A prototype differential atom interferometer for fundamental physics

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The discovery of gravitational waves by the LIGO and Virgo laser-interferometer experiments 10 has opened a new window on the Universe, with prospects for breakthroughs in fundamental physics, astrophysics and cosmology. Just as observations of electromagnetic waves over a wide range of frequencies have provided insights into physical processes within and beyond our Galaxy and in the primordial Universe, it is expected that observing gravitational waves over a wide range of frequencies will offer complementary insights into an equally rich spectrum of phenomena. The operating terrestrial laser-interferometer detectors—LIGO, Virgo and KAGRA—are sensitive to gravitational waves at frequencies around 10 1  Hz to 10 3  Hz (refs. 5 , 11 , 12 ), and the Laser Interferometer Space Antenna experiment, now under construction, will be most sensitive to gravitational waves with frequencies around 10 −4  Hz to 10 −1  Hz (ref. 6 ), leaving unexplored an intermediate range of frequencies around 10 −1  Hz to 10 1  Hz.

Important sources of gravitational waves in this frequency range are mergers of intermediate-mass black holes that are heavier than those detected by ground-based laser interferometers and lighter than those targeted by the Laser Interferometer Space Antenna. Such intermediate-mass black holes are thought to be the building blocks for the supermassive black holes 13 at the hearts of most galaxies, so measurements of their mergers using long-baseline atom interferometers 14 , 15 could reveal how supermassive black holes are formed 16 . Further, observations of the slowly evolving inspiral stages of solar-mass mergers would be possible for days or weeks instead of seconds, which would enable multi-messenger astronomy by pinpointing the locations of gravitational-wave sources in the sky 17 .

Atom interferometers, which use lasers to split and recombine the wavefunctions of atoms, have optimal sensitivities to gravitational waves with frequencies \({\mathcal{O}}(1)\) Hz (refs. 1 , 2 ) and, hence, are well suited to explore the frequency gap between terrestrial and space-borne laser interferometers, as seen in Fig. 1 . With the gradiometer configuration shown in Fig. 2 , a differential, single-photon, pair of atom interferometers separated by a baseline L of approximately 1 km could have sufficient sensitivity to detect gravitational waves 18 , 19 with frequencies of approximately 1 Hz, which, at present, cannot be measured. Such detectors are also sensitive to theorized interactions between atomic constituents and bosonic dark matter fields with masses of approximately 10 −15  eV (ref. 8 ), with potential resolution significantly beyond that of existing experiments 1 .

Fig. 1: The parameter space of black hole mergers probed by various gravitational-wave detectors, both operational and planned. The alternative text for this image may have been generated using AI.

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The horizontal axis gives the mass M of the black hole merger causing the gravitational wave, in units of the solar mass. The vertical axis is the distance to the gravitational-wave source, expressed as the redshift z . The cyan dots are gravitational-wave signals from a simulation of a 1-year data sample of black hole mergers generated using a hierarchical model of the formation of supermassive black holes 13 , resulting in 6 × 10 4  simulated events. The orange dots are gravitational-wave signals from a simulated sample of stellar-mass black hole mergers. The violet dots are gravitational-wave signals from a hypothetical population of primordial black holes (see Methods for details). Also shown are the prospective sensitivities of different detectors, including laser-interferometer detectors 5 , 6 , 52 and the AION-km 9 and AEDGE 4 atom-interferometer detectors, which have baselines of 1 km and 40,000 km respectively. This figure was inspired by the Cosmic Explorer proposal 52 . IMBHs, intermediate-mass black holes. ET, Einstein Telescope.

Source Data

Fig. 2: An illustration of the sensitivity of the detector to gravitational waves. The alternative text for this image may have been generated using AI.

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a – d , In the moments before the final π/2 beam-splitter pulse (Fig. 3 ), the two atom interferometers can be treated as freely falling atomic clocks ( a ) accruing phase at a rate ω 0 ; the pulse halts this accrual of phase for the lower cloud ( b ), resulting in an accrual of differential phase ( d ) that continues until the pulse reaches the second cloud ( c ). In the proper frame of the bottom cloud (as pictured), the atoms are displaced by a transient gravitational wave (GW). This has the effect of delaying (or hastening) this second interaction, imparting (at leading order) a detectable differential phase of \({\rm{\delta }}{\phi }_{\mathrm{GW}}=\pm \frac{{\rm{\delta }}L}{c}{\omega }_{0}\) (ref. 36 ). d , Differential phase accumulated between the two interferometers throughout the sequence, shown with (red)…