Fundamental Interactions

Precision interferometry and polarimetry for dark sector searches and quantum electrodynamics

Members of this research group develop cavity-based polarimetry and long-baseline interferometry to explore axion-like particles, vacuum magnetic birefringence, and related fundamental physics signatures, while contributing core optical metrology to large-scale experiments.

Our mission

We build experiments for ultra-stable optical measurements to look for new physics. Our tools are high-finesse cavities, precision polarimetry and heterodyne interferometry.

With them we

  • explore the dark sector and search for axion-like particles, which are a compelling dark matter candidate,
  • aim to detect vacuum magnetic birefringence, an extremely small quantum electrodynamics effect predicted in the 1930s, and
  • investigate methods for a path to observing ultra-high frequency gravitational waves.

Our metrology contributes to large scale experiments such as ALPS II at DESY and potentially to future long baseline ground and space based instruments.

Research

Our research integrates robust hardware, sophisticated controls, and innovative analysis methods to achieve background-limited sensitivity in the three distinct yet interconnected fields:

Photo of a very long, straight tunnel with several tubes running toward a point at the back right.

Dark sector

ALPS II and APE: two complementary routes to search for axion-like particles – very light bosons that couple to photons.

Square glass with rainbow colors, held by a silhouetted hand.

Vacuum magnetic birefringence

Testing the non-linear optics of the quantum vacuum using strong magnetic fields and ultra-stable polarimetry.

Red arrows and green and yellow waves converge at a central white starburst on a gradient blue backdrop.

Ultra-high frequency gravitational waves

Concept studies for detecting ultra-high frequency gravitational waves using optical metrology.

Dark sector / axion searches
We follow two complementary routes to search for very light bosons that couple to photons.
  • ALPS II (DESY): light-shining-through-a-wall experiment to probe generic photon couplings with dual 120 m cavities and heterodyne readout.
  • APE: a compact cavity-based polarimeter for narrow-line dark matter searches sharing controls and analysis with ALPS II.
Vacuum magnetic birefringence (VMB)

We conduct experiments that test the non-linear optics of the quantum-mechanical vacuum using strong magnetic fields and ultra-stable polarimetry. Our experiments include the use of high-finesse optical cavities in two complementary configurations:

  • Tabletop cavity-based polarimetry with rotating/fast-switching Halbach magnets on active magnetic bearings and
  • An interferometric approach using the long-baseline ALPS II magnet string for a macroscopic test of QED.
UHF gravitational waves (pathfinder)

We explore new methods for the detection of ultra-high frequency gravitational waves using optical metrology.

  • They rely on a three-field heterodyne scheme: a static magnetic field, an optical carrier, and a passing gravitational wave.
  • The readout of sideband frequencies is linearly proportional to strain in the optical band (1064 nm laser carrier).
  • Feasibility studies will be done within the Cluster of Excellence QuantumFrontiers.

 

Research highlights

COSMIC WISPers Prize 2024 for Laura Roberts

COSMIC WISPers Prize 2024 for Laura Roberts

June 27, 2024

PhD student receives presentation award and invitation to speak at general meeting more

‘Light shining through a wall’ experiment ALPS starts searching for dark matter

‘Light shining through a wall’ experiment ALPS starts searching for dark matter

May 23, 2023

World’s most sensitive instrument of its kind is to produce axions more

More about our research

Workers in a tunnel installing a massive cylindrical structure with metallic and mechanical components.

Any Light Particle Search (ALPS) II

ALPS II (Any Light Particle Search II) is looking for a new, light-weight, set of particles called axion-like-particles. These could explain dark matter and other phenomena that the standard model of particle physics cannot describe. more
Complex setup om an optical laboratory table with various scientific instruments, cables, and a large chamber.

Differential Reflection Phase Experiment

A small cavity inside a vacuum tank uses mirrors with the ALPS II reflective coating in order to determine effects in the so called differential reflection phase and project the impact for ALPS II. more

Deep-dive explainers

Why do we think that there is a “dark sector”?

