↗

arXiv2021

Defect polaritons from first principles

Precise control over the electronic and optical properties of defect centers in solid-state materials is necessary for their applications as quantum sensors, transducers, memories, and emitters. In this study, we demonstrate, from first principles, how to tune these properties via the formation of defect polaritons. Specifically, we investigate three defect types -- CHB, CB-CB, and CB-VN -- in monolayer hexagonal boron nitride (hBN). The lowest-lying electronic excitation of these systems is coupled to an optical cavity where we explore the strong light-matter coupling regime. For all defect systems, we show that the polaritonic splitting that shifts the absorption energy of the lower polariton is much higher than can be expected from a Jaynes-Cummings interaction. In addition, we find that the absorption intensity of the lower polariton increases by several orders of magnitude, suggesting a possible route toward overcoming phonon-limited single photon emission from defect centers. Finally, we find that initially localized electronic transition densities can become delocalized across the entire material under strong light-matter coupling. These findings are a result of an effective continuum of electronic transitions near the lowest-lying electronic transition for both pristine hBN and hBN with defect centers that dramatically enhances the strength of the light-matter interaction. We expect our findings to spur experimental investigations of strong light-matter coupling between defect centers and cavity photons for applications in quantum technologies.

quant-ph↗

arXiv2021

Simple Exchange-Correlation Energy Functionals for Strongly Coupled Light-Matter Systems based on the Fluctuation-Dissipation Theorem

Recent experimental advances in strongly coupled light-matter systems has sparked the development of general ab-initio methods capable of describing interacting light-matter systems from first principles. One of these methods, quantum-electrodynamical density-functional theory (QEDFT), promises computationally efficient calculations for large correlated light-matter systems with the quality of the calculation depending on the underlying approximation for the exchange-correlation functional. So far no true density-functional approximation has been introduced limiting the efficient application of the theory. In this paper, we introduce the first gradient-based density functional for the QEDFT exchange-correlation energy derived from the adiabatic-connection fluctuation-dissipation theorem. We benchmark this simple-to-implement approximation on small systems in optical cavities and demonstrate its relatively low computational costs for fullerene molecules up to C$_{180}$ coupled to 400,000 photon modes in a dissipative optical cavity. This work now makes first principle calculations of much larger systems possible within the QEDFT framework effectively combining quantum optics with large-scale electronic structure theory.

physics.chem-ph↗

arXiv2020

Hybridized defects in solid-state materials as artificial molecules

Two-dimensional materials can be crafted with structural precision approaching the atomic scale, enabling quantum defects-by-design. These defects are frequently described as artificial atoms and are emerging optically-addressable spin qubits. However, interactions and coupling of such artificial atoms with each other, in the presence of the lattice, is remarkably underexplored. Here we present the formation of artificial molecules in solids, introducing a new degree of freedom in control of quantum optoelectronic materials. Specifically, in monolayer hexagonal boron nitride as our model system, we observe configuration- and distance-dependent dissociation curves and hybridization of defect orbitals within the bandgap into bonding and antibonding orbitals, with splitting energies ranging from $\sim$ 10 meV to nearly 1 eV. We calculate the energetics of $cis$ and $trans$ out-of-plane defect pairs CH$_\textrm{B}$-CH$_\textrm{B}$ against an in-plane defect pair C$_\textrm{B}$-C$_\textrm{B}$ and find that in-plane defect pair interacts more strongly than out-of-plane pairs. We demonstrate an application of this chemical degree of freedom by varying the distance between C$_\textrm{B}$ and V$_\textrm{N}$ of C$_\textrm{B}$V$_\textrm{N}$ and observe changes in the predicted peak absorption wavelength from the visible to the near-infrared spectral band. We envision leveraging this chemical degree of freedom of defect complexes to precisely control and tune defect properties towards engineering robust quantum memories and quantum emitters for quantum information science.

quant-ph↗

arXiv2020

Weak-to-Strong Light-Matter Coupling and Dissipative Dynamics from First Principles

Cavity-mediated light-matter coupling can dramatically alter opto-electronic and physico-chemical properties of a molecule. Ab initio theoretical predictions of these systems need to combine non-perturbative, many-body electronic structure theory-based methods with cavity quantum electrodynamics and theories of open quantum systems. Here we generalize quantum-electrodynamical density functional theory to account for dissipative dynamics and describe coupled cavity-molecule interactions in the weak-to-strong-coupling regimes. Specifically, to establish this generalized technique, we study excited-state dynamics and spectral responses of benzene and toluene under weak-to-strong light-matter coupling. By tuning the coupling we achieve cavity-mediated energy transfer between electronic excited states. This generalized ab initio quantum-electrodynamical density functional theory treatment can be naturally extended to describe cavity-mediated interactions in arbitrary electromagnetic environments, accessing correlated light-matter observables and thereby closing the gap between electronic structure theory and quantum optics.

quant-ph↗