WO2012047087A1

WO2012047087A1 - Apparatus for flipping the polarization state of an optical pulse between a transmitter and a receiver in quantum key distribution and method thereof

Info

Publication number: WO2012047087A1
Application number: PCT/MY2010/000295
Authority: WO
Inventors: Mohd Fared Abdul Khir; Zalhan Md. Yusof; Dr. Suhairi Saharudin; Suryadi Dr; Mohd. Nor Zaliman Mohd. Zain; Dr. Mohamed Ridza Wahiddin; Dr. Sahbudin Saari
Original assignee: Mimos Berhad
Priority date: 2010-10-07
Filing date: 2010-11-25
Publication date: 2012-04-12
Also published as: MY152705A

Abstract

An apparatus for flipping the polarization state of an optical pulse between a transmitter and a receiver in a quantum key distribution system comprising: a passive non flipping component that preserves the polarization state of said optical pulse; a passive flipping component that rotates the polarization state of said optical pulse orthogonally; and an active optical switch that receives said optical pulse and sends said optical pulse to either the said passive non flipping component or the said passive flipping component whereby the said optical pulse upon entering said active optical switch is directed either towards the passive non flipping component where its polarization state is preserved or towards the passive flipping component where its polarization state is rotated orthogonally.

Description

Apparatus For Flipping The Polarization State Of An Optical Pulse Between A Transmitter And A Receiver In Quantum Key Distribution And Method

Thereof

FIELD OF INVENTION

The present invention relates generally to a quantum key distribution apparatus and more specifically to an apparatus for distributing a sequence of symbols between a transmitter and a receiver and detecting an eavesdropper in a quantum key distribution system.

BACKGROUND OF INVENTION

The technology of quantum cryptography provides an unprecedented level of enciphering data. Elementary particles transferred on communication channels carry the information, and also the cipher keys at the same time. Einstein-Podolsky-Rozen paradox (EPR) and Jon Bell's theorem (1964) have served the conceptual foundation for quantum encoding. Quantum encoding is based on the entanglement of the pair of pure single-photon impulses, the source for which is a depressed laser ray. Henceforth the theorem by Jon Bell was extended by J.F.CIauser and .A. Home (1974), which made it possible to check experimentally the foundation of quantum encoding. The result surpassed all expectations - quantum encoding theoretically guarantees absolute privacy and security, that is it ensures data transmission secure from unauthorized access (interception and decoding).

Quantum encoding makes it possible to use so-called a distributed key with polarized photons or phase encoding, which is only used for one communication session. Taking the analogy of Alice and Bob as two communicating parties, in a QKD (Quantum Key Distribution) system, through several processes of an agreed upon protocol based on exchanging quantum states realized with a single photon, Alice and Bob, will ultimately share a secret key without having to meet or rely on any third party such as in a conventional cryptography system. Several protocols have been proposed by several groups which includes the two way QKD protocol. In two way QKD communication protocol or the so called deterministic protocol the Transmitter (Bob) prepares and sends a qubit, which is a unit of quantum information, embodied by a single photon optical pulse, to the other end (Alice) who will either flip or not flip the qubit and resend it back to Bob. Using the same basis as the qubit was prepared, Bob is able to measure and infer with certainty the qubit returned by Alice. Taking the example of polarization encoded LM05 protocol, Bob randomly prepares a qubit in any of four polarization states, the Vertical (V) and Horizontal (H) from the '+' basis and the Diagonal right (+45) and Diagonal Left (- 45) from the 'x' basis. Each qubit is sent to Alice over a quantum channel. Quantum channels can be optical paths or free space that allows the propagation of the optical pulses. Upon receiving the qubit, Alice randomly flips or does not flip the qubit. Any "flip" operation will represent a bit value "1 " while "Not flip" operation represents a bit value 0. The returning qubit is then measured by Bob using the same basis as the qubit was prepared which let Bob infers with certainty, the bit value sent by Alice. The resulted bit values become the raw shared key between Alice and Bob. The said process is depicted in the Figure 1. Figure 2 shows a diagram of an example implementation of a four states two way protocol that is known in the art. Referring to Figure 2, the two way QKD protocol implementation includes a source (1200) and detector (1300) apparatus located at Bob and a flipper (3300) and control mode (3200) apparatus located at Alice. All optical apparatus are controlled by a control apparatus (1100) located at Alice and control apparatus (3100) at Bob using synchronization channel (2100). A single photon is emitted from source (1200) located at Bob and sent through quantum channel (2200), proceed through control mode apparatus (3200) and flipper (3300) located at Alice before returned back to Bob via quantum channel (2200) to be measured using detector (1300) located at Bob.

