METHOD THEREOF

Field of the Invention [0001] The present invention relates quantum cryptography. More specifically, the present invention relates to an apparatus and method for measuring six polarization states of optical pulse and a method thereof.

Background

[0002] Polarization states of optical pulse have been used in many recent applications such as quantum cryptography and photonic sensors. There exist six degenerate polarization states being commonly used in these applications, the vertical (V), horizontal (H), diagonal (D), anti-diagonal (A), circular right (R) and circular left (L) polarization states. In quantum cryptography, having six polarization states would increase the security of the shared key as compared to having lesser. This advantage is hindered due to the difficulties in handling the circular left and right together with the linear polarization states. One may for example need to use a separate wavelength for the circular left and right and therefore, different set of apparatus are required to handle them. Reference is made to J.S. Shaari, M. Lucamarini, M.R.B. Wahiddin, Physics Letters A Volume 358 (2) :85 and A.P. Shurupov, S.S. Straupe, S.P. Kulik, M. Gharib, M.R.B. Wahiddin, "Quantum state engineering with ququarts: Application for deterministic QKD protocol", Europhysics Letter 87 (2009) 10008. [0003] In one of the prior arts, the optical pulse with circular left and right polarization states was separately measured. They were assigned with different wavelengths to the linear one and are guided to their respective detector sets by a dichroic beam splitter. Such implementation is not only complex but also uneconomical.

Summary

[0004] In accordance with one aspect of the present invention, there is provided an apparatus for measuring six degenerated polarization state of an input optical pulse. The apparatus comprises a first polarization rotator having a first non-linear material orientated at 45 degree in relation to the axis of the input optical pulse; a second polarization rotator having a second non-linear material orientated at 22.5 degree from the axis, wherein the second polarization rotator is connected to the first polarization rotator in series; a horizontal detector adapted for detecting a horizontal pulse; a vertical detector adapted for detecting a vertical pulse; and a polarization beam splitter that operably splits a polarized input optical pulse to either horizontal or vertical pulse. The input optical pulse first passes through the first polarization rotator and then through the second polarization rotator, then the polarization beam splitter, and finally the polarized input optical pulse is directed to either the horizontal or the vertical detector based on the polarization state of the input pulse to obtain an X, iY and Z basis measurement in relation to the six degenerated polarization state.

[0005] In one embodiment, the first non-linear material is operably triggered with a quarter wave voltage to serve as a quarter wave plate and the second non-linear material is operably triggered with a half wave voltage to serve as a half wave plate. The X basis measurement is obtained by triggering only the second non-linear material but not the first non-linear material. The Z basis measurement is obtained by not triggering both the first non-linear material and the second non-linear material. The iY basis measurement is obtained by triggering both the first non-linear material and the second non-linear material.

[0006] In another further embodiment, the six polarization states of optical pulse includes a horizontal state (H), a vertical state (V), a diagonal state (D), a anti-diagonal state (A), a circular right state (R) and a circular left state (L) with respect to the optical axis of the non-linear materials. The respective polarization states can be represented by the following Jones vector:

[0007] Vertical, ^{V =} U ; Horizontal, ^{H =} J ; Diagonal, ^{D ~} Ϊ W ; Anti-

4 =— i ¹ ) R =—( ¹ ) L =—( ¹ ) diagonal, Circular right, J2 V ^J ; and Circular left, V-i ^-i^. The corresponding Jones matrix for the X, iY and Z basis measurements are accordingly derived from Jones Calculus. [0008] In a further embodiment, the determination of X, iY or Z basis measurement may be selected based on a predefined sequence or in random.

Brief Description of the Drawings

[0009] Preferred embodiments according to the present invention will now be described with reference to the figures accompanied herein, in which like reference numerals denote like elements; [0010] FIG. 1 illustrates a schematic diagram of an apparatus for measuring six degenerated polarization states of an optical pulse in accordance with one embodiment of the present invention; and

[0011] FIG. 2 illustrates a measuring process for measuring the polarization states of an optical input pulse in accordance with one embodiment of the present invention.

Detailed Description

[0012] Embodiments of the present invention shall now be described in detail, with reference to the attached drawings. It is to be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

[0013] In the embodiments of the present invention, there is provided an apparatus for measuring six polarization states (i.e. horizontal, vertical, diagonal, anti- diagonal, circular right and circular left) of an optical pulse. The apparatus comprises two non-linear materials with different functions and orientations. Two serially positioned non-linear materials in which the first behaves as quarter wave plate at 45 degree with respect to the optical axis of the incoming optical pulse when triggered with quarter wave voltage and the second behaves as half wave plate at 22.5 degree with respect to the optical axis of the incoming optical pulse when triggered with half wave voltage. The X, iY and Z basis measurement for the six polarization states is realized by having the optical pulse with any of the six polarization states to pass through the two serially positioned non-linear materials such that the optical pulse will pass through the first non-linear material before passing through the second non-linear material before it is being analyzed by a polarization beam splitter and a set of two detectors. Such approach reduces the complexity of the system and is relatively economical as compared to the existing system as it removes the requirement for separate wavelength.

