Quantum Fluctuations Research Articles

Anomalous suppression of large-scale density fluctuations in classical and quantum spin liquids

Classical spin liquids (CSLs) are intriguing states of matter that do not exhibit long-range magnetic order and are characterized by an extensive ground-state degeneracy. Adding quantum fluctuations, which induce dynamics between these different classical ground states, can give rise to quantum spin liquids (QSLs). QSLs are highly entangled quantum phases of matter characterized by fascinating emergent properties, such as fractionalized excitations and topological order. One such exotic quantum liquid is the [Formula: see text] QSL, which can be regarded as a resonating valence bond (RVB) state formed from superpositions of dimer coverings of an underlying lattice. In this work, we unveil a hidden large-scale structural property of archetypal CSLs and QSLs known as hyperuniformity, i.e., normalized infinite-wavelength density fluctuations are completely suppressed in these systems. In particular, we first demonstrate that classical ensembles of close-packed dimers and their corresponding quantum RVB states are perfectly hyperuniform in general. Subsequently, we focus on a ruby-lattice spin liquid that was recently realized in a Rydberg-atom quantum simulator, and show that the QSL remains effectively hyperuniform even in the presence of a finite density of spinon and vison excitations, as long as the dimer constraint is still largely preserved. Moreover, we demonstrate that metrics based on the framework of hyperuniformity can be used to distinguish the QSL from other proximate quantum phases. These metrics can help identify potential QSL candidates, which can then be further analyzed using more advanced, computationally intensive quantum numerics to confirm their status as true QSLs.

Transparency, nonclassicality, and nonreciprocity in chiral waveguide quantum electrodynamics

We examine quantum statistical properties of transmission and reflection from a chiral waveguide coupled to qubits for arbitrary input powers. We report on several remarkable features of output fields such as transparency, quantum nonreciprocity, and the second-order correlation function g(2)(0) values less than unity. In particular, for two qubits detuned antisymmetrically with respect to the central waveguide frequency, we find transparency in forward transmission and in photon numbers for arbitrary values of the input powers provided the phase separation between qubits is an integer multiple of π. Values of g(2)(0) less than unity can be reached even for nonzero value of the intrinsic damping by using phase separation different from integer multiple of π, marking the transition from classical to quantum light. We also uncover a different type of critical coupling regime for qubits that enables complete suppression of forward-propagating amplitude transmission at specific driving powers, giving rise to enhanced nonreciprocal effects in both transmission and quantum fluctuations in amplitudes. Forward propagation amplifies the quantum fluctuations in amplitudes, while backward propagation significantly suppresses them. These findings open pathways for controlling light-matter interactions in chiral quantum electrodynamics, with potential applications in quantum information and nonreciprocal quantum devices. Published by the American Physical Society 2025

Probing the magnetic ground state of stretched diamond lattices NdTaO4 and NdNbO4: impact of spin–orbit coupling and crystal electric field

Magnetic systems, wherein competing degree of freedoms arising from spin orbit coupling and crystal electric field lead to non-trivial magnetic ground states, remains in the forefront of research in condensed matter physics. Here, we present a comprehensive investigation on three-dimensional rare-earth based spin systems NdTaO4and NdNbO4, where the Nd ions sit on a stretched diamond lattice. No signatures of long-range ordering and spin freezing are observed down to 1.8 K, in both cases. The low temperature Curie-Weiss analysis indicate towards the dominance of antiferromagnetic interactions between Nd spins. A three-level CEF model clearly explain the nature of susceptibility curve. At low temperatures, heat capacity data exhibit two-level Schottky anomaly associated with ground state Kramer's doublet. Additionally, the low temperature magnetic behaviour is found reliable to effective spin (Jeff) = ½ ground state, suggesting the presence of quantum fluctuations in both cases. First-principle calculations reveal a significant value of orbital moment with inclusion of spin orbit coupling and reinforce theJeff= ½ nature of the ground state.

Weighing the vacuum with the Archimedes experiment

Exploring the complex connection between vacuum fluctuations and gravitational field is the goal of Archimedes experiment. Tiny changes in the weight of the vacuum contained in high-temperature superconductors can be measured using a high-precision balance. These structures can absorb or expel vacuum energy depending on their normal or superconductive status. In the measurement, the state of the sample is modulated into two states, normal and superconducting, with a reversible transition achieved by periodically varying the temperature of the thermal bath. The paper provides the current status of the experiment, highlighting the study and construction of the balance with its thermal modulation and cryogenic systems, and the investigation of high-temperature superconductor candidates.

