QKD methods and systems have been developed which enable two parties to share random data in a way that has a very high probability of detecting any eavesdroppers. This means that if no eavesdroppers are detected, the parties can have a high degree of confidence that the shared random data is secret. QKD methods and systems are described, for example, in U.S. Pat. No. 5,515,438, U.S. Pat. No. 5,999,285.
Whatever particular QKD system is used, QKD methods typically involve QKD transmitting apparatus 10 (see
In most known QKD systems, the quantum signal is embodied as a stream of randomly polarized photons sent from the transmitting apparatus to the receiving apparatus either through a fiber-optic cable or free space; such systems typically operate according to the well-known BB84 quantum coding scheme (see C. H. Bennett and G. Brassard “Quantum Cryptography: Public Key Distribution and Coin Tossing”, Proceedings of IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, December 1984, pp 175-179).
In such systems, the QKD transmitter 12 provides the optical components for selectively polarizing photons, and the QKD receiver 22 provides the optical components for receiving photons and detecting their polarization. Typically, these optical components establish two pairs of orthogonal polarization axes, the two pairs of polarization axes being offset by 45° relative to each other. Conventionally, these two pairs of polarization axes are referred to as vertical/horizontal and diagonal/anti-diagonal respectively. An example QKD transmitter 12 and QKD receiver 22 will now be described with reference to
The QKD transmitter 12 of
The
The beam splitter 31 is depicted in
Operation of the apparatus of
Alice randomly generates (using source 11) a multiplicity of pairs of bits, typically of the order of 108 pairs. Each pair of bits consists of a data bit and a basis bit, the latter indicating the pair of polarization axes to be used for sending the data bit, be it vertical/horizontal or diagonal/anti-diagonal. A horizontally or diagonally polarized photon indicates a binary 1, while a vertically or anti-diagonally polarized photon indicates a binary 0. The data bit of each pair is thus sent by Alice over the quantum signal channel 5 encoded according to the pair of polarization directions indicated by the basis bit of the same pair. When receiving the quantum signal from Alice, Bob randomly chooses, by virtue of the action of the half-silvered mirror 31, which paired-detector unit 32, 33 and thus which basis (pair of polarization directions) it will use to detect the quantum signal during each time slot and records the results. The sending of the data bits of the randomly-generated pairs of bits is the only communication that need occur using the quantum channel 5.
Next, Bob sends Alice, via the classical channel 6, partial reception data comprising the time slots in which a signal was received, and the basis (i.e. pair of polarization directions) thereof, but not the data bit values determined as received.
Alice then determines a subset m of its transmitted data as the data bit values transmitted for the time slots for which Bob received the quantum signal and used the correct basis for determining the received bit value. Alice also sends Bob, via the classical channel 6, information identifying the time slots holding the data bit values of m. Bob then determines the data bit values making up the received data. The next phase of operation is error correction of the received data by an error correction process involving messages passed over the classical channel 6; after error correction, Bob's received data should match the data m held by Alice and this can be confirmed by exchanging encrypted checksums over the classical channel 6.
A requirement for the successful transmission of the quantum signal over the quantum signal channel 5 is that the quantum signal is correctly aligned with the quantum signal detector arrangement of the receiving apparatus 20, both directionally and such that the polarization directions of the transmitting and receiving apparatus 10, 20 have the same orientation. Where both the transmitting and receiving apparatus 10, 20 are fixed in position, this is not a major issue as alignment need only be effected once, that is, at the time the apparatus is installed. However, where one or both apparatus 10, 20 are movable, alignment is a greater issue as it will need to be done repeatedly.
For example, the QKD transmitting apparatus may take the form of a hand-held device intended to cooperate with fixed receiving apparatus; one possible scenario where this could be the case is depicted in
In cases, such as that depicted in
Instead of using a cradle, an active alignment system can be provided that uses an alignment channel between the transmitting and receiving apparatus to generate alignment adjustment signals for use in aligning the transmitting apparatus 2 and the receiving apparatus 4; example active alignment systems for a hand-held QKD transmitting apparatus are disclosed in US published application 20070025551 (Assignees: Hewlett-Packard Development Company, and The University of Bristol, UK).
Embodiments of the invention will now be described, by way of non-limiting example, with reference to the accompanying diagrammatic drawings of the prior art and of an embodiment of the invention, in which:
The device 10, which for example is similar in constitution to the apparatus of
Ideally, the longitudinal axis of the quantum signal emitted by the QKD transmitter of device 10 will be aligned with the input optical axis of a cooperating QKD receiving apparatus and the polarization axes of the signal and the detector of the QKD receiving apparatus will be in angular alignment. Absent any intervening compensatory mechanism, this would require the optical and polarization axes of the QKD transmitting device 10 to be aligned with the optical and polarization axes of the QKD receiving apparatus, the only unconstrained degree of freedom of the device being translation along the Z axis (though, of course, there are limits as to how far away the device 10 can be moved from the receiving apparatus 20). Such a tight requirement on the positioning and orientation of the QKD transmitting device 10, is a practical impossibility where the device is held in the hand of a human user.