In the prior arts, the flipping operation at Alice is done using a combination of serially connected half wave plates. A device such as Pockets Cell can be used to function as a half wave plate by setting and triggering the driver to the half wave voltage. With this setup, any four polarization states can be 'Hipped" - that is their polarization states are orthogonally rotated. It is worth noting that in actual implementation, since Alice does not measure the polarization state of the qubit sent by Bob, Alice has no means of knowing what polarization state has been sent. Alice relies on its "flipping" operation to do either execute the "flip" or "not flip" unitary transformation.

Also, since half wave plates are used, the setup can only be used for specific polarization states of incident optical pulse due to the rotation applied depends on the tilting of the half wave plates. This point is clearer when the polarization states of incident optical pulses are more than four such as in 6 states protocol where 6 polarization states are in use which may include Circular Right and Circular Left. In the present invention, rather than specifically choosing, rotating and tilting the wave plates, a simple active assignment of corresponding active optical switch ports that leads to the passive flip or not flip component provides the same functionality.

What is desired is a quantum key distribution apparatus or method that allows any number of polarization states to be used.

SUMMARY OF INVENTION

The present invention aims at providing an apparatus and method for distributing a sequence of symbols between a transmitter and a receiver and detecting an eavesdropper in a quantum key distribution system that overcomes the problems and disadvantages of the prior art. As opposed to the conventional techniques using half wave plates, in the present invention a Mirror/Reflector, a Faraday mirror and an optical pulse switching component are combined to provide the same capability of selectively 'Hipping" - or rotating the polarization state of - the qubit. The present invention relates to an apparatus for flipping the polarization state of an optical pulse between a transmitter and a receiver in a quantum key distribution system comprising: a passive non flipping component, such as a mirror or reflector, that preserves the polarization state of said optical pulse; a passive flipping component, such as a Faraday mirror, that rotates the polarization state of said optical pulse orthogonally; and an active optical switch that receives said optical pulse and sends said optical pulse to either the said passive non flipping component or the said passive flipping component whereby the said optical pulse travels through an optical path before entering said active optical switch whereupon it is directed either towards the passive non flipping component where its polarization state is preserved or towards the passive flipping component where its polarization state is rotated orthogonally. A controller controls the timing and voltage of the active optical switch. The optical paths connecting the various components are single mode optical fiber.

The present invention also relates to a method for distributing a sequence of symbols between a transmitter and a receiver and detecting an eavesdropper in a quantum key distribution system, comprising the steps of:

a) the transmitter preparing a randomly polarized single photon optical pulse and noting the polarization state of said optical pulse and then sending said optical pulse to the receiver over an optical path;

b) the receiver either rotating or preserving the polarization state of the optical pulse;

c) the receiver sending the optical pulse back to the transmitter; and d) the transmitter comparing the received optical pulse to the one sent earlier so as to determine if an eavesdropper has accessed the said optical pulse signal

characterized in that the rotating of the polarization state is performed by a passive flipping component, such as a Faraday mirror, that rotates the polarization state of said optical pulse orthogonally and the said preserving of the polarization state is performed by a passive non flipping component, such as a mirror or reflector, that preserves the polarization state of said optical pulse. An active optical switch sends the optical pulse to either the passive non flipping component or the passive flipping component.

These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows an example coding scheme of two way deterministic protocol in a quantum key distribution method.

Figure 2 shows a diagram of a two way deterministic protocol optical setup in a prior art example.

Figure 3 shows a flow chart of a selectable optical polarization flipping apparatus in an embodiment of this invention.

Figure 4 shows a diagram of a selectable optical polarization flipping apparatus in a first embodiment of this invention. Figure 5 shows a diagram of a selectable optical polarization flipping apparatus in a second embodiment of this invention.

DETAILED DESCRIPTION OF INVENTION

It should be noted that the following detailed description is directed to an apparatus and method for distributing a sequence of symbols between a transmitter and a receiver and detecting an eavesdropper in a quantum key distribution system and is not limited to any particular size or configuration but in fact a multitude of sizes and configurations within the general scope of the following description.

