Free space quantum system based on quantum secure direct communication GE Hua*a,b, LIU Wenyua Department of Electronics and Information Engineering, Huazhong University of Science and Technology, Wuhan, 430074 b School of Information Engineering, Wuhan University of Technology, Wuhan, 430070
a
ABSTRACT A new practical free space quantum communication system is proposed, it is based on the principle of quantum secure direct communication. Quantum cryptography is a practical method to realize free space optical communication. Different from the existing free space quantum key distribution communication system, the free space quantum secure direct communication is deterministic and the system is more simple. In this paper, a new structure of four transmitting channels is prensented, this structure could increase the security and capacity of system. The setup of transmitter and receiver of each transmitting channel is discussed in detail, and the communication process is analyzed. The result shows that this free space quantum system could be realized and can work over long distance. Keywords: free space quantum system, quantum secure direct communication, quantum cryptography, free space optical communication
1. INTRODUCTION Nowadays, the technology of free space optical communication(FSO) is developed rapidly, it has the advantage of both optical fiber communication and wireless communication[1,2]. But it is not insecure and unreliable because the information is transmitted through free atmosphere in this communication style. It is affected easily by existed nature noise and other disturbances. So it is important to increase the security of FSO system. In the present technology, quantum cryptography(QC), the first practical application of quantum information technology, is a good choice to increase the reliability of FSO system. There are some different quantum cryptography communication schemes that have been implemented in FSO system. In the present quantum cryptography systems[3-7], the quantum key distribution(QKD) is used in these systems. Making use of the principle of quantum mechanics, the basic advantage of QKD is its unconditional security. Some studies have proved that distribution of secret keys over long distance using either faint laser or entangled photon pairs is possible. The existing QC systems are usually using BB84 protocol or B92 protocol that are typical works based on the principle of QKD. In the QKD communication system, it is necessary to create a key and encrypt the secret message before the beginning of transmission. The system will be more complex. And it is impossible to provide high security transmission in free space because they are insecure if the quantum system is operated in noisy environment. On the other hand, BB84 and B92 protocol are not deterministic. In this paper a new practical free space quantum communication system that based on the principle of quantum secure direct communication(QSDC) is discussed in detail. Different from QKD, QSDC is deterministic and it offers the option of secure communication without first establishing a shared key[8]. So the communication systems are simplified. The most typical protocols of QSDC are ping-pong protocol[9] and one-time pad protocol[8]. In these protocols Bob and Alice exchange message with single photon in certain two qubit states. In our free space QSDC system, we use photons in four qubit states to represent the classical binary bit ‘0’ and ‘1’ after two typical operations. It could be immuned to some kinds of noisy during the communication. So it is harder for Eve to have effective attacks to quantum transmission link, and the probability of acquiring useful message from legitimate users Bob and Alice by Eve is smaller than the existed QKD systems. When the quantum messages are transmitted in the free space, it is more simple and secure and is easier to realize. In the existing QC system, the style of one transmitting channel is always used, so the bit rate is relatively low and *
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the capacity is relatively small. The stability is affected easily by nature existing disturbances. So in this paper, we propose a new QSDC system that has four transmission channels and each channel is the same. This system could increase the overall bit rate and the capacity is the four times to other only one channel system. It realizes high security, high bit rate and large capacity transmission.
2. THE STRUCTURE DESIGN OF THE FOUR CHANNEL QSDC SYSTEM 2.1 The structure of overall system In our quantum cryptography scheme, each terminal, Alice and Bob, have four transmitting parts and receiving parts, and each part could transmit and receive secret message at the same time. It is similar to the principle of wavelength divided multiplex(WDM). The structure of each terminal is as figure1, it is different from the existing QC systems. Every channel is working based on the principle of quantum secure direct communication. So each channel could not need to form the transmitted key first in the beginning of transmission. It is more difficult for eavesdropper(Eve) to acquire useful information from the whole transmission link. So it is more security in our QSDC system. In our system, it is important to pointing between transmitting part and receiving part, so the technology of automatic tracking pointing(ATP) is required. And the axis of one terminal will be rotate 45 degrees from the other terminal in order to make the transmitting part of one terminal pointing the corresponding receiving part of the other terminal correctly. Four transmitting parts
ATP part
Four receiving parts
Figure 1 the structure of each terminal
In our QSDC system, the secret information are encoded to one polar state by the pulses of the new type laser, vertical cavity surface emitting laser(VCSEL)[10]. The wavelength of VCSEL is easily satisfied with the low loss window of atmosphere and this laser has high power to give feasibility of a long distance QC scheme. The typical wavelength of each VCSEL is 850nm. The communication system that using this wavelength could be avoid the influence of free space’s temperature and some weather condition or disturbance because the wavelength 850nm is not long and belongs to near infrared light, and it could not be observed by both Eve and other people too. On the other hand, this wavelength is easily satisfied the requirement of detector in receiving part. So this wavelength is very suit to use in the transmission in atmosphere. The transmitting part of each terminal is consist of transmitting module and transmitting optical antenna. Transmtting antenna is optical system that always using inversion optical Galileo telescope system, this system could reduce optical diffusing angle in order to control the radius of optical beam after long distance transmission. If the radius of beam is too large, it is difficult to receive the optical beam in receiving part. The transmitting module is provide the emitting of the secret message that based on the quantum protocol that we proposed. The receiving part always composes of receiving optical antenna, that always is Cassegrain telescope, and optical filter etc. In our system, because the four transmission links are used, it is necessary to use mux equipment in receiving part to simplify the structure.