The most that can be expected of a human user is that the user points the device 10 generally at the receiving apparatus (that is, the Z axis of the device points towards the receiving apparatus). Even this will generally require some form of feedback to the user (for example, by means of a laser pointer showing where the Z axis of the device is currently intersecting the front of the receiving apparatus).
If the user were able to keep the QKD transmitting device positioned on the optical axis of the receiving apparatus while pointing at the latter (that is, no positioning errors along the X and Y axes), the only misalignments requiring compensation would be related to undesired rotation of the device 10 about the axes X, Y and Z; that is, errors in pitch, yaw and roll). However, generally when a user has the device 10 pointed at the receiving apparatus, the device will be offset along its X and Y axes from the optical axis of the receiving apparatus and the user compensates for such offsets by yaw and pitch adjustments of the device. These pitch and yaw adjustments of the device can be compensated for at the QKD receiving apparatus by making complementary adjustments, that is, by orientating the input optical axis of the receiving apparatus such that it points at the transmitting device. Such compensation does not, of course, account for pitch and yaw errors associated with inaccurate pointing of the device at the receiving apparatus.
Recapping, generally where a user seeks to interface a hand-held QKD transmitting device with a fixed QKD receiving apparatus by pointing the device at the QKD receiving apparatus, it will be necessary to implement compensatory measures in order for the quantum signal to strike the detector of the receiving apparatus along (or substantially along, that is, juxtaposed and parallel to) its optical axis with the polarization axes of the quantum signal and detector aligned. In particular, the following compensations will generally be required:
To effect such compensation, an alignment correction system can be provided, preferably with at least the majority of its active elements in the fixed receiving apparatus; the above mentioned US published application 20070025551 discloses an example of such an alignment correction system.
An alignment correction system comprises, at least conceptually:
Having reviewed the considerations applicable to an alignment correction system for a hand-held QKD transmitting device and cooperating QKD receiving apparatus, a preferred embodiment of such an alignment correction system will now be described with reference to
The other elements of Alice and Bob (corresponding to elements 11, 13, 14, 23 & 24 of
The quantum signal from the source 40 enters the QKD receiver 22 through an optical port 200 and is guided along an optical channel defined by optical components (in this embodiment, three mirrors 44-46) to the quantum signal detector 42. As will be more fully described below, the mirrors 44 and 46 are tip-tilt mirror (that is rotatable about two orthogonal axes lying in the plane of the mirror); the mirror 45 is a dichroic beam splitter, reflecting the quantum signal photons but passing photons of an alignment beam. The optical channel along which the quantum signal passes between the optical port 200 and detector 42 can be differently arranged from that depicted in
The QKD receiver 22 is provided with an alignment correction system comprising a misalignment measuring sub-system 50 for taking measures of the alignment corrections required to optimize the entry of the quantum signal to the detector 42, and a misalignment compensation sub-system 60 for effecting the required alignment corrections using an appropriate alignment compensator for each type of alignment correction to be effected. The misalignment measuring subsystem 50 and the misalignment compensation sub-system 60 are respectively depicted in
The misalignment measuring sub-system 50 (
Considering the misalignment measuring sub-system 50 in more detail, an alignment beam source 51 of receiver 22 generates a bright, wide-angled alignment beam at a wavelength different to that of the quantum signal; this beam after passage through a partially transmitting mirror 52 and the dichroic beam splitter 45 (transparent at the wavelength of the alignment beam), is reflected by mirror 53 (the mirror 42 of
Assuming the QKD transmitter 12 is roughly in the expected direction from the QKD receiver 22, a portion of the alignment beam will strike the transmitter and be reflected by retro-reflector 54 back to the transmitter 20. As is well known, a retro-reflector is a device or surface that reflects a wave front back along a vector that is parallel to but opposite in direction from the angle of incidence; a number of different forms of retro-reflection units are known (for example, a corner cube with a set of three mutually perpendicular mirrors that form a corner). In the present embodiment, the retro-reflector 54 actually comprises three retro-reflection units arranged in a predetermined configuration, for example an isosceles triangle, when viewed along the optical axis of the transmitter 20 (that is, the direction of emission of the quantum signal a.k.a. the direction of pointing of the transmitter 20); as a result, the reflected alignment beam is actually made up of three sub-beams that give the overall beam a predetermined cross-sectional pattern which corresponds to said predetermined configuration when the retro-reflected beam returned to the receiver 22 is aligned with the direction of pointing of the transmitter; more generally, the cross-sectional pattern presented by the retro-reflected alignment beam will be dependent both on the aforesaid predetermined configuration of the retro-reflection units, and on any offset between the direction of pointing of the transmitter and the actual direction to the receiver, that is, any transmitter pointing misalignment.
