Controlling the Interaction of Electron and Nuclear Spins in a Tunnel-Coupled Quantum Dot

We present a technique for manipulating the nuclear spins and the emission polarization from a single optically active quantum dot. When the quantum dot is tunnel coupled to a Fermi sea, we have discovered a natural cycle in which an electron spin is repeatedly created with resonant optical excitation. The spontaneous emission polarization and the nuclear spin polarization exhibit a bistability. For a ${\sigma }^{+}$ pump, the emission switches from ${\sigma }^{+}$ to ${\sigma }^{-}$ at a particular detuning of the laser. Simultaneously, the nuclear spin polarization switches from positive to negative. Away from the bistability, the nuclear spin polarization can be changed continuously from negative to positive, allowing precise control via the laser wavelength.

Figure 1

Top: Photoluminescence (PL) at 4.2 K from a single quantum dot versus bias driven with excitation at the $X^0$ energy. $X^{1-}$ PL appears in a narrow range of voltage, the hybridization regime. Bottom: Energy dependence versus bias for the quantum dot vacuum state $|0⟩$ and the single electron state $|e⟩$, showing a crossing where the ground state changes. $X^0$ and $X^{1-}$ cross at lower bias on account of the hole. Within the hybridization region, automatic cycling takes place when a laser is tuned to the $|0⟩\leftrightarrow |X^0⟩$ transition. An electron tunneling from the Fermi sea turns the $|X^0⟩$ into $|X^{1-}⟩$; recombination leaves the system in state $|e⟩$; tunneling out returns the dot to $|0⟩$.

Figure 2

Left (right) panels: Experimentally measured (calculated) signal intensity, polarization degree, and Overhauser shift versus laser energy (laser detuning) for a ${\sigma }^{+}$ pump and an external field of $+0.5\mathrm{T}$ at fixed bias in the center of the hybridization region. In the experiment, the laser is tuned close to the $|0⟩\leftrightarrow |X^0⟩$ transition and the dot (at 4.2 K) is the same as in Fig. 1.

Figure 3

Top: Bistability between state I (${\sigma }^{+}$ PL:nuclear spins up) and state II (${\sigma }^{-}$ PL: nuclear spins down). $P$ and ${\Delta }_n$ were measured as the laser was tuned. The laser was blocked for 30 s between each point. Bottom: Demonstration of switching with gate voltage pulses (40 mV, 5 s duration) by measuring the ${\sigma }^{+}$ PL; strong PL signifying state I, weak PL signifying state II. In between voltage pulses, the laser was turned off. Both curves were recorded at $+0.5\mathrm{T}$, 4.2 K, using a ${\sigma }^{+}$ pump and the same dot as in Figs. 1, 2.

Figure 4

Top: The five quantum states in the simulation showing an optical coupling (Rabi energy $\hslash \Omega$) and a coherent coupling (energy $\hslash {\omega }_{\mathrm{fs}}$) between the two neutral exciton states. Decay processes are drawn with dashed lines. Bottom: The calculated nuclear spin dynamics as a function of laser detuning for a ${\sigma }^{+}$ pump, $+0.5\mathrm{T}$ external field, and the parameters described in the text. The solid (dashed) line shows the stable (unstable) solution for $\frac{d}{dt}⟨I_z⟩=0$.