Galactic rotation curves: In our solar system, outer planets at larger distances from the central mass (our Sun) move slower. We expected galaxies to behave in the same way. Instead, outer stars race at higher speeds. Researchers explain this behavior by the existence of invisible dark matter in galaxies that adds mass and leads to the observed higher speeds of stars on outer orbits.

The strong CP problem: The current mathematical formulation of quantum chromodynamics allows for violations of CP (charge conjugation parity) symmetry. Yet, experiments involving only the strong interaction have never demonstrated a violation of CP-symmetry. The electric dipole moment of the neutron is near zero. Why is nature so perfectly fine-tuned?

The solution – enter the axion

The axion: Peccei and Quinn (1977) proposed a new field to relax CP violation to zero. The particle associated with this field is the axion.

What are the properties of the axion? It is an extremely light-weight pseudo-Nambu–Goldstone boson with a mass in the sub-eV range. It is weakly interacting. In magnetic fields, axions can be converted into photons.

The axion solves the strong CP problem and is a leading candidate for dark matter, solving two longstanding puzzles in physics simultaneously.

Searching for the axion with ALPS II

Photons to axions and back: ALPS II is a “light shining through a wall” experiment that aims to generate axions in a lab by model-independent conversion of photons into axions in a strong magnetic field. In a production cavity, high-power laser light in a strong magnetic field is converted into axions. These weakly-interacting particles travel through an opaque wall and enter a regeneration cavity behind the wall. Inside this second cavity axions are converted back into photons in a second strong magnetic field.

Photo taken inside a large laboratory. On the left and right, a pipe runs into the depths of the room; in the middle, there is an area covered with white protective walls.

Key technologies:

  • Dual high-finesse cavities: Two 120 m long optical cavities are locked to the same laser frequency, amplifying the probability of photon-to-axion and axion-to-photon conversion.
  • Heterodyne readout: Interferes regenerated photons with a local oscillator to detect single photons amidst noise.
  • Leakage-aware estimators: Statistical calibration distinguishes true axion signals from light leaking through the system.

Dark matter polarimetry with APE

The Axion Precision Experiment (APE) probes the “wind” of dark matter passing through an experiment in a laboratory at the institute. It searches for a narrow spectral line caused by the oscillation of the local dark matter field, which induces polarization effects even without an external magnetic field.

The measurement principle: axion dark matter acts as a birefringent medium. The oscillation of the local dark matter field rotates the polarization of light circulating in the cavity, even without an external magnetic field.

\Theta (t) \approx g_{a\gamma\gamma} \sqrt{2\rho_\textrm{DM}} \cos(m_at)

Status: At the moment, a 1.5 m cavity is used to establish the “low loss polarimeter” baseline. The optical setup includes quarter wave plates near the mirrors to convert polarization rotation into measurable ellipticity (or vice versa). The experiment is operating in a wide frequency range from millihertz to kilohertz. No strong static magnetic field is required for this oscillation search.

Vacuum magnetic birefringence

Magnetized vacuum in quantum electrodynamics: In classical physics, a vacuum is simply “nothing”. Light travels through a vacuum unchanged. According to the predictions from quantum electrodynamics (QED), however, a vacuum containing a strong magnetic field behaves like a birefringent crystal.

When linearly polarized light passes through this “magnetized vacuum”, its polarization components travel at different speeds, converting it into elliptically polarized light. This effect is called vacuum magnetic birefringence (VMB).

The science case: VMB probes light-by-light scattering of macroscopic electromagnetic fields, an as yet unobserved prediction of QED in the low-energy, strong-field regime. A measurement of VMB at the QED predicted amplitude, or stringent limits close to it, would place strong constraints on Born-Infeld type nonlinear electrodynamics and other beyond the Standard Model scenarios that predict refractive indices n>1 in a magnetized vacuum.

VMB physics

The physics of vacuum magnetic birefringence is governed by the Euler-Heisenberg effective action (formulated in 1936). It describes the interaction between photons and virtual electron-positron pairs in the presence of an external magnetic field. These loop fluctuations give the quantum vacuum an effective refractive index.