The retro-reflected beam returned to the receiver 22 enters the optical port 200 and, after reflection by the mirror 53/44 and passage through the dichroic beam splitter 45, is reflected by the partially transmitting mirror 52 to strike a position-sensing detector 55. The detector 55 is part of a detector arrangement that further comprises a measurement processor 56 for processing the output of the detector 55 to generate several different misalignment measures.
More particularly, by determining where each of the three alignment sub-beams strikes the detector 55, the detector arrangement is arranged to generate on output 57 signals indicative of each of the following three misalignment measures:
The misalignment measures are passed to a controller 61 of the misalignment compensation sub-system 60 (
The receiver-pointing misalignment compensator 63 serves as one of the optical components defining the quantum-signal optical channel (in this case the tip-tilt mirror 44) and controls the direction of pointing of the longitudinal axis of the quantum-signal optical channel at the receiver optical port 200 (that is, the direction of pointing of the receiver). The angling of the compensator 63 (tip-tilt mirror 44) is set by the drive 62, the latter being controlled by the controller 61 in dependence on the receiver-pointing misalignment measure from subsystem 50 such as to point the receiver 22 at the transmitter 12. In the present embodiment, as adjusting the direction of pointing of the receiver by adjusting the angling of the compensator 63, affects not only the incoming quantum signal but also the alignment beam, the elements that measure the receiver-pointing misalignment and compensate for this misalignment form a closed-loop control system with the misalignment measure reducing to zero as the compensator 63 is adjusted to point the receiver at the transmitter. Although not preferred, it would alternatively be possible to arrange for the alignment beam to be unaffected by the compensator 63 resulting in open-loop control of the latter.
The transmitter-pointing misalignment compensator 67 serves as another of the optical components defining the quantum-signal optical channel (in this case the tip-tilt mirror 46) and controls the direction of pointing of the longitudinal axis of the quantum-signal optical channel at the quantum-signal detector 42. The angling of the compensator 67 (tip-tilt mirror 46) is set by the drive 66, the latter being controlled by the controller 61 in dependence on the transmitter-pointing misalignment measure from subsystem 50 such that the quantum signal strikes the detector 42 substantially orthogonally. In the present embodiment, as adjusting the angling of the compensator 67 does not affect the alignment beam, the elements that measure the transmitter-pointing misalignment and compensate for this misalignment form an open-loop control system with the misalignment measure being unaffected by adjustment of the compensator 67. It would alternatively be possible to provide a closed-loop control system for the compensator 67 by, for example, mounting the position sensing detector 55 on a tip-tilt table mechanically coupled to tip/tilt with the tip-tilt mirror forming the compensator 67. Furthermore, rather than the transmitter-pointing misalignment compensator 67 being formed by one of the optical components defining the path 41, the transmitter-pointing misalignment compensator 67 can be implemented as an arrangement for adjusting the direction of pointing of the optical axis of the quantum-signal detector 42.
The polarization misalignment compensator 65 takes the form of a polarization rotator (for example, a half-wave plate) located in the quantum-signal optical channel and controls the orientation of the axes of polarization of the quantum signal arriving at the detector 42. The angling of the compensator 65 is set by the drive 64, the latter being controlled by the controller 61 in dependence on the polarization misalignment measure from subsystem 50 such that the polarization axes of the quantum signal entering the detector 42 are aligned with the polarization axes of the detector 42. In the present embodiment, as adjusting the compensator 65 has no effect on the sub-beam incidence pattern at the position-sensing detector 55, the elements that measure the polarization misalignment and compensate for this misalignment form an open-loop control system with the misalignment measure being unaffected by adjustment of the compensator 65. It would alternatively be possible to provide a closed-loop control system for the compensator 65 by, for example, arranging for the position-sensing detector 55 to rotate in its plane in coordination with rotation of the polarization rotator forming the compensator 65. Furthermore, rather than the polarization misalignment compensator 65 being formed by a polarization rotator located in the path 41, the polarization misalignment compensator 67 can be implemented as an arrangement for rotating the quantum-signal detector 42 to adjust the orientation of its polarization axes.
It will be appreciated that many variants are possible to the above described embodiment of the invention. For example, the number and configuration of the individual retro-reflection units making up the retro-reflector 54 can be varied; indeed, the division of the alignment beam into multiple sub-beams with a predetermined cross-sectional pattern can be effected at the receiver 22 on the outgoing beam (for example, using a beam splitter or providing separate sources for each sub-beam) in which case the retro-reflector 54 need only comprise a single retro-reflection unit.
Furthermore, the configuration of the paths followed by the alignment beam and the quantum signal within the receiver 22 can be varied from that described above.
Number | Date | Country | Kind |
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0809227.2 | May 2008 | GB | national |