Dispersion effects in thermal emission from temporal metamaterials: high-frequency cutoffs

The latest breakthroughs in time-varying photonics are fueling novel, to the best of our knowledge, thermal emission phenomena, e.g., showing that the dynamic amplification of quantum vacuum fluctuations, induced by the time modulation of material properties, enables a mechanism to surpass the blackbody spectrum. So far, this issue has only been investigated under the assumption of non-dispersive time modulations. In this work, we identify the existence of a non-physical diverging behavior in the time-modulated emission spectra at high frequencies and prove that it is actually attributed to the simplistic assumption of a non-dispersive (temporally local) response of the time modulation associated with memory-less systems. Accordingly, we upgrade the theoretical formalism by introducing a dispersive response function, showing that it leads to a high-frequency cutoff, thereby eliminating the divergence and hence allowing for the proper computation of the emission spectra of time-modulated materials.

Light's hidden power: Vacuum fluctuations reshape superconductivity from within.

Light's hidden power: Vacuum fluctuations reshape superconductivity from within.

Manipulating intermolecular interactions with the coupling between quantum fluctuations and the vacuum field

Manipulating intermolecular interactions with the coupling between quantum fluctuations and the vacuum field

Particle production and density fluctuations of nonclassical inflaton in coherent squeezed vacuum state of flat FRW universe

In this paper, we study nonclassical inflaton, which is minimally coupled to the semi-classical gravity in FRW universe in Coherent Squeezed Vacuum State (CSVS). We have determined the Oscillatory phase of inflaton, power-law expansion, scale factor, density fluctuations, quantum fluctuations and particle production for CSVS. We have obtained an estimated leading solution of scale factor in CSVS and found it proportional to [Formula: see text] and showing similar diversification as demonstrated by Semi-classical Einstein Equation (SCEE) of gravity in matter dominated universe. We have also studied the validity of SCEE in CSVS. We have computed validity of uncertainty relation for FRW Universe by determining the quantum fluctuation for CSVS. Our results show that Quantum fluctuations don’t depend on coherent parameter [Formula: see text] as the uncertainty relation doesn’t effect by the displacement of [Formula: see text] in phase space. We have also studied the production of particles in CSVS for oscillating massive inflaton in flat FRW universe.

Signatures of longitudinal spin pumping in a magnetic phase transition.

A particle current generated by pumping in the absence of gradients in potential energy, density or temperature1 is associated with non-trivial dynamics. A representative example is charge pumping that is associated with the quantum Hall effect2 and the quantum anomalous Hall effect3. Spin pumping, the spin equivalent of charge pumping, refers to the emission of a spin current by magnetization dynamics4-7. Previous studies have focused solely on transversal spin pumping arising from classical dynamics, which corresponds to precessing atomic moments with constant magnitude. However, longitudinal spin pumping arising from quantum fluctuations, which correspond to a temporal change in the atomic moment's magnitude, remains unexplored. Here we experimentally investigate longitudinal spin pumping using iron-rhodium (FeRh), which undergoes a first-order antiferromagnet-to-ferromagnet phase transition during which the atomic moment's magnitude varies over time. By injecting a charge current into a FeRh/platinum bilayer, we induce a rapid phase transition of FeRh in nanoseconds, leading to the emission of a spin current to the platinum layer. The observed inverse spin Hall signal is about one order of magnitude larger than expected for transversal spin pumping, suggesting the presence of longitudinal spin pumping driven by quantum fluctuations and indicating its superiority over classical transversal spin pumping. Our result highlights the significance of quantum fluctuations in spin pumping and holds broad applicability in diverse angular momentum dynamics, such as laser-induced ultrafast demagnetization8, orbital pumping9,10 and quantum spin transfer11-13.

Swampland bounds on magnetized extra dimensions

We consider a six-dimensional U(1) gauge theory compactified on two-dimensional manifolds. The number of chiral fermions is determined by the flux quantization number on the two-dimensional compact manifolds. Using the Swampland Conjectures, we find constraints among the parameters of the theory: the flux quantization number, the compactification scale, and the string scale. Specifically, the Weak Gravity Conjecture and the Trans-Planckian Censorship Conjecture give non-trivial bounds.

Percolation renormalization group analysis of confinement in Z2 lattice gauge theories

The analytical study of confinement in lattice gauge theories (LGTs) remains a difficult task to this day. Taking a geometric perspective on confinement, we develop a real-space renormalization group (RG) formalism for Z2 LGTs using percolation probability as a confinement order parameter. The RG flow we analyze is constituted by both the percolation probability and the coupling parameters. We consider a classical Z2 LGT in two dimensions, with matter and thermal fluctuations, and analytically derive the confinement phase diagram. We find good agreement with numerical and exact benchmark results and confirm that a finite matter density enforces confinement at T<∞ in the model we consider. Our RG scheme enables future analytical studies of Z2 LGTs with matter and quantum fluctuations and beyond. Published by the American Physical Society 2025

