Quantum Interference Research Articles

Magnetically shielded high-resolution visual stimulation for OPM-MEG applications.

Many magnetoencephalography (MEG) experiments require visual
stimulation (VS) inside a magnetically shielded room (MSR). For conventional MEG
utilizing superconducting quantum interference devices (SQUIDs), the participant's
head must stay within the semi-spherical surface of a cryogenic storage Dewar. This
design allows to have many SQUID sensors as close as possible to the head in order
to achieve good signal quality. Because Dewars have very restricted mobility, VS is
usually realized using a projector outside of the MSR, some optical elements and a
back-projection screen in the line of sight of the participant.
Recently, the feasibility of MEG using optically pumped magnetometers (OPMs)
was demonstrated. These sensors can be attached directly to the head because
they operate near room temperature. OPM-MEG therefore offers more experimental
freedom including different postures, movements or hyperscanning, creating the need
for a more flexible kind of VS setup.
In this paper, we present a compact, high-resolution VS setup which is enclosed by
a portable magnetic shield with an opening for the projection. The VS setup is based
on a single-board computer which acts as experiment control device to create visual
stimuli, process inputs, log participant activity and set off trigger signals. This setup
supports the new possibilities of OPM-MEG and can be easily installed into any MSR.
We investigate if the shielded VS inside the MSR generates distortion signals above
the noise floor of the OPMs. We also show that visual cortex activity can be evoked
with our setup and recorded with a custom-made OPM-MEG cap. By applying two
well-established visual stimulation paradigms, we demonstrate the ability of our setup
to elicit brain activity in different frequency ranges.

A Stable Open‐Shelled Au26 Nanocluster with Remarkable Performance in Selective Oxidation of Benzyl Alcohol

AbstractOpen metal sites are crucial in catalysis. We have used a “loose coordination strategy” (LCS) to preorganize open metal sites in gold cluster catalysts. A gold nanocluster with composition of [Au26(3,4‐Me2‐Ph‐form)9(iPr2‐imy)3(Me2S)](BF4)2 (iPr2‐imy=1,3‐Diisopropylimidazolium tetrafluoroborate, 3,4‐Me2‐Ph‐form=N,N′‐Di(3,4‐dimethyl‐phenyl)formamidine) (Au26) has been obtained by one pot synthesis, i.e. the direct reduction of Me2SAuCl in the presence of N‐heterocyclic carbenes and amidinate ligands. ESI‐TOF‐MS reveals that the Me2S ligand is detached from the cluster to form open sites. The accessibility of the exposed Au atoms has been confirmed quantitatively by luminescent titration with 2‐naphthalenethiol. Surprisingly, Au26 has 15 valence electrons, and the presence of an unpaired electron is confirmed by superconducting quantum interference device (SQUID) and electron paramagnetic resonance (EPR). This open‐shelled Au26 not only shows unexpected high stability but also exhibits excellent catalytic performance toward the selective oxidation of benzyl alcohol to benzaldehyde, achieving a remarkable turnover number up to 100670.

Programmable quantum circuits in a large-scale photonic waveguide array

Over the past decade, integrated quantum photonic technologies have shown great potential as a platform for studying quantum phenomena and realizing large-scale quantum information processing. Recently, there have been proposals for utilizing waveguide lattices to implement quantum gates, providing a more compact and robust solution compared to discrete implementation with directional couplers and phase shifters. We report on the first demonstration of precise control of single photon states on an 11-dimensional continuously-coupled programmable waveguide array. Through electro-optical control, the array is subdivided into decoupled subcircuits and the degree of on-chip quantum interference can be tuned with a maximum visibility of 0.962 ± 0.013. Furthermore, we show simultaneous control of two subcircuits on a single device. Our results demonstrate the potential of using this technology as a building block for quantum information processing applications.

Minimal Damage and Tunable Fabrication of Atomic‐Scale Ultrathin YBa2Cu3O7‐δ Nanowires with High Uniformity

The growing demand for deep‐space optical communication and remote sensing has highlighted the need for high‐temperature superconducting nanowire single photon detectors (SNSPDs). However, fabricating ultrathin and ultranarrow high‐temperature superconducting nanowires remains significant challenges due to the extreme instability of their films. Herein, an effective approach is presented for fabricating high‐quality YBa2Cu3O7−δ (YBCO) nanowires by utilizing in situ protective layers that shield ultrathin films from environmental and processing‐induced degradation, coupled with low‐temperature etching techniques to achieve precise, and tunable etching while minimizing damage. Thus, YBCO nanowires are successfully fabricated with a minimum width of 68 nm, an atomic‐scale thickness of 5 nm, and lateral damage limited to ≈15 nm. These nanowires exhibit robust I–V hysteresis and a minimum switching current (Is) of about 120 μA. The coefficient of variation for the Is less than 6% and for the critical temperature (Tc0) is below 1%, confirming the exceptional uniformity of the nanowires. Electrical transport measurements reveal that voltage switching in these nanowires is governed by phase slip and hotspot effects. These advancements open new avenues for YBCO‐based high‐temperature SNSPDs, addressing a key challenge in the broader deployment of high‐temperature superconducting devices, including SNSPDs, superconducting quantum interference devices, and superconducting diodes.

