Showing papers on "Quantum channel published in 2004"
TL;DR: It is shown that 2log3N is the maximal perfect communication distance for hypercube geometries if one allows fixed but different couplings between the qubits, then perfect state transfer can be achieved over arbitrarily long distances in a linear chain.
Abstract: We propose a class of qubit networks that admit the perfect state transfer of any quantum state in a fixed period of time. Unlike many other schemes for quantum computation and communication, these networks do not require qubit couplings to be switched on and off. When restricted to N-qubit spin networks of identical qubit couplings, we show that 2log3N is the maximal perfect communication distance for hypercube geometries. Moreover, if one allows fixed but different couplings between the qubits, then perfect state transfer can be achieved over arbitrarily long distances in a linear chain.
1,014 citations
TL;DR: Deterministic quantum-state teleportation between a pair of trapped calcium ions is reported, demonstrating unequivocally the quantum nature of the process.
Abstract: Teleportation of a quantum state encompasses the complete transfer of information from one particle to another. The complete specification of the quantum state of a system generally requires an infinite amount of information, even for simple two-level systems (qubits). Moreover, the principles of quantum mechanics dictate that any measurement on a system immediately alters its state, while yielding at most one bit of information. The transfer of a state from one system to another (by performing measurements on the first and operations on the second) might therefore appear impossible. However, it has been shown that the entangling properties of quantum mechanics, in combination with classical communication, allow quantum-state teleportation to be performed. Teleportation using pairs of entangled photons has been demonstrated, but such techniques are probabilistic, requiring post-selection of measured photons. Here, we report deterministic quantum-state teleportation between a pair of trapped calcium ions. Following closely the original proposal, we create a highly entangled pair of ions and perform a complete Bell-state measurement involving one ion from this pair and a third source ion. State reconstruction conditioned on this measurement is then performed on the other half of the entangled pair. The measured fidelity is 75%, demonstrating unequivocally the quantum nature of the process.
983 citations
TL;DR: It is conjecture that a particular kind of group-covariant SIC–POVM exists in arbitrary dimensions, providing numerical results up to dimension 45 to bolster this claim.
Abstract: We consider the existence in arbitrary finite dimensions d of a positive operator valued measure (POVM) comprised of d2 rank-one operators all of whose operator inner products are equal. Such a set is called a “symmetric, informationally complete” POVM (SIC–POVM) and is equivalent to a set of d2 equiangular lines in Cd. SIC–POVMs are relevant for quantum state tomography, quantum cryptography, and foundational issues in quantum mechanics. We construct SIC–POVMs in dimensions two, three, and four. We further conjecture that a particular kind of group-covariant SIC–POVM exists in arbitrary dimensions, providing numerical results up to dimension 45 to bolster this claim.
933 citations
TL;DR: Un unconditional teleportation of massive particle qubits using atomic (9Be+) ions confined in a segmented ion trap is reported, which achieves an average fidelity of 78 per cent, which exceeds the fidelity of any protocol that does not use entanglement.
Abstract: Quantum teleportation1 provides a means to transport quantum information efficiently from one location to another, without the physical transfer of the associated quantum-information carrier. This is achieved by using the non-local correlations of previously distributed, entangled quantum bits (qubits). Teleportation is expected to play an integral role in quantum communication2 and quantum computation3. Previous experimental demonstrations have been implemented with optical systems that used both discrete and continuous variables4,5,6,7,8,9, and with liquid-state nuclear magnetic resonance10. Here we report unconditional teleportation5 of massive particle qubits using atomic (9Be+) ions confined in a segmented ion trap, which aids individual qubit addressing. We achieve an average fidelity of 78 per cent, which exceeds the fidelity of any protocol that does not use entanglement11. This demonstration is also important because it incorporates most of the techniques necessary for scalable quantum information processing in an ion-trap system12,13.
912 citations
TL;DR: The direct observation of entanglement between stationary and ‘flying’ qubits is accomplished without using cavity quantum electrodynamic techniques or prepared non-classical light sources and it is envisioned that this source of entangling may be used for a variety of quantum communication protocols and for seeding large-scale entangled states of trapped ion qubits for scalable quantum computing.