In the present invention, a Mirror/Reflector, a Faraday mirror and an optical pulse switching component are combined to provide the capability of selectively "flipping" - or rotating the polarization state of - the qubit. Figure 3 shows a flow chart of an embodiment of the present invention. First Embodiment

Figure 4 shows an optical layer setup of a selectable optical polarization flipper apparatus in a first embodiment of this invention. It is coupled to several other optical components to show its example usage. In the example embodiment of Figure 4, two vertically (not restricted to) polarized single photon optical pulses (further also referred to as optical pulse) are prepared in and sent from laser source (300) through optical path (100) into port (31 1) of a one by two optical directional element (310). The optical pulses then leave through port (312) to enter the present invention through the optical path (101 ).

Referring to Figure 4, this first embodiment of the present invention is an apparatus that comprises three main components: the active optical switch (320) with timing and voltage controlled by controller (330); the passive non flipping component (340) also referred to as Mirror/Reflector (340), optical path (102A) comprised of single mode fiber connecting port (322) of active optical switch (320) with the passive non flipping component (340); the passive flipping component (350) also referred to as Faraday Mirror (350) and optical path (102B) comprised of single mode fiber connecting port (323) of active optical switch (320) with the passive flipping component (350).

The active optical switch (320) is a one by two optical switch with one port (321) at one end and two ports (322, 323) at the other end. The one port end (321 ) is connected to the quantum channel or optical path (101 ). The two port ends, that is port (323) and port (322), are connected to the passive flipping (350) and non flipping (340) components, respectively. All three ports (321 , 322, 323) are bidirectional in that they accept light from both directions.

Each optical pulse enters the present invention through port (321) of active optical switch (320) and leave the optical switch (320) through either port (322) or port (323) depending on the control voltage set by the voltage and timing controller (330). However, when the said optical pulse enters through port (322) or port (323), it will only leave active optical switch (320) through port (321 ), as it is the only incoming and outgoing port in this first embodiment of the present invention. The one by two active optical switch (320) is an optical component that preserves any polarization state of incident light. All optical paths (101 , 102A and 102B) are single mode (S ) optical fiber. Optical path (101 ) is the path that leads to port (321 ) which is the only input and output port of the apparatus in this first embodiment of the present invention. Optical path (102A) is the path that leads the said optical pulse to and from the passive non flipping component Mirror/Reflector (340). Optical path (102B) is the path that leads optical pulse to and from the passive flipping component Faraday mirror (350). Due to the same optical path taken by each optical pulse on its way in and out of the flipping apparatus, the polarization transformation introduced by the single mode optical fiber (S F) path to the polarization state of each optical pulse is auto- compensated. It is the passive non flipping component Mirror/Reflector (340) and the passive flipping component Faraday mirror (350) that do the flip or not flip unitary transformation.

The passive flipping component is a Faraday mirror (350) connected with a single mode optical fiber (102B) to port (323) of the active optical switch. Any polarized optical pulse entering the Faraday mirror (350) will be reflected with its polarization state rotated orthogonally. In the example illustration of Figure 4, the vertical polarization state (201 A) is rotated to horizontal polarization state (201 B) and reflected back through path (102B) to again enter the active optical switch (320) through port (323). The optical pulse proceeds out of the active optical switch (320) via port (321 ) and through path (101 ) back to the optical directional element (310).

The non flipping component is a Mirror/Reflector (340) connected with a single mode optical fiber (102A) to port (322) of the active optical switch (320). Any incident polarized optical pulse entering the Mirror/Reflector (340) will be reflected with its polarization state preserved to as when it entered the Mirror/Reflector (340). In the example illustration of Figure 4, the vertical polarization state (202A) is reflected with its polarization state preserved as vertical polarization state (202B). The reflected optical pulse (202B) proceeds back through optical path (102A) to again enter the active optical switch (320) through port (322). The optical pulse proceeds out of the active optical switch (320) via port (321 ) through path (101 ) back to the optical directional element (310).

