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Asia Pacific Physics Newsletter

Teleporting a Macroscopic Ensemble of Spins

Alexey N. Pyrkov, Tim Byrnes

Published Oct 23, 2015• 0 citations

Abstract

Quantum teleportation is one of the most enigmatic protocols realized so far in the area of quantum information, allowing for the transfer of quantum information assisted by an entangled state. It was a subject of science fiction 20 years ago, but now is a fundamental experimental protocol and quantum primitive for quantum information processing tasks. There have been many successful realizations of teleportation ranging from those using photons, atoms, and hybrid systems and long-range teleportation beyond 100km has already been achieved multiple times. These however typically involve explicitly single particle systems or effective few-particle systems. For larger (visible to the naked eye) objects involving many particles, teleportation is typically more difficult due to the fast decoherence such macroscopic objects suffer. In this case, the entanglement disappears almost as soon as it is created, making it useless for such tasks. More recently, teleportation was accomplished in the continuous variables (CV) framework between two atomic ensembles [1]. As remarkable as these achievements are, teleportation schemes that are known today (qubit/CV) do not allow us to teleport more complex quantum states at the macroscopic level. For example, while it is possible in principle to extend teleportation to multiple qubits, on a macroscopic level this is prohibitively complicated and highly susceptible to decoherence. For atomic ensemble CV based schemes, the teleported states are restricted to small displacements from the completely Sx-polarized state on the Bloch sphere.

DOI: 10.1142/s2251158x15000247

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Quantum teleportation allows for the transfer of arbitrary, in principle, unknown quantum states from a sender to a spatially distant receiver, who share an entangled state and can communicate classically. It is the essence of many sophisticated protocols for quantum communication and computation. In order to realize flying qubits in these schemes, photons are an optimal choice. However, teleporting a photonic qubit has been limited due to experimental inefficiencies and restrictions. Major disadvantages have been the probabilistic nature of both entangled resource states and linear-optics Bell-state measurements (BSM), as well as the need for post-selecting the successful events by destroying the teleported qubit. Here, all the above limitations are overcome through a totally distinct, “hybrid” approach: continuous-variable (CV) quantum teleportation of a discrete-variable, photonic qubit. The strength of CV teleportation lies in the on-demand availability of the quadrature-entangled states and the completeness of a BSM in the quadrature bases using linear optics and homodyne detections; this enables fully deterministic, unconditional teleportation of photonic qubits. We implement this kind of “hybrid” teleportation by utilizing a very recent, advanced technology: a broadband CV teleporter and a narrow-band qubit compatible with the teleporter.

Quantum teleportation and quantum memory are two crucial elements for large-scale quantum networks. With the help of prior distributed entanglement as a "quantum channel," quantum teleportation provides an intriguing means to faithfully transfer quantum states among distant locations without actual transmission of the physical carriers [Bennett CH, et al. (1993) Phys Rev Lett 70(13):1895-1899]. Quantum memory enables controlled storage and retrieval of fast-flying photonic quantum bits with stationary matter systems, which is essential to achieve the scalability required for large-scale quantum networks. Combining these two capabilities, here we realize quantum teleportation between two remote atomic-ensemble quantum memory nodes, each composed of ∼10(8) rubidium atoms and connected by a 150-m optical fiber. The spin wave state of one atomic ensemble is mapped to a propagating photon and subjected to Bell state measurements with another single photon that is entangled with the spin wave state of the other ensemble. Two-photon detection events herald the success of teleportation with an average fidelity of 88(7)%. Besides its fundamental interest as a teleportation between two remote macroscopic objects, our technique may be useful for quantum information transfer between different nodes in quantum networks and distributed quantum computing.

Atomic ensembles containing a large number of atoms have been proved to be an effective medium for quantum-state (quantum information) engineering and processing via their coupling with multi-photon light pulses. The general mechanism of this coupling, which involves continuous quantum variables for light and atoms, is described. The efficient quantum interface between light and atoms has led to the recent demonstration of an entangled state of two macroscopic atomic objects, more precisely two caesium gas samples. Based on this result, a proposal for teleportation of an entangled state of two atomic samples (entanglement swapping) is presented.

We introduce a quantum teleportation scheme that transfers a spin coherent state between two locations using entanglement. In the scheme an unknown spin coherent state lying on the equator of the Bloch sphere, such as realized in a coherent two-component Bose–Einstein condensate, is teleported onto a distant spin coherent state using only elementary operations and measurements. The scheme works in the regime beyond the standard continuous variables approximation based on the Holstein–Primakoff transformation. We analyze the error of the protocol with the number of particles N in the spin coherent state under decoherence and find that it scales favorably with N . The simplicity of the operations involved and the robustness under decoherence should make the protocol suitable under realistic experimental conditions.

Quantum teleportation is the transfer of quantum information between two locations by the use of shared entanglement. Current teleportation schemes broadly fall under one of two categories, either qubit or continuous-variable teleportation. Spin coherent states on spin ensembles can be teleported under the continuous-variable approximation for states with small deviations from a given polarization. However, for spin-coherent states with large deviations, no convenient teleportation protocol exists. Recently, we introduced a teleportation scheme where a spin-coherent state lying on the equator of the Bloch sphere is teleported between distant parties [A. N. Pyrkov and T. Byrnes, New J. Phys. 16, 073038 (2014)]. Here we generalize the protocol to a spin-coherent state with an arbitrary position on the Bloch sphere. Our proposed scheme breaks classical bounds based on communication in the unconditional case and quantum state estimation with postselection.

Quantum teleportation, a transfer protocol of quantum states, is the essence of many sophisticated quantum information protocols. There have been two complementary approaches to optical quantum teleportation: discrete variables (DVs) and continuous variables (CVs). However, both approaches have pros and cons. Here we take a “hybrid” approach to overcome the current limitations: CV quantum teleportation of DVs. This approach enabled the first realization of deterministic quantum teleportation of photonic qubits without post-selection. We also applied the hybrid scheme to several experiments, including entanglement swapping between DVs and CVs, conditional CV teleportation of single photons, and CV teleportation of qutrits. We are now aiming at universal, scalable, and fault-tolerant quantum computing based on these hybrid technologies.