Abstract: An outstanding goal in quantum information science is the faithful mapping of quantum information between a stable quantum memory and a reliable quantum communication channel. This would allow, for example, quantum communication over remote distances, quantum teleportation of matter and distributed quantum computing over a 'quantum internet'. Because quantum states cannot in general be copied, quantum information can only be distributed in these and other applications by entangling the quantum memory with the communication channel. Here we report quantum entanglement between an ideal quantum memory--represented by a single trapped 111Cd+ ion--and an ideal quantum communication channel, provided by a single photon that is emitted spontaneously from the ion. Appropriate coincidence measurements between the quantum states of the photon polarization and the trapped ion memory are used to verify their entanglement directly. Our direct observation of entanglement between stationary and 'flying' qubits is accomplished without using cavity quantum electrodynamic techniques or prepared non-classical light sources. We envision that this source of entanglement may be used for a variety of quantum communication protocols and for seeding large-scale entangled states of trapped ion qubits for scalable quantum computing.
610 citations
TL;DR: A proof-of-principle demonstration of five-photon entanglement and open-destination teleportation and can be used for investigations of measurement-based quantum computation and multi-party quantum communication.
Abstract: Quantum-mechanical entanglement of three or four particles has been achieved experimentally, and has been used to demonstrate the extreme contradiction between quantum mechanics and local realism. However, the realization of five-particle entanglement remains an experimental challenge. The ability to manipulate the entanglement of five or more particles is required for universal quantum error correction. Another key process in distributed quantum information processing, similar to encoding and decoding, is a teleportation protocol that we term 'open-destination' teleportation. An unknown quantum state of a single particle is teleported onto a superposition of N particles; at a later stage, this teleported state can be read out (for further applications) at any of the N particles, by a projection measurement on the remaining particles. Here we report a proof-of-principle demonstration of five-photon entanglement and open-destination teleportation (for N = 3). In the experiment, we use two entangled photon pairs to generate a four-photon entangled state, which is then combined with a single-photon state. Our experimental methods can be used for investigations of measurement-based quantum computation and multi-party quantum communication.
533 citations
TL;DR: In principle, the approach enables a quantum state to be maintained by means of repeated error correction, an important step towards scalable fault-tolerant quantum computation using trapped ions.
Abstract: Scalable quantum computation1 and communication require error control to protect quantum information against unavoidable noise. Quantum error correction2,3 protects information stored in two-level quantum systems (qubits) by rectifying errors with operations conditioned on the measurement outcomes. Error-correction protocols have been implemented in nuclear magnetic resonance experiments4,5,6, but the inherent limitations of this technique7 prevent its application to quantum information processing. Here we experimentally demonstrate quantum error correction using three beryllium atomic-ion qubits confined to a linear, multi-zone trap. An encoded one-qubit state is protected against spin-flip errors by means of a three-qubit quantum error-correcting code. A primary ion qubit is prepared in an initial state, which is then encoded into an entangled state of three physical qubits (the primary and two ancilla qubits). Errors are induced simultaneously in all qubits at various rates. The encoded state is decoded back to the primary ion one-qubit state, making error information available on the ancilla ions, which are separated from the primary ion and measured. Finally, the primary qubit state is corrected on the basis of the ancillae measurement outcome. We verify error correction by comparing the corrected final state to the uncorrected state and to the initial state. In principle, the approach enables a quantum state to be maintained by means of repeated error correction, an important step towards scalable fault-tolerant quantum computation using trapped ions.
485 citations
TL;DR: It is proved the existence of gapped quantum Hamiltonians whose ground states exhibit an infinite entanglement length, as opposed to their finite correlation length, and it is reported on evidence that the ground state of an antiferromagnetic chain can be used as a perfect quantum channel if local measurements on the individual spins can be implemented.
Abstract: We prove the existence of gapped quantum Hamiltonians whose ground states exhibit an infinite entanglement length, as opposed to their finite correlation length. Using the concept of entanglement swapping, the localizable entanglement is calculated exactly for valence bond and finitely correlated states, and the existence of the so-called string-order parameter is discussed. We also report on evidence that the ground state of an antiferromagnetic chain can be used as a perfect quantum channel if local measurements on the individual spins can be implemented.
