WO2007041096A2

WO2007041096A2 - Two-way qkd system with faraday effect compensation

Info

Publication number: WO2007041096A2
Application number: PCT/US2006/037456
Authority: WO
Inventors: Michael J. Lagasse; Alexei Trifonov; Ekaterina Rogacheva; Anton Zavriyev; A. Craig Beal
Original assignee: Magiq Technologies, Inc.
Priority date: 2005-09-29
Filing date: 2006-09-27
Publication date: 2007-04-12
Also published as: WO2007041096A3

Abstract

Systems and methods for compensating the effects of polarization-mode dispersion (PMD) and polarization rotation due to the Faraday effect induced by the Earth's magnetic field in the transmission optical fiber (3) in a two-way QKD system (1) are disclosed. An example system includes a polarization-adjusting device that includes a Faraday rotator (100), polarization modulator (150) or polarization scrambler (120). The system is adapted to make the quantum pulses (P1 and P2) co-polarized when they travel over the transmission optical fiber connecting the Alice and Bob QKD stations. The amount of compensation provided by the polarization-adjusting device can be deduced empirically based on either the qubit error rate (BER) or detector counts, or by calculations based on the length and orientation of the transmission optical fiber relative to the Earth's magnetic field.

Description

TWO-WAY QKD SYSTEM WITH FARADAY EFFECT COMPENSATION

Claim of Priority

This patent application claims priority from U.S. Provisional Patent Application No. 60/721 ,702, filed September 29, 2005.

Field of the Invention

The present invention relates to and has industrial utility with respect to quantum cryptography, and in particular relates to and has industrial utility with respect to apparatus and methods for compensating Faraday effects in the transmission fiber of a two-way quantum key distribution (QKD) system such as caused by the Earth's magnetic field.

Background Art

Quantum key distribution involves establishing a key between a sender ("Alice") and a receiver ("Bob") by using weak (e.g., 0.1 photon on average) optical signals transmitted over a "quantum channel." Unlike classical cryptography that relies on computational impracticality, the security of quantum key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper ("Eve") that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.

The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article "Quantum Cryptography: Public key distribution and coin tossing," IEEE Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, December 10- 12, 1984, pp. 175-179. Specific QKD systems are described in the publication by CH. Bennett et al., entitled "Experimental Quantum Cryptography," J. Cryptology 5: 3-28 (1992), in the publication by CH. Bennett, entitled "Quantum Cryptography Using Any Two Non-Orthogonal States", Phys. Rev. Lett. 68 3121 (1992), and in U.S. Patent No. 5,307,410 to Bennett (the '410 patent). The general process for performing QKD is described in the book by Bouwmeester et al., "The Physics of Quantum Information," Springer-Verlag 2001 , in Section 2.3, pages 27-33.

The Bennett-Brassard article and the '410 patent each describe a so- called "one-way" QKD system wherein Alice randomly encodes the polarization of single photons, and Bob randomly measures the polarization of the photons. The one-way system described in the '410 patent is based on a two-part optical fiber Mach-Zehnder interferometer. Respective parts of the interferometer are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized during transmission to compensate for thermal drifts. This is generally inconvenient for practical applications involving transmission distances measured in kilometers.

U.S. Patent No. 6,438,234 to Gisin (the '234 patent'), which patent is incorporated herein by reference, discloses a so-called "two-way" QKD system that employs an autocompensating interferometer of the type invented by Dr. Joachim Meier of Germany and published in 1995 (in German) as "Stabile lnterferometrie des nichtlinearen Brechzahl-Koeffizienten von Quarzglasfasern der optischen Nachrichtentechnik," Joachim Meier. - AIs Ms. gedr.. - Dϋsseldorf : VDI-Verl., Nr. 443, 1995 (ISBN 3-18-344308-2). Because an autocompensated interferometer naturally compensates for polarization and/or thermal variations, a QKD system based thereon is generally less susceptible to environmental effects than a one-way system.

