There are three major ways to build quantum computing hardware. Google and IBM use the superconducting qubit technology, where Josephson junction works as the foundation of engineered, macro-scale quantum bit (qubit). Another popular approach is ion trap, where energy sub-levels of atoms serve as qubit. The last approach is called linear optics quantum computing (LOQC), which is the subject of our research group.

Among different these approaches for quantum computing, integrated photonics stands out in its large-scale manufacturability. This manufacturing scalability is what promises the ultimate goal of quantum computing: fault-taulerant, error-corrected machine. However, LOQC is approach has the major challenge - it does not even have a single reliable qubit yet due to its non-deterministic nature.

The path to overcome this challenge has been laid out already. By multiplexing non-deterministic components, it is possible to make nearly-deterministic component. This scheme requires integration of many components: probabilistic single photons sources, single-photon detector, fast electronic circuit and optical switch. With the recent progress in the manufacturing capability of integrated photonic circuit and electronics, this goal seems to be within reach.

Based on this idea, our group works on producing single-photons by using nonlinear optical interaction in integrated photonics platform. For example, strong laser pulses can produce quantum photon pairs via nonlinear optical phenomena such as spontaneous parametric downconversion (SPDC) or spontaneous four-wave mixing (SFWM). With proper engineering of photonic circuit, it would be possible to obtain pure and efficient single photons which are good enough to be used in photonic quantum computation.

Realizing robust quantum memory has been challenging due to its fragile nature. Fluctuations of environment are origins of short lifetime of qubits therein. However, electrons of artificial atoms in a good host material, such as silicon vacancy (SiV) color center in diamond, have relatively long lifetime thanks to low magnetic field noise. Conventionally, reading and writing of such spin qubit have been addressed by electro-magnetic waves, such as microwave or optical fields.

However, using magnetic field for the control of electron spin at quantum level has been futile due to fundamental physical constant called Bohr magneton. Under this limit, flipping spin qubit cannot be done by sending single microwave photon because interaction strength is too low. Due to this problem, using magnetic field of single microwave photon to flip single electron spin is practically infeasible.

In our group, we take a novel approach that uses acoustic wave in solid to control electron spin qubit. The principle of the operation is based on spin-orbit coupling, where stress wave can generate effective magnetic fields. We aim to control electron spin qubit with the energy as low as that of single-phonon, by engineering acoustic circuits.