317 citations
TL;DR: In this article, it was shown that the conjectures of additivity of the minimum output entropy of a quantum channel, the Holevo expression, and strong superadditivity of entanglement of formation are either all true or all false.
Abstract: We reduce the number of open additivity problems in quantum information theory by showing that four of them are equivalent. Namely, we show that the conjectures of additivity of the minimum output entropy of a quantum channel, additivity of the Holevo expression for the classical capacity of a quantum channel, additivity of the entanglement of formation, and strong superadditivity of the entanglement of formation, are either all true or all false.
307 citations
TL;DR: This work describes the high-fidelity teleportation of photons over a distance of 600 metres across the River Danube in Vienna, with the optimal efficiency that can be achieved using linear optics.
Abstract: A real-world experiment marks a step towards worldwide quantum communication. Efficient long-distance quantum teleportation1 is crucial for quantum communication and quantum networking schemes2. Here we describe the high-fidelity teleportation of photons over a distance of 600 metres across the River Danube in Vienna, with the optimal efficiency that can be achieved using linear optics. Our result is a step towards the implementation of a quantum repeater3, which will enable pure entanglement to be shared between distant parties in a public environment and eventually on a worldwide scale.
304 citations
TL;DR: A novel scheme for secure direct communication between Alice and Bob is proposed, where there is no need for establishing a shared secret key and teleportation transmits Bob’s message without revealing any information to a potential eavesdropper.
Abstract: A novel scheme for secure direct communication between Alice and Bob is proposed, where there is no need for establishing a shared secret key. The communication is based on Einstein-Podolsky-Rosen (EPR) pairs and teleportation between Alice and Bob. After insuring the security of the quantum channel (EPR pairs), Bob encodes the secret message directly on a sequence of particle states and transmits them to Alice by teleportation. In this scheme teleportation transmits Bob’s message without revealing any information to a potential eavesdropper. Alice can read out the encoded messages directly by the measurement on her qubits. Because there is not a transmission of the qubit which carries the secret message between Alice and Bob, it is completely secure for direct secret communication if perfect quantum channel is used.
TL;DR: This work proves the equivalence of the cluster-state-based quantum computational model and the teleportation-based model, and shows that all stabilizer states have a very simple interpretation in terms of valence-bond solids, which allows to understand their entangled properties in a transparent way.
Abstract: We propose a way of universal quantum computation by doing joint measurements on distributed singlets. We show how these joint measurements become local measurements when the singlets are interpreted as the virtual components of a large valence-bond state. This proves the equivalence of the cluster-state-based quantum computational model and the teleportation-based model, and we discuss several features and possible extensions. We show that all stabilizer states have a very simple interpretation in terms of valence-bond solids, which allows to understand their entanglement properties in a transparent way.
TL;DR: This work reports the experimental realization of a tripartite quantum teleportation network for quantum states of continuous variables (electromagnetic field modes) and demonstrates teleportation of a coherent state between three different pairs in the network, unambiguously demonstrating its tri partite character.
Abstract: Quantum teleportation involves the transportation of an unknown quantum state from one location to another, without physical transfer of the information carrier. Although quantum teleportation is a naturally bipartite process, it can be extended to a multipartite protocol known as a quantum teleportation network. In such a network, entanglement is shared between three or more parties. For the case of three parties (a tripartite network), teleportation of a quantum state can occur between any pair, but only with the assistance of the third party. Multipartite quantum protocols are expected to form fundamental components for larger-scale quantum communication and computation. Here we report the experimental realization of a tripartite quantum teleportation network for quantum states of continuous variables (electromagnetic field modes). We demonstrate teleportation of a coherent state between three different pairs in the network, unambiguously demonstrating its tripartite character.
IBM1
TL;DR: By considering the question of whether entanglement is "monogamous," Charles Bennett's influence on modern quantum information theory is illustrated and the recent answers to this entanglements question are reviewed.
Abstract: In this paper I discuss some of the early history of quantum information theory. By considering the question of whether entanglement is "monogamous," I illustrate Charles Bennett's influence on modern quantum information theory. Finally, I review our recent answers to this entanglement question and its relation to Bell inequalities.
