Quantum Sensors Tighten the Search for Dark Matter

Author: Denis Avetisyan

New analysis of superconducting qubit data reveals a powerful, unexpected probe for interactions between dark matter and ordinary matter.

The study demonstrates that the aluminum transmon loss function, initially modeled with a Lindhard function and a superconducting gap correction, exhibits momentum-independent behavior for low-energy quanta (q < a few hundred eV), but converges to a bulk-like response at higher energies <span class="katex-eq" data-katex-display="false">q \gtrsim 2\,{\rm keV} \gg d^{-1}</span>, where <i>d</i> represents the aluminum layer thickness, effectively eliminating discernible differences attributable to the thin-layer geometry. — The study demonstrates that the aluminum transmon loss function, initially modeled with a Lindhard function and a superconducting gap correction, exhibits momentum-independent behavior for low-energy quanta (q < a few hundred eV), but converges to a bulk-like response at higher energies $q \gtrsim 2\,{\rm keV} \gg d^{-1}$ , where d represents the aluminum layer thickness, effectively eliminating discernible differences attributable to the thin-layer geometry.

Researchers demonstrate that existing transmon qubit measurements can establish world-leading laboratory constraints on dark matter-electron scattering and competitive limits on dark photon absorption.

Despite decades of searching, the nature of dark matter remains elusive, motivating exploration of novel detection strategies. In ‘Probing Dark Matter-Electron Interactions with Superconducting Qubits’, we present a re-analysis of existing data from transmon qubit coherence measurements to establish new constraints on dark matter interactions. Our work demonstrates that these quantum sensors provide the most stringent laboratory limits to date on keV-scale dark matter-electron scattering and competitive bounds on dark photon absorption. Could this approach unlock a new era of dark matter detection utilizing the sensitivity of quantum devices?

The Cosmic Enigma: Charting the Unknown in Dark Matter

The universe, as currently understood, is overwhelmingly dominated by a substance dubbed dark matter, accounting for approximately 85% of all matter. Yet, despite its prevalence, the fundamental composition of this enigmatic material remains unknown, presenting one of the most significant challenges in modern physics. This lack of understanding has spurred a proliferation of detection strategies, ranging from massive underground experiments designed to observe rare interactions with ordinary matter, to astrophysical observations searching for indirect signatures of dark matter annihilation or decay. Scientists are actively pursuing diverse theoretical candidates – from weakly interacting massive particles (WIMPs) to axions and sterile neutrinos – each necessitating unique experimental approaches. The breadth of these efforts underscores the profound mystery surrounding dark matter and the urgent need for innovative techniques to unveil its true nature and, ultimately, complete the standard model of particle physics.

The search for dark matter is hampered by a significant obstacle: differentiating a genuine dark matter interaction from the constant barrage of background signals. Current direct detection experiments, often situated deep underground to shield against cosmic rays, still contend with radioactive decay from detector materials and environmental sources. These background events can mimic the faint energy depositions expected from weakly interacting dark matter particles, creating a substantial challenge for signal extraction. Researchers employ sophisticated techniques – including careful material selection, active veto systems, and advanced data analysis – to reduce this noise, but the extremely weak interaction expected of dark matter necessitates extraordinarily sensitive detectors and robust methods to confidently identify a true signal amidst the pervasive background.

The search for dark matter is increasingly focused on the sub-GeV mass range – particles lighter than a proton – yet these candidates pose exceptional detection challenges. Traditional methods, optimized for heavier Weakly Interacting Massive Particles (WIMPs), struggle with the exceedingly small energy depositions expected from lighter dark matter. This necessitates a shift towards innovative detector technologies and techniques; researchers are exploring novel materials with enhanced sensitivity, such as cryogenic detectors and specialized target nuclei. Furthermore, background discrimination becomes paramount, requiring advanced shielding and signal processing to isolate potential dark matter interactions from ambient radiation and other noise sources. The development of directional detection capabilities, aiming to identify the source of dark matter signals, also represents a promising avenue for distinguishing true interactions from background events, ultimately pushing the boundaries of sensitivity in the quest to unveil the nature of this elusive substance.

Superconducting Sensors: A Platform for Precision Measurement

Superconducting targets offer a novel approach to dark matter detection due to the materials’ exceptionally high sensitivity to energy deposition from particle interactions. Unlike traditional detectors relying on scintillation or ionization, superconducting targets measure the minute temperature increases resulting from energy absorption. These materials exhibit a sharp transition to a zero-resistance state below a critical temperature; any energy deposited by a weakly interacting massive particle (WIMP) or other dark matter candidate breaks Cooper pairs, creating quasiparticles and a measurable temperature change. This allows for a very low energy threshold, potentially enabling detection of dark matter particles with masses and interaction strengths inaccessible to other experimental techniques. The sensitivity is further enhanced by operating the target at millikelvin temperatures, minimizing thermal noise and maximizing the signal-to-noise ratio for rare event detection.

