Decoherence as a way to measure extremely soft collisions with dark matter

Abstract

A new frontier in the search for dark matter (DM) is based on the idea of detecting the decoherence caused by DM scattering against a mesoscopic superposition of normal matter. Such superpositions are uniquely sensitive to very small momentum transfers from new particles and forces, especially DM with a mass below 100 MeV. Here we investigate what sorts of dark sectors are inaccessible with existing methods but would induce noticeable decoherence in the next generation of matter interferometers. We show that very soft, but medium range (0.1 nm -1 µm) elastic interactions between nuclei and DM are particularly suitable. We construct toy models for such interactions, discuss existing constraints, and delineate the expected sensitivity of forthcoming experiments. The first hints of DM in these devices would appear as small variations in the anomalous decoherence rate with a period of one sidereal day. This is a generic signature of interstellar sources of decoherence, clearly distinguishing it from terrestrial backgrounds. The OTIMA experiment under development in Vienna will begin to probe Earth-thermalizing DM once sidereal variations in the background decoherence rate are pushed below one part in a hundred for superposed 5-nm gold nanoparticles. The proposals by Bateman et al. and Geraci et al. could be similarly sensitive, although they would require at least a month of data taking. DM that is absorbed or elastically reflected by the Earth, and so avoids a greenhouse density enhancement, would not be detectable by those three experiments. On the other hand, the aggressive proposals of the MAQRO collaboration and Pino et al. would immediately open up many orders of magnitude in DM mass, interaction range, and coupling strength, regardless of how DM behaves in bulk matter.

Pure nanodiamonds for levitated optomechanics in vacuum

“…At approximately 10 −2 -10 −3 mbar, black-body radiation becomes the dominant heat dissipation mechanism and the temperature stabilizes at 1400 K. At atmospheric pressure, nanodiamond graphitizes between 940 and 1070 K [48,49]. Therefore, low absorption grade (850 K) or electronic grade (400 K) material might be required to reach the pressures required for proposals [1][2][3][4][5][6][7][8][9].…”

“…Optically levitated nanodiamonds containing nitrogen vacancy (NV − ) centre spin defects have been proposed as probes of quantum gravity [1,2], mesoscopic wavefunction collapse [3][4][5][6], phonon mediated spin coupling [7], and the direct detection of dark matter [8,9]. The NV − centre is a point defect in diamond that has a single electron spin which has long coherence times at room temperature and can be both polarized and read out optically [10,11].…”

Exploring the ultra-light to sub-MeV dark matter window with atomic clocks and co-magnetometers

“…A possibility to explore smaller masses is to use electronic instead of nuclear targets to capture a larger fraction of the DM's kinetic energy, although these too have a lower threshold on mass of order MeV [10,22,23]. Other alternatives using different physical phenomena to detect sub-MeV DM are currently under study [22,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Moving to the 'ultra-light' regime requires yet a new battery of techniques (including relaxing the energy threshold by studying absorption, and not only scattering, of DM), e.g.…”

“…Thus, it can be used to explore masses from the lowest values allowed to a few keV, the upper limit arising from the fact that our expected constraints become worse than existing ones. Other complementary ideas to use quantum devices to detect light DM include [37][38][39].…”

Quantum enhanced estimation of diffusion

“…While our results will be presented in the context of arXiv:1901.02497v3 [quant-ph] 2 Sep 2019 collapse models, observing similar momentum diffusion processes could aid detection of certain dark-matter candidates [42][43][44]. Since excess heating of wavepackets is also a consequence of momentum diffusion [20,45,46], our results imply a quantum enhanced estimation of heating.…”