TL;DR: A way to teleport multiqubit quantum information from a sender to a distant receiver via the control of many agents in a network is presented, in such a way that the required auxiliary qubit resources, local operation, and classical communication are considerably reduced.
Abstract: We present a way to teleport multiqubit quantum information from a sender to a distant receiver via the control of many agents in a network. We show that the original state of each qubit can be restored by the receiver as long as all the agents collaborate. However, even if one agent does not cooperate, the receiver cannot fully recover the original state of each qubit. The method operates essentially through entangling quantum information during teleportation, in such a way that the required auxiliary qubit resources, local operation, and classical communication are considerably reduced for the present purpose.
TL;DR: These proposals offer practical and realistic alternatives to existing schemes for quantum key distribution over optical fibers without resorting to interferometry or two-way quantum communication, thereby circumventing, respectively, the need for high precision timing and the threat of Trojan horse attacks.
Abstract: We present two polarization-based protocols for quantum key distribution. The protocols encode key bits in noiseless subspaces or subsystems and so can function over a quantum channel subjected to an arbitrary degree of collective noise, as occurs, for instance, due to rotation of polarizations in an optical fiber. These protocols can be implemented using only entangled photon-pair sources, single-photon rotations, and single-photon detectors. Thus, our proposals offer practical and realistic alternatives to existing schemes for quantum key distribution over optical fibers without resorting to interferometry or two-way quantum communication, thereby circumventing, respectively, the need for high precision timing and the threat of Trojan horse attacks.
TL;DR: It is shown that only preferred pointer states of the system can leave a redundant and therefore easily detectable imprint on the environment.
Abstract: We study the emergence of objective properties in open quantum systems. In our analysis, the environment is promoted from a passive role of a reservoir selectively destroying quantum coherence to an active role of amplifier selectively proliferating information about the system. We show that only preferred pointer states of the system can leave a redundant and therefore easily detectable imprint on the environment. Observers who---as is almost always the case---discover the state of the system indirectly (by probing a fraction of its environment) will find out only about the corresponding pointer observable. Many observers can act in this fashion independently and without perturbing the system. They will agree about its state. In this operational sense, preferred pointer states exist objectively.
TL;DR: A set of linearly independent quantum states, where U(m,n) are generalized Pauli matrices, cannot be discriminated deterministically or probabilistically by local operations and classical communication, but any l maximally entangled states from this set are locally distinguishable if l(l-1)< or =2d.
Abstract: It is well known that orthogonal quantum states can be distinguished perfectly. However, if we assume that these orthogonal quantum states are shared by spatially separated parties, the distinguishability of these shared quantum states may be completely different. We show that a set of linearly independent quantum states [formula: see text] where U(m,n) are generalized Pauli matrices, cannot be discriminated deterministically or probabilistically by local operations and classical communication. On the other hand, any l maximally entangled states from this set are locally distinguishable if l(l-1)< or =2d. The explicit projecting measurements are obtained to locally discriminate these states. As an example, we show that four Werner states are locally indistinguishable.
TL;DR: A novel construction is presented that allows the legitimate parties to get equal bit strings out of correlated variables by using a classical channel, with as little leaked information as possible, which opens the way to securely correcting nonbinary key elements.
Abstract: Two parties, Alice and Bob, wish to distill a binary secret key out of a list of correlated variables that they share after running a quantum key distribution (QKD) protocol based on continuous-spectrum quantum carriers. We present a novel construction that allows the legitimate parties to get equal bit strings out of correlated variables by using a classical channel, with as little leaked information as possible. This opens the way to securely correcting nonbinary key elements. In particular, the construction is refined to the case of Gaussian variables as it applies directly to recent continuous-variable protocols for QKD.
TL;DR: In this paper, a quantum key agreement protocol for quantum teleportation is presented. But the key bits are determined only by random measurement outcomes and are independent of the states transmitted over the channel.
Abstract: By replacing a classical channel with a quantum one during quantum teleportation, a quantum key agreement protocol is presented. The key bits are determined only by the random measurement outcomes and are independent of the states transmitted over the channel.