Transmon qubits function as highly sensitive kinetic inductance detectors (KIDs) within superconducting target experiments, enabling the detection of individual particle interactions. These qubits, a type of superconducting circuit, exhibit a quantized energy level structure responsive to changes in kinetic inductance caused by the deposition of even minimal energy from interacting particles. The energy deposited by an incident particle alters the qubit’s resonant frequency, which can be precisely measured using microwave readout techniques. This allows for the discrimination of single energy depositions, providing the necessary granularity for rare event searches, such as those targeting dark matter interactions. The sensitivity is achieved through careful control of the qubit’s geometry and materials, optimizing its response to these subtle energy shifts while minimizing background noise.

The transmon qubit’s layered structure, a consequence of its thin-film geometry, directly influences its sensitivity to particle interactions. These structures typically consist of a superconducting thin film-aluminum being a common choice-deposited on a substrate, often sapphire or silicon. The film’s thickness, typically on the scale of tens to hundreds of nanometers, and the patterning of this film into Josephson junctions and capacitor plates, create a resonant circuit. Incoming particles deposit energy into the superconducting film, causing quasi-particle excitations which alter the transmon’s resonant frequency. The specific response-magnitude and frequency shift-is dependent on the energy deposited and the spatial distribution of that energy within the layered structure, making precise control of film thickness and junction geometry crucial for optimizing detector performance and energy resolution.

Quasiparticle Dynamics: An Inherent Limitation in Detection

Quasiparticles are generated in superconducting detectors through the breaking of Cooper pairs, a process induced by background radiation or, hypothetically, dark matter interactions. These quasiparticles, consisting of unbound electrons and holes, exhibit current-like behavior that can be indistinguishable from the signals expected from weakly interacting massive particles (WIMPs). This mimicry constitutes a significant background source, limiting the sensitivity of dark matter searches. Furthermore, an elevated density of quasiparticles degrades detector performance by introducing excess noise and reducing the energy resolution of the superconducting sensors. The creation of quasiparticles therefore represents a fundamental limitation in low-background experiments relying on superconducting technology.

Quasiparticle (QP) poisoning represents a significant limitation in the performance of superconducting dark matter detectors utilizing transmon qubits. An elevated density of QPs introduces energy fluctuations that disrupt the quantum coherence of these qubits, effectively shortening their relaxation time $T_1$ and reducing their energy resolution. This degradation directly impacts the detector’s ability to discriminate between faint dark matter signals and background noise; a higher QP density necessitates a higher energy threshold for signal detection, diminishing sensitivity. The effect is proportional to the QP density and the duration of data acquisition, requiring careful control of environmental factors contributing to QP generation and sophisticated data analysis techniques to mitigate their impact.

Successful dark matter detection with superconducting detectors is fundamentally limited by quasiparticle dynamics. These excitations, generated by breaking Cooper pairs, introduce noise and degrade detector performance; current measurements indicate a residual quasiparticle fraction of $5.6 \times 10^{-{10}}$ . Accurate modeling of quasiparticle behavior, utilizing the Master Equation, is therefore essential for discriminating between true dark matter interactions and spurious signals. This modeling allows researchers to establish thresholds and place increasingly stringent constraints on dark matter interaction parameters, effectively defining the sensitivity limits of these detectors.

Modeling Material Response: A Mathematical Foundation for Detection

The dielectric function, denoted as $\epsilon(\omega)$ , is a complex quantity characterizing a material’s polarization response to an applied electric field of frequency ω. This response arises from the material’s electronic and atomic structure, dictating how it interacts with electromagnetic fields. For dark matter detection, the dielectric function is critical because it governs the electromagnetic response of the target material – typically a superconductor – to energy deposited by dark matter particle interactions. Specifically, it determines the induced currents and electromagnetic fields, which subsequently influence the signal observed in sensitive detectors. Variations in the dielectric function with frequency and material composition directly affect the detector’s sensitivity and ability to discriminate between dark matter signals and background noise.

The Lindhard function, $L(v) = \frac{1}{\pi} \in t_0^\in fty \frac{Im[\epsilon(\omega)]}{v} \frac{1}{1 + (v/\omega)^2} d\omega$ , provides a means to calculate the energy loss of a particle traversing a material, specifically detailing the fraction of energy deposited as phonons versus electronic excitations. In the context of dark matter detection using superconducting targets, this function is crucial because the ratio of these excitations directly influences the detector’s sensitivity; maximizing phonon production enhances the signal strength available for detection by transition edge sensors (TES). The Lindhard function, coupled with knowledge of the material’s dielectric function $\epsilon(\omega)$ and the incoming particle’s velocity $v$ , allows detector designers to optimize target materials and thicknesses to maximize the observable signal from potential dark matter interactions and minimize background noise.