2.2 The structure of transmitter Because our quantum cryptography system is based on the principle of quantum secure direct communication. The structure of transmitting module (Alice) of each channel is Figure2.
Figure2 the structure of transmitting module(Alice)
During the existing QSDC protocols, the most two typical protocols are ping-pong protocol and one-time pad protocol. In our schematic the later, one-time pad protocol is utilized. Alice uses two measuring bases: the rectilinear
1 ( 0 + 1 ) , ϕ1 = 1 ( 0 − 1 )⎫⎬ that be produced by four pulsed 2 2 ⎩ ⎭ VCSELs and polarizers as in Figure2. Similar to the BB84 QKD protocol, the 0 and ϕ0 states represent the binary
basis
{ 0 , 1 } and diagonal basis ⎧⎨ ϕ
0, and the 1 and
ϕ1
0
=
states represent the binary 1. The four polarization are recombined by beam splitter and
transmitted by inversion Galileo telescope. 2.3 The structure of receiver The structure of receiving module of each transmitting channel is as figure3:
Figure3 the structure of receiving module(Bob)
The Bob module is directly connected to the end of the Cassegrain telescope. The receiving module is consist of a non-polarising beam splitter, a set of two polarizing beam splitters, a half-wave retarder and a quarter-wave retarder.
These equipments are used to determine the polarization of the receiving single photons. The single photons are detected by the single photon detector, that always is avalanche photon diodes(APD). In the receiving module, it should be considered that the message transmitting from four receiving channel should be muxed correctly. It could be avoided to lose the useful message and be detected easily. 2.4 The communication link Because our QSDC system is working in free space, the transmitting media is atmosphere. The influence of free space is considered enough. It is important to distinguish between natural noise or disturbance and Eve. So the security check of communication is necessary.
3. THE ANALYSIS OF COMMUNICATIN BETWEEN ALICE AND BOB Based on the system’s structure that be discussed above in detail. The communication between Alice and Bob based on the principle of QSDC protocol mainly contains the following steps: a. Alice prepares a batch of one single photon and their states is one of four polarization states: 0 , 1 , ϕ0 and
ϕ1 randomly. Then Alice transmits these single qubits states to Bob and defines measuring basis, basis R is basis D is
{ϕ
0
{ 0 , 1 },
, ϕ1 } .
b. Bob receives these transmitted secret messages and selects some states of them as checking qubits. Bob measures the selected states on one basis R or D. He remembers the positions of selected states, the selected measuring basis and the measuring results. Then Bob transmits these information to Alice. c. After comparing the receiving results with his original knowledge, Alice could know whether there are bit error. So he can determine whether there is eavesdropper or not. If Eve is existed then the communication is halted, otherwise communication is continued. This is the stage of security checking. d. Finishing security checking, Bob encodes the other qubit states of this prepared batch using the operation I (encode the bit ‘0’) or σ x (encode the bit ‘1’), he sends the encoded states to Alice. Alice compares these encoded states with original states in the same position of the batch, then decoded them after corresponding operation. So Alice and Bob finish one batch qubit communication and prepares the next batch qubit transmission. So the communication between Alice and Bob is continue. Comparing this QSDC system with QKD system, we could find that the probability of successful attack by Eve is very low. So the security of our system is increased greatly. And the structure of our system is simple and could be realized easily.
4. CONCLUSION In this paper, a new four channels free space optical communication system is proposed. Each transmitting channel is designed and it works on the principle of QSDC protocol. The structure of overall system, transmitting system and receiving system is discussed in detail. At last, the communication between Alice and Bob is analyzed. According the discussion it could be concluded that the proposed free space quantum system based on QSDC is more secure and simple than other existing QC system. And our system has large communication capacity, high stability and fast transmitting bit rate. It could be used widely.
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