TL;DR: This work argues that in the definition of the so-called “quantum privacy,” Holevo quantities should be used instead of classical mutual informations and shows that this modified quantum privacy is the optimum achievable rate of secure transmission.
Abstract: Following Schumacher and Westmoreland, we address the problem of the capacity of a quantum wiretap channel. We first argue that, in the definition of the so-called "quantum privacy," Holevo quantities should be used instead of classical mutual informations. The argument actually shows that the security condition in the definition of a code should limit the wiretapper's Holevo quantity. Then we show that this modified quantum privacy is the optimum achievable rate of secure transmission.
TL;DR: It is demonstrated that spontaneous parametric down conversion can be used to generate four-photon states which enable the encoding of one qubit in a decoherence-free subspace and the immunity against noise is verified by quantum state tomography of the encoded qubit.
Abstract: Decoherence-free states protect quantum information from collective noise, the predominant cause of decoherence in current implementations of quantum communication and computation. Here we demonstrate that spontaneous parametric down conversion can be used to generate four-photon states which enable the encoding of one qubit in a decoherence-free subspace. The immunity against noise is verified by quantum state tomography of the encoded qubit. We show that particular states of the encoded qubit can be distinguished by local measurements on the four photons only.
TL;DR: In this paper, it was shown that one-dimensional rings of qubits with fixed (time-independent) interactions, constant around the ring, allow high-fidelity communication of quantum states.
Abstract: It has been recently suggested that the dynamics of a quantum spin system may provide a natural mechanism for transporting quantum information. We show that one-dimensional rings of qubits with fixed (time-independent) interactions, constant around the ring, allow high-fidelity communication of quantum states. We show that the problem of maximizing the fidelity of the quantum communication is related to a classical problem in Fourier wave analysis. By making use of this observation we find that if both communicating parties have access to limited numbers of qubits in the ring (a fraction that vanishes in the limit of large rings) it is possible to make the communication arbitrarily good.
TL;DR: A deterministic secure direct communication scheme via entanglement swapping, where a set of ordered maximally entangled three-particle states (GHZ states), initially shared by three spatially separated parties, Alice, Bob and Charlie, functions as a quantum information channel.
Abstract: We present a deterministic secure direct communication scheme via entanglement swapping, where a set of ordered maximally entangled three-particle states (GHZ states), initially shared by three spatially separated parties, Alice, Bob and Charlie, functions as a quantum information channel. After ensuring the safety of the quantum channel, Alice and Bob apply a series local operations on their respective particles according to the tripartite stipulation and the secret message they both want to send to Charlie. By three Alice, Bob and Charlie's Bell measurement results, Charlie is able to infer the secret messages directly. The secret messages are faithfully transmitted from Alice and Bob to Charlie via initially shared pairs of GHZ states without revealing any information to a potential eavesdropper. Since there is not a transmission of the qubits carrying the secret message between any two of them in the public channel, it is completely secure for direct secret communication if perfect quantum channel is used.
TL;DR: In this paper, the authors proposed a controlled quantum teleportation protocol, in which quantum information of an unknown state of a 2-level particle is faithfully transmitted from a sender (Alice) to a remote receiver (Bob) via an initially shared triplet of entangled particles under the control of the supervisor Charlie.
Abstract: We present a controlled quantum teleportation protocol. In the protocol, quantum information of an unknown state of a 2-level particle is faithfully transmitted from a sender (Alice) to a remote receiver (Bob) via an initially shared triplet of entangled particles under the control of the supervisor Charlie. The distributed entangled particles shared by Alice, Bob and Charlie function as a quantum information channel for faithful transmission. We also propose a controlled and secure direct communication scheme by means of this teleportation. After insuring the security of the quantum channel, Alice encodes the secret message directly on a sequence of particle states and transmits them to Bob supervised by Charlie using this controlled quantum teleportation. Bob can read out the encoded message directly by the measurement on his qubit. In this scheme, the controlled quantum teleportation transmits Alice's message without revealing any information to a potential eavesdropper. Because there is not a transmission of the qubit carrying the secret message between Alice and Bob in the public channel, it is completely secure for controlled and direct secret communication if perfect quantum channel is used. The feature of this scheme is that the communication between two sides depends on the agreement of the third side.