\mathcal{L} = \frac{\alpha^2\hbar^3}{90m_e^4c^5}\left[\left(F_{\mu\nu}F^{\mu\nu}\right)^2+\frac{7}{4}\left(F_{\mu\nu}\widetilde{F}^{\mu\nu}\right)^2\right]

Birefringence: The predicted effect is extremely small, of the order 4x10-23 in a magnetic field of 1 Tesla, and it scales quadratically with the magnetic field strength:

\Delta n = k_\textrm{CM}B^2,\quad \textrm{where } k_\textrm{CM} \approx 4.03\times10^{-23}\,T^{-2}

What we measure: The experimentally observable effect is the induced ellipticity:

\psi = \frac{\pi L}{\lambda}\Delta n

Born-Infeld and beyond-the-standard-model context: While Born-Infeld theory (1934) aimed to solve infinite self-energy and predicts no variations of the refractive index in its original form, modern string and brane models predict Born-Infeld-like structures with n>1 in a magnetized vacuum. Measuring VMB, or setting upper limits near the QED prediction, constrains Born–Infeld-type models and other new physics.

VMB experimental frontiers

The challenge is detecting a ψ=10-11 rad signal amidst wide-band noise. We employ high-finesse Fabry-Pérot cavities to increase the path length. Effectively the light is bouncing back and forth thousands of times.

Dual experimental approach: We are pursuing two complementary strategies for a VMB search:

  • High B2L (slow modulation): The ALPS II experiment at DESY (212 m, 5.3 Tesla) implements a cavity-based VMB measurement scheme with an exceptionally high (B2L) figure of merit at very low modulation frequencies (~1 mHz).
  • High frequency (lower B2L): The polarimetric experiment at AEI uses levitated permanent magnets spinning up to 500 Hz and a high-finesse cavity to access higher modulation frequencies and avoid (1/f) noise at a more modest (B2L).

VMB in astrophysical objects

An artistic representation of a magnetar, an extremely dense type of neutron star with an extremely strong magnetic field. The magnetar emits intense blue light, surrounded by complex magnetic field lines that extend through space. Numerous stars in the universe can be seen in the background.

The universe provides magnetars, strongly magnetized, rotating neutron stars with field strengths of up to 1011 Tesla. In this regime, vacuum magnetic birefringence can significantly affect radiative transport. As light travels through the strong magnetic field of a magnetar, the magnetized vacuum imprints characteristic polarization signatures on the escaping radiation.

In 2016, first evidence for this effect was reported by Mignani et al. using optical polarimetry of the magnetar RX J1856.5−3754.

The IXPE Mission (Imaging X-ray Polarimetry Explorer) observes X-rays from these stars and has also seen first evidence of this effect.

Physics beyond QED

Photon-axion mixing: In vacuum magnetic birefringence experiments, beyond-QED effects can manifest as photon-axion mixing. If the axion has non-zero mass, it travels slower than the speed of light. The resulting lag between photons and axions in optical cavities would present itself as a characteristic signature of new physics.

Ultra-high-frequency gravitational-wave detection

Gravitons to photons: In the inverse Gertsenshtein effect, a gravitational wave propagating through a static transverse magnetic field can spontaneously convert into a photon. The interaction is purely geometric: the gravitational stretches and compresses spacetime, which modulates the existing magnetic field and creates an oscillating electromagnetic field. This, in turn, leads to the creation of photons.

Conversion probability: The chance of converting gravitons into photons via the inverse Gertsenshtein effect depends on the magnetic field strength (B), the travel length of the laser beam (L), and the gravitational-wave strain (h):

p_{\mathrm{g}\rightarrow\gamma} \propto B^2 L^2 h^2

Heterodyne mixing: It is possible to “boost” the signal by interfering the tiny GW-induced electromagnetic field with a strong co-propagating probe beam from a local oscillator to create beat notes.

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