Mapping Spin Coherence of a Single Rare-Earth Ion in a Crystal onto a Single Photon Polarization State

We report on optical detection of a single photostable ${\mathrm{Ce}}^{3+}$ ion in an yttrium aluminium garnet (YAG) crystal and on its magneto-optical properties at room temperature. The spin quantum state of the emitting level of a single cerium ion in YAG can be initialized by a circularly polarized laser pulse. Coherent precession of the electron spin is read out by observing temporal behavior of circularly polarized fluorescence of the ion. This implies direct mapping of the spin quantum state of ${\mathrm{Ce}}^{3+}$ ion onto the polarization state of the emitted photon and represents the quantum interface between a single spin and a single photon.

Figure 1

Detection of a single ${\mathrm{Ce}}^{3+}$ ion in a YAG crystal. (a) Scanning confocal image of ${\mathrm{Ce}}^{3+}$ centers in a nominally pure YAG crystal. (b) Photon correlation signal taken on the marked spot on (a) with a pulsed laser excitation. Antibunching at zero time delay indicates that the emitter is indeed a single quantum object. (c) Fluorescence decay signal taken on the same spot reveals the lifetime of 64 ns in agreement with the known lifetime of the lowest $5d$ state of ${\mathrm{Ce}}^{3+}$ in YAG. (d) The spectrum of the emission of the same spot (lower curve, black line) well correlates with the fluorescence spectrum of a bulk Ce:YAG crystal (upper curve, red line).

Figure 2

Electronic level structure of ${\mathrm{Ce}}^{3+}$ ion in a YAG crystal. (a) $4f^1$ and $5d^1$ shells are split by a combined action of spin-orbit coupling and the crystal field. While in the $4f^1$ state spin-orbit coupling dominates, in the $5d^1$ states the major contribution to the splitting is from the crystal field. (b) Strengths of the transition dipoles between the lowest $4f$ and the lowest $5d$ Kramer’s doublets. The quantization axis is chosen along the $y$ axis of the local frame of the cerium site [10]. (c) Calculated time traces of the fluorescence corresponding to the ZPL of ${\mathrm{Ce}}^{3+}$ once the ion is pumped into one of the lowest $5d$ spin sublevels. The magnetic field is along the $y$ axis of the local frame of cerium site. The ${\sigma }_{-}$ polarization (red curve) builds up due to spin relaxation while the ${\sigma }_{+}$ polarization (black curve) declines until it reaches the level of ${\sigma }_{-}$ polarization. (d) Calculated fluorescence oscillations with the external magnetic field applied along the $x$ axis of the local frame of cerium site. The curves corresponding to ${\sigma }_{+}/{\sigma }_{-}$ start at maximum/minimum, respectively.

Figure 3

Quantum beats of the fluorescence of a single ${\mathrm{Ce}}^{3+}$ ion. Solid lines represent fitting curves. (a) ${\sigma }_{+}$ and ${\sigma }_{-}$ decay signals after excitation with a circularly polarized laser pulse. The external magnetic field of 500 G is along the laser propagation direction parallel to the (111) crystal axis. (b) Larmor precession of the excited electron spin visualized by oscillating behavior of the ${\sigma }_{+}$ and ${\sigma }_{-}$-polarized emissions. The magnetic field of 250 G is perpendicular to the (111) crystal axis and to the excitation laser direction. (c) Nonexponential decay of the ${\sigma }_{+}$ and ${\sigma }_{-}$-polarized fluorescent signals reveals strong hyperfine coupling to the surrounding ${}^{27}\mathrm{Al}$ nuclei.