[0014] FIG. 1 illustrates a schematic diagram of an apparatus for measuring six degenerated polarization states of an optical pulse in accordance with one embodiment of the present invention. The six polarization states incudes horizontal, vertical, diagonal, anti-diagonal, circular right and circular left polarization states, is adaptable on many applications such as quantum cryptography and photonic sensors. For easy reference, the six degenerate polarization states is herein referred and denoted as the vertical (V), horizontal (H), diagonal (D), anti-diagonal (A), circular right (R) and circular left (L) polarization states. The two non-linear materials with different configurations are used to achieve the automated measurement apparatus for optical pulse with any of the six polarization states.

[0015] The apparatus comprises a first non-linear material 221, a second nonlinear material 222, a polarization beam splitter (PBS) 310, a horizontal detector 421 and a vertical detector 422. The first non-linear material 21 is disposed in its enclosure and driver circuit forming a first polarization rotator 231 behaves as a quarter wave plate at 45 degrees orientation when it is triggered with a quarter wave voltage. The second non-linear material 222 is disposed in its enclosure and driver circuit forming a second polarization rotator 232 behaves as a half wave plate at 22.5 degrees orientation when triggered with half wave voltage. When an input optical pulse is fed in at an optical path 110 with either one of the six polarization states passes, it first passes through the first non-linear material 221 and then the second non-linear material 222 before entering the PBS 310. The input pulse is then directed to either to the horizontal detector 421 or vertical detector 422 accordingly. For a Z basis measurement, both nonlinear materials are not triggered leaving the horizontal and vertical optical pulses enter the PBS 310 without any transformation. For an X basis measurement, the first nonlinear material 221 is not being triggered while the second non-linear material 222 is triggered. In this way the diagonal or anti-diagonal polarization state of the optical pulse is polarized into both horizontal and vertical direction. The polarized diagonal or anti-diagonal will then enter the PBS 310 for further detection. For an iY basis measurement, the first non-linear material 221 and also the second non-linear material 222 are triggered. In this way the optical pulse with circular left and circular right polarization state are polarized to horizontal and vertical directions respectively and will then enter the PBS 310 for further detection. The detected pulses captured by the horizontal and vertical detectors 421 and 422 are further processes to achieve the polarization states measurements.

[0016] FIG. 2 illustrates a measuring process for measuring the polarization states of an optical input pulse in accordance with one embodiment of the present invention. The process can be carried out through the apparatus of FIG. 1. In the measurement process, the apparatus determines which basis of measurement should be proceeded, X, Z or iY at step 252. The iY, Z and X are determined accordingly at steps 254, 256 and 258 respectively. At the step 254, i.e. when iY is to be measured, the nonlinear material with a quarter wave voltage PCI and a half wave voltage PC2 are to be triggered. At the step 256, i.e. when Z is to be measured, both non-linear materials with neither voltage PCI nor PC2 are triggered. At the step 258, i.e. when X is to be measured, only one non-linear material with the half wave voltage PC2 is triggered leaving the non-linear material with the quarter wave voltage PCI not triggered. The polarized optical pulse is then fed through the polarization beam splitter (PBS) 310 at step 260. Accordingly, either the horizontal or vertical detector detects the optical pulse in the six individual states at step 262.

[0017] Referring to the X, iY and Z basis determination sequence, it can be selected randomly or it can also be selected based on a predetermined sequence.

[0018] The polarization states measurement can be described through a method known in the art. For example, Jones calculus (see E. Collett, "Field Guide to Polarization ", SPIE Press Book 2005 for example), can be used to describe this measurements. Denoting the six polarization states of concern using Jones vector, their representations shall be presented as follows:

[0019] Vertical,

[0020] Horizont

[0021] Diagonal

[0022] Anti-diag

J? =

[0023] Circular right, 72 ^; and

[0024] Circular left, [0025] Pockels cell at non-triggered state is equivalent to identity matrix (I),

H) v , which in turn facilitates the Z basis measurement. The Jones matrix for the half wave voltage PCI 221 and the quarter wave voltage PC2 222 rotatable half- wave plate (HWP) and a quarter- wave plate (QWP) are as follows:

HWP = ( ^{COS26 Sin 29} )

[0026] _siT1 2t? -coslQJ

[0028] The Jones matrix for PCI 221 and PC2 222 can be obtained by replacing π π

9 = -

^~ 8 and 4 respectively. For the X basis measurement, the input Jones vector for either D or A is first multiplied by the identity matrix I, and then the Jones matrix HWP resulting the H and V respectively. For the iY basis measurement, the input Jones vector either L or R is multiplied by first the Jones matrix QWP and then the Jones matrix HWP resulting in H and V respectively. For the Z basis measurement, the input Jones vector either H or V is multiplied by first the identity matrix I and then also the identity matrix I resulting in H and V respectively. [0029] While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations, and combinations thereof could be made to the present invention without departing from the scope of the invention.