Lithium niobate (LN) exhibits many exciting material properties, one of which is the electro-optic effect. Applying an electric field across the LN changes its refractive index. This change in refractive index can be mapped onto a phase shift, and therefore a frequency shift, of light passing through the crystal. By modulating an applied voltage on-chip, such as with a GHz-frequency RF tone, one can rapidly modulate this phase shift, generating sidebands at new optical frequencies, shifted from the main laser tone by the frequency of modulation.
While these “electro-optic modulators” have existed for many years in bulk or “free-space” setups, high-efficiency modulators have also been developed on-chip. Compared with acousto-optic or piezo-electric vibrational modulation (scattering between phonons and photons), electro-optic performance is near-instantaneous and exhibits much larger bandwidths of modulation (on the order of tens of GHz). These frequency shifts can be applied to a variety of technologies, including the development of new types of classical and quantum optical sensors, optical computation, on-chip laser sources, frequency combs (see nonlinear optics), optical quantum gates, and even all-optical control for on-chip quantum applications.
LINQS has demonstrated an efficient quantum electro-optic converter, leveraging frequency-shifting techniques at single-photon levels and cryogenic temperatures, to convert microwave photons (in the GHz regime) into optical photons (THz regime). Through this conversion process, the quantum properties of the microwave photons are mapped onto optical photons, enabling efficient room-temperature transmission of quantum information. Whereas superconducting circuits promise scalable quantum processors, these technologies are restricted to operating within the confines of dilution refrigerators. Space limitations block scalability. Microwave-to-optical conversion provides for a quantum “internet,” converting quantum information into optical frequencies, which are more robust to information degradation at room temperature, and allowing transmission of quantum signals from one dilution refrigerator to another.
A related ongoing project is our group’s collaboration with Prof. Shanhui Fan’s group to demonstrate, for the first time, an integrated circulator on lithium niobate. Optical isolators are two-port devices in which an input signal is passed in one direction, but not another. These devices enable filtering of back-reflecting signals and are vital to scaling optical infrastructure. A circulator is a three-port device in which any two ports act as an isolator. If we number the ports as “1,” “2,” and “3”, signals are passed between the ports in the sequence 1>2>3>1. The reverse direction of propagation is blocked. Our device operates in the frequency regime, exhibiting non-reciprocal frequency conversion, which can be leveraged to multiplex and filter large numbers of propagating signals along a single optical channel. This device can route signals into different channels on-chip, filtered by their frequencies.
Electro-optic studies in LINQS also focus on designing new technologies for quantum optical and tunable photonic control. Ongoing collaborations exist between LINQS and the groups of Profs. Fan, Vuckovic, Fejer, and Schleier-Smith.