Remote Entanglement Research Articles

Enhancing and expanding remote photonic entanglement via local filtering operations

We present an entanglement distillation scheme for enhancing remote two-photon polarization entanglement of mixed states. Although the main idea of the current scheme is based on Gisin׳s work (Phys. Lett. A 210 (1996) 151 [21]), there are new advantages in our new scheme, which are guaranteed by the nondemolition measurement of photonic state and the re-distillation of the garbage states. This entanglement distillation scheme not only can enhance the remote entanglement of mixed states, but also can expand two-photon entangled states to four-photon entangled states. So this scheme is an apparently feasible way for preparing multi-photon entangled states. The main idea is based on the principle of the cross-Kerr nonlinearity and the parity-check measurements (a nondemolition measurement) on photonic states. Two distant users Alice and Bob first start with one shared but less entangled photon pair, and with the help of local auxiliary photons, parity-check measurements and classical communication they can get a four-photon highly entangled states with a high success probability. For the fail result, although the garbage state is less entangled than the initial one, there is still entanglement in it. So these garbage states can be re-collected and distilled again instead of being discarded. In this sense, we can see that this protocol has a high yield, and the fidelity (with respect to the Bell state) of the initial state is not required to be bigger than 1/2 (a common threshold of the standard entanglement purification theory). In addition, post-selection measurements on the entangled photons are not needed here because of the nondemolition measurement. The nondemolition character of the measurement allows further processing of the resulting states. These advantages make the current scheme more feasible within the current technology.

A study of teleportation and super dense coding capacity in remote entanglement distribution

In this work we consider a quantum network consisting of nodes and entangled states connecting them. In every node there is a single player. The players at the intermediate nodes carry out measurements to produce an entangled state between the initial and final node. Here we address the problem that how much classical as well as quantum information can be sent from initial node to final node. In this context, we present strong theorems which state that how the teleportation capability of this remotely prepared state is linked up with the fidelities of teleportation of the resource states. Similarly, we analyze the super dense coding capacity of this remotely prepared state in terms of the capacities of the resource entangled states. However, we first obtain the relations involving the amount of entanglement of the resource states with the final state in terms concurrence. These relations are quite similar to the bounds obtained in references [G. Gour, Phys. Rev. A 71, 012318 (2005); G. Gour, B.C. Sanders, Phys. Rev. Lett. 93, 260501 (2005)]. More specifically, in an arbitrary quantum network when two nodes are not connected, our result shows how much information, both quantum and classical can be transmitted between these nodes. We show that the amount of transferable information depends on the capacities of the inter connecting entangled resources. These results have a tremendous future application in the context of determining the optimal path in a quantum network to send the maximal possible information.

Remote entanglement preparation

We introduce a multipartite communication scheme, with the aim to enable the senders to remotely and obliviously provide the receivers with an arbitrary amount of multipartite entanglement. The scheme is similar to remote state preparation (RSP). However, we show that even though the receivers are restricted to local unitary operations, the required resources for remote entanglement preparation are less than for RSP. In order to do so we introduce a canonical form of arbitrary multipartite pure states describing an arbitrary number of qubits. Moreover, we show that if the receivers are enabled to perform arbitrary local operations and classical communication, the required resources can drastically be reduced. We employ this protocol to derive robust entanglement purification protocols for arbitrary pure states and show that it can also be used for sending classical information.

Direct measurement of the concurrence for two-photon polarization entangled pure states by parity-check measurements

We present a protocol for directly measuring the concurrence of a two-photon polarization entangled pure or mixed state without prior quantum state tomography. By parity-check measurements and simple operations on two copies of the two-photon polarization entangled pure state, the concurrence is encoded in the total probability of picking up the odd parity states from the signal states. This protocol makes use of highly efficient homodyne detection, and it could be feasible in the near future with the help of the weak cross-Kerr nonlinearity. Moreover, our protocol can be used in a distributed fashion to directly determine the entanglement of remote states, which may find its important applications in quantum communication. • Direct measurement of two-photon polarization entanglement is proposed. • The concurrence can be obtained without prior quantum state tomography. • The entangled photons donʼt annihilate during the concurrence measuring process. • The protocol is feasible with the help of the weak cross-Kerr nonlinearity. • The direct measurement scheme works for both local and remote entanglement.

Entanglement Swapping with Local Certification: Application to Remote Micromechanical Resonators

We propose a protocol for entanglement swapping which involves tripartite systems. The generation of remote entanglement induced by the Bell measurement can be easily certified by additional local measurements. We illustrate the protocol in the case of continuous variable systems where the certification is effective for an appropriate class of three-mode gaussian states. We then apply the protocol to optomechanical systems, showing how mechanical entanglement between two remote micromechanical resonators can be generated and certified via local optical measurements.