FIG. 1 is a schematic diagram of a prior art QKD system 1 according to the '234 patent, and further including an optical delay line DL, the role of which is discussed below. FIG. 2 is a close-up schematic diagram of Bob according to another example embodiment in the prior art, and discussed in greater detail below. The QKD system 1 of FIG. 1 includes Alice and Bob connected by an transmission optical fiber 3. Bob includes a 2 x 2 coupler 12. In principle, Bob's side is an unbalanced Michelson interferometer with one long arm going to Alice. Bob's side includes a pulsed laser 10, a first coupler 11, a Faraday mirror 16, a second coupler 12, a phase modulator 13, a second Faraday mirror 14 and a single photon detector 17. The laser 10 may be, e.g., a DFB laser and produces e.g. 300 picosecond (ps) long pulses at 1300 nm, with a repetition rate of e.g. 1 MHz. Alice's side includes a coupler 20, a "normal" detector 23 (i.e., a non-single- photon detector), a phase modulator 21 , a Faraday mirror 22 and an attenuator 24 controlled by the detector 23.

In the operation of QKD system 1, Bob initiates transmission by sending a short, relatively strong laser pulse towards Alice. The pulse arriving in the coupler 12 is split into two parts (pulses), P1 and P2. P1 goes directly towards Alice over transmission optical fiber 3, and P2 is first delayed by one bounce in the mirrors 14 and 16 (delay line). Pulses P1 and P2, travel down the transmission optical fiber to Alice. The two pulses are split at coupler 20, with the majority of the pulse going to detector 23 so that weak pulses are sent through phase modulator 21. In order to encode her bits, Alice lets the first pulse P1 be reflected by mirror 22, but modulates the phase (phase shift ΦA) of the second pulse P2 using phase modulator 21 situated in front of Faraday mirror 22 The two pulses then travel back to Bob.

Detection on Bob's side is done by delaying part of pulse P1 in the same delay line 14-16. Bob lets pulse P2 pass unaltered but modulates the phase of the first pulse P1 with the phase modulator 13 situated in front of the mirror 14 (phase shift ΦB). This pulse then interferes with P2. If the phase modulators at both Alice's and Bob's are off, or if the difference Φ_A - ΦB- = 0 (same phase shift applied to the two pulses P1 and P2), then the interference will be constructive (the two pulses follow exactly the same path). If however Alice or Bob change their phase setting between the two pulses, the interference may become destructive. Totally destructive interference is obtained when ΦA - ΦB = TΓ. In this case no light is detected at single photon detector 17. Note that it is essential that the interference obtained when the phase shifts are different is totally destructive. This ensures that, when Bob obtains a detection event, he can be certain that Alice did not use a different phase, and thus that she used the same phase as Bob.

As discussed in the article by Gisin et al., entitled "Quantum Cryptography," Rev. Mod. Phys., Vol. 74, No. 1 , January 2002, on pages 172- 173, the intrinsically bi-directional nature of the reflective QKD system makes Rayleigh backscattering problematic. With continuing reference to FIG. 1 , light pulses P1 and P2 emitted by Bob into optical fiber 3 undergo scattering by inhomogeneities in the optical fiber material, and a small fraction of this light (~ 1%) is recaptured by the fiber and travels backwards towards Bob. The backward traveling light can combine with phase-encoded signals returning to Bob from Alice, causing false counts at Bob.

To solve this problem, the QKD system described in the '234 and shown in FIG. 1 needs to further include an optical delay line DL in Alice in which trains of pulses sent by Bob are stored. This ensures that pulses traveling to and from Bob are not simultaneously present in optical fiber 3 connecting Bob and Alice. However, such a delay line increases the effective length of the transmission optical fiber 3.

While the two-way QKD system generally compensates for environmental effects such as temperature variations, it does not compensate for all environmental effects. In particular, one subtle environmental effect that is not compensated in a two-way QKD system is the Faraday effect induced in the transmission optical fiber by the Earth's magnetic field. The small magnetic field of the Earth (which is about 50 μT) can impart small amounts of circular birefringence in the transmission optical fiber when the fiber is oriented in the North-South direction. One effect of this birefringence is to interact with polarization mode dispersion (PMD), wherein optical signals of different polarization experience a different refractive index and thus travel at different speeds through the fiber, thereby introducing a phase delay between the different pulses. The circular nature of the birefringence also leads to polarization rotation. These effects cannot be compensated by the Faraday mirror at Alice and lead to an increase in the quantum bit-error rate (QBER).

