Implementation of Quantum Channel

The quantum channel plays a major role in the proposed QKD Wi-Fi protocol. The accuracy of transmission always helps to recover identical secret keys at either end much faster and enables the rest of the Wi-Fi communication to proceed smoothly.

As of now, there are no commercially available Wi-Fi specific wireless devices that are equipped with quantum transmitters or receivers. Hence in this research work the quantum transmission has been established as a separate project aligned to an existing QKD research work [4], [22], [23], [24]. In this particular research, the quantum transmissions have been practically demonstrated under various scenarios over large distances over free space. Since this approach provides much needed quantum transmission, the same set up has been chosen to implement the quantum channel of QKD based Wi-Fi protocol. This has no impact on the experimental set up as the quantum and classical channels operate independently. Basic high level set up of quantum channel between AP and STA is shown in Figure 29. Just for this illustration, the channel

switches have been shown as located outside AP and STA. In reality the channel switches reside inside STA and AP itself.

Figure 29 : High Level Set Up of Quantum Channel [10]

As shown in Figure 29, the first step of QKD is the quantum transmission established between AP and STA. During this one way communication, STA sends a series of polarized photon representing the key towards AP. As soon as the quantum transmission completes, the quantum channel is switched to classical channel where the 4-phase handshake protocol takes place to recover the secret key. The classical channel is two-way communication in which both AP and STA interact with each other to obtain the key.

Figure 30 shows the schematic diagram of the QKD system set up with illustrations to internal hardware. At STA, laser pulses are generated by vertical cavity surface emitting lasers (VCSELs) and attenuated into a single photon level [19]. The polarization states of photons are set by polarisers as per the corresponding protocol (B92, BB84 etc).

Figure 30 : Schematic Diagram of QKD System [19], [30]

Afterwards photons are combined and sent through a non-polarizing beam splitter (NPBS). If the chosen QKD protocol uses four states (e.g.: BB84), the polarisers Pol. 0A, 0B, 1A, and 1B are oriented to 0°, 90°, +45°, and -45° respectively. If the QKD protocol contains only two states (e.g.: B92), polarisers 0A and 1A, will be used. At AP, polarization controllers recover the polarization state of photons with respect to their original state at STA. The 3-dB coupler randomly chooses the detection base and the polarization beam splitter (PBS) helps to determine the key value. Finally the photons are detected by single photon detectors (APDs). Two APDs, 0A and 1A, are used for B92, while four APDs are used for BB84. The scope of this research is limited to SARG04 QKD protocol (which is a slight variation of BB84), but can be extended to support other QKD protocols.

In the proposed QKD based Wi-Fi protocol, when the STA is about to start photon transmission, the state gets changed to STAQCSTART. At this moment the software sends a signal to the quantum controlling device to initiate the photon transmission. This software API call includes the number of photons to be transmitted in order to recover the final key that would have sufficient length to retrieve key hierarchy.

Before the quantum channel begins, both parties agreed on set of QKD specific parameters such as QKD protocol, transmission rate etc that are required to set up the quantum transmission.

These parameters are negotiated during the 802.11 association as described in section 3.3.6.1.

During the photon transmission, the photon transmitter and receiver maintain their line-of-sight. The hardware apparatus performs the transmission with clocks synchronized so that the STA and AP can sample bases accurately. As soon as the photons transmission completes, the quantum channel seizes. At this stage, the quantum hardware hands over the control back to the classical channel, which is wireless channel in this case, to proceed with the rest of the communication. This marks the exit of new transition state STAQCSTART.

There are several outcomes at the end of the quantum transmission. At the STA side, it records all the random bases it used as well as the original bit string that sent across to AP. In this implementation, both these types of information are stored in an in-memory buffers written in C++ language. One of the buffers stores the key bit string used in the form of unsigned integer.

For example, say the transmitted bit string is: 10010111001001010, the buffer stores the bit pattern as it is, but with a chosen delimiter separating each bit. While recording the bits in this implementation, coma (,) has been used as the delimiter.

Thus the buffer will hold the bit string as: 1,0,0,1,0,1,1,1,0,0,1,0,0,1,0,1,0

The second buffer holds the bases used to polarise these bits. This too stores the bases as unsigned integer with mapping to the respective bases.

The SARG04 uses four non-orthogonal bases to polarise the photons. Table 13 shows the mapping used to represent bases in the buffer containing base used.

Table 13 : Bases Representation in STA Buffer

Thus this buffer would hold random bases that used to map those integer values. Similar to the former buffer, this too uses a delimiter to separate the bases.

As an example, this buffer would hold: 2,1,4,4,3,1,1,4,4,3,3,4,2,1

The AP too maintains similar buffers: one to hold the random bases used while the other holds retrieved bit string. The buffer holding bases is very similar to the one used at STA. However the buffer used to hold the retrieved bits could contain empty spaces separated by the delimiter used (commas). This is because, due to the errors introduced during the transmission and also due to the mismatch of bases used, there could be instances where STA is unable to decide a particular bit specifically. Such occurrences will result in empty spaces (shown as “ ” below).

Thus the buffer will look like: 0, ,1,0,0,1, , ,1,0,0, ,1, , , ,1,0,1 Thus, there are four main outputs at the end of the quantum transmission:

1. Random bases used by AP

In addition to the main QKD link that has been running over long distance in free space, University of Canberra optical laboratory also consists of the experimental setup as shown in Figure 31. This set up has been used to generate more specific test data with various error levels that has been fed into the simulation model described in section 4.3. A filter has been used to simulate environmental conditions such as dust, moisture etc.

Figure 31 : QKD Experimental Setup