A unitary transformation operation consists of two types of operation, the "flip" operation or "not flip" operation. For either type of operation, the active optical switch (320) must be controlled in synchronism with the incident optical pulses (201 A) and (202A). Referring to Figure 4, to direct optical path (202A) to flipping component (350), the active optical switch (320) will provide an internal path that connects port (321 ) to port (323). The said internal path will remain connected for a certain period of time delay unless the next request is of a different type of operation, that is, in this example, the "not flip" operation. Connecting to the other type of operation requires a new internal path to be provided and this will also disconnect the current internal path. A delay of a certain amount of time is required to complete an operation that is if in the case of the present example, to allow for the optical pulse to return from the passive flipping component (350) through path (102B) and out of the active optical switch (320).

In the example embodiment of Figure 4, to show the example usage of the present invention, the returning optical pulses noted as 201 B and 202B, enter the optical directional element (310) through port (312) and leave through port (313) to be split by the polarization beam splitter (360) based on their polarization state. In the case of Diagonal right +45 and Diagonal Left -45 polarization state, a half wave plate rotated at 22.5 ° may be used to transform them to Horizontal H or Vertical V polarization state respectively before entering polarization beam splitter (360).

Second Embodiment

In the second embodiment, the configuration of the present invention includes two ports (321 , 324) where one serves as the input port (321 ) and the other one serves as the output port (324). As compared to the first embodiment whereby one port serves as input and also output to the present invention, in this second embodiment, the input and output port is made to be separated.

Figure 5 shows an optical layer setup of a selectable optical polarization flipper apparatus in a second embodiment of this invention. Referring to Figure 5, an optical pulse enters the present invention through optical path (101) that leads to port (321 ) of optical switch (320) to be routed either to the "flip" or "not flip" path. In returning to the optical switch (320), regardless from either path (102A) through port (322) or path (102B) through port (323), the optical pulse is directed to output port (324) as it is the only output port in this second embodiment of the present invention.

Referring to Figure 5, this second embodiment of the present invention is an apparatus that comprises three main components: the active optical switch (320) with timing and voltage controlled by controller (330); the passive non flipping component (340) also referred to as Mirror/Reflector (340), optical path (102A) comprised of single mode fiber connecting port (322) of active optical switch (320) with the passive non flipping component (340); the passive flipping component (350) also referred to as Faraday Mirror (350) and optical path (102B) comprised of single mode fiber connecting port (323) of active optical switch (320) with the passive flipping component (350).

The active optical switch (320) is provided with one input port (321 ) and one output port (324) at one end and two ports (322, 323) at the other end. The input port (321 ) is connected to the quantum channel or optical path (101 ) and receives the optical pulse from the optical path (101 ). The output port (324) is connected to the quantum channel or optical path (103) and sends the optical pulse into the optical path (103). The two port ends, that is port (323) and port (322), are connected to the passive flipping (350) and non flipping (340) components, respectively. Ports (322, 323) are bidirectional in that they accept light from both directions.

Each optical pulse enters the present invention from optical path (101 ) via input port (321 ) of active optical switch (320) and leaves the optical switch (320) through either port (322) or port (323) depending on the control voltage set by the voltage and timing controller (330). However, when the said optical pulse enters through port (322) or port (323), it will only leave active optical switch (320) through output port (324) and into optical path (103), as it is the outgoing port in this second embodiment of the present invention. The active optical switch (320) is an optical component that preserves any polarization state of incident light. All optical paths (101 , 102A, 102B and 103) are single mode (SM) optical fiber. Optical path (101 ) is the path that leads to input port (321 ) which is the only input port of the apparatus in this second embodiment of the present invention. Optical path (102A) is the path that leads the said optical pulse to and from the passive non flipping component Mirror/Reflector (340). Optical path (102B) is the path that leads optical pulse to and from the passive flipping component Faraday mirror (350). Optical path (103) is the path that leads to output port (324) which is the only output port of the apparatus in this second embodiment of the present invention. Due to the same optical path taken by each optical pulse on its way in and out of the flipping apparatus, the polarization transformation introduced by the single mode optical fiber (SMF) path to the polarization state of each optical pulse is auto- compensated. It is the passive non flipping component Mirror/Reflector (340) and the passive flipping component Faraday mirror (350) that do the flip or not flip unitary transformation.

The passive flipping component is a Faraday mirror (350) connected with a single mode optical fiber (102B) to port (323) of the active optical switch. Any polarized optical pulse entering the Faraday mirror (350) will be reflected with its polarization state rotated orthogonally.