Description of the Invention

The present invention includes systems and methods for compensating the effects of polarization-mode dispersion (PMD) and polarization rotation due to the Faraday effect induced by the Earth's magnetic field in the transmission optical fiber in a two-way QKD system. An example system includes a polarization-adjusting device that includes a Faraday rotator, a polarization modulator or a polarization scrambler. The system is adapted to make the quantum pulses (P1 and P2) co-polarized when they travel over the transmission optical fiber connecting the Alice and Bob QKD stations to mitigate the effects of polarization rotation and PMD from the Earth's magnetic field. The amount of polarization compensation provided by the polarization-adjusting device can be deduced empirically based on either the qubit error rate (BER) or detector counts, or by calculations based on the length and orientation of the transmission optical fiber relative to the Earth's magnetic field.

Brief Description of the Drawings

FIG. 1 is schematic diagram of a prior art folded QKD system as described in the '234 patent, and further including a delay line to reduce Rayleigh scattering;

FIG. 2 is a close-up schematic diagram of an example prior art bulk-optics embodiment of Bob for the QKD system of FIG. 1 ;

FIG. 3 is a schematic diagram of an example embodiment of Bob according to the present invention that utilizes an adjustable Faraday rotator and a polarizer to compensate for the induced Faraday effect in the transmission fiber;

FIG. 4 is a schematic diagram of an example embodiment of Bob according to the present invention that utilizes a polarization scrambler and a polarizer to compensate for the induced Faraday effect in the transmission fiber;

FIG. 5 is a schematic diagram of an example embodiment of Bob according to the present invention that utilizes a high-speed polarization modulator to compensate for the induced Faraday effect in the transmission fiber;

FIG. 6 is a schematic diagram of an example embodiment of Bob according to the present invention that utilizes a three-port circulator and a delay line to compensate for the induced Faraday effect in the transmission fiber;

FIG. 7 is a schematic diagram of an example embodiment of Alice according to the present invention that includes a Faraday rotator used in combination with the Faraday mirror; FIG. 8 is a schematic diagram of an example embodiment of Bob according to the present invention that utilizes polarization control to compensate for the induced Faraday effect in the transmission fiber; and

FIG. 9 is an example embodiment of a Faraday rotator apparatus according to an aspect of the invention suitable for use in the QKD stations Alice and Bob of the present invention.

In the Figures, like elements are identified with like reference numbers.

Detailed Description of the Best Mode of the Invention

FIG. 2 is a close-up schematic diagram of an example prior art bulk-optics embodiment of Bob for the two-way QKD system of FIG. 1. This prior art embodiment is described here for the sake of illustration so that the example embodiments of the present invention set forth below are more easily understood.

The Bob of FIG. 2 includes a variable optical attenuator (VOA) 56 arranged downstream of laser 10, a circulator 40 arranged downstream of VOA 56, a 50/50 beam splitter 50 downstream of circulator 40, and a polarizing (PBS) beam splitter 52 downstream of beam splitter 50. The beam splitters are coupled by a first polarization maintaining (PM) optical fiber section F1 coupled to respective first faces of the beam splitters. Phase modulator 13 is arranged in a second PM optical fiber section F2 connecting beam splitters 50 and 52 at respective second faces of the beam splitters. Beam splitters 50 and 52 and PM optical fiber sections F1 and F2 form an interferometer loop L that forms orthogonally polarized light pulses from an initial polarized light pulse, as described below.

Bob of FIG. 2 also includes two SPDs 17A and 17B respectively optically coupled to beam splitter 50 and circulator 40. A controller 70 is operatively coupled to VOA 56, SPDs 17A.17B and laser 10, and controls the operation of these elements. Controller 70 is also operatively coupled to a controller (not shown) at Alice so that Bob and Alice can synchronize their operation and otherwise publicly communicate. In the operation of Bob of FIG. 2, laser 10 generates (initial) optical pulses PO. First and second orthogonally polarized light pulses P1 and P2 are created from each optical pulse PO by the interferometer loop L made up of beam splitters 50 and 52 and PM optical fiber sections F1 and F2. The polarizations of light pulses P1 and P2 are flipped by 90° at Alice and are returned to Bob, where they take opposite paths through loop L and interfere at beam splitter 50 to form an interfered pulse that is detected by either SPD 17A or 17B.