A versatile Hong–Ou–Mandel interference experiment in optical fiber for the undergraduate laboratory

Hong–Ou–Mandel (HOM) interference is a quantum optics laboratory experiment that has recently become more accessible to undergraduate students. The experiment consists of two identical photons simultaneously entering a nonpolarizing beamsplitter, where the photon wavefunctions interfere. As a result, the photon pair exits from the same beamsplitter output, whereas classically, the two photons are equally likely to exit from the two different outputs. This effect is demonstrated by observation of a dip in coincidence counts measured between the two beamsplitter outputs. Due to the precision needed to achieve photon indistinguishability, the setup and alignment of this experiment is often considered to be too difficult and time consuming to be appropriate for the undergraduate instructional laboratory. Here, we present an alternative optical-fiber-based apparatus that gives a consistently reproducible experiment. In our approach, quantum interference occurs within a fused-fiber coupler, instead of within a traditional beamsplitter. We use a commercially available fiber-coupled biphoton source that requires minimal alignment and increases the interference coherence length. In addition, our biphoton source provides direct temperature-based control of the frequency degeneracy of the photon pairs, allowing students to investigate physical properties of HOM interference such as coherence length and interference visibility. Through use of standard optomechanical parts, combined with the commercially available fiber-integrated biphoton source, our apparatus is positioned midway between a completely built-from-scratch and a pre-aligned setup, making it ideal for the advanced instructional laboratory.

Controlling quantum interference in a four-level atomic system using a driving field

Controlling quantum interference in a four-level atomic system using a driving field

Flat Band Generation Through Interlayer Geometric Frustration in Intercalated Transition Metal Dichalcogenides.

Electronic flat bands can lead to rich many-body quantum phases by quenching the electron's kinetic energy and enhancing many-body correlation. The reduced bandwidth can be realized by either destructive quantum interference in frustrated lattices, or by generating heavy band folding with avoided band crossing in Moiré superlattices. Here a general approach is proposed to introduce flat bands into widely studied transition metal dichalcogenide (TMD) materials by dilute intercalation. A flat band with vanishing dispersion is observed by angle-resolved photoemission spectroscopy (ARPES) over the entire momentum space in intercalated Mn1/4TaS2. Polarization-dependent ARPES measurements combined with symmetry analysis reveal the orbital characters of the flat band. Supercell tight-binding simulations suggest that such flat bands arising from destructive interference between Mn and Ta on S through hopping pathways, are ubiquitous in a range of TMD families as well as for different intercalation configurations. The findings establish a new material platform to manipulate flat band structures and explore their corresponding emergent correlated properties.

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.

A Stable Open-Shelled Au26 Nanocluster with Remarkable Performance in Selective Oxidation of Benzyl Alcohol.

Open metal sites are crucial in catalysis. We have used a "loose coordination strategy" (LCS) to preorganize open metal sites in gold cluster catalysts. A gold nanocluster with composition of [Au26(3,4-Me2-Ph-form)9(iPr2-imy)3(Me2S)](BF4)2(iPr2-imy = 1,3-Diisopropylimidazolium tetrafluoroborate, 3,4-Me2-Ph-form = N,N'-Di(3,4-dimethyl-phenyl)formamidine) (Au26) has been obtained by one pot synthesis, i.e. the direct reduction of Me2SAuCl in the presence of N-heterocyclic carbenes and amidinate ligands. ESI-TOF-MS reveals that the Me2S ligand is detached from the cluster to form open sites. The accessibility of the exposed Au atoms has been confirmed quantitatively by luminescent titration with 2-naphthalenethiol. Surprisingly,Au26has 15 valence electrons, and the presence of an unpaired electron is confirmed by superconducting quantum interference device (SQUID) and electron paramagnetic resonance (EPR). This open-shelledAu26not only shows unexpected high stability but also exhibits excellent catalytic performance toward the selective oxidation of benzyl alcohol to benzaldehyde, achieving a remarkable turnover number up to 100670.

