A photonic integrated circuit for time-bin qubits*

Scalability is a significant challenge for expanding quantum networks. In the classical internet, information is encoded in the presence or absence of light in an optical fiber at a given time. A natural analog for quantum information is time-bin encoding: a photon can be in one (or a coherent superposition) of two (or more) time bins, measured by the relative delay in arrival time at a detector. Fiber-based and free-space optical time-bin systems have a significant system footprint, and require repeated precise alignment or stabilization to counteract drift and maintain performance. We use a lithium niobate photonic integrated circuit (PIC) to analyze time-bin qubits at 1560 nm. The PIC contains a balanced Mach-Zehnder interferometer (MZI) and an unbalanced MZI. The path imbalance in the latter is such that the 'early' and 'late' time-bins are separated by 200 ps. Sending such a two time-bin state, generated with an equivalent interferometer with a matching path imbalance, as an input to the device yields three time-bins at the output. The final middle time bin in this case corresponds to a superposition of the long-short and short-long path combinations through the two unbalanced interferometers, which exhibit interference when the path length imbalances are matched. By adjusting the switching ratio in the first MZI, we can equate the amplitudes of these processes, thus maximizing the achievable interference visibility. Varying the relative phase, and with polarization filtering, we observe a maximum visibility of ~83%. Improved performance is expected with the inclusion of active phase stabilization, using a narrowband CW laser. These measurements are an important step toward scalable quantum networking.