Faraday effects induced in optical fiber 3 due to the Earth's magnetic field introduce an additional source of PMD in the fiber, which changes the phase relationship between orthogonally polarized pulses P1 and P2. Likewise, the circular birefringence causes polarization rotation of the two pulses. Both of these effects lead to an increase in the QBER.

Modification of QKD station Bob

The present invention modifies Bob to make pulses P1 and P2 co- polarized, i.e., polarized in the same direction rather than being orthogonally polarized. As pulses P1 and P2 propagate from Bob to Alice and then back to Bob, they experience polarization rotation and PMD due to the Faraday effect in the fiber induced by the terrestrial magnetic field. However, since the pulses are co-polarized, they experience essentially the same amount of phase shift so that there is a negligible amount of relative phase shift between them. The remaining polarization rotation is compensated using the different example embodiments of Bob discussed below.

In the case of orthogonally polarized pulses, upon returning from Alice's station, the two pulses can be passively separated by the polarizing beam splitter 52 and made to take the opposite paths through loop L from which they left Bob's device. This is not the case with co-polarized pulses because there is no way to passively route the two pulses in different directions. Therefore both pulses are split equally into the two paths in loop L. There are four different paths the pulses can take, they can exit through F1 and return through F1, they can exit through F2 and return through F2, they can exit through F1 and return through F2, and they can exit though F2 and return through F1. It is the two pulses which travel though the last two paths that produce interference. Pulses traveling though the first two paths will not interfere but will produce so-called "ghost" pulses which are temporally (and spatially) separated from the interfering pulses and must be time gated away to prevent high error rate.

First example embodiment

FIG. 3 is a schematic diagram of a first example embodiment of QKD station Bob according to the present invention. Bob of FIG. 3 is similar to Bob of FIG. 2, but PBS 52 is replaced by a 50/50 beam splitter 54, and a polarizer 90 and an adjustable Faraday rotator 100 are arranged downstream of beam splitter 54. A current controller 104 is operably coupled to Faraday rotator 100 and controller 70. Faraday rotator 100 is adapted to change the amount of polarization rotation of polarized light passing therethrough based on an input current signal S104 from current controller 104. A typical Faraday rotator includes a magneto-optic material that is transparent at the wavelength of interest and that has a relatively high Verdet constant, and also includes a source of a magnetic field (e.g., a current-carrying coil) used to induce the Faraday effect in the material. The amount of polarization rotation imparted by the Faraday rotator is a function of the Verdet constant (V), the applied magnetic field (H) and the length of the Faraday rotator (L).

In the operation of Bob of FIG. 3 as part of the larger QKD system, co- polarized light pulses P1 and P2 are created by interferometer loop L from each polarized initial optical pulse PO generated by light source 10, rather than orthogonally polarized light pulses as in the Bob of FIG. 2. Polarizer 90 is oriented to pass the co-polarized light pulses.

Assume for the moment that there is no Faraday effect in transmission optical fiber 3. In this situation, Faraday rotator 100 rotates the polarization of outgoing pulses P1 and P2 by 45°. Pulses P1 and P2 travel over to Alice, where the polarization is flipped by 90° by the Faraday mirror (not shown), and then returned to Bob. The pulses enter the Faraday rotator, where their polarizations are flipped again by 45°. The pulses thus experience a total polarization rotation of 180, so that they pass through polarizer 90, whereupon they enter loop L and are interfered and detected in the usual manner. Now consider the Faraday effect being present in optical fiber 3 due to the Earth's magnetic field. The co-polarized pulses P1 and P2 now experience polarization rotation of Δ° over the course of their round trip from Bob to Alice and back to Bob. If this change in polarization rotation is not accounted for, polarizer 90 will block some of the returned pulses. Here it is worth remembering that pulses P1 and P2 are quantum pulses, so that polarizer 90 will either pass or block a pulse, as opposed to transmitting a fraction of the pulse as is the case for multi-photon pulses. Thus, the effect of the polarization rotation Δ° on pulses P1 and P2 is to reduce the number of pulses, passing through polarizer 90, which reduces the transmission rate and increases the QBER.

