In-Situ Fiber-Coupling at Sub-Kelvin Temps

• In-situ fiber-coupling is a technology that enables stable, low-loss optical interfacing in sub-Kelvin environments through advanced mechanical, thermal, and optical engineering.
• Techniques such as self-aligned lithography, adiabatic fiber tapering, and nanopositioned evanescent coupling achieve high efficiency and precise mode matching, with efficiencies exceeding 50% at 1550 nm.
• Robust thermal management and integrated diagnostics maintain performance across extreme temperature cycles, supporting applications in quantum optics, superconducting detectors, and hybrid quantum systems.

In-situ fiber-coupling at sub-Kelvin temperatures is a technology-driven methodology for robust, low-loss optical interfacing between fiber and photonic devices inside cryogenic environments—crucial for quantum optics, nanophotonics, and hybrid quantum systems. The term encompasses mechanical, thermal, and optical engineering strategies for achieving stable, high-efficiency optical coupling despite the extreme mechanical and thermal constraints imposed by sub-Kelvin operation.

1. Mechanical and Optical Principles of In-Situ Fiber Coupling

At sub-Kelvin temperatures, thermal contractions, material mismatches, and mechanical vibrations impose stringent requirements on the alignment and stability of fiber–device interfaces. Techniques for in-situ fiber-coupling fall into several primary regimes:

• Self-aligned lithographic interfacing: Example implementations use a lithographically defined, circularly-etched silicon chip that precisely matches the ferrule diameter of a standard telecom fiber, enabling the detector to self-align within a fiber sleeve (Dorenbos et al., 2011). In such a design, alignment is defined during room temperature assembly and does not require further adjustment during cooldown.
• Adiabatic and rigid integration: Rigid, low-loss couplers based on adiabatically tapered fibers are permanently fixed to photonic waveguides via UV-curable adhesives optimized for cryogenic compatibility. The adiabatic expansion of the on-chip waveguide and controlled fiber shaping are critical for mode matching and minimizing reflection, yielding coupling efficiencies exceeding 50% at 1550 nm that are robust against both vibration and temperature cycling (&&&1&&&).
• Nanopositioned free-space and evanescent coupling: For high-Q microcavities or solid-state emitter interfaces, sub-wavelength-diameter fiber tapers (≤500 nm) are positioned near devices with piezo-driven nanopositioners, providing nm-precision control; this facilitates evanescent coupling or direct excitation of cavity modes (Fujiwara et al., 2012, Fujiwara et al., 2016).

Key implementation strategies include matching the thermal expansion of interfacing materials, careful adhesive selection, and utilizing UHV-compatible feedthroughs and mounting systems with low outgassing and high vibration damping. Relevant optical modeling frequently employs overlap integrals for mode-matching efficiency:

$$\eta = |\langle \psi_{\mathrm{fiber}} | \psi_{\mathrm{waveguide}} \rangle|^2$$

and exponential efficiency scaling for adiabatic tapers:

$\eta \approx 1 - \exp(-\alpha L)$

where $L$ is taper length and $\alpha$ is a geometry- and index-contrast-dependent parameter.

2. Thermal Management and Stability Across Temperature Cycles

Maintaining fiber alignment and coupling efficiency during thermal cycling from room temperature to sub-Kelvin regimes requires engineered thermal pathways and mechanical decoupling. Strategies include:

• Material selection and matching: High-conductivity materials such as OFHC copper and gold coatings optimize cooling rates and minimize differential contraction (Korsch et al., 27 Aug 2025).
• Contraction-managed feedthroughs and mandrels: Feedthroughs using epoxies (e.g., Stycast) with tailored contraction properties compress the fiber–chamber interface during cooldown, ensuring hermetic seals against vacuum and superfluid helium leakage. Copper mandrels store and guide fibers inside the chamber with minimal bend-induced loss.
• Fixed, rigid designs: Mechanically rigid (glued) fiber–chip interfaces ensure that the coupling is preserved across repeated thermal cycles and vibration events, avoiding misalignment without requiring in-situ adjustment (Zhao et al., 2023).

Vibration sensitivity is controlled either by eliminating moving parts inside the cryostat (allowing external positioning) (Eikelmann et al., 8 Oct 2025) or by incorporating floating baseplates, damping layers, or high-resonance motion stages to decouple device motion from the ambient vibration spectrum (Herrmann et al., 3 Sep 2024).

3. Quantum Efficiency, Measurement, and Device Characterization

Fiber-coupled devices at sub-Kelvin temperatures are typically characterized using an array of measurement strategies to establish the quantum (or system) detection efficiency, stability, and operational robustness:

• Calibrated photon flux methods: Attenuated, wavelength-tunable sources provide a known photon flux, used to calculate the efficiency via:

$\mathrm{SDE} = \frac{N_c - N_{dc}}{N_{ph}}$

where $N_c$ is the count rate, $N_{dc}$ the dark count rate, and $N_{ph}$ the photon flux (Dorenbos et al., 2011).

