An attenuator is a device used to reduce the intensity of a signal in a free space or optical fiber. Because the number of photons per pulse of the laser is very large, we attenuate the number of photons per pulse to an appropriate small value for quantum experiments.

The beam splitter divides incident light into reflected light and transmitted light. The ratio of reflected light and transmitted light varies depending on the reflectance and transmission. Usually, a 50:50 beam splitter with the same reflectance and transmittance is used.

An optical circulator is a device that allows light to pass only in a specific direction. The light incident on port1 is output only to port2, the light incident on port2 is output only to port3, and the light incident on port3 is output only to port1.

A photodetector sensitive to optical signals, also known as a single photon detector (SPD). Because it detects a very weak optical signal, it can result in a dark count that even noise is mistaken for an optical signal, and the detection rate varies depending on the detector component.

Delay lines (DLs) are used to delay signals and typically use optical cables. The degree of delay of the signal depends on the pulse width and repetition rate of the input signal.

A polarizing beam splitter divides incident light into P-polarized light and S-polarized light. Here, P-polarization refers to polarization parallel to the plane of incidence, and S-polarization refers to polarization perpendicular to the plane of incidence.

Quantum systems require a single photon generator with only one photon at a time, but we are currently using the existing Laser due to the absence of that part. Traditional Laser requires the use of attenuators after photon generation because of the large number of photons being output.

A phase modulator is a device that changes the phase of light. When the voltage is applied to the phase modulator, birefringence is induced and phase shift is generated by the induced birefringence. An intensity modulator is a device that modulates the intensity of light. As with the phase modulator, voltage is applied to induce birefringence, and phase shift is generated by induced birefringence. The light produced by the phase shift is interfered by a Mach-Zehner interferometer within the light intensity modulator, resulting in a change in the intensity of the light.

SL(Storage line) is used to delay signals, just like DL(Delay line). It is used to apply encoding through a phase modulator (PM) only to the front signal so that external inputs can be applied to only one signal through the delay of the signal.

As a step to model photon reception on Bob, it largely handles no-click events, dark-count events, and double-click events.

Outputs the key value for which the post-processing process has been completed.

Correct errors in the QKD transfer process and errors caused by double-click events.

The phase error rate estimation determines how much correction should be made in the phase error rate estimation, which is the step of deleting the amount of information leaked in the IR stage and the phase error in the process of transmitting it through quantum channels.

Use click-event in Detector to create a raw key for post-processing.

A polarizer passes light of a particular polarization and blocks light of polarization perpendicular to that light.

The Faraday rotator rotates polarization. The Faraday Mirror is an element that connects a Faraday rotator and a mirror that rotates polarization by 45°. The polarized light rotated by 45° by the Faraday rotator is reflected by the mirror and rotated by 45° more as it passes through the Faraday rotator, sending out the polarized light rotated by 90° from the incident polarized light.

A wave plate is a device that changes the polarization state of light by phase shifting between two vertical polarization components, called a phase retarder. The wave plate with the relative phase difference between two vertical polarization components being π radians is called the Half-Wave Plate, and the wave plate with π/2 radians is called the Quarter-Wave Plate.

Optical fiber is a fiber used as a means of transmitting light. The diameter of the core, the central part of the fiber through which light passes, is so small that fiber with only one propagation mode of light is called a single-mode fiber. Polarization-Maintaining Fiber is a type of single-mode fiber that is designed to keep polarization aligned on fast or slow axes unchanged, unlike single-mode fiber optics that transmit randomly polarized light.

The Polarization Controller is a device that changes the polarization state of light. It has the same effect as a combination of multiple wave plates over the wire.