QuGrav: Bringing gravitational waves to light with Qumodes

TL;DR

QuGrav introduces qumodes—fixed-photon-number cavity states—as a means to enhance high-frequency gravitational-wave detection via the inverse Gertsenshtein effect. By resonantly converting GWs into photons in a magnetized cavity and leveraging Bose enhancement with an $n$-photon qumode, the graviton–photon conversion rate scales with $n+1$, enabling substantially improved sensitivity provided the qumode can be continuously prepared and measured within its coherence time. The approach yields both narrowband and broadband sensitivity gains, with microwave-frequency implementations potentially approaching within $1.7$ orders of magnitude of the cosmological bound set by $\Delta N_{\rm eff}$, and optical frequencies offering an additional order-of-magnitude improvement for existing detectors. While promising, the scheme requires robust qumode regeneration, precise control of decay and noise, and advanced microwave-to-optical cavity technology to realize broad-band reach and potential single-graviton sensitivity.

Abstract

We propose using qumodes, quantum bosonic modes, for detecting high-frequency gravitational waves via the inverse Gertsenshtein effect, where a gravitational wave resonantly converts into a single photon in a magnetized cavity. For an occupation number $n$ of the photon field in a qumode, the conversion probability is enhanced by a factor of $n+1$ due to Bose-Einstein statistics. Unlocking this increased sensitivity entails the ability to continuously prepare the qumode and perform non-demolition measurement on the qumode-qubit system within the qumode coherence time. Our results indicate that, at microwave frequencies and with existing technology, the proposed setup can attain sensitivities within 1.7 orders of magnitude of the cosmological bound. With anticipated near-future improvements, it has the potential to surpass this limit and pave the way for the first exploration of high-frequency cosmological gravitational wave backgrounds. At optical frequencies, it can enhance the sensitivity of current detectors by one order of magnitude. That further enhances their potential in reaching the single-graviton level.

Figures (2)

• Figure 1: Left: the inverse Gertsenshtein effect, and right: toy model of Qumode detector.
• Figure 2: Strain sensitivity of HFGW detectors based on photon regeneration. The area below the orange line corresponds to gravitational fields that behave like quantum particles (gravitons). The dashed gray curve denotes the $\Delta N_{\text{eff}}$ cosmological bound. Hatched regions indicate projected improvements in sensitivity from current experiments and proposals employing a 100-photon qumode ( $\ket{Q_{100}}$ in \ref{['eq:Qu-1']}) within the cavity. The checkered (black-blue) represents a sensitivity that relies on technological progress in cavity development anticipated in the near future, i.e., increasing S by 1.7 orders of magnitude (see \ref{['eq:def-S']}). The crossed regions indicate the enhanced sensitivity to broadband signals achievable with the futuristic broadband 100-photon Qumode $\overline{\ket{Q_{100}}}$ described in \ref{['eq:Qu-2']}.