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The implementation of polarisation encoded quantum key distribution in fibre.

Sharmini Pillay

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

Quantum Key Distribution (QKD) employs the laws of quantum mechanics for the purpose of cryptography. Two parties, commonly called Alice and Bob, are able to share a random key which is used to encrypt a message. Any eavesdropper trying to intercept their key will have to make measurements, thereby disturbing the system. This can be detected by Alice and Bob and they will then discard their key. Polarisation encoded QKD protocols use the polarisation of single photons as qubits to generate a cryptographic key. This can be implemented using a fibre optic link between Alice and Bob but the polarisation of light is altered when passed through a fibre due to birefringence caused by asymmetries in the fibre. This causes refractive differences for orthogonal components of the state of polarisation of light, so the polarisation is rotated as the photon is transmitted through the fibre. If the fibre is fixed, the change of polarisation will be unique and constant. This can be compensated by rotating each photon appropriately to its original state. Under typical environmental conditions, such as temperature changes and vibrations, the birefringence effects vary and should be compensated in real time. Therefore, an active polarisation controller is needed in order to maintain the state of polarisation of each qubit. An investigation was done to first track how the state of polarisation changes over time in a natural environment. Both wavelength-division multiplexing and time-division multiplexing were investigated as testing methods for the compensation system. A timedivision multiplexed system was developed to compensate the changes in polarisation. Since QKD protocols such as BB84 and B92 utilise two non-orthogonal bases, two polarisation controllers are usually used for compensation. However, by using a search algorithm, one polarisation controller was able to isolate the plane on the Poincare sphere that passes through both bases, thus compensating non-orthogonal states with one device.

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Towards polarisation-encoded quantum key distribution in optical fibre networks

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Quantum key distribution – a process that encodes digital information – often utilises fibre optic technologies for commercial applications. Fibre provides the benefit of a dark channel as well as the convenience of independence of a line-of-sight connection between the sender and receiver. In order to implement quantum key distribution protocols utilising polarisation encoding, the birefringence effects of fibre must be compensated for. Birefringence is caused by manufacturing impurities in the fibre or a change in environmental conditions and results in a rotation of the state of polarisation of light as it is propagated through the fibre. With dynamic environmental conditions, the birefringence effects should be monitored with a test signal at regular time intervals so that the polarisation of each photon can be appropriately compensated to its original state. Orthogonal states are compensated simultaneously, but most protocols, such as BB84 and B92, require non-orthogonal basis sets. Instead of using a compensator for each basis, the presented scheme fixes the polarisation controller onto the plane on the Poincaré that passes through both bases, compensating both non-orthogonal bases simultaneously.

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In 1946, C. E. Shannon proved that One-Time Pad is truly unbreakable [1].However, stringent conditions pose difficulties such as the key distribution problem which limited its practicability.Fortunately, public key cryptosystem widely used today was developed to solve the key distribution problem.Its security is based on the assumption that an adversary has limited computational power to factor a number with large prime factors.With the increase in computational power and technological advancement, this security may one day be compromised.In contrast, quantum key distribution (QKD) offers a platform for secure key distribution and everlasting secrecy.Unlike conventional cryptography, quantum key distribution's security is governed by the laws of quantum mechanics.The basic principle of quantum key distribution is to encode classical binary bit information onto the properties of quantum states such as the polarisation of a photon.Because of the quantum no-cloning theorem, an eavesdropper is unable to simply duplicate these photons.Moreover, by intercepting these photons, the eavesdropper will leave detectable trace which will reveal its presence.In 1989, the first experimental demonstration of QKD using the polarisations of single photons based on the BB84 protocol occurred through 32cm of air [2].Since then, QKD over hundreds of kilometres of optical fibre Abstract iv and free space have been reported [3][4][5].Key generation rate of a few megahertz have also been demonstrated [6][7][8].However, photons travelling through optical fibres are subjected to random polarisation drifts.Moreover, clock drifts often causes inaccuracy during the detection of these photons.If these performance issues were not addressed, the reliability and availability of QKD systems and their cryptographic keys will be affected.Therefore, polarisation recovery schemes have been implemented to mitigated polarisation drifts [9][10][11][12][13][14].However, these schemes often limit the transmission distance, slow down or even disrupt the key generation process for polarisation recovery.These limitations lead to the proposed development of a polarisation-encoded QKD system based on an adaptive polarisation state monitoring and recovery scheme that adapts the system to the existing polarisation drift condition in the transmission link to enhance its reliability and availability.On the other hand, current high-speed single-photon detection schemes are often designed to work with idealised parameters such as fixed gating rate and operating temperature.Therefore, such schemes are unable to accommodate changes in gating frequency induced by clock drifts which results in the reduction of detection efficiency.Hence, a proposed robust high-speed singlephoton avalanche diode with tunable sinusoidal gate frequency was developed to mitigate the effect caused by clock drift in order to maintain the detection efficiency over varying operating conditions.

