Efficient Optical Quantum State Engineering

We discuss a novel method of efficiently producing multiphoton states using repeated spontaneous parametric down-conversion. Specifically, by attempting down-conversion several times, we can pseudodeterministically add photons to a mode, producing various several-photon states, e.g., Fock states and $N00N$ states (number-path entangled states of the form $|N_A,0_B⟩+|0_A,N_B⟩$). This scheme is exponentially more efficient than previous proposals; we discuss expected performance and experimental limitations.

Figure 1

Diagram of proposed Fock-state source. A pulsed laser pumps a down-conversion crystal (DC). The signal photon of each created pair is detected, and the idler photon is emitted into a storage cavity. Photons are allowed to accumulate in the cavity until the desired number is reached. The light can be switched out by rotating its polarization with a Pockels cell (PC1), so the polarizing beam splitter (PBS) reflects rather than transmits it.

Figure 2

Theoretical performance of Fock-state source versus cavity transmission. Curves are shown for several values of $N$ (labeled), and several detector efficiencies (black solid, $\eta =1$; blue dashed, $\eta =0.95$; red dotted, $\eta =0.9$).

Figure 3

Diagram of proposed $N00N$-state source. Similar to the setup for the Fock-state source (Fig. 1), with the addition of a Pockels cell (PC2) in the cavity to rotate the polarization of the photons as they are created, and a polarization-independent switch [made up of a beam splitter (BS), half-wave plate (HWP), and Pockels cell (PC1)] in place of a polarization-dependent switch. The inset shows the linear polarization of 4 photons in the desired state.

Figure 4

Theoretical performance of $N00N$-state source. Curves are shown for several values of $N$ (labeled), and for several different detector efficiencies (black solid, $\eta =1$; blue dashed, $\eta =0.95$; red dotted, $\eta =0.9$).