Direct observation of chiral edge current at zero magnetic field in a magnetic topological insulator

The chiral edge current is the boundary manifestation of the Chern number of a quantum anomalous Hall (QAH) insulator. The van der Waals antiferromagnet MnBi2Te4 is theorized to be a QAH in odd-layers but has shown Hall resistivity below the quantization value at zero magnetic field. Here, we perform scanning superconducting quantum interference device (sSQUID) microscopy on these seemingly failed QAH insulators to image their current distribution. When gated to the charge neutral point, our device exhibits edge current, which flows unidirectionally on the odd-layer boundary both with vacuum and with the even-layers. The edge current chirality reverses with the magnetization of the bulk. Surprisingly, we find the edge channels coexist with finite bulk conduction even though the bulk chemical potential is in the band gap, suggesting their robustness under significant edge–bulk scattering. Our result establishes the existence of chiral edge currents in a topological antiferromagnet and offers an alternative for identifying QAH states.

Coherent Photocurrent Injection through Quantum Interference between Stimulated Electronic Raman Scattering and Single-Photon Absorption in Intrinsic Graphene.

The physical picture for photocurrent injection and coherent control in intrinsic graphene under two-color laser excitation remains obscure. Previously, photocurrent injection of intrinsic graphene was attributed to the quantum interference between two electronic transition pathways of single-photon and two-photon absorptions as well as layer-to-layer coupling. Here, we show that quantum interference between stimulated electronic Raman scattering and single-photon absorption plays a very important role in contributing to the total photocurrent, while interlayer coupling does not sufficiently affect the photocurrent injection, which is in contrast to the previous interpretation of the experimental results on photocurrent injection and coherent control. Our findings may change the basic understanding of the physical picture of electron transitions and photocurrent injection, which may benefit the design and fabrication of graphene-based optoelectronic devices.

Simulation and Measurement of Stray Fields for the Manipulation of Spin Qubits in One- and Two-Dimensional Arrays.

The inhomogeneous magnetic stray field of micromagnets has been extensively used to manipulate electron spin qubits. By means of micromagnetic simulations and scanning superconducting quantum interference device microscopy, we show that the polycrystallinity of the magnet and nonuniform magnetization significantly impact the stray field and corresponding qubit properties. The random orientation of the crystal axis in polycrystalline Co magnets alters the qubit frequencies by up to 0.5 GHz, compromising single qubit addressability and single gate fidelities. We map the stray field of Fe micromagnets with an applied magnetic field of up to 500 mT, finding field gradients above 1 mT/nm. The measured gradients and the lower magnetocrystalline anisotropy of Fe demonstrate the advantage of using Fe instead of Co as magnets in spin qubit devices. These properties of Fe also enabled us to design a 2D arrangement of nanomagnets for driving spin qubits distributed on a 2D lattice.

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.

Anomalous and large topological Hall effects in β-Mn chiral compound Co6.5Ru1.5Zn8Mn4: electron electron interaction facilitated quantum interference effect

β-Mn-type chiral cubic CoxZnyMnz(x+y+z= 20) alloys present a intriguing platform for exploring topological magnetic orderings with promising spintronic potential. This study examines the magnetotransport properties of Co6.5Ru1.5Zn8Mn4, a skyrmion-hostingβ-Mn-type chiral compound. The longitudinal resistivity (ρxx) exhibits field-insensitive low-temperature minima due to quantum interference effects, driven byT1/2-dependent electron-electron interactions. We observe a substantial intrinsic anomalous Hall conductivity, unaffected by quantum interference. Additionally, a pronounced topological Hall effect is observed at the metastable skyrmionic state, persisting up toTCand achieving notable magnitudes for stoichiometric compounds. These results position the CoxZnyMnzfamily favourably to leverage the rich pallete of emergent magnetotransport properties for spintronic applications.

Quantum control in size selected semiconductor quantum dot thin films

Abstract We introduce a novel technique for coherent control that employs resonant internally generated fields in CdTe quantum dot (QD) thin films at the L-point. The bulk band gap of CdTe at the L-point amounts to 3.6 eV, with the transition marked by strong Coulomb coupling. Third harmonic generation (λ 3 = 343 nm, hν = 3.61 eV) for a fundamental wavelength of λ 1 = 1,030 nm is used to control quantum interference of three-photon resonant paths between the valence and conduction bands. Different thicknesses of the CdTe QDs are used to manipulate the phase relationship between the external fundamental and the internally generated third harmonic, resulting in either suppression or strong enhancement of the resonant third harmonic, while the nonresonant components remain nearly constant. This development could pave the way for new quantum interference–based applications in ultrafast switching of nanophotonic devices.

