Searching for ultralight bosons with Josephson junction interferometry

This paper proposes using Josephson junction interferometry to detect long-range potentials generated by ultralight bosons through three experimental scenarios targeting scalar, Lorentz-violating, and axion-mediated interactions.

The Gist

The Cosmic "Ghost" Hunt: Using Super-Sensitive Quantum Detectors to Find Hidden Particles

Imagine you are standing in a room full of people. You can see them, hear them, and feel them if they bump into you. But imagine there was also a "ghost" in the room—something you couldn't see or hear, but which exerted a tiny, invisible tug on everything it passed. If you had a sensitive enough scale, you might notice that a cup of coffee on the table shifted just a fraction of a millimeter every time the ghost walked by.

That is essentially what physicists are trying to do with this paper. They are looking for "ultralight bosons"—tiny, nearly weightless particles that act like cosmic ghosts. These particles are candidates for Dark Matter, the mysterious "glue" that holds galaxies together but remains invisible to our telescopes.

Here is a breakdown of how they plan to catch these ghosts.

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1. The Tool: The Josephson Junction (The "Quantum Tuning Fork")

To catch a ghost that barely touches anything, you need a detector that is incredibly sensitive. The researchers propose using a Josephson Junction (JJ).

Think of a Josephson Junction like a super-sensitive musical string made of superconducting material. In a normal wire, electricity flows like water through a pipe. But in a superconductor, electricity flows in a perfectly synchronized "dance" of pairs of electrons called Cooper pairs. Because they are all dancing in perfect unison, even the tiniest "nudge" from an invisible particle will cause the whole dance to stumble or shift its rhythm.

By measuring how this "dance" (the electrical current) changes, scientists can detect the presence of these invisible particles.

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2. The Three "Ghost" Scenarios

The paper describes three different ways these particles might interact with our world. You can think of these as three different "flavors" of ghosts:

Scenario A: The Magnetic Magnet (Photophilic Scalars)

Imagine a giant, powerful magnet. This paper suggests that if certain particles exist, this magnet won't just pull on iron; it will actually "leak" a field of these invisible particles.

• The Experiment: They place a magnetized sphere near the Josephson Junction. If the "ghost" particles are there, the sphere will create a tiny, invisible "wind" that pushes on the electrons in the junction, changing their rhythm.

Scenario B: The Cosmic Compass (Lorentz-Violating Scalars)

Some theories suggest that space itself might have a "preferred direction"—like a cosmic grain in a piece of wood. This would mean that the laws of physics might look slightly different depending on which way you are facing in the universe.

• The Experiment: They use a slab of material where all the electrons are spinning in the same direction (like a tiny, organized army of compass needles). As the Earth rotates, the direction of this "army" changes relative to the stars. If the "ghost" particles are tied to a cosmic direction, the signal in the detector will pulse in time with the Earth's rotation (a "sidereal" rhythm).

Scenario C: The One-Sided Tug (Axion Monopole-Dipole)

Some particles, called Axions, are very strange. They might interact differently depending on whether a particle is "spinning" or not.

• The Experiment: They use a source that has a "monopole" quality (a simple charge) and a "dipole" quality (a spin). This creates a very specific, lopsided tug. It’s like a ghost that only pulls on you if you are spinning in a certain direction. The Josephson Junction is the perfect "spin-sensitive" detector to feel this lopsided pull.

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3. Why does this matter?

Right now, we know Dark Matter is out there, but we are essentially looking at a dark room with a tiny flashlight. We can see the shadows, but we can't see the objects casting them.

This paper provides a blueprint for a new kind of flashlight. By using the extreme precision of quantum mechanics (the "dance" of the electrons), they are proposing a way to search for these particles at much smaller scales (centimeters to micrometers) than ever before.

In short: They are building a way to listen to the "whispers" of the universe to see if we can finally hear the secret language of Dark Matter.

Technical Summary

Technical Summary: Searching for Ultralight Bosons with Josephson Junction Interferometry

Authors: Djuna Croon and Tanmay Kumar Poddar
Affiliation: Institute for Particle Physics Phenomenology (IPPP), Durham University
Subject Area: Particle Physics Phenomenology / Quantum Technologies

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1. The Problem: The "Short-Range" Gap in New Physics Searches

The search for ultralight bosons (such as scalars, pseudoscalars/axions, or higher-spin particles) is a primary frontier in dark matter research and tests of gravity. These particles mediate "fifth forces" with a range $\lambda_c$ determined by their Compton wavelength.

