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6CCVD Research

Ultrastrong magnetic light-matter interaction with cavity mode engineering

At a Glance

Section titled ā€œAt a Glanceā€
MetadataDetails
Publication Date2023-05-18
JournalCommunications Physics
AuthorsHyeongrak Choi, Dirk Englund
Citations2
AnalysisFull AI Review Included

Executive Summary

Section titled ā€œExecutive Summaryā€

This research details novel mode engineering techniques for hollow metallic cavities to achieve unprecedentedly strong magnetic light-matter interaction, crucial for quantum technologies.

  • Ultrastrong Coupling Achieved: The magnetic interaction figure of merit (Q/VB) was enhanced by more than 1016 times compared to free space, representing an improvement of five orders of magnitude over existing cavity designs.
  • Arbitrarily Small Mode Volume: The effective magnetic mode volume (VB) can be reduced arbitrarily, limited only by the materialā€™s electromagnetic penetration depth (e.g., 40 nm for Niobium).
  • Three Core Methods: VB reduction is achieved via (1) Longitudinal Mode Squeezing, (2) Current Engineering (using inverse-tapered reentrances for current crowding), and (3) Magnetic Field Expulsion (using thin metallic plates).
  • Superconducting Advantage: Utilizing superconducting materials (Niobium) allows for ultrahigh quality factors (Q > 109) by minimizing ohmic losses, enabling the maximum Q/VB ratio.
  • Experimental Validation: Proof-of-principle experiments using diamond Nitrogen-Vacancy (NV) spin ensembles confirmed the theoretical predictions for VB, Q, and resonance frequency (f ā‰ˆ 2.87 GHz).
  • Enabling Technology: These methods pave the way for high-cooperativity microwave-spin coupling, compact Electron Paramagnetic Resonance (EPR) sensors, and fundamental science applications like dark matter searches.

Technical Specifications

Section titled ā€œTechnical Specificationsā€
ParameterValueUnitContext
Maximum Q/VB (Theoretical)3 x 1016Ī»-3Combined current engineering and field expulsion (Niobium, 1 Āµm plate)
Maximum Q (Theoretical)1.06 x 109N/AAchieved using Niobium (Rs = 10 nĪ©)
Minimum VB (Theoretical)3.6 x 10-9Ī»3Achieved using Niobium (Rs = 10 nĪ©)
Niobium (Nb) Surface Resistance (Rs)1 - 10nĪ©Superconducting material loss (T < 4.2 K)
Copper (Cu) Surface Resistance (Rs)1mĪ©Normal conductor loss (at 1 K)
Niobium London Penetration Depth (Ī»L)40nmUltimate limit for VB reduction
Copper Skin Depth (1 GHz)2ĀµmLimit for VB reduction in normal conductors
Experimental VB (Doubly Reentrant)1.75 x 10-3Ī»3Measured using NV centers (f = 2.929 MHz Rabi frequency)
Experimental Q (Doubly Reentrant)2,421N/AMeasured at f = 2.871 GHz (Copper cavity)
NV Center Coherence Time (T2)Up to 1.5msRoom temperature diamond
Calculated Single-Spin Cooperativity (C)12.5N/ASingle NV center at 10 GHz, VB = 3.62 x 10-8 Ī»3
Sapphire Dielectric Loss (Qdielectric)1.45 x 1010N/AEstimated additional loss from substrate in field-expulsion cavity

Key Methodologies

Section titled ā€œKey Methodologiesā€

The study employed three primary methods, often combined, to manipulate the electromagnetic fields within hollow metallic cavities, focusing on the Transverse Magnetic (TM) modes.

  1. Longitudinal Mode Squeezing (TM010 Mode):

    • Principle: The TM010 modeā€™s resonant frequency is independent of the cavity height (h).
    • Implementation: Decreasing h (longitudinal squeezing) reduces VB proportionally to h, achieving VB less than the diffraction limit (Ī»3).
    • Limitation: VB reduction is ultimately limited by the metalā€™s penetration depth (Ī“).

  2. Current Engineering (Reentrant Cavities):

    • Principle: Locally increasing the surface current density (Js) enhances the magnetic field (B), thereby reducing VB (VB āˆ 1/B2).
    • Designs: Framework integrates split-mode, loop-gap, and reentrant cavities.
    • Current Crowding via Inverse-Tapering: Modified reentrant cavities were designed with inverse-tapering (smaller bottom radius Rā€™ < R) to combine large capacitance (charge storage) with a narrow conducting channel, maximizing current density and VB reduction.

  3. Magnetic Field Expulsion (Meissner Effect Analogy):

    • Principle: Good conductors and superconductors expel magnetic fields (Faradayā€™s law/Meissner effect), concentrating the field at sharp edges or gaps.
    • Implementation: A thin metallic plate (thickness t, down to 10 Āµm) was inserted into the cavity. The external field is circumvented, creating magnetic ā€œhot spotsā€ at the edges.
    • Scaling: VB was found to be linearly proportional to the plate thickness (t). Crucially, Q did not degrade when VB was reduced by thinning the plate.

  4. Numerical Simulation and Validation:

    • Simulation: All designs were modeled using the Finite Element Method (FEM) in COMSOL, ensuring mesh sizes were smaller than half the minimum feature size (e.g., plate thickness).
    • Experimental Setup: A doubly reentrant copper cavity (h = 1.9 cm) was fabricated and tested using an ensemble of 300 ppb NV centers in diamond as a magnetic field sensor.
    • Measurement: Optically Detected Magnetic Resonance (ODMR) was used to measure the Rabi oscillation frequency, which directly correlates to the single-photon magnetic field (Bs) and thus VB.

Commercial Applications

Section titled ā€œCommercial Applicationsā€

The ultrastrong magnetic light-matter interaction enabled by these engineered cavities is critical for advancing several high-tech fields:

  • Quantum Computing:
    • High-cooperativity coupling between microwave photons and solid-state spin qubits (e.g., NV centers, electron spins) or superconducting flux qubits.
    • Enabling compact, high-coherence 3D circuit Quantum Electrodynamics (cQED) architectures.
  • Quantum Networks and Transduction:
    • High-fidelity, thresholdless single MW-photon to optical photon transduction, essential for connecting quantum processors via optical fibers.
  • Sensing and Metrology:
    • Development of compact, high-sensitivity Electron Paramagnetic Resonance (EPR) sensors.
    • Low-noise, continuous-wave room-temperature masers (MW amplifiers).
    • Nuclear Magnetic Resonance (NMR) and ultra-precision metrology.
  • Fundamental Science:
    • High-sensitivity searches for fundamental particles, such as dark matter (axions), using high-Q microwave-spin coupling systems.
  • Microwave Devices:
    • Active microwave devices and magnetic nonlinear devices (e.g., Kerr blockade, nonlinear bistability) operating at high cooperativity.

Tech Support

Section titled ā€œTech Supportā€

Original Source

Section titled ā€œOriginal Sourceā€

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