“…On the other hand, key distribution is a primitive that can only be implemented with classical resources if one is willing to constrain the power of the eavesdropper. Even though there exist practical QKD schemes which enable secure communication over large distances with high key rates [13][14][15][16], some of the fundamental questions about the capacity to transmit secure correlations remain unanswered.There are essentially two quantities that describe the ability of the channel to send secure messages to the receiver, and consequently, generate secret keys. The first one is called private capacity P [6,17].…”
The quantum capacity of a quantum channel is always smaller than the capacity of the channel for private communication. Both quantities are given by the infinite regularization of the coherent and the private information, respectively, which makes their evaluation very difficult. Here, we construct a family of channels for which the private and coherent information can remain strictly superadditive for unbounded number of uses, thus demonstrating that the regularization is necessary. We prove this by showing that the coherent information is strictly larger than the private information of a smaller number of uses of the channel. This implies that even though the quantum capacity is upper bounded by the private capacity, the nonregularized quantities can be interleaved. Efficient information transmission is the cornerstone of all information processing tasks in our interconnected world. In the most basic scenario, two parties, linked by a fixed communication channel wish to exchange messages with each other. What is the maximum rate at which they can reliably transmit information?Classical information theory gives an exhaustive answer to this question [1]. There exists an efficient convex optimization algorithm which takes the description of a channel and calculates its capacity to convey information. This is the consequence of a particularly simple analytic expression for the classical capacity of a channel. Our world is inherently quantum and when we turn to the channels that transmit quantum information we are able to perform many novel information processing tasks which are impossible in the classical theory, such as establishing entanglement between sender and receiver. Presently, when confronted with the above question for the quantum channels, there is no known efficient algorithm that takes the description of an arbitrary channel and calculates its capacity. Different types of capacity of the quantum channel are defined as regularized quantities [2-9], which implies that in order to compute them it is necessary to perform an unbounded optimization over the number of the copies of the channel. In practice it means that to estimate the capacity for n uses of the channel the dimension of the state space which one has to optimize over may increase exponentially in n.Arguably, the biggest practical success of quantum information theory to date is the possibility of quantum key distribution (QKD) [10][11][12]. QKD allows two distant parties to agree on a secret key independent of any eavesdropper. The required assumptions are access to a quantum channel with positive private capacity and the validity of quantum physics. However, in practice one does not know the quantum channel exactly, and to characterize it one uses a public authentic classical channel. On the other hand, key distribution is a primitive that can only be implemented with classical resources if one is willing to constrain the power of the eavesdropper. Even though there exist practical QKD schemes which enable secure communication over large dis...
“…On the other hand, key distribution is a primitive that can only be implemented with classical resources if one is willing to constrain the power of the eavesdropper. Even though there exist practical QKD schemes which enable secure communication over large distances with high key rates [13][14][15][16], some of the fundamental questions about the capacity to transmit secure correlations remain unanswered.There are essentially two quantities that describe the ability of the channel to send secure messages to the receiver, and consequently, generate secret keys. The first one is called private capacity P [6,17].…”
The quantum capacity of a quantum channel is always smaller than the capacity of the channel for private communication. Both quantities are given by the infinite regularization of the coherent and the private information, respectively, which makes their evaluation very difficult. Here, we construct a family of channels for which the private and coherent information can remain strictly superadditive for unbounded number of uses, thus demonstrating that the regularization is necessary. We prove this by showing that the coherent information is strictly larger than the private information of a smaller number of uses of the channel. This implies that even though the quantum capacity is upper bounded by the private capacity, the nonregularized quantities can be interleaved. Efficient information transmission is the cornerstone of all information processing tasks in our interconnected world. In the most basic scenario, two parties, linked by a fixed communication channel wish to exchange messages with each other. What is the maximum rate at which they can reliably transmit information?Classical information theory gives an exhaustive answer to this question [1]. There exists an efficient convex optimization algorithm which takes the description of a channel and calculates its capacity to convey information. This is the consequence of a particularly simple analytic expression for the classical capacity of a channel. Our world is inherently quantum and when we turn to the channels that transmit quantum information we are able to perform many novel information processing tasks which are impossible in the classical theory, such as establishing entanglement between sender and receiver. Presently, when confronted with the above question for the quantum channels, there is no known efficient algorithm that takes the description of an arbitrary channel and calculates its capacity. Different types of capacity of the quantum channel are defined as regularized quantities [2-9], which implies that in order to compute them it is necessary to perform an unbounded optimization over the number of the copies of the channel. In practice it means that to estimate the capacity for n uses of the channel the dimension of the state space which one has to optimize over may increase exponentially in n.Arguably, the biggest practical success of quantum information theory to date is the possibility of quantum key distribution (QKD) [10][11][12]. QKD allows two distant parties to agree on a secret key independent of any eavesdropper. The required assumptions are access to a quantum channel with positive private capacity and the validity of quantum physics. However, in practice one does not know the quantum channel exactly, and to characterize it one uses a public authentic classical channel. On the other hand, key distribution is a primitive that can only be implemented with classical resources if one is willing to constrain the power of the eavesdropper. Even though there exist practical QKD schemes which enable secure communication over large dis...
“…One fiber link and one DPS-QKD system was employed for this demonstration, with the transmitter first operating as Bob and then subsequently Charlie in a time-multiplexed configuration. The sender and receiver were located in the NICT laboratories at Koganei, Tokyo, Japan.Figure 2The underlying QKD system that underpins this QDS experiment is based on a DPS system developed by NTT 33 . The channel between senders Bob/Charlie and receiver Alice was composed of 45âkm of installed optical fiber in a loop-back configuration between NICT laboratories at Koganei and Otematchi to give a total optical path length of 90âkm, and additional attenuation introduced by a variable ND filter.
…”
Ensuring the integrity and transferability of digital messages is an important challenge in modern communications. Although purely mathematical approaches exist, they usually rely on the computational complexity of certain functions, in which case there is no guarantee of long-term security. Alternatively, quantum digital signatures offer security guaranteed by the physical laws of quantum mechanics. Prior experimental demonstrations of quantum digital signatures in optical fiber have typically been limited to operation over short distances and/or operated in a laboratory environment. Here we report the experimental transmission of quantum digital signatures over channel losses of up to 42.8â±â1.2âdB in a link comprised of 90âkm of installed fiber with additional optical attenuation introduced to simulate longer distances. The channel loss of 42.8â±â1.2âdB corresponds to an equivalent distance of 134.2â±â3.8âkm and this represents the longest effective distance and highest channel loss that quantum digital signatures have been shown to operate over to date. Our theoretical model indicates that this represents close to the maximum possible channel attenuation for this quantum digital signature protocol, defined as the loss for which the signal rate is comparable to the dark count rate of the detectors.
“…In the commercial arena, in particular, great progresses have been recently achieved in the area of high-speed QKD and long distance quantum network systems [14][15][16][17][18][19][20][21][22][23][24][25]. Many studies on QKD security have been also conducted uninterruptedly since 1984, because providing security is essential for the commercialization of such systems.…”
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