Superradiant phase transition in a large interacting driven atomic ensemble in free space

Atoms strongly interacting with light constitute rich quantum-optical systems with the potential for observing cooperative effects and dissipative nonequilibrium phase transitions. We analyze the conditions under which a driven atomic array, characterized by strong dipole–dipole interactions and a large spatial extent, can undergo a superradiant phase transition, also known as cooperative resonance fluorescence. We find that the array can exhibit completely cooperative decay that conserves the collective pseudospin, resulting in a second-order quantum phase transition, with a key hallmark being an abrupt shift from total light reflection by the atoms to rapidly increasing transmission and significant quantum fluctuations. We compare the results with decay mechanisms that fail to conserve pseudospin, leading to a discontinuous first-order phase transition at a critical finite atom number.

Gate- and flux-tunable sin(2φ) Josephson element with planar-Ge junctions

Hybrid superconductor-semiconductor Josephson field-effect transistors (JoFETs) function as Josephson junctions with gate-tunable critical current. Additionally, they can feature a non-sinusoidal current-phase relation (CPR) containing multiple harmonics of the superconducting phase difference, a so-far underutilized property. Here we exploit this multi-harmonicity to create a Josephson circuit element with an almost perfectly π-periodic CPR, indicative of a largely dominant charge-4e supercurrent transport. We realize such a Josephson element, recently proposed as building block of a protected superconducting qubit, using a superconducting quantum interference device (SQUID) with low-inductance aluminum arms and two nominally identical JoFETs. The latter are fabricated from a SiGe/Ge/SiGe quantum-well heterostructure embedding a high-mobility two-dimensional hole gas. By carefully adjusting the JoFET gate voltages and finely tuning the magnetic flux through the SQUID close to half a flux quantum, we achieve a regime where the sin(2φ) component accounts for more than 95% of the total supercurrent. This result demonstrates a new promising route towards parity-protected superconducting qubits.

Quantum Brownian motion induced by a scalar field in Einstein’s universe under Dirichlet and Neumann boundary conditions

In this paper, the quantum Brownian motion of a point particle induced by the quantum vacuum fluctuations of a real massless scalar field in Einstein’s universe under Dirichlet and Neumann boundary conditions is studied. Using the Wightman functions, general expressions for the renormalized dispersion of the physical momentum are derived. Distinct expressions are found for the dispersion associated with each component of the particle’s physical momentum, indicating that the global properties of homogeneity and isotropy of space are lost, as a consequence of the introduced boundary conditions. Divergences also arise and are related to the compact nature of Einstein’s universe and the introduced boundary conditions.

Scalable superconductor neuron with ternary synaptic connections for ultra-fast SNN hardware

Abstract A novel high-fan-in differential superconductor neuron structure designed for ultra-high-performance Spiking Neural Network (SNN) accelerators is presented. Utilizing a high-fan-in neuron structure allows us to design SNN accelerators with more synaptic connections, enhancing the overall network capabilities. The proposed neuron design is based on superconductor electronics fabric, incorporating multiple superconducting loops, each with two Josephson Junctions. This arrangement enables each input data branch to have positive and negative inductive coupling, supporting excitatory and inhibitory synaptic data. Compatibility with synaptic devices and thresholding operation is achieved using a single flux quantum (SFQ) pulse-based logic style. The neuron design, along with ternary synaptic connections, forms the foundation for a superconductor-based SNN inference. To demonstrate the capabilities of our design, we train the SNN using snnTorch, augmenting the PyTorch framework. After pruning, the demonstrated SNN inference achieves an impressive 96.1% accuracy on MNIST images. Notably, the network exhibits a remarkable throughput of 8.92 GHz while consuming only 1.5 nJ per inference, including the energy consumption associated with cooling to 4K. These results underscore the potential of superconductor electronics in developing high-performance and ultra-energy-efficient neural network accelerator architectures.

Testing the lepton content of the proton at HERA and EIC

Although protons are baryons with an overall vanishing lepton number, they possess a non-trivial leptonic content arising from quantum fluctuations which can be described by lepton parton distribution functions (PDFs) of the proton. These PDFs have been recently computed and can be used to define lepton-induced processes at high-energy colliders. In this article, we propose a novel way to test the computation of lepton PDFs of the proton by analyzing both non-resonant di-lepton and resonant Z gauge boson production processes induced by leptons within the proton at proton-electron colliders like HERA and EIC. Despite the fact that lepton PDFs of the proton are known to be small, this work demonstrates that both processes imply a measurable yield of events at HERA and EIC, which could be used to test these PDFs.