While existing experimental constraints are robust at galactic and cosmological scales, there is a significant gap in sensitivity at centimeter to micrometer length scales. In this regime, the mediator mass $m_\phi$ ranges from $10^{-6}$ to $10^{-2}$ eV. Traditional precision gravitational experiments, such as torsion balances, lose sensitivity at these shorter distances due to the exponential suppression of the Yukawa-type potential. There is a critical need for a new class of tabletop experiments capable of probing these specific mass and distance regimes.

2. Methodology: Josephson Junction (JJ) Interferometry

The authors propose using Superconductor-Insulator-Superconductor (S-I-S) Josephson Junctions as highly sensitive quantum detectors. The core physical mechanism relies on the following chain of events:

1. Sourcing: A macroscopic object (a magnetized sphere or a polarized fermion slab) generates a scalar or pseudoscalar field profile.
2. Potential Induction: This field induces a position-dependent potential $V_\phi(r)$ that acts on the Cooper pairs within the superconducting electrodes.
3. Phase Accumulation: Because the two electrodes of a JJ are located at slightly different positions relative to the source, they acquire different macroscopic quantum phases $\phi_1$ and $\phi_2$ over an integration time $T$.
4. Current Modulation: The resulting phase difference $\Delta\phi$ induces a measurable modulation in the Josephson current ($I \simeq I_c \sin \Delta\phi$).

The authors utilize a SQUID (Superconducting Quantum Interference Device) in a flux-locked loop (FLL) for readout, which allows for extremely high-precision detection of these phase shifts. They identify quantum number-phase uncertainty as the dominant irreducible noise source, which sets the fundamental sensitivity limit.

3. Key Contributions: Three Experimental Scenarios

The paper develops a unified framework to probe three distinct types of novel interactions:

• Scenario I: Photophilic Scalar Interactions (Spin-Independent)

  • Setup: A magnetized sphere placed near a JJ.
  • Mechanism: The magnetic energy density of the sphere acts as an effective scalar charge density $\rho_\phi \propto g_{\phi\gamma\gamma}(B^2 - E^2)$.
  • Goal: To probe the mixed coupling between electrons and photons ($g_{\phi ee}g_{\phi\gamma\gamma}$).

• Scenario II: Lorentz-Violating Scalar Interactions (Spin-Dependent)

  • Setup: A slab of polarized fermions (electrons or neutrons) placed near a JJ.
  • Mechanism: A Lorentz-violating scalar field couples to the polarized fermion spins in the slab and the Cooper pairs in the JJ.
  • Goal: To probe the mixed coupling $g_{\phi ee}\tilde{J}_e$, where $\tilde{J}_e$ is a Lorentz-violating coefficient.

• Scenario III: Axion-Mediated Monopole-Dipole Interactions (Spin-Dependent)

  • Setup: A polarized fermion slab near a JJ.
  • Mechanism: An axion mediates an interaction between the scalar charge (monopole) and the fermion spin (dipole).
  • Goal: To probe the mixed coupling $g_{Se}g_{Pe}$ (scalar-electron and pseudoscalar-electron couplings).

4. Results and Projected Sensitivities

The authors provide detailed sensitivity projections (as seen in the paper's figures) across various mass ranges:

• Photophilic Scalars: The setup is most sensitive when the scalar mass $m_\phi \sim 1/(R+d)$. They demonstrate that the JJ-based approach can reach competitive levels with existing Eot-Wash bounds, particularly for mixed couplings.
• Lorentz Violation: The proposed setup is highly effective in the $10^{-6}$ eV to $5$ eV range. For $m_\phi \sim 0.1$ eV, the projected sensitivity to $g_{\phi ee}\tilde{J}_e$ is approximately three orders of magnitude stronger than current conservative benchmarks derived from red giant stellar cooling.
• Axion Monopole-Dipole: For axion masses $m_a \sim 0.1$ eV, the setup improves upon existing astrophysical limits by approximately one order of magnitude.

The authors also propose modulation schemes (varying the distance $d$ or the magnetization $B_0$) and sidereal modulation analysis (exploiting Earth's rotation) to distinguish the signal from terrestrial systematic backgrounds and electromagnetic noise.

5. Significance

This paper establishes a theoretical foundation for a new generation of quantum-technology-based dark matter searches. By shifting the focus from mechanical force detection to superconducting quantum phase evolution, the authors demonstrate that Josephson junctions can probe "new physics" in a distance/mass regime that is currently inaccessible to conventional gravity experiments. This work motivates the development of dedicated SQUID-embedded tabletop experiments to explore the dark sector at micrometer scales.