WINTER: WISP Interferometer for Axion Detection

• WINTER is a broadband experiment that uses a Mach–Zehnder interferometer with a Fabry–Perot cavity and strong magnetic field to detect photon–axion conversion in vacuum.
• The design employs dark-fringe operation with dual modulation techniques to register conversion-induced amplitude losses, achieving sensitivities around 5.5×10⁻¹⁵ GeV⁻¹ for axion masses up to 84.8 μeV.
• Its setup features precise path-length stabilization, cavity locking, and noise suppression in a 9 T magnetic field, enabling a robust investigation of axion-like particles.

Searching arXiv for recent and relevant papers on WINTER/WISP interferometer and closely related experiments. WISP Interferometer (WINTER) is a proposed broadband laboratory experiment for detecting photon–axion conversion in vacuum with a free-space Mach–Zehnder-type interferometer. In its full configuration, one interferometer arm contains a Fabry–Perot cavity inside a strong transverse magnetic field and vacuum chamber, while the second arm serves as a reference; the observable is a conversion-induced amplitude loss read out near a dark fringe. WINTER targets axions and axion-like particles as weakly interacting slim particles without assuming a dark-matter origin, and the design is stated to be sensitive to photon–axion couplings $$g_{a\gamma\gamma}\gtrsim 5.5\times10^{-15}\,\mathrm{GeV}^{-1}$$ for axion masses up to $84.8\,\mu\mathrm{eV}$ (Batllori et al., 20 Sep 2025).

1. Concept and physical basis

WINTER is framed as a single-stage photon-to-axion conversion experiment in vacuum. Unlike haloscopes, it does not assume that axions constitute the local dark matter density, and unlike light-shining-through-walls experiments it does not require photon regeneration in a second magnetized region. Its central observable is a small amplitude loss in one interferometer arm produced by the Primakoff effect in an external magnetic field. The proposal therefore treats the experiment as broadband and model-independent in the sense used in the design study (Batllori et al., 20 Sep 2025).

The underlying interaction is the standard axion–photon coupling,

$$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$

with photon–axion mixing induced by a static transverse magnetic field $B_{\mathrm{ext}}$. In the plane-wave approximation, the coupled system is written for

$$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$

and the conversion probability over a distance $z$ takes the familiar form

$$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$

In the weak-mixing, short-distance limit this reduces to

$$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$

where $G=\frac{1}{2}g_{a\gamma\gamma}B_{\mathrm{ext}}$.

A mass-dependent coherence scale limits the accessible parameter space. For the quoted magnet length of $\sim 10\,\mathrm{m}$, coherent conversion is maintained up to $84.8\,\mu\mathrm{eV}$0, with the stated full-design reach extending to $84.8\,\mu\mathrm{eV}$1. The proposal therefore occupies a lower-mass regime than fiber-based resonant interferometric concepts in the tens-of-meV range, while remaining distinct from narrow-band microwave cavity scans.

2. Interferometric architecture

WINTER is based on a free-space Mach–Zehnder interferometer. A laser beam is split by a 50:50 beam splitter into a sensitive arm and a reference arm, then recombined at a second beam splitter to form a bright port and a dark port. The sensitive arm traverses a long Fabry–Perot cavity inside a 9 T dipole magnet and vacuum chamber; the reference arm remains outside the magnet and includes a piezo-controlled mirror for path-length stabilization. The dark port is the principal signal channel, while auxiliary photodiodes monitor cavity locking, interferometer locking, and polarization balance (Batllori et al., 20 Sep 2025).

The full setup uses a laser wavelength of $84.8\,\mu\mathrm{eV}$2, a cavity length $84.8\,\mu\mathrm{eV}$3, and a finesse $84.8\,\mu\mathrm{eV}$4. High-reflectivity mirrors with transmission losses of $84.8\,\mu\mathrm{eV}$5–$84.8\,\mu\mathrm{eV}$6 ppm each define the cavity. For $84.8\,\mu\mathrm{eV}$7, the free spectral range is $84.8\,\mu\mathrm{eV}$8, and $84.8\,\mu\mathrm{eV}$9 implies $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$0. The cavity enhances both the effective interaction length and the circulating power in the magnetized region.

