The Quantum Channel

The laser (1) at Bob is a fiber coupled laser diode. From this laser, a strong pulse is send, which has a width of less than 1 ns. In this context, a strong pulse means a pulse which contains a lot of photons, whereas a faint pulse has low probability of containing more than one photon.

The pulse is split into two by a 50/50 beam splitter (2). Each pulse runs through a polarization controller (3), which are used for adapting the polarization of the pulses to the PBS (6), in such a way that the pulses run to Alice. The polarization controllers are adjusted, such that the transmission through the PBS is optimal, when the laser is in CW mode.

The pulse in the upper arm is also delayed before encountering the PBS. This is achieved by 150 m of optical fiber (4), which gives a delay of 750 ns, since the travel speed of light in optical fiber is 0.2 m/ns. The phase modulator (5) in the upper arm is not used until the pulse returns from Alice, since it then would be easy for an eavesdropper to detect the encoded signal, because the pulses are strong. The two pulses separated by 750 ns then travel 20 km, though standard single mode fiber (7), on the way to Alice.

As the pulses arrive at Alice, they are again split by a 10/90 beam splitter (8). This allows Alice to measure when they arrive with her detector (9) in the 90 arm.

Then the pulses are attenuated with an optical attenuator (10) and send into a delay/storage line (11), since it may provide some problems sending a strong pulse in one direction of the fiber and a faint pulse in the other (such as dispersion of the pulses). Furthermore, Alice has the time needed for her electronics to respond to the pulses (if Bob does not only send one pulse, but a train of pulses, then entire train should fit into this storage line).

Alice does not modulate the first of the two pulses, she only reflects it on the Faraday mirror (FM) (13). The second pulse does not arrive at the phase modulator (12) until the first pulse has passed it on the way back, since the second pulse is 150 m behind the first. The phase of the second pulse may therefore be modulated without disturbing the first pulse, and this is the encoding of Alice.

Before sending the pulses back to Bob, Alice attenuates them further with the attenuator (10), in such a way they are faint pulses, i.e. they have low possibility of containing more than one photon. The strength of the pulses is set such that the secure bit rate R is as large as possible.

When the pulses encounter the PBS (6), the first pulse is send into the long arm, and the last pulse is send into the short arm. The reason for this, is the fact that the pulses are reflected on a Faraday mirror, since the polarization of the pulses is rotated 90 degrease. Furthermore, the system is self-adjusting, since the pulses travel the same path with the same polarization, which among other things means that birefringence is of no concern.

The first pulse is therefore send to the phase modulator (5), which may modulate the phase. The phase modulation is the encoding of Bob. The pulse now runs through the 150 m of optical fiber (4) before encountering the polarization controller (3) and the beam splitter (2). The second pulse travels directly form the PBS (6) to the beam splitter through the polarization controller. The two pulses have traveled equally distances except for the phase modulations, and they will interfere on the beam splitter (2). This interference is at last measured with an avalanche photo diode (14).

The computers at Alice (15) and Bob (16) set the phase shifts at random, such that they are unknown to an Eavesdropper Eve. The settings are recorded such that they are known for every pulse, the times of measurements performed by the APD are also recorded. After the key has been exchanged, the bases are compared for every pulse measured by the APD, in order to create the secret key.