Analysis of photon-mediated entanglement between distinguishable matter qubits

We theoretically evaluate establishing remote entanglement between distinguishable matter qubits through interference and detection of two emitted photons. The fidelity of the entanglement operation is analyzed as a function of the temporal- and frequency-mode matching between the photons emitted from each quantum memory. With a general analysis, we define limits on the absolute magnitudes of temporal- and frequency-mode mismatches in order to maintain entanglement fidelities greater than 99$%$ with two-photon detection efficiencies greater than 90$%$. We apply our analysis to several selected systems of quantum memories. Results indicate that high fidelities may be achieved in each system using current experimental techniques, while maintaining acceptable rates of entanglement. Thus, it might be possible to use two-photon-mediated entanglement operations between distinguishable quantum memories to establish a network for quantum communication and distributed quantum computation.

Psi-Lines, Chaos, Spirals, Magnetism, and Entanglement

This paper develops existing knowledge regarding psi-lines. The minimum size width and length that a psi-line can attain has been re-examined. This has led to a connection to the well established Megalithic Yard and to Feigenbaum’s Constant (δ), which leads to chaos theory and the ability of the mind to interact with space-time and the laws of physics. Two different types of spiral that terminate psi-lines have been quantified. For example, each has a different orientated entry point, and they have different properties. They are shown to be 3-dimensional helical coaxial bicones with one apex angle involving the Golden Ratio (φ), and the other sine 1/3; both universal angles. The mathematics of the different types of spirals has been explored. In the course of this research new discoveries have been made regarding the effect on psi-lines of the earth’s gravity, spin, and magnetism. In particular, magnetism has an important effect both on the brain and on the entry point of terminal psi-line spirals. The latter has led to a convincing example of remote macro entanglement. NeuroQuantology | September 2012 | Volume 10 | Issue 3 | Page 416-427

Efficient fluorescence collection and ion imaging with the “tack” ion trap

Trapped, laser-cooled ions produce intense fluorescence. Detecting this fluorescence enables efficient measurement of quantum state of qubits based on trapped atoms. It is desirable to collect a large fraction of the photons to make the detection faster and more reliable. Additionally, efficient fluorescence collection can improve speed and fidelity of remote ion entanglement and quantum gates. Here we show a novel ion trap design that incorporates metallic spherical mirror as the integral part of the trap itself, being its RF electrode. The mirror geometry enables up to 35% solid angle collection of trapped ion fluorescence; we measure a 25% effective solid angle, likely limited by imperfections of the mirror surface. We also study properties of the images of single ions formed by the mirror and apply aberration correction. Owing to the simplicity of its design, this trap structure can be adapted for micro-fabrication and integration into more complex trap architectures.

Surface plasmons in a metal nanowire coupled to colloidal quantum dots: Scattering properties and quantum entanglement

We investigate coherent single surface-plasmon transport in a metal nanowire strongly coupled to two colloidal quantum dots. Analytical expressions are obtained for the transmission and reflection coefficients by solving the corresponding eigenvalue equation. Remote entanglement of the wave functions of the two quantum dots can be created if the interdot distance is equal to a multiple half-wavelength of the surface plasmon. Furthermore, by applying classical laser pulses to the quantum dots, the entangled states can be stored in metastable states that are decoupled from the surface plasmons.

Entangling unstable optically active matter qubits

In distributed quantum computation, small devices composed of a single or a few qubits are networked together to achieve a scalable machine. Typically, there is an optically active matter qubit at each node, so that photons are exploited to achieve remote entanglement. However, in many systems the optically active states are unstable or poorly defined. We report a scheme to perform a high-fidelity entanglement operation even given severe instability. The protocol exploits the existence of optically excited states for phase acquisition without actually exciting those states; it functions with or without cavities and does not require number-resolving detectors.

Remote Entanglement between a Single Atom and a Bose-Einstein Condensate

Entanglement between stationary systems at remote locations is a key resource for quantum networks. We report on the experimental generation of remote entanglement between a single atom inside an optical cavity and a Bose-Einstein condensate (BEC). To produce this, a single photon is created in the atom-cavity system, thereby generating atom-photon entanglement. The photon is transported to the BEC and converted into a collective excitation in the BEC, thus establishing matter-matter entanglement. After a variable delay, this entanglement is converted into photon-photon entanglement. The matter-matter entanglement lifetime of 100 μs exceeds the photon duration by 2 orders of magnitude. The total fidelity of all concatenated operations is 95%. This hybrid system opens up promising perspectives in the field of quantum information.

Efficient fluorescence collection from trapped ions with an integrated spherical mirror

Efficient collection of fluorescence from trapped ions is crucial for quantum optics and quantum computing applications, specifically, for qubit state detection and in generating single photons for ion-photon and remote ion entanglement. In a typical setup, only a few per cent of ion fluorescence is intercepted by the aperture of the imaging optics. We employ a simple metallic spherical mirror integrated with a linear Paul ion trap to achieve photon collection efficiency of at least 10% from a single Ba$^+$ ion. An aspheric corrector is used to reduce the aberrations caused by the mirror and achieve high image quality.