Distribution of flux states in a hysteretic dc SQUID

Abstract This work investigates the distribution of flux states of hysteretic dc superconducting quantum interference devices (SQUIDs) obtained from standard current ramp measurement at 2.50 K and 0.35 K. Each flux state corresponds to a specific number of flux quanta trapped in the superconducting loop, associated with a metastable state in the two-dimensional potential energy of the hysteretic SQUID. The flux states can be distinguished by the multiple switching currents. The observed distribution is different from the distribution caused only by thermal excitation at a specific temperature. It shows good agreement with a quasi-Boltzmann distribution with multiple different inverse temperatures, dependent on the bottom energy of the potential wells and featuring a short equilibrium time. The standard Boltzmann distribution is insufficient to accurately describe the observed results. The finding suggests that the observed distribution of flux states is established during the SQUID’s transition from the dissipative state to the zero-voltage state, and multiple equivalent temperatures may be involved in the process.

Non-adiabatic quantum electrodynamic effects on electron-nucleus-photon systems: Single photonic mode vs infinite photonic modes.

The quantum-electrodynamic non-adiabatic emission (QED-NAE) is a type of radiatively assisted vibronic de-excitation due to electromagnetic vacuum fluctuations on non-adiabatic processes. Building on our previous work [Tsai et al., J. Phys. Chem. Lett. 14, 5924 (2023)], we extend the theory of the QED-NAE rate from a single cavity photonic mode to infinite photonic modes and calculate the QED-NAE rates of 9-cyanoanthracene at the first-principles level. To avoid the confusion, the quantum electrodynamic internal conversion process is renamed as "QED-NAE" in our present work. According to our theory, we identify three key factors influencing the QED-NAE processes: light-matter coupling strength (mode volume), mass-weighted orientation factor, and photonic density of states. The mode volume is the primary factor causing rate differences between the two scenarios. In a single cavity with a small mode volume, strong light-matter coupling strength boosts QED-NAE rates. In contrast, in free space with infinite photonic modes, weak coupling strength significantly reduces these rates. From a single cavity photonic mode to infinite photonic modes, the mass-weighted orientation factor only causes an 8π/3-fold increase in the QED-NAE rate. In free space, the photonic density of state exhibits a flat and quadratic distribution, which slightly reduces the QED-NAE rate. Our study shows that cavities can significantly enhance non-adiabatic QED effects while providing a robust analysis demonstrating that QED vibronic effects can be safely ignored in free space.

Superconducting Quantum Oscillations and Anomalous Negative Magnetoresistance in a Honeycomb Nanopatterned Oxide Interface Superconductor

The extremely low superfluid density and unprecedented tunability of oxide interface superconductors provide an ideal platform for studying fluctuations in two-dimensional superconductors. In this work, we fabricate an LaAlO3/KTaO3 interface superconductor patterned with a nanohoneycomb array of insulating islands. Little-Parks-like magnetoresistance oscillations are observed, which are dictated by the superconducting flux quantum h/2e. Moreover, an anomalous negative magnetoresistance (ANMR) appears under a weak magnetic field, suggesting magnetic-field-enhanced superconductivity. By examining their dependences on temperature, measurement current, and electrical gating, we conclude that both phenomena are associated with superconducting order parameter: The h/2e oscillations provide direct evidence of Cooper-pair transport, and the ANMR is interpreted as a consequence of multiple connected narrow superconducting paths with strong fluctuations. Published by the American Physical Society 2025

Full Quantum Dynamics Study for H Atom Scattering from Graphene.

This study deals with the understanding of hydrogen atom scattering from graphene, a process critical for exploring C-H bond formation and energy transfer during atom surface collision. In our previous work [Shi, L.; J. Chem. Phys. 2023, 159, 194102], starting from a cell with 24 carbon atoms treated periodically, we have achieved quantum dynamics (QD) simulations with a reduced-dimensional model (15D) and a simulation in full dimensionality (75D). In the former work, the H atom attacked the top of a single C atom, enabling a comparison of QD simulation results to classical molecular dynamics (cMD). Our approach required the use of sophisticated techniques such as Monte Carlo canonical polyadic decomposition (MCCPD) and multilayer multiconfiguration time-dependent Hartree (ML-MCTDH), as well as further development of quantum flux calculations. We could benchmark our calculations by comparison to cMD calculations. We now refined our method to better mimic experimental conditions. Specifically, rather than sending the H atom to a specific position on the surface, we employed a plane wave for the H atom in directions parallel to the surface. Key findings for these new simulations include the identification of discrepancies between classical molecular dynamics (cMD) simulations and experiments, which are attributed to both the potential energy surface (PES) and quantum effects. Additionally, this study sheds light on the role of classical collective normal modes during collisions, providing insights into energy transfer processes. The results validate the robustness of our simulation methodologies and highlight the importance of considering quantum mechanical effects in the study of hydrogen-graphene interactions.