The quantum computer is considered to be able to compute specific tasks extremely faster than classical computers. However, in order to run a variety of algorithms in reality, the quantum computer must be able to handle various basic computations in the fault-tolerant manner.

It is well known that classical computers can be constructed of only NAND gates. Similarly, there is also a minimum set of “quantum gates” constituting the quantum computer. Particularly, in optical quantum computing with the continuous variable scheme, any quantum computation can be composed of combinations of five quantum gates [1]. In our laboratory, four of the gates have been realized experimentally [2], and the last gate, called “cubic phase gate”, is the target of our group.

This gate corresponds to a “third-order nonlinear-optical effect”, which cannot be easily applied to quantum states because of very low-power signals.

Quantum teleportation enables us to implement such a difficult gate in the fault-tolerant manner [3]. In the quantum teleportation, we exploit entangled ancillary states to obtain the same quantum state as the input state at the output. If we modify this procedure by preparing special ancillary states, called “cubic phase state”, and applying similar operation to the quantum teleportation, the output state will be a desired quantum state that a cubic phase gate has been applied to the input state. In other words, the cubic phase gate can be implemented indirectly by separating its difficulty into two parts: preparation of ancillary states and advanced technique of quantum teleportation. In addition, the auxiliary cubic phase state used here is a magic state in continuous variable quantum information processing, which is essential for error correction. Therefore, the realization of cubic phase gates by this method is directly related to the realization of an error-tolerant universal quantum computer.

We have proposed a method to implement the cubic phase gate in an error-tolerant way with minimum resources [4], collaborating with Dr. Petr Marek and Dr. Radim Filip of Palacký University in Czech Republic. We are now constructing experimental setups based on the proposal, such as generation of the special quantum state or verification of the advanced teleportation techniques.