Simultaneous photon blockade and bunching in giant atom-cavity system

Abstract We propose an innovative model predicated on giant atom-cavity system, wherein three cavity modes are interconnected via nearest-neighbor photon tunneling, and a giant atom is simultaneously coupled with two single-mode cavities on each side. This model demonstrates both photon blockade and photon bunching effects simultaneously. Specifically, when the giant atom is weakly driven, the central cavity displays unconventional photon blockade with g b ( 2 ) ( 0 ) ≈ 1 0 − 5.97 , while the side cavities exhibit photon bunching effects with g a ( 2 ) ( 0 ) = g c ( 2 ) ( 0 ) ≈ 1 0 4.85 due to the quantum interference and the anharmonicity of energy eigenvalues. This model introduces a fresh perspective for investigating intricate photon behaviors in multi-cavity systems.

Transition Metal Based Spin-Tuned Charge Transfer in Single-Molecule Junctions.

Investigating the correlation between metal coordination and molecular conductivity in single-molecule systems is essential for advancing our knowledge of molecular electronics, particularly in the realm of spintronics. In the present study, we developed two complex wires utilizing the bipyridine ligand and two transition metal ions, Co2+ and Zn2+, aiming to study the impact of different spin characters on single-molecule charge transport properties. Single-molecule conductance was investigated using scanning tunnelling microscope breaking junctions (STM-BJ) technique and the underlying mechanism was analysed by density functional theory (DFT) calculations. We demonstrated that the conductance predominantly increases after inserting Zn2+, indicating a conducting channel has been constructed. In contrast, the conductance of the analogue coordinated with Co2+ does not change significantly, this can be explained by distinct disparities between spin-up and spin-down channels, in which destructive quantum interference in spin-down state counteracts the enhancement of molecular conductance. Our study establishes the groundwork for a systematic approach to the design, fabrication, and implementation of single-complex conductance explorations.

DUV Double‐Resonant Raman Spectra and Interference Effect in Graphene: First‐Principles Calculations

ABSTRACTWe calculate double‐resonance Raman (DRR) spectra of monolayer graphene by first‐principles density functional calculation, for wide laser excitation energies from the near‐infrared (1.58 eV) to the deep‐ultraviolet (DUV, 5.41 eV) region. When laser excitation energy, , goes into the DUV region, Raman peak wavenumber for G band switches from red‐shift to blue‐shift and for 2D band switches from red‐shift to constant, in contrast to the continuous blue‐shift of G band. Raman intensity of the three bands generally decreases with increasing , except for around 4.08 eV where Raman intensity diverges due to van Hove singularity of electron density of states. The combined two‐phonon modes change with for both G and G bands (e.g., from 2LO to 2TO and back to 2LO for G and from LA + LO/TO to TA + LO/TO for G) but remain 2LO for 2D band. Further, the dominant DRR scattering process of G band changes from the electron‐hole ( or ) scattering processes to the scattering processes as goes into the DUV region, since the Dirac energy bands become asymmetric between and band that suppresses the process and the Raman intensity. Another factor to suppress the Raman intensity is the quantum interference effect between four scattering processes () which changes from constructive to destructive interference and finally to no interference with increasing . We calculate ‐dependent Raman tensor of the three bands and polarized Raman spectra, which further support the interference effect. The calculated results are directly compared and consistent with the experimental results.

Crossing exceptional points in non-Hermitian quantum systems.

Exceptional points facilitate peculiar dynamics in non-Hermitian systems. Yet, in photonics, they have mainly been studied in the classical realm. In this work, we reveal the behavior of two-photon quantum states in non-Hermitian systems across the exceptional point. We probe the lossy directional coupler with an indistinguishable two-photon input state and observe distinct changes of the quantum correlations at the output as the system undergoes spontaneous breaking of parity-time symmetry. Moreover, we demonstrate a switching in the quantum interference of photons directly at the exceptional point, where Hong-Ou-Mandel dips are transformed into peaks by a change of basis. These results show that quantum interference and exceptional points are linked in curious ways that can now be further explored.

All-optical switch based on two-dimensional asymmetric electromagnetically induced grating in nanohybrid systems.

This study investigates (EIG) in a nanohybrid configuration involving a semiconductor quantum dot (SQD) and a core-shell bimetallic nanoparticle coated with graphene. The goal is to optimize interactions between plasmons and excitons. This is achieved by utilizing nanoparticles covered with graphene, which enhances control over surface plasmons. These interactions decrease light absorption by quantum dots. At the same time, they enhance the presence of coherent states and quantum interference. The innovative aspect of this model lies in its ability to produce a two-dimensional asymmetric diffraction grating. This is accomplished by modulating the phase within a closed-loop structure and utilizing the nonlinear multi-wave mixing phenomenon, without needing to adjust other system parameters. More specifically, altering the phase of the incident fields produces an asymmetric diffraction grating with an efficiency exceeding 50%. Similarly, varying the frequency of the probing field results in an asymmetric diffraction grating with efficiencies exceeding 40%. This technology has the potential to enhance optical systems, such as all-optical switches in communications, by simplifying the alteration of laser beam phases and probe field frequencies.