Dark matter searches using superconducting qubits

Knocked down by a gust of "dark breeze"

Superconducting qubits are very fragile to noises. Things like electric noise, magnetic noise, irregular crystal structures in the chip interfaces, or even cosmic rays can easily disturb the qubit state, causing the "errors". Although this vulnerability is completely unwelcome from the quantum computation point of view, it implies that they can be good sensors for feebly interacting particles or fields. Specifically, our group has been pursing the application of superconducting qubits for "dark matter sensing".

Direct detection of massive dark matter in the frame work called WIMP (Weakly Interacting Massive Particle) has been widely attempted for the past few decades, and the absence of its discovery has set a very stringent limits. On the other side, the low mass regime remains relatively under-explored and therefore has gained more and more importance these days. Dark matter with a mass below O(eV) is referred to as "wave-like dark matter" because of their macroscopic de Broie wavelength (>O(mm)) such that the wave functions of a macroscopic number of dark matter particles are coherent. The most motivated candidates include axion, axion-like particle, or dark photon, which are predicated from the extensions of the Standard Model of particle physics. While they are known to be able to be converted to photons though at a very low rate, superconducting qubits are supposed to be very good at detecting such photons thanks to its huge electric dipole moment, 6 order of magnitude larger than a usual single atom. As for axion, the mass range of O(µeV-meV) is most preferred for it to be the dark matter, corresponding to a frequency of O(0.1-100GHz) for the converted photons. This coincides with the operational frequency for superconducting qubits, opening up a straightforward application of the looming quantum technologies to searching wave-like dark matters (it even provokes a speculative expectation that quantum computers can be straightforwardly dark matter detectors...!).

Catch the photons from the dark wind

Suppose the dark photon dark matter (DPDM) as the simplest example. Since the dark photon (DP) kinetically mixes with the ordinary photon, if DPDM permeates the space there should be a feeble harmonic electromagnetic field everywhere at the frequency corresponding to the mass. One of the most successful experiments of detecting such feeble coherent photons from DPDM is "haloscope" using high-Q microwave cavities. In this setup, the photons are piled up to the Q-value, and the power is read out as a RF signal through a strongly-coupled antenna . However since the readout signal is too low (typically the average photon number in the cavity is << 1), the sensitivity is limited by the quantum measurement back-action characterized by the uncertainty relation, called "Standard Quantum Limit" (SQL). Qubits are anticipated for the game-changer as they can circumvent the SQL by the performing projection measurements to the photon number eigenstates. The first demonstration experiment has been already reported [1] relying on the precise detection of the qubit frequency shift in the presence of ambient photons ("ac Stark shift"). This broke through towards the next generation haloscope experiments using quantum technology. Our group's focus is to extend their demonstration setup towards a physics search with a wide bandwidth, as well as the R&D for the even more advanced quantum application.

An alternative approach is to utilize the direct excitation of qubit (|0> →|1>) [2], caused by photons resonant to qubit frequency. While there are numerous number of noise sources expediting qubits' deexcitations (|1>→|0>), there is only remarkably limited number of noises that can cause the excitations.
With a SQUID-based transmon the frequency tunability can be also easily acquired, allowing to scan a few order of magnitude for the dark matter mass. The absolute sensitivity is yet not as good as the cavity-based method due to the lack of the high-Q enhancement, this method has an advantage in searching high frequency regime (>10GHz) where the cavity-based method start to suffer from the dwinding detection volume as a result of smaller size of resonant cavities, suggesting a good complementarity between the two methods.

Status of our experiment

Using the facility of Takeda Super Cleanroom (Center of The University of Tokyo for The Advanced Research Infrastructure for Materials and Data Hub Nanofabrication Division) and the Mili-Kelvin Quantum Platform (Cryogenic Research Center of University of Tokyo), the first pass of fabricating and characterizing superconducting qubits have been already completed in autumn 2022, achieving a coherence time of T1~10µs and T2~3µs. A very intense campaign for the improvement has been carried out in 2023 aiming an extension of coherence time for an order of magnitude. The development of the DAQ system is also in progress, targeting the coming first physics data taking.

References

• Searching for Dark Matter with a Superconducting Qubit
  Akash V. Dixit et. al, Phys. Rev. Lett. 126, 141302 (2021)
• Detection of hidden photon dark matter using the direct excitation of transmon qubits
  Shion Chen, Hajime Fukuda, Toshiaki Inada, Takeo Moroi, Tatsumi Nitta, Thanaporn Sichanugrist, Phys. Rev. Lett. 131, 211001 (2023).