In one example embodiment, the amount of polarization rotation -Δ° needed to compensate for the added rotation of Δ° is determined according to the distance between Bob and Alice and the orientation of the transmission optical fiber relative to the Earth's magnetic field.

In another example embodiment that provides for more accurate compensation, the amount of compensating polarization rotation is determined empirically at Bob based on obtaining a minimum QBER during system operation. In particular, controller 70 processes the number of clicks received from SPDs 17A and 17B and during system operation, and based thereon provides a control signal S70 to current controller 104, which in turn provides a current signal S104 to Faraday rotator 100. The current in signal S104 generates a corresponding magnetic field H in the Faraday rotator, thereby affecting the amount of polarization rotation it induces. By noting the change in QBER as a function of the value of the current supplied to the Faraday rotator, the minimum QBER can be found and maintained. Alternatively, the compensation of polarization rotation can be achieved by maximizing the counts obtained at SPDs 17A and 17B rather than actually calculating the QBER.

Second example embodiment

FIG. 4 is a second example embodiment of QKD station Bob according to the present invention. Bob of FIG. 4 is similar to Bob of FIG. 3, except that instead of an adjustable Faraday rotator 100 (and the associated current controller 104), Bob of FIG. 4 includes a polarization scrambler 120 downstream of polarizer 90. Polarization scrambler is operably coupled to controller 70 and is activated thereby via a control signal S 120 from the controller. When activated, polarization scrambler 120 randomly changes the polarization of polarized pulses passing therethrough. In the present invention, polarization controller 120 is activated so as to scramble the polarization of pulses P1 and P2 incoming to Bob. The polarization scrambler 120 scrambles polarization at the rate much slower than the separation between pulses P1 and P2 so that the polarization scrambler imparts the same polarization to both pulses. At the same time, the rate of polarization scrambler 120 should be fast enough compared to signal round trip time between Alice and Bob.

Because of the 90° polarization rotation by the Faraday mirror at Alice, incoming pulses P1 and P2 are polarized such that they will not pass through polarizer 90. Polarization scrambler 120 insures that at least 50% of polarized pulses P1 and P2 will be transmitted by polarizer 90 and be processed by Bob, even if there is polarization rotation due to the Faraday effect in optical fiber 3 due to the Earth's magnetic field. The trade-off here is that half of the quantum signals are lost in the process.

Third example embodiment

FIG. 5 is a third example embodiment of QKD station Bob according to the present invention. Bob of FIG. 5 is similar to Bob of FIG. 2, except that the Bob of FIG. 5 includes a high-speed polarization modulator 150 operably coupled to controller 70 and controlled thereby via a control signal S150. When activated, polarization modulator imparts a 90° polarization to light passing therethrough.

In operation, polarization modulator 150 applies a 90° polarization rotation to one of the outgoing pulses P1 and P2 via timed control signal S150 so that they are co-polarized. Pulses P1 and P2 travel to Alice, where the polarization of both pulses is flipped by 90°. The pulses then return to Bob. During their round trip, pulses P1 and P2 accrue essentially the same amount of polarization rotation due to the Faraday effect in transmission optical fiber 3. Upon return of the pulses to Bob, polarization modulator 150 is again activated via timed control signal S150 to flip the polarization (by 90°) of the same pulse P1 and P2 which was flipped when exiting Bob bound for Alice. In this way the two pulses are orthogonally polarized as they exit the polarization modulator whereupon they pass through loop L in the direction opposite from which they left.

While pulses P1 and P2 were co-polarized, their respective polarizations will have been rotated by Δ° due to the Faraday effect in transmission optical fiber 3. Accordingly, the effect of the polarizing beam splitter 52 is to block a fraction of the pulses in proportion to the amount of rotation Δ°, by causing this proportion to appear as ghost pulses. By applying a small additional modulation to both pulses (in addition to flipping one by 90°) on the return trip it is possible to eliminate the loss of signals caused by the Δ° mismatch. The level of this additional polarization modulation can be determined empirically through iterative adjustment or via calculation as described above. Note that the effect of PMD is mitigated as compared to a two-way QKD system that uses the Bob of FIG. 2.

