Dual-Species Matter Qubit Entangled with Light

We propose and demonstrate an atomic qubit based on a cold ${}^{85}\mathrm{Rb}\mathrm{\text{−}}{}^{87}\mathrm{Rb}$ isotopic mixture, entangled with a frequency-encoded optical qubit. The interface of an atomic qubit with a single spatial light mode, and the ability to independently address the two atomic qubit states, should provide the basic interferometrically robust element of a quantum network.

Figure 1

(a) Schematic of the experimental setup showing the geometry of the addressing and scattered fields from the co-trapped isotope mixture of ${}^{85}\mathrm{Rb}\mathrm{\text{−}}{}^{87}\mathrm{Rb}$. The write and read laser fields generate signal and idler fields, respectively, detected at D1 and D2; E1 and E2 are optical frequency filters. PM1-4 are light phase modulators; ${\varphi }_s$ and ${\varphi }_i$ are relative phases of the driving rf fields. The insets shows the relevant atomic levels. (b),(c) Diagrams of the light field frequencies of the write, read, signal and idler fields, see text for details.

Figure 2

Measured $C_{si}({\varphi }_s,{\varphi }_i)$ as a function of ${\varphi }_i$ for ${\varphi }_s=0$, diamonds and for ${\varphi }_s=-\pi /2$, circles. The angle ${\varphi }_0$ is absorbed into the arbitrary definition of the origin, i.e., ${\varphi }_0$ is defined to be zero. Solid lines are sinusoidal fringes based on Eq. (5) with ${\rm Y}=0.86$. Single channel counts of D1 and D2 show no dependence on the phases.