Generation of remote multi-photon entangled state from Einstein–Podolsky–Rosen photon pairs

We describe a protocol for generation of remote multi-photon entanglement using Einstein–Podolsky–Rosen (EPR) photon pairs via entanglement swapping. According to the requests of users, Quantum Switch (QS) can prepare three-photon W entangled states or Greenberger–Horne–Zeilinger (GHZ) states based on independent, spatially separated EPR pairs among three distant users. Only Bell states measurement (BSM) is needed utilizing a CPHASE gate and PAs. This protocol can also generate remote N-photon GHZ entangled states.

Dynamics of remote entanglement in one-dimensional Ising chains with Dzyaloshinskii–Moriya interactions

With the introduction of Dzyaloshinskii–Moriya (DM) interaction, dynamics of the remote entanglement in one-dimensional Ising chains is investigated. It is found that the DM interaction can excite the remote entanglement from an initial Néel state. For a given strength of DM interaction, the concurrence between the end spins oscillates and decreases simultaneously with the increase of the chain’s length, and drops to zero at a critical length. For the chains with two and three spins, it is very interesting that the dynamics of the staggered magnetization (or the chiral parameter) can be used to qualitatively estimate the evolution of the remote concurrence between the end spins. At last, we discuss the generation of W state from the Ising chain with DM interactions, and it is obtained that W state can only be prepared in the three-qubit and four-qubit chains with a specific strength of DM interaction.

Extracting high fidelity quantum computer hardware from random systems

An overview of current status and prospects of the development of quantum computer hardware based on inorganic crystals doped with rare-earth ions are presented. Major parts of the experimental work in this area have been done in two places, Canberra, Australia and Lund, Sweden, and the present description follows more closely the Lund work. Techniques will be described that include optimal filtering of the initially inhomogeneously broadened profile down to well separated and narrow ensembles, as well as the use of advanced pulse-shaping in order to achieve robust arbitrary single-qubit operations with fidelities above 90%, as characterized by quantum state tomography. It is expected that full scalability of these systems will require the ability to determine the state of single rare-earth ions. It has been proposed that this can be done using special readout ions doped into the crystal and an update is given on the work to find and characterize such ions. Finally, a few aspects on possibilities for remote entanglement of ions in separate rare-earth-ion-doped crystals are considered.

QUANTUM INFORMATION PROCESSING IN AN ARRAY OF SUPERCONDUCTING TRANSMISSION LINE RESONATORS

We consider a one-dimensional array of superconducting transmission line resonators (TLRs). The TLRs are coupled by current-biased Josephson junctions, which act as tunable couplers between each two nearest TLRs, and a superconducting qubit is fabricated in the center of each TLR. We show that some important quantum information processing, such as quantum state transfer and preparation of remote entanglement, can be achieved in this system, and we also propose a scheme for generating the W-class states.

Giant optical Faraday rotation induced by a single-electron spin in a quantum dot: Applications to entangling remote spins via a single photon

We propose a quantum nondemolition method---a giant optical Faraday rotation near the resonant regime to measure a single-electron spin in a quantum dot inside a microcavity where a negatively charged exciton strongly couples to the cavity mode. Left-circularly and right-circularly polarized lights reflected from the cavity obtain different phase shifts due to cavity quantum electrodynamics and the optical spin selection rule. This yields giant and tunable Faraday rotation that can be easily detected experimentally. Based on this spin-detection technique, a deterministic photon-spin entangling gate and a scalable scheme to create remote spin entanglement via a single photon are proposed.

Remote atom entanglement in a fibre-connected three-atom system

An Ising-type atom-atom interaction is obtained in a fibre-connected three-atom system. The interaction is effective when Δ ≈ γ0 ≫ g. The preparations of remote two-atom and three-atom entanglements governed by this interaction are discussed in a specific parameter region. The overall two-atom entanglement is very small because of the existence of the third atom. However, the three-atom entanglement can reach a maximum very close to 1.

Implementing remote controlled-NOT gates and entanglement swapping via geometric phase gates in ion-trap systems

We propose a scheme for the implementation of remote controlled-NOT gates and entanglement swapping via geometric phase gates in ion-trap systems. The proposed scheme uses the two ground states of the Λ-type ions as memory instead of the vibrational mode. And the system is robust against the spontaneous radiation and the dephasing.

Deterministic entanglement of assistance in quantum networks

We present a powerful theorem for tripartite remote entanglement distribution protocols, which provides an operational interpretation of concurrence as a type of entanglement capacity, and we establish that concurrence of assistance, which we show is an entanglement monotone, identifies capabilities of and limitations to producing pure bipartite entangled states from pure tripartite entangled states. In addition, we show that, if concurrence of assistance for the pure tripartite state is at least as large as the concurrence of the desired pure bipartite state, then the former may be transformed to the latter via local operations and classical communication, and we calculate the maximum probability for this transformation when this condition is not met.PACS Nos.: 03.67.Mn, 03.67.Hk, 03.65.Ud