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Quantum key distribution (QKD) using polarisation encoding can be hard to implement over deployed telecom fibres because the routing geometry and the birefringence of the fibre link can alter the polarisation states of the propagating photons. These alterations cause a basis mismatch, leading to an increased quantum bit error rate (QBER). In this work we demonstrate a technique for a dynamically compensating fibre-induced state alteration in a QKD system. This compensation scheme includes a feedback loop that minimizes the QBER using a stochastic optimization algorithm. The effectiveness of this technique is implemented and verified in a polarisation entanglement QKD system over a deployed telecom fibre.

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Quantum cryptographic key distribution (QKD) uses extremely faint light pulses to carry quantum information between two parties (Alice and Bob), allowing them to generate a shared, secret cryptographic key. Autocompensating QKD systems automatically and passively compensate for uncontrolled time-dependent variations of the optical fibre properties by coding the information as a differential phase between orthogonally polarized components of a light pulse sent on a round trip through the fibre, reflected at mid-course using a Faraday mirror. We have built a prototype system based on standard telecom technology that achieves a privacy-amplified bit generation rate of ~1000 bits s -1 over a 10 km optical fibre link. Quantum cryptography is an example of an application that, by using quantum states of individual particles to represent information, accomplishes a practical task that is impossible using classical means.

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In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40π rad s–1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.

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Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

GUILHERME BARRETO XAVIER, N Walenta, G Vilela de Faria +4 more

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40π rad s −1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.

