Bosonic particles beyond the Standard Model, such as axions, are among the leading candidates for dark matter. If such particles have a small mass, they are usually described as a purely classical field. However, it was recently shown that this description is incomplete: their inherent gravitational self-interactions can squeeze the field, producing a quantum feature beyond the classical description. Here we analyze squeezing of light dark matter in the presence of decoherence. We show that squeezing is continuously generated, and rapidly enough to survive dissipation, and may also remain present under thermal fluctuations and dephasing. Our results demonstrate that dark matter might possess persistent quantum features and motivate further investigation of their implications for dark matter detection.
Bosonic particles beyond the Standard Model, such as axions, are among the leading candidates for dark matter. If such particles have a small mass, they are usually described as a purely classical field. However, it was recently shown that this description is incomplete: their inherent gravitational self-interactions can squeeze the field, producing a quantum feature beyond the classical description. Here we analyze squeezing of light dark matter in the presence of decoherence. We show that squeezing is continuously generated, and rapidly enough to survive dissipation, and may also remain present under thermal fluctuations and dephasing. Our results demonstrate that dark matter might possess persistent quantum features and motivate further investigation of their implications for dark matter detection.
Direct detection of dark matter remains an important outstanding problem since abundant astrophysical evidence points towards its existence, but no experiment has succeeded in detecting it. Axions and axion-like-particles are some of the most compelling candidates for dark matter given their appearance in many theories of physics beyond the Standard Model and their relatively unexplored parameter space compared to other candidates. Recently, the Cosmic Axion Spin Precession Experiment-Electric (CASPEr-e) has used nuclear magnetic resonance (NMR) to search for effective magnetic fields created by axionic dark matter. By decreasing technical noise sources, CASPEr-e is projected to reach the standard quantum limit where spin projection noise is the dominant noise source limiting sensitivity. However, some axion models predict axion couplings to normal matter that would be too small for even a quantum limited CASPEr-e experiment to detect. This creates a need for surpassing the spin projection noise limit in NMR dark matter searches. In this thesis, I explore the prospects for surpassing the quantum limit in NMR by using spin squeezed states, entangled states with variance in one projection reduced below the standard quantum limit. First, I propose an experimental scheme for generating squeezed states by coupling the spins to an off-resonant circuit to create a One-Axis-Twist Hamiltonian. Then, using exact results and numerical simulations, I determine the amount of squeezing that can be achieved given decoherence and noise. Next, I perform modeling to show that squeezing can accelerate dark matter searches despite earlier results that argued squeezing cannot improve experimental sensitivity when subject to decoherence. Finally, I apply these results to the CASPEr-e experiment and show that at axion frequencies near 100MHz, squeezing can speed up the experiment by a factor of up to 30, corresponding to a sensitivity improvement by a factor of over 5.
Decoherence describes the tendency of quantum sub-systems to dynamically lose their quantum character. This happens when the quantum sub-system of interest interacts and becomes entangled with an environment that is traced out. For ordinary macroscopic systems, electromagnetic and other interactions cause rapid decoherence. However, dark matter (DM) may have the unique possibility of exhibiting naturally prolonged macroscopic quantum properties due to its weak coupling to its environment, particularly if it only interacts gravitationally. In this work, we compute the rate of decoherence for light DM in the galaxy, where a local density has its mass, size, and location in a quantum superposition. The decoherence is via the gravitational interaction of the DM overdensity with its environment, provided by ordinary matter. We focus on relatively robust configurations: DM perturbations that involve an overdensity followed by an underdensity, with no monopole, such that it is only observable at relatively close distances. We use non-relativistic scattering theory with a Newtonian potential generated by the overdensity to determine how a probe particle scatters off of it and thereby becomes entangled. As an application, we consider light scalar DM, including axions. In the galactic halo, we use diffuse hydrogen as the environment, while near the earth, we use air as the environment. For an overdensity whose size is the typical DM de Broglie wavelength, we find that the decoherence rate in the halo is higher than the present Hubble rate for DM masses m a ≲ 5 × 10 −7 eV and in earth based experiments it is higher than the classical field coherence rate for m a ≲ 10 −6 eV . When spreading of the states occurs, the rates can become much faster, as we quantify. Also, we establish that DM BECs decohere very rapidly and so are very well described by classical field theory.
Quantum mechanics allows for states in macroscopic superpositions, but they ordinarily undergo rapid decoherence due to interactions with their environment. A system that only interacts gravitationally, such as an arrangement of dark matter (DM), may exhibit slow decoherence. In this letter, we compute the decoherence rate of a quantum object in general relativity. For axion DM in a superposition of the field's phase, we find that DM in the Milky Way is robust against decoherence, while a spatial superposition is not. This novel phase behavior may be relevant to direct detection experiments.
We provide a fully quantum-mechanical description of the continuous measurement of wavelike dark matter. Our focus is cavity haloscopes, although our results inform a broad range of axion searches. We extend the input-output formalism by treating the axion as a quantum field, finding how its quantum state is imprinted onto higher-order moments of observables of the outgoing transmission-line modes. We further develop a novel measurement-record formalism, which computes the joint probability distribution of all measurement outcomes and is well suited to studying the axion in the time domain. With it, we compute the exact axion-induced number distribution for the first time, demonstrating that it is not Poisson. We argue that virialized axion dark matter is expected to be in a Gaussian quantum state, and show that number or quadrature measurements can distinguish this from the predictions of a coherent state or even a Bose-Einstein condensate. Finally, we show that intrinsically quantum effects, such as those from Fock or squeezed axion states, are far too weak to detect.
In particle physics, axions and axion-like particles are ubiquitous. Remarkably, ultra-light axions could constitute dark matter or dark energy. Therefore, it is important to detect axions experimentally. In the presence of a magnetic field, a photon can be converted into an axion, and vice versa. Utilizing the conversion phenomenon, several methods for detecting axions have been proposed. To improve detectability, it is desirable to use quantum sensing. However, since the conversion process is usually treated as classical wave dynamics, it is unclear how to incorporate quantum effects such as entanglement. In this work, we formulate the photon-axion conversion in a quantum field theoretical manner. As a result, we succeed in evaluating the conversion probability from a photon quantum state to an axion quantum state. In particular, it turns out that squeezed coherent states can enhance the conversion probability.
Quantum mechanics allows for states in macroscopic superpositions, but they ordinarily undergo rapid decoherence due to interactions with their environment. A system that only interacts gravitationally, such as an arrangement of dark matter (DM), may exhibit slow decoherence. In this Letter, we compute the decoherence rate of a quantum object within general relativity, focusing on superposed metric oscillations; a rare quantum general relativistic result. For axion DM in a superposition of the field's phase, we find that DM in the Milky Way is robust against decoherence, while a spatial superposition is not. This novel phase behavior may impact direct detection experiments.