Quantum Science and the Search for Axion Dark Matter

The dark matter puzzle is one of the most important open problems in modern physics. The ultra-light axion is a well-motivated dark matter candidate, conceived to resolve the strong-CP problem of quantum chromodynamics. Numerous precision experiments are searching for the three non-gravitational interactions of axion-like dark matter. Some of the searches are approaching fundamental quantum limits on their sensitivity. This Perspective describes several approaches that use quantum engineering to circumvent these limits. Squeezing and single-photon counting can enhance searches for the axion-photon interaction. Optimization of quantum spin ensemble properties is needed to realize the full potential of spin-based searches for the electric-dipole-moment and the gradient interactions of axion dark matter. Several metrological and sensing techniques, developed in the field of quantum information science, are finding natural applications in this area of experimental fundamental physics

Effective electric field: quantifying the sensitivity of searches for new P,T-odd physics with EuCl3⋅_3\cdot6H2_2O

Laboratory-scale precision experiments are a promising approach to searching for physics beyond the standard model. Non-centrosymmetric solids offer favorable statistical sensitivity for efforts that search for new fields, whose interactions violate the discrete parity and time-reversal symmetries. One example is the electric Cosmic Axion Spin Precession Experiment (CASPEr-e), which is sensitive to the defining interaction of the QCD axion dark matter with gluons in atomic nuclei. The effective electric field is the parameter that quantifies the sensitivity of such experiments to new physics. We describe the theoretical approach to calculating the effective electric field for non-centrosymmetric sites in ionic insulating solids. We consider the specific example of the EuCl$_3\cdot$6H$_2$O crystal, which is a particularly promising material. The optimistic estimate of the effective electric field for the $^{153}$Eu isotope in this crystal is 10 MV/cm. The calculation uncertainty is estimated to be two orders of magnitude, dominated by the evaluation of the Europium nuclear Schiff moment

A Precessing Ferromagnetic Needle Magnetometer

A ferromagnetic needle is predicted to precess about the magnetic field axis at a Larmor frequency $\Omega$ under conditions where its intrinsic spin dominates over its rotational angular momentum, $N\hbar \gg I\Omega$ ($I$ is the moment of inertia of the needle about the precession axis and $N$ is the number of polarized spins in the needle). In this regime the needle behaves as a gyroscope with spin $N\hbar$ maintained along the easy axis of the needle by the crystalline and shape anisotropy. A precessing ferromagnetic needle is a correlated system of $N$ spins which can be used to measure magnetic fields for long times. In principle, by taking advantage of rapid averaging of quantum uncertainty, the sensitivity of a precessing needle magnetometer can far surpass that of magnetometers based on spin precession of atoms in the gas phase. Under conditions where noise from coupling to the environment is subdominant, the scaling with measurement time $t$ of the quantum- and detection-limited magnetometric sensitivity is $t^{-3/2}$. The phenomenon of ferromagnetic needle precession may be of particular interest for precision measurements testing fundamental physics.Comment: Main text: 6 pages, 2 figures; Supplementary material: 3 pages, 1 figur

Floquet-engineered quantum state manipulation in a noisy qubit

Adiabatic evolution is a common strategy for manipulating quantum states and has been employed in diverse fields such as quantum simulation, computation and annealing. However, adiabatic evolution is inherently slow and therefore susceptible to decoherence. Existing methods for speeding up adiabatic evolution require complex many-body operators or are difficult to construct for multi-level systems. Using the tools of Floquet engineering, we design a scheme for high-fidelity quantum state manipulation, utilizing only the interactions available in the original Hamiltonian. We apply this approach to a qubit and experimentally demonstrate its performance with the electronic spin of a Nitrogen-vacancy center in diamond. Our Floquet-engineered protocol achieves state preparation fidelity of $0.994 \pm 0.004$, on the same level as the conventional fast-forward protocol, but is more robust to external noise acting on the qubit. Floquet engineering provides a powerful platform for high-fidelity quantum state manipulation in complex and noisy quantum systems

Probing dynamics of a two-dimensional dipolar spin ensemble using single qubit sensor

Understanding the thermalization dynamics of quantum many-body systems at the microscopic level is among the central challenges of modern statistical physics. Here we experimentally investigate individual spin dynamics in a two-dimensional ensemble of electron spins on the surface of a diamond crystal. We use a near-surface NV center as a nanoscale magnetic sensor to probe correlation dynamics of individual spins in a dipolar interacting surface spin ensemble. We observe that the relaxation rate for each spin is significantly slower than the naive expectation based on independently estimated dipolar interaction strengths with nearest neighbors and is strongly correlated with the timescale of the local magnetic field fluctuation. We show that this anomalously slow relaxation rate is due to the presence of strong dynamical disorder and present a quantitative explanation based on dynamic resonance counting. Finally, we use resonant spin-lock driving to control the effective strength of the local magnetic fields and reveal the role of the dynamical disorder in different regimes. Our work paves the way towards microscopic study and control of quantum thermalization in strongly interacting disordered spin ensembles

Understanding the dynamics of randomly positioned dipolar spin ensembles

Dipolar spin ensembles with random spin positions are attracting much attention because they help us to understand decoherence as it occurs in solid-state quantum bits in contact with spin baths. Also, these ensembles are systems which may show many-body localization, at least in the sense of very slow spin dynamics. We present measurements of the autocorrelations of spins on diamond surfaces at infinite temperature in a doubly rotating frame which eliminates local disorder. Strikingly, the timescales in the longitudinal and the transversal channel differ by more than one order of magnitude, which is a factor much greater than one would have expected from simulations of spins on lattices. A previously developed dynamic mean-field theory for spins (spinDMFT) fails to explain this phenomenon. Thus, we improve it by extending it to clusters (CspinDMFT). This theory does capture the striking mismatch up to two orders of magnitude for random ensembles. Without positional disorder, however, the mismatch is only moderate with a factor below 4. The pivotal role of positional disorder suggests that the strong mismatch is linked to precursors of many-body localization

Understanding the dynamics of randomly positioned dipolar spin ensembles

Dipolar spin ensembles with random spin positions attract much attention currently because they help to understand decoherence as it occurs in solid state quantum bits in contact with spin baths. Also, these ensembles are systems which may show many-body localization, at least in the sense of very slow spin dynamics. We present measurements of the autocorrelations of spins on diamond surfaces in a doubly-rotating frame which eliminates local disorder. Strikingly, the time scales in the longitudinal and the transversal channel differ by more than one order of magnitude which is a factor much greater than one would have expected from simulations of spins on lattices. A previously developed dynamic mean-field theory for spins (spinDMFT) fails to explain this phenomenon. Thus, we improve it by extending it to clusters (CspinDMFT). This theory does capture the striking mismatch up to two orders of magnitude for random ensembles. Without positional disorder, however, the mismatch is only moderate with a factor below 4. The pivotal role of positional disorder suggests that the strong mismatch is linked to precursors of many-body localization.Comment: 21 pages, 12 figure