arXiv:0905.0394•New Journal of Physics•Apr 30, 2009•98 citations
Quantum Key Distribution (QKD) employs the laws of quantum mechanics for the purpose of cryptography. Two parties, commonly called Alice and Bob, are able to share a random key which is used to encrypt a message. Any eavesdropper trying to intercept their key will have to make measurements, thereby disturbing the system. This can be detected by Alice and Bob and they will then discard their key. Polarisation encoded QKD protocols use the polarisation of single photons as qubits to generate a cryptographic key. This can be implemented using a fibre optic link between Alice and Bob but the polarisation of light is altered when passed through a fibre due to birefringence caused by asymmetries in the fibre. This causes refractive differences for orthogonal components of the state of polarisation of light, so the polarisation is rotated as the photon is transmitted through the fibre. If the fibre is fixed, the change of polarisation will be unique and constant. This can be compensated by rotating each photon appropriately to its original state. Under typical environmental conditions, such as temperature changes and vibrations, the birefringence effects vary and should be compensated in real time. Therefore, an active polarisation controller is needed in order to maintain the state of polarisation of each qubit. An investigation was done to first track how the state of polarisation changes over time in a natural environment. Both wavelength-division multiplexing and time-division multiplexing were investigated as testing methods for the compensation system. A timedivision multiplexed system was developed to compensate the changes in polarisation. Since QKD protocols such as BB84 and B92 utilise two non-orthogonal bases, two polarisation controllers are usually used for compensation. However, by using a search algorithm, one polarisation controller was able to isolate the plane on the Poincare sphere that passes through both bases, thus compensating non-orthogonal states with one device.
Quantum key distribution – a process that encodes digital information – often utilises fibre optic technologies for commercial applications. Fibre provides the benefit of a dark channel as well as the convenience of independence of a line-of-sight connection between the sender and receiver. In order to implement quantum key distribution protocols utilising polarisation encoding, the birefringence effects of fibre must be compensated for. Birefringence is caused by manufacturing impurities in the fibre or a change in environmental conditions and results in a rotation of the state of polarisation of light as it is propagated through the fibre. With dynamic environmental conditions, the birefringence effects should be monitored with a test signal at regular time intervals so that the polarisation of each photon can be appropriately compensated to its original state. Orthogonal states are compensated simultaneously, but most protocols, such as BB84 and B92, require non-orthogonal basis sets. Instead of using a compensator for each basis, the presented scheme fixes the polarisation controller onto the plane on the Poincaré that passes through both bases, compensating both non-orthogonal bases simultaneously.
In 1946, C. E. Shannon proved that One-Time Pad is truly unbreakable [1].However, stringent conditions pose difficulties such as the key distribution problem which limited its practicability.Fortunately, public key cryptosystem widely used today was developed to solve the key distribution problem.Its security is based on the assumption that an adversary has limited computational power to factor a number with large prime factors.With the increase in computational power and technological advancement, this security may one day be compromised.In contrast, quantum key distribution (QKD) offers a platform for secure key distribution and everlasting secrecy.Unlike conventional cryptography, quantum key distribution's security is governed by the laws of quantum mechanics.The basic principle of quantum key distribution is to encode classical binary bit information onto the properties of quantum states such as the polarisation of a photon.Because of the quantum no-cloning theorem, an eavesdropper is unable to simply duplicate these photons.Moreover, by intercepting these photons, the eavesdropper will leave detectable trace which will reveal its presence.In 1989, the first experimental demonstration of QKD using the polarisations of single photons based on the BB84 protocol occurred through 32cm of air [2].Since then, QKD over hundreds of kilometres of optical fibre Abstract iv and free space have been reported [3][4][5].Key generation rate of a few megahertz have also been demonstrated [6][7][8].However, photons travelling through optical fibres are subjected to random polarisation drifts.Moreover, clock drifts often causes inaccuracy during the detection of these photons.If these performance issues were not addressed, the reliability and availability of QKD systems and their cryptographic keys will be affected.Therefore, polarisation recovery schemes have been implemented to mitigated polarisation drifts [9][10][11][12][13][14].However, these schemes often limit the transmission distance, slow down or even disrupt the key generation process for polarisation recovery.These limitations lead to the proposed development of a polarisation-encoded QKD system based on an adaptive polarisation state monitoring and recovery scheme that adapts the system to the existing polarisation drift condition in the transmission link to enhance its reliability and availability.On the other hand, current high-speed single-photon detection schemes are often designed to work with idealised parameters such as fixed gating rate and operating temperature.Therefore, such schemes are unable to accommodate changes in gating frequency induced by clock drifts which results in the reduction of detection efficiency.Hence, a proposed robust high-speed singlephoton avalanche diode with tunable sinusoidal gate frequency was developed to mitigate the effect caused by clock drift in order to maintain the detection efficiency over varying operating conditions.
Quantum key distribution (QKD) using polarisation encoding can be hard to implement over deployed telecom fibres because the routing geometry and the birefringence of the fibre link can alter the polarisation states of the propagating photons. These alterations cause a basis mismatch, leading to an increased quantum bit error rate (QBER). In this work we demonstrate a technique for a dynamically compensating fibre-induced state alteration in a QKD system. This compensation scheme includes a feedback loop that minimizes the QBER using a stochastic optimization algorithm. The effectiveness of this technique is implemented and verified in a polarisation entanglement QKD system over a deployed telecom fibre.
Quantum cryptographic key distribution (QKD) uses extremely faint light pulses to carry quantum information between two parties (Alice and Bob), allowing them to generate a shared, secret cryptographic key. Autocompensating QKD systems automatically and passively compensate for uncontrolled time-dependent variations of the optical fibre properties by coding the information as a differential phase between orthogonally polarized components of a light pulse sent on a round trip through the fibre, reflected at mid-course using a Faraday mirror. We have built a prototype system based on standard telecom technology that achieves a privacy-amplified bit generation rate of ~1000 bits s -1 over a 10 km optical fibre link. Quantum cryptography is an example of an application that, by using quantum states of individual particles to represent information, accomplishes a practical task that is impossible using classical means.
In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40π rad s–1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.
In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40π rad s −1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.