Quantum Computers Based on Electron Spins Controlled by Ultrafast Off-Resonant Single Optical Pulses

We describe a fast quantum computer based on optically controlled electron spins in charged quantum dots that are coupled to microcavities. This scheme uses broadband optical pulses to rotate electron spins and provide the clock signal to the system. Nonlocal two-qubit gates are performed by phase shifts induced by electron spins on laser pulses propagating along a shared waveguide. Numerical simulations of this scheme demonstrate high-fidelity single-qubit and two-qubit gates with operation times comparable to the inverse Zeeman frequency.

Figure 1

Sketch of a loop-qubus quantum computer. The switch introduces, ejects, and provides displacement operations on the coherent-state pulse.

Figure 2

(a) Energy level diagram for a charged quantum dot in an in-plane $B$ field. Light with ${\sigma }^{+}$ polarization incident along the growth direction ($x$ axis) couples the $|m_e=+1/2{⟩}_x=1/\sqrt{2}(|m_e=1/2{⟩}_z+|m_e=-1/2{⟩}_z)\leftrightarrow |m_h=+3/2{⟩}_x$ transitions, isolating a three-level system. (b) Energy level picture of two pairs of frequencies contained within the applied pulse that will induce transitions between states $|0⟩$ and $|1⟩$. (c) Frequency domain picture of the optical pulse, showing the two pairs of frequency components shown in (b).

Figure 3

Rotations about various axes induced by pulse delays, (a) $x$-pulse train (b) $y$-pulse train, and (c) $-x$-pulse train.

Figure 4

(a) Fidelity of single-qubit rotations for $\pi$ and $\pi /2$ pulses vs Rabi frequency. (b) Fidelity of two-qubit gates vs ${\Gamma }_{\mathrm{L}}/{\Gamma }_0$ for different values of ${\kappa }_{\mathrm{c}}/\kappa$ and $\alpha$.