The optical train includes a Faraday isolator, an electro-optic amplitude modulator, a Pockels cell for polarization modulation, and an electro-optic phase modulator for Pound–Drever–Hall locking. A motorized linear polarizer sets the polarization angle, with feedback from a photodiode used to balance power in the two linear polarization states. The polarization component parallel to $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$1 is the one that mixes with the axion field, so polarization control is integral rather than ancillary.

Dark-fringe operation is central to the design. Destructive interference at the dark port is enforced by controlling the differential path length,

$$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$2

so that the carrier is strongly suppressed. The proposal states that the interferometer is stabilized close to a dark fringe with dark-port power at $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$3 of full power. This reduces shot noise and makes the residual dark-port signal linearly sensitive to conversion-induced amplitude mismatches.

3. Modulation, demodulation, and signal formation

WINTER does not rely on a static dark-port offset alone. The design introduces both amplitude modulation and polarization modulation so that the axion-sensitive signal is transferred to controlled sidebands and then recovered by lock-in-style demodulation. The amplitude modulation frequency is $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$4, chosen to stay above low-frequency noise and below electronic limitations. Polarization modulation is applied at $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$5, which satisfies the cavity build-up constraint derived from the photon lifetime. The Pound–Drever–Hall sideband frequency is $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$6, equal to half the free spectral range (Batllori et al., 20 Sep 2025).

In the sensitive arm, photon–axion conversion reduces the photon amplitude by a factor $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$7. The carrier field therefore no longer cancels perfectly at the dark port, and the amplitude-modulated sidebands produce a term proportional to the conversion probability. In the signal model adopted for the proposal, the dark-port cross-term contains

$$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$8

where $$\mathcal{L}_{\mathrm{int}}=-\frac{1}{4}g_{a\gamma\gamma}aF_{\mu\nu}\tilde{F}^{\mu\nu},$$9 is the amplitude-modulation depth.

After mixing with a local oscillator at $B_{\mathrm{ext}}$0 and low-pass filtering, the time-averaged signal becomes

$B_{\mathrm{ext}}$1

Polarization modulation introduces the dependence on the projection $B_{\mathrm{ext}}$2, so the measurable signal is periodically switched by the overlap between the electric field and the external magnetic field. The design therefore uses double demodulation: one stage to extract the amplitude-modulated interferometric sidebands, and another to tag the axion-sensitive polarization state.

The proposal states that for resonant conversion the axion-induced phase shift is zero, so the leading observable is amplitude loss rather than a first-order phase signature. That choice differentiates WINTER from birefringence and ellipticity searches, even though the instrumental vocabulary—dark fringe, lock-in detection, polarization control, and cavity locking—overlaps strongly with precision metrology.

4. Noise model and projected sensitivity

The dominant quoted noise terms are the dark noise of the dark-port photodiode and shot noise from the residual light at the dark fringe. The photodiode is assigned $B_{\mathrm{ext}}$3. For residual dark-port power $B_{\mathrm{ext}}$4, the shot-noise contribution is quoted as

$B_{\mathrm{ext}}$5

The paper then quotes a total effective

$B_{\mathrm{ext}}$6

and for a 10 Hz detection bandwidth gives a noise power

$B_{\mathrm{ext}}$7

These values are the basis of the published sensitivity estimate rather than an independent recalculation (Batllori et al., 20 Sep 2025).

For the full design, the assumed operating point is:

• $B_{\mathrm{ext}}$8
• $B_{\mathrm{ext}}$9
• $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$0
• $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$1
• $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$2
• $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$3
• $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$4
• $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$5

Under these assumptions, the stated coupling reach is

$$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$6

for axion masses up to about $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$7. The scaling presented in the design study is inverse in $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$8 and $$\psi(z)= \begin{pmatrix} E(z)\ a(z) \end{pmatrix},$$9, proportional to $z$0, and proportional to $z$1. Integration time, noise-equivalent power, and bandwidth enter with weaker fractional exponents.

