The primary focus of the project was to build quantum logic gates programmable by single photons as a building blocks of quantum computers. Particularly, the phase-shift gate with single-quantum-bit program and the sign-flip gate programmable by fully quantum software were developed. The former applies a unitary phase shift operation to a data quantum bit (qubit) with the value of the phase shift being fully determined by the state of a program qubit [1]. The latter applies a sign flip operation to data qubit in an arbitrary basis fully specified by the quantum entangled state of a two-qubit program register [2].

Quantum circuit (top) and experimental scheme (right) for sign-flip programmable quantum logic gate. Circuit description: The single wires carry quantum bits (qubits) and the double wires carry classical bits. Qubits 2 and 3 form a program register prepared in a particular entangled state Phi_p. Bell-state measurement (BSM) is applied to data qubit 1 in an unknown quantum state psi_d and qubit 2 of the program register. For singlet Bell state detected in channels 1 and 2 the measurement yields x=y=1 which heralds the successful gate operation leaving the remaining qubit 3 in the target state. The unitary operation Sigma_phi is determined by the state of the program register.

Our experimental implementation was based on the encoding of qubits into polarization states of single photons, multiphoton interference, and photon coincidence detection. Linear and non-linear bulk as well as fiber optics components were employed during the realization. Particularly, sign-flip gate was built almost completely using optical fibers that enables trasferring the gate onto optical chips.

Photo of the experimental setup used for the preparation of single photons, quantum teleportation, and sign-flip programmable quantum logic gate.

Functionality of the new gates was demonstrated with fidelity higher than 97% for single-qubit-program gate and 90% for two-qubit-program gate. The realized devices were characterized thoroughly using quantum tomography.

Visualization of quantum teleportation (first column) and sign-flip gate for different program states. The outer shell stands for the input qubit and its various possible states represented by a color distribution. The inner shell visualizes the action of the gate by color changes depending on the program. Deformations come from experimental imperfections which degrade the gate operation.