TL;DR: The concept of Localizable entanglement (LE) was introduced in this paper, where the authors consider systems of interacting spins and study the entanglements that can be localized, on average, between two separated spins by performing local measurements on the remaining spins.
Abstract: We consider systems of interacting spins and study the entanglement that can be localized, on average, between two separated spins by performing local measurements on the remaining spins This concept of Localizable Entanglement (LE) leads naturally to notions like entanglement length and entanglement fluctuations For both spin-1/2 and spin-1 systems we prove that the LE of a pure quantum state can be lower bounded by connected correlation functions We further propose a scheme, based on matrix-product states and the Monte Carlo method, to efficiently calculate the LE for quantum states of a large number of spins The virtues of LE are illustrated for various spin models In particular, characteristic features of a quantum phase transition such as a diverging entanglement length can be observed We also give examples for pure quantum states exhibiting a diverging entanglement length but finite correlation length We have numerical evidence that the ground state of the antiferromagnetic spin-1 Heisenberg chain can serve as a perfect quantum channel Furthermore, we apply the numerical method to mixed states and study the entanglement as a function of temperature
IBM1
TL;DR: This paper describes the family of quantum protocols, aNoiseless qubit channel, noiseless classical bit channel and pure ebit (EPR pair) that reflect their classical-quantum and dynamic-static nature.
Abstract: We introduce three new quantum protocols involving noisy quantum channels and entangled states, and relate them operationally and conceptually with four well-known old protocols. Two of the new protocols (the mother and father) can generate the other five "child" protocols by direct application of teleportation and superdense coding, and can be derived in turn by making the old protocols "coherent." This gives very simple proofs for two famous old protocols (the hashing inequality and quantum channel capacity) and provides the basis for optimal trade-off curves in several quantum information processing tasks.
TL;DR: It is shown that global operations of the senders do not increase the information transfer capacity, in the case of a single receiver, and a nontrivial upper bound for the capacity is derived.
Abstract: We introduce the notion of distributed quantum dense coding, i.e., the generalization of quantum dense coding to more than one sender and more than one receiver. We show that global operations (as compared to local operations) of the senders do not increase the information transfer capacity, in the case of a single receiver. For the case of two receivers, using local operations and classical communication, a nontrivial upper bound for the capacity is derived. We propose a general classification scheme of quantum states according to their usefulness for dense coding. In the bipartite case (for any dimensions), bound entanglement is not useful for this task.
TL;DR: Clock recovery techniques at 1.25 Gbps enable continuous quantum key distribution at demonstrated sifted-key rates up to 1.0 Mbps, two orders of magnitude faster than has been reported previously.
Abstract: We have demonstrated the exchange of sifted quantum cryptographic key over a 730 meter free-space link at rates of up to 1.0 Mbps, two orders of magnitude faster than previously reported results. A classical channel at 1550 nm operates in parallel with a quantum channel at 845 nm. Clock recovery techniques on the classical channel at 1.25 Gbps enable quantum transmission at up to the clock rate. System performance is currently limited by the timing resolution of our silicon avalanche photodiode detectors. With improved detector resolution, our technique will yield another order of magnitude increase in performance, with existing technology.
TL;DR: An improved version of this teleportation scheme using more ancillas is the building block of the recent Knill, Laflamme, and Milburn proposal for efficient linear optics quantum computation.
Abstract: We report the experimental demonstration of a quantum teleportation protocol with a semiconductor single photon source. Two qubits, a target and an ancilla, each defined by a single photon occupying two optical modes (dual-rail qubit), were generated independently by the single photon source. Upon measurement of two modes from different qubits and postselection, the state of the two remaining modes was found to reproduce the state of the target qubit. In particular, the coherence between the target qubit modes was transferred to the output modes to a large extent. The observed fidelity is 80%, in agreement with the residual distinguishability between consecutive photons from the source. An improved version of this teleportation scheme using more ancillas is the building block of the recent Knill, Laflamme, and Milburn proposal for efficient linear optics quantum computation.