The non flipping component is a Mirror/Reflector (340) connected with a single mode optical fiber (102A) to port (322) of the active optical switch (320). Any incident polarized optical pulse entering the Mirror/Reflector (340) will be reflected with its polarization state preserved to as when it entered the Mirror/Reflector (340).

It is clear to anyone in the field of quantum cryptography that the present invention is able to function as an optical polarization flipper which in the case of two way communication protocol such as LM05 deterministic QKD protocol is achieved conventionally using combinations of half wave plates. In the present invention, a voltage controlled active optical switch (320), the passive flipper component comprising a Faraday mirror (350) and a passive non flipping component comprising a Mirror/Reflector (340) are used instead. It is also clear to anyone in the field that the apparatus can be used with optical medium that uses either free space or optical fiber. While several particularly preferred embodiments of the present invention have been described and illustrated, it should now be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are intended to embrace such changes, modifications, and areas of application that are within the spirit and scope of this invention.

Claims

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system comprising:

a passive non flipping component (340) that preserves the polarization state of said optical pulse;

a passive flipping component (350) that rotates the polarization state of said optical pulse orthogonally; and

an active optical switch (320) that receives said optical pulse and sends said optical pulse to either the said passive non flipping component (340) or the said passive flipping component (350)

whereby the said optical pulse travels through an optical path (101 ) before entering said active optical switch (320) whereupon it is directed either towards the passive non flipping component (340) via an optical path (102A) where its polarization state is preserved or towards the passive flipping component (350) via an optical path (102B) where its polarization state is rotated orthogonally.

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to claim 1 wherein the said passive non flipping component (340) is a mirror or reflector.

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to claim 1 or 2 wherein the said passive flipping component (350) is a Faraday mirror.

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to any of the preceding claims further comprising a controller (330) for controlling the timing of the said active optical switch (320).

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to any of the preceding claims wherein the said optical paths (101 , 102A, 102B) are single mode optical fiber.

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to any of the preceding claims further comprising a means of assessing the amount of information an eavesdropper having access to the optical pulse could have obtained.

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to any of claims 1 to 6 wherein the said active optical switch (320) receives the said optical pulse from said optical path (101 ) via a port (321 ) and sends the said optical pulse, after polarization state of said optical pulse has either been rotated or preserved, out via the same port (321 ).

An apparatus for flipping the polarization state of an optical pulse between a transmitter (2) and a receiver (4) in a quantum key distribution system according to any of claims 1 to 6 wherein the said active optical switch (320) receives the said optical pulse from said optical path (101 ) via a port (321 ) and sends the said optical pulse, after polarization state of said optical pulse has either been rotated or preserved, out via a second port (324).

A method for distributing a sequence of symbols between a transmitter (2) and a receiver (4) and detecting an eavesdropper (6) in a quantum key distribution system, comprising the steps of:

e) the said transmitter (2) preparing a randomly polarized single photon optical pulse and noting the polarization state of said optical pulse and then sending said optical pulse to said receiver (4) over an optical path (101 ); f) the said receiver (4) either flipping or not flipping the polarization state of said optical pulse;

g) the said receiver (4) sending said optical pulse back to said transmitter (2); and

h) the transmitter (2) comparing the received optical pulse to the one sent earlier so as to determine if an eavesdropper has accessed the said optical pulse

characterized in that said flipping is performed by a passive flipping component (350) that rotates the polarization state of said optical pulse orthogonally and the said not flipping is performed by a passive non flipping component (340) that preserves the polarization state of said optical pulse.

10. A method for distributing a sequence of symbols between a transmitter (2) and a receiver (4) and detecting an eavesdropper (6) in a quantum key distribution system according to claim 9 wherein the said passive non flipping component (340) is a mirror or reflector.

1. A method for distributing a sequence of symbols between a transmitter (2) and a receiver (4) and detecting an eavesdropper (6) in a quantum key distribution system according to claim 9 or 10 wherein the said passive flipping component (350) is a Faraday mirror.

2. A method for distributing a sequence of symbols between a transmitter (2) and a receiver (4) and detecting an eavesdropper (6) in a quantum key distribution system according to any of claims 9 to 1 1 wherein an active optical switch (320) sends the optical pulse to either the said passive non flipping component (340) or the said passive flipping component (350).