Fourth example embodiment

FIG. 6 is a fourth example embodiment of QKD station Bob according to the present invention. Bob of FIG. 6 includes a polarization insensitive three-port circulator 180 optically coupled via fiber F3 to a first input/output side 54A of beam splitter 54. Light exiting through the output side 54A of beamsplitter 54 passes through the three-port PM circulator from port 180A to 180B, propagates to Alice, is rotated by 90° by the Faraday mirror at Alice, and returns back to three-port PM circulator 180. The circulator 180 routes this light from port 180B to port 180C and then to polarization controller 181. Polarization controller 181 is optically coupled to polarizer 182, which is optically coupled to the input side 54B of beamsplitter 54 via PM fiber F4. Polarization controller 181 removes the 90° rotation imparted by the Faraday mirror in Alice as well as the additional rotation angle, Δ° caused by the Faraday effect in the transmission optical fiber 3. Thu, the polarization state input through the input side 54B of beam splitter 54 is identical to the one leaving the output side 54A. A control algorithm run in Bob's controller 70 used to produce control signals S181 serves to maximize counts on the two single photon detectors 17A and 17B and/or minimize QBER in a similar fashion as described in the first example embodiment.

Light exiting through output side 54B of beam splitter 54 is routed through polarizer 182 and polarization controller 181 to port 180C of the three-port circulator where it is absorbed (these devices do not pass optical signals from port C to port A). In this manner, the QKD system is made co-polarized, thereby mitigating the induced Faraday effects in transmission optical fiber 3.

Fifth example embodiment

FIG. 7 is a fifth example embodiment of a QKD station Alice according to the present invention. Alice of FIG. 7 is similar to Alice of FIG. 1 , except for the absence of detector 23 and the addition of an adjustable Faraday rotator 100 is arranged adjacent Faraday mirror 22. Also included is Faraday rotator current source 140 operably coupled to the Faraday rotator and to Alice's controller 200, which is in operative communication with Bob's controller 70 so that the operation of the QKD system is properly synchronized.

Alice of FIG. 7 can be used together with QKD station Bob of FIG. 2 or FIG. 3, for example. In response to a control signal S200 from controller 200, current source 104 proves a current signal S104 to Faraday rotator 100 (e.g., to an induction coil, not shown). Current signal S104 is selected so as to tune the rotation of polarization applied by Faraday rotator to compensate for the rotation of polarization caused by Earth's magnetic field. As discussed above, the proper amount of compensation can be calculated based on the length of optical transmission fiber 3, or deduced empirically via the QBER or SPD clicks. This configuration can be used both for the cross-polarized (FIG. 2) or the co- polarized (FIG. 3) versions of the Bob QKD station.

Sixth example embodiment

FIG. 8 is a sixth example embodiment of QKD station Bob according to the present invention. Bob of FIG. 8 is similar to Bob of FIG. 5, except that instead of having a polarization beam splitter 52, Bob in FIG. 8 has a 50/50 beam splitter 54. The polarization modulator 150 is activated by control signal S150 from controller 70. Control signal S150 is timed so that the activation of polarization modulator 150 coincides with the arrival of pulses P1 and P2 back to Bob from Alice. Polarization controller 70 is adjusted similar to the manner described above (e.g., empirically through iterative adjustment or via calculation) until the Faraday effects from the Earth's magnetic field are compensated. This scheme will cause half of the signal to be lost to "ghost" pulses.

Faraday rotator example embodiment

With reference to FIG. 9, there is shown an example embodiment of a Faraday rotator apparatus 300 suitable for use in QKD stations Alice and Bob of the present invention. Apparatus 300 utilizes a Faraday rotator (e.g., a magneto- optical material that exhibits the Faraday effect) placed in a magnetic field that varies spatially in a given direction. Instead of changing the magnetic field applied to the Faraday rotators at the Bob or Alice QKD station by changing current signal 104, the Faraday rotator 330 of apparatus 300 is placed in a static magnetic field H that spatially varies. The position of the Faraday rotator is then changed to subject it to either a stronger or weaker magnetic field, which in turn results in either a stronger or weaker polarization rotation.