A smaller prototype is described as under construction with a $z$2 laser of $z$3, magnetic field $z$4, magnet length $z$5, cavity length $z$6, finesse $z$7, measurement time $z$8, and detector $z$9. Its projected reach is $$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$0 up to $$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$1. This suggests a staged development path in which interferometric control, cavity locking, and noise budgeting are validated before the full-scale implementation.

5. Relation to neighboring WISP searches

WINTER belongs to a broader family of laboratory WISP searches, but its architecture is not interchangeable with either resonant light-shining-through-walls experiments or fiber-based resonant interferometers. The closest related interferometric concept in the supplied literature is WISPFI, which uses a Mach–Zehnder-type fiber interferometer with a hollow-core photonic crystal fiber inside a 9 T, 100 m magnetic field, dual telecom lasers at 1535 nm and 1570 nm, and pressure tuning to scan resonant axion masses from $$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$2 to $$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$3 (Batllori et al., 2023). WINTER, by contrast, is a free-space vacuum interferometer with a Fabry–Perot cavity and no refractive-index tuning medium.

Experiment	Architecture	Quoted reach
WINTER	Free-space Mach–Zehnder with vacuum Fabry–Perot cavity in one arm	$$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$4, up to $$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$5
WISPFI	Fiber Mach–Zehnder with HC-PCF in a 9 T, 100 m magnet	$$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$6 baseline; mass range $$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$7–$$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$8
CROWS	Resonant microwave light-shining-through-walls experiment	$$P_{\gamma\rightarrow a}=\sin^2(2\theta)\,\sin^2(k_{\mathrm{osc}}z).$$9 at $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$0; $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$1 at $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$2

WISPFI emphasizes resonant enhancement through a tunable effective photon mass in the fiber core. Its signal is modulated by alternating two laser wavelengths at $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$3, and its long-term configuration adds a Fabry–Pérot cavity with finesse $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$4. That design directly probes an unexplored meV-scale region, whereas WINTER is targeted at lower masses in vacuum. The two experiments therefore differ in medium, resonance mechanism, and mass window, although both use interferometric suppression of common-mode backgrounds and sideband-based readout.

CROWS provides a contrasting benchmark. It is a resonant microwave light-shining-through-walls experiment rather than an interferometer, and it reported lower bounds $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$5 for axion-like particles at $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$6 and $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$7 for hidden sector photons at $$P_{\gamma\rightarrow a}\simeq (Gz)^2=\frac{g_{a\gamma\gamma}^2B_{\mathrm{ext}}^2}{4}z^2,$$8 (Betz et al., 2013). Its relevance to WINTER lies in cavity engineering, narrow-band detection, and shielding methodology rather than in the observable itself. A plausible implication is that WINTER can be read as an attempt to combine the metrological advantages of dark-fringe interferometry with the resonant path-length enhancement more familiar from cavity experiments.

6. Terminology and naming ambiguity

The acronym “WINTER” is ambiguous in current literature. In laboratory axion physics it denotes the WISP Interferometer, expanded in the supplied description as “Weak Interacting Slim Particle INTERferometer,” a proposed free-space Mach–Zehnder-type experiment for photon–axion conversion in vacuum (Batllori et al., 20 Sep 2025). In observational astronomy, however, WINTER denotes the Wide-field Infrared Transient Explorer, a robotic near-infrared survey telescope at Palomar Observatory.

The astronomical WINTER is explicitly described as a single-aperture, wide-field, near-infrared time-domain survey instrument rather than an interferometer. Its hardware is a 1 m telescope with a six-sensor InGaAs fly’s-eye camera for seeing-limited imaging, and the design documentation states that there is no baseline, fringe measurement, or multi-aperture combination involved (Frostig et al., 2021). Confusion between the two usages is therefore substantive, not merely stylistic.

For clarity in technical writing, “WISP Interferometer (WINTER)” should be reserved for the axion-conversion experiment, whereas “Wide-field Infrared Transient Explorer (WINTER)” refers to the astronomical facility. The two projects are unrelated in scientific target, instrumental principle, and operating regime.