Quantum-optical phenomena in magnetoelectric crystals

Since Star Wars, lasers have been known from sci-fi movies and more recently from everyday applications such as DVD players and laser pointers. Unlike conventional lamps, lasers produce a coherent beam of light that can be described by a single wavelength and a single wave function of position and time. The laser medium is located between a pair of mirrors, a so - called optical cavity resonator. The cavity length selects the only possible light mode that determines the wavelength of the output beam. However, lasers are sensitive to small changes in the length of the optical cavity resonator, which limits their use in high-precision applications such as atomic clocks. This disadvantage is overcome in quantum optics. The phase memory of the system is stored in the collective quantum state of the laser material and not in the geometric parameters of the resonator. The collective state of the laser material guarantees the high stability of the laser beam when isolated from the environment. Experimental realizations of such systems typically work near absolute zero temperature, limiting their investigation and making practical applications impossible. In our project we plan to realize quantum optical effects at routinely accessible standard laboratory temperatures. Independent up or down directed magnetic moments (spins) o f rare earth metal ions give the ensemble of two-state quantum systems of our study. The wave that couples them is provided by the oscillations (spin waves) of the ordered magnetic iron moments surrounding them. Thus, we concentrate on realizing the magnetic alternative of conventional quantum optics of the electrical domain. Besides the fundamental novelty, the main practical advantages of our approach are the higher, easily accessible characteristic temperatures, and, due to the longer wavelength of the spin waves, the insensitivity to small changes in sample size.

Since the debut of Star Wars, lasers have captured the imagination through sci-fi movies and found their way into everyday applications like DVD players and pen laser pointers. Unlike conventional lamps, lasers emit coherent light beams characterized by a single wavelength and a uniform wave-function of position and time. This coherence arises from optical amplification, where the dimensions of the optical cavity containing the lasing medium select a single light mode corresponding to the wavelength of the output beam. However, lasers are susceptible to minute changes in the length of the optical cavity resonator, which limits their use in precision applications such as atomic clocks. In quantum optics, this limitation is addressed by storing the phase memory of the system in the collective state of the lasing material rather than relying on the resonator's geometric parameters. When isolated from the environment, the collective state of the lasing material ensures the high stability of the laser beam. Experimental realizations of such superradiant systems typically operate at temperatures close to absolute zero, which restricts their study and hampers practical applications. In our project, we aimed to achieve quantum-optical effects at standard laboratory temperatures routinely. We utilized independent magnetic moments (spins) of rare-earth metal ions, which act as two-state quantum systems in our study. The wave connecting these spins is facilitated by the oscillations (spin waves) of the surrounding magnetically ordered magnetic moments. Thus, our focus lies on realizing the magnetic counterpart of conventional electric-domain quantum optics. Apart from its fundamental novelty, our approach offers practical advantages such as higher, easily attainable characteristic temperatures and insensitivity to minor changes in sample size due to the longer wavelength of spin waves. While our attempt yielded partial success, achieving a coupling strength only 60% of that necessary for the superradiant state, our findings represent a significant advancement compared to the scientific state-of-the-art. Furthermore, our results demonstrate the promising potential of our approach for investigating quantum-optical phenomena. As a byproduct of our investigations into the superradiant phase transition, we first uncovered the role of collective effects in the apparent damping of magnetic resonances. Additionally, we also discovered exotic magnetoelectric multiple-magnon resonances in magnetolectric crystals.