Power injection into transmon qubits serves as the primary detection method by leveraging the qubit’s sensitivity to changes in its electromagnetic environment. Dark matter interactions within the superconducting target deposit minute amounts of energy, causing a shift in the qubit’s energy levels. This shift alters the resonant frequency of the qubit, which is detected by monitoring changes in the power required to maintain the qubit in a specific quantum state. The transmon qubit, designed with a relatively large Josephson energy, minimizes charge dispersion, enhancing sensitivity to these subtle energy depositions. The measurement principle relies on precisely controlling and monitoring the microwave power injected into the qubit, allowing for the detection of energy depositions on the order of $10^{-{15}}$ eV, which is crucial for probing weakly interacting dark matter candidates.

Expanding the Search: Novel Interactions and Emerging Constraints

The elusive nature of dark matter has prompted exploration into potential interactions beyond the standard model, and a leading hypothesis centers on the dark photon. This hypothetical particle serves as a mediator, a bridge allowing dark matter to interact, however weakly, with ordinary matter through electromagnetic forces. Unlike direct detection experiments searching for dark matter’s collision with atomic nuclei, the dark photon model proposes that dark matter might interact with electrons via this intermediary particle. This offers a distinct detection pathway, as even a small coupling between dark matter, the dark photon, and electrons could produce measurable signals. Current research leverages highly sensitive detectors to search for these subtle interactions, effectively opening a new window into the dark sector and expanding the scope of dark matter detection beyond traditional methods.

The elusive nature of dark matter necessitates exploration beyond standard interaction paradigms, and a promising avenue lies in the concept of kinetic mixing. This phenomenon proposes that dark photons – hypothetical particles mediating interactions within the dark sector – can subtly ‘mix’ with ordinary photons, the fundamental particles of light. This mixing doesn’t create new particles, but rather allows dark matter particles to indirectly interact with standard model particles, specifically electrons, through this intermediary dark photon. The strength of this coupling, governed by the kinetic mixing parameter, directly influences the probability of detecting dark matter interactions in laboratory experiments. A larger mixing value significantly enhances the signal, making detection more feasible. Consequently, researchers are actively investigating the implications of kinetic mixing, refining experimental designs to maximize sensitivity to these faint interactions, and establishing increasingly stringent constraints on the parameter space that governs this potential coupling mechanism.

The search for dark matter extends beyond traditional Weakly Interacting Massive Particle (WIMP) candidates, increasingly focusing on alternative interaction mechanisms with ordinary matter. This research significantly expands the scope of this search by establishing world-leading constraints on dark matter-electron scattering for particle masses below 30 keV, exceeding the sensitivity of all previous terrestrial experiments. Through meticulous data analysis, the study not only refines the understanding of low-mass dark matter but also delivers competitive limitations on dark photon dark matter with masses under 0.1 eV. By probing these previously less-explored interaction channels – including both scattering and potential absorption processes – this work dramatically broadens the parameter space considered in the ongoing quest to directly detect the elusive substance that comprises a substantial portion of the universe.

The pursuit of constraints on dark matter interactions, as detailed in this study, embodies a similar rigor. The researchers leverage existing transmon qubit data-data already collected for other purposes-to establish leading laboratory limits on dark matter-electron scattering. This resourceful approach mirrors a commitment to demonstrable truth rather than mere empirical observation. As Jean-Paul Sartre noted, “Existence precedes essence.” In this context, the existence of readily available qubit data precedes the essence of its application to a novel dark matter search, demonstrating that a solution’s validity isn’t merely in its function, but in the provable foundation upon which it rests. The application of coherence measurements, central to the study, further exemplifies this dedication to mathematical purity and provable results.

The Path Forward

The demonstrated sensitivity of transmon qubits to interactions beyond the standard model is, predictably, not the endpoint. While the current work establishes a novel, and arguably elegant, method for constraining dark matter interactions, the limitations inherent in interpreting coherence measurements demand careful consideration. The Lindhard function, a necessary component in translating qubit behavior to scattering cross-sections, relies on assumptions about material properties and dark matter velocity distributions-assumptions that introduce a systematic uncertainty that, while acknowledged, requires continued refinement. A truly robust detection necessitates a deeper understanding of these parameters, ideally through independent validation.

The reliance on quasiparticle poisoning as a signal mechanism also presents a challenge. While the observed coherence losses are compelling, the precise relationship between dark matter interactions and the generation of these quasiparticles remains somewhat empirical. A theoretical framework that definitively connects the two would strengthen the interpretability of future results. Furthermore, exploring alternative signal mechanisms within the same qubit platform-perhaps leveraging higher-order transitions or more complex circuit designs-could broaden the search space for dark matter candidates.

Ultimately, the true test will lie in the ability to move beyond simply setting limits. The potential for unambiguous detection hinges not on increased sensitivity alone, but on a rigorous, mathematically sound framework that can definitively distinguish a dark matter signal from the inevitable noise inherent in any physical system. The pursuit of such precision is, after all, the only worthwhile endeavor.

Original article: https://arxiv.org/pdf/2601.02474.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/