• Coincidence-based absolute efficiency: SPDC-generated photon pairs are used for absolute calibration, deriving detector efficiency from coincidence rates and losses (Dorenbos et al., 2011).
• Fluorescence channeling and emission studies: In nanofiber/nanodiamond systems, efficient channeling is validated by comparing guided and free-space collected fluorescence, and by observing sharp, lifetime-limited zero-phonon lines (ZPLs) at low temperature (Fujiwara et al., 2016).

Active microcavity platforms also quantify optomechanical coupling (e.g., using the “membrane-in-the-middle” configuration), using formulas such as:

$$G = \frac{\omega_{\mathrm{cav}}}{L_{\mathrm{cav}}},\qquad g_0 = g_m^{\mathrm{max}} x_{\mathrm{zpf}}$$

where $g_0$ is the single-photon optomechanical coupling strength and $x_{\mathrm{zpf}}$ the zero-point motion amplitude (Zhong et al., 2016).

4. Optical, Microwave, and Hybrid Integration

Many in-situ fiber-coupling schemes are embedded within hybrid systems integrating optical, microwave, or mechanical control:

• Spin-photon interfaces: Systems with embedded gold striplines enable microwave access for optically detected magnetic resonance (ODMR) on qubits such as NV centers, without sacrificing cavity stability or photon collection efficiency (Herrmann et al., 3 Sep 2024).
• Cryogenic imaging and probe access: Imaging systems employing fixed, in-cryostat 8f relays provide high-resolution (1.1 μm) visualization for alignment and testing of fiber-coupled nanophotonic devices, supporting large working distances for additional probes without in-situ mechanical adjustment (Eikelmann et al., 8 Oct 2025).
• Electrical and optical access in superfluid platforms: Hermetically sealed fiber and sub-D electrical feedthroughs based on indium wire and compatible epoxies enable combined optomechanical and electronic measurements of quantum fluids at millikelvin temperatures (Korsch et al., 27 Aug 2025).

Spatial and spectral tunability, as well as hybrid integration with atomic, mechanical, or phononic platforms, are enabled by precision nanopositioning, closed-cycle cryostats, and large-volume sample chambers with optical access.

5. Controlling and Characterizing Polarization and Mode Properties

Polarization fidelity is crucial for many cryogenic quantum applications. Distortions or rotations induced by fibers, window birefringence, or temperature-dependent contraction can degrade system performance. Studies of standard polarizing components (half-wave plates, beamsplitter cubes, dichroic polarizers) at 4 K show that polarization can be maintained at the $10^{-4}$ level over the telecom band, with contraction- and refractive-index-induced phase shifts found to be on the order of 10⁻⁶ K⁻¹ or less (Chanelière et al., 3 Dec 2024). For example, for a birefringent plate,

$R = \frac{2\pi \Delta n L_T}{\lambda},$

and contraction is quantified by:

$\epsilon(T) = \frac{L_{293K} - L_T}{L_{293K}}.$

Such stability supports the use of compact, fiber-based cryogenic probes for quantum optical experimentation.

6. Calibration, Modulation, and In-Situ Diagnostics

Integrated fiber-coupled beam steering and calibration systems allow for precise, spatially localized delivery of photons to detectors under cryogenic conditions (Tabassum et al., 9 Apr 2025). Using broadband LED or laser sources, reflective collimators, parabolic mirrors, and MEMS-based beam steerers, optical pulses of controlled energy (1.2–4.5 eV) and duration (down to a few μs) can be generated and coupled via output fibers to superconducting or photonic devices at sub-Kelvin temperatures. Key design relationships include:

$$d_{\mathrm{ext}} \approx M \times \sqrt{d_f^2 + \left(\frac{1.22 \lambda}{2 \mathrm{NA}}\right)^2}$$

and

$$t_{\mathrm{pulse}} \gtrsim 4.0\,\mu\mathrm{s} \cdot \sqrt{1 + (0.31\lambda\cdot 10^6)^2},$$

where $M$ is the magnification and $d_f$ the fiber core diameter. This facilitates robust in-situ validation of device response and spatial mapping, with special attention to maintaining low loss through cryogenic-compatible fibers and careful management of transmission and alignment challenges during cooldown.

7. Implications, Limitations, and Applications

State-of-the-art in-situ fiber-coupling approaches at sub-Kelvin temperatures provide the foundational optical interface for experiments in quantum optics, superconducting photodetectors, hybrid quantum systems, and quantum metrology. Advantages include:

• Immunity to vibration and thermal stress with rigid or self-aligned couplers;
• No in-situ micropositioning required for many geometries, minimizing complexity in dilution refrigerators or UHV environments;
• Compatibility with quantum information and sensing applications, including high-efficiency detection and coherent light–matter interaction.

Limitations persist in long-term mechanical stability (especially for repeated thermal cycling of glued interfaces), mode-matching to ultra-small or multi-mode fibers, and managing residual polarization changes or birefringence noise in advanced cavities. Nonetheless, ongoing work extends these technologies to sub-Kelvin operation, large quantum networks, superfluid optomechanics, and hybrid systems requiring electrical and microwave co-integration.

Advances in thermal engineering, materials science, and system integration promise further improvements in both efficiency and operational robustness, underpinning the scaling of quantum photonic technologies and precision measurement tools in the sub-Kelvin domain.