In an example embodiment, apparatus 300 includes a conducting coil 310 operably connected to a current source 320. A current, represented by current signal S320, is provide to conducting coil 310 by current source 320 so as to create a steady magnetic field H. Current source 320 is, in turn coupled to the controller (e.g., controller 70 or 100) and is operated thereby via a control signal S70 from the controller. Controller 70 is used in the present example embodiment for the sake of illustration. Conducting coil 310 is arranged so as to create a magnetic field that varies spatially, preferably mainly along a single direction (e.g., the X-direction as indicated FIG. 9).

Faraday rotator 330 has a first (left) side 332 and a second (right) side 334. Faraday rotator 330 is placed on or is otherwise supported by a positioner 340 adapted to move with micrometer accuracy. The movements of the positioner 340 that adjust the position of Faraday rotator within magnetic field H are determined by control signals S340 from controller 70 and are determined by the controller according to the maximum counts obtained from SPDs or minimum QBER.

In an example embodiment where apparatus 300 is incorporated into the Alice QKD station of FIG. 7, the controller is Alice's controller 100, and side 332 of Faraday rotator 300 is attached to Faraday mirror 22 (FIG. 7) while side 334 is connected to Alice phase modulator 21.

In an example embodiment where apparatus 300 is incorporated into the Bob QKD station (.e.g., the Bob of FIG. 3), the Faraday rotator side 332 is operably coupled to the output of Bob QKD station, while side 334 is operably coupled to polarizer 90.

Claims

What is claimed is:

1. In a two-way QKD system having two QKD stations Bob and Alice optically coupled by a transmission optical fiber, a method of compensating for the Faraday effect induced in the optical fiber from the Earth's magnetic field, comprising: causing quantum pulses traveling over the transmission optical fiber to be co-polarized so as to mitigate effects from polarization-mode dispersion (PMD) associated with the induced Faraday effect; and rotating the polarization of the co-polarized pulses using a polarization- adjusting device to compensate for a polarization-rotation effect associated with the induced Faraday effect.

2. The method of claim 1 , wherein the QKD system has a qubit error rate (QBER), and including determining an amount of polarization rotation compensation by operating the QKD system and minimizing the QBER.

3. The method of claim 1 , wherein the QKD system has one or more single- photon detectors at Bob, and including determining an amount of polarization rotation compensation by operating the QKD system and optimizing a number of clicks generated by the one or more single-photon detectors.

4. The method of claim 1 , wherein the polarization-adjusting device is located at Bob.

5. The method of claim 1 , wherein the polarization-adjusting device includes a Faraday rotator and a polarizer.

6. A QKD station Bob for a two-way QKD system having an optical fiber link that connects QKD station Bob to a QKD station Alice, comprising: a laser adapted to generate polarized initial light pulses; an interferometer loop operably coupled to the laser and adapted to generate from each initial light pulse first and second co-polarized light pulses; a polarization-adjusting device arranged downstream of the interferometer loop and adapted to adjust the polarization of the first and second co-polarized light pulses so as to compensate for polarization rotation and/or polarization mode dispersion caused by the earth's magnetic field.

7. The QKD station Bob of claim 6, including: first and second single-photon detectors (SPDs) operably coupled to the interferometer loop so as to detect an interfered optical pulse formed by first and second co-polarized light pulses P1 and P2 upon returning to Bob from Alice; a controller operably coupled to the first and second SPDs and to the polarization-adjusting device, the controller adapted to determine a qubit error rate (QBER) and adjust the polarization-adjusting device to maximize the QBER.

8. The QKD station Bob of claim 6, wherein the polarization-adjusting device includes a Faraday rotator and a polarizer.

9. The QKD station Bob of claim 6, wherein the polarization-adjusting device includes a polarization scrambler and a polarizer.

10. The QKD station Bob of claim 6, wherein the polarization-adjusting device includes a polarization modulator.

11. A QKD station Alice for a two-way QKD system that includes a QKD station Bob optically coupled to Alice via an optical fiber link, comprising along an optical axis: a Faraday mirror; a Faraday rotator; a phase modulator; and wherein the Faraday rotator is coupled to a current source adapted to provide a current to the Faraday rotator to compensate for polarization rotation caused by the earth's magnetic field.