Delayed-Choice Quantum Eraser with Thermal Light

We report a random delayed-choice quantum eraser experiment. In a Young’s double-slit interferometer, the which-slit information is learned from the photon-number fluctuation correlation of thermal light. The reappeared interference indicates that the which-slit information of a photon, or wave packet, can be “erased” by a second photon or wave packet, even after the annihilation of the first. Different from an entangled photon pair, the jointly measured two photons, or wave packets, are just two randomly distributed and randomly created photons of a thermal source that fall into the coincidence time window. The experimental observation can be explained as a nonlocal interference phenomenon in which a random photon or wave packet pair, interferes with the pair itself at distance.

Figure 1

Schematic of delayed choice quantum eraser. The He-Ne laser beam spot on the rotating ground glass has a diameter of $\sim 2\mathrm{mm}$. A double-slit, with slit-width $150\mu \mathrm{m}$ and slit-separation 0.7 mm, is placed $\sim 25\mathrm{cm}$ away from the GG. The spatial coherence length of the pseudothermal field on the double-slit plane is calculated from Eq. (1), $l_c=\lambda /\mathrm{\Delta }\theta \sim 160\mu \mathrm{m}$, which guarantees the two fields $E_A$ and $E_B$ are spatially incoherent. Under this experimental condition, the photon number fluctuates correlatively only within slit $A$ or slit $B$. All beam splitters are nonpolarizing and $50/50$. The two fields from the two slits may propagate to detector $D_0$ which is transversely scanned on the focal plan of lens $f$ for observing the interference patten of the double-slit interferometer; and may also pass along a Mach-Zehnder-like interferometer and finally reach $D_1$ or $D_4$ ($D_2$ or $D_3$). The rise time of detector $D_0$ is less than 1 ns, $(d_{\mathrm{BS}}-d_0)/c\approx 5\mathrm{ns}$ ensures that a “delayed choice” is made at ${\mathrm{BS}}_B$. A positive-negative fluctuation-correlation protocol is followed to evaluate the photon-number fluctuation correlations from the coincidences between $D_0\text{−}{D}_1$ and $D_0\text{−}{D}_4$ (or $D_0\text{−}{D}_2$ and $D_0\text{−}{D}_3$).

Figure 2

The measured $\mathrm{\Delta }{\mathcal{R}}_{04}$ by scanning $D_0$ on the observation plane of the Young’s double-slit interferometer. The black dots are experimental data, the red line is the theoretical fitting with Eq. (12).

Figure 3

The measured $\mathrm{\Delta }{\mathcal{R}}_{01}$ as a function of the transverse coordinate of $D_0$. The black dots are experimental data, the red line is the theoretical fitting with Eq. (13).