Experimental realization of long-distance entanglement between spins in antiferromagnetic quantum spin chains

Abstract

Entanglement is a concept that has defied common sense since the discovery of quantum mechanics. Two particles are said to be entangled when the quantum state of each particle cannot be described independently, no matter how far apart in space and time the two particles are. We demonstrate experimentally that unpaired spins separated by several hundred ångström entangle through a collection of spin singlets made up of antiferromagnetic spin-1/2 chains in a bulk material. Low-temperature magnetization and specific heat studies as a function of magnetic field reveal the occurrence of very dilute spin dimers and at least two quantum phase transitions related to the breaking of excited local triplets. The mechanism at the origin of the unpaired spins inside the quantum chains is the inter-modulation potential between two sublattices, and may be replicated using well-designed synthetic multilayers. E ntanglement is an essential feature of the macroscopic world 1 , despite the fact that human perception has difficulty accepting such strange predictions. A very fruitful area of research today is quantum computing technology, where entanglement is being harnessed for eventual use in quantum computation and communications 2,3 . Scientists are also developing new areas of research where entanglement could lie at the origin of the interactions in ever larger objects, ranging from photons to clusters of atoms to molecules-and even to cosmological objects.Most of the protocols for quantum communication rely on entangled photons 4 , as they are weakly interacting and can be easily manipulated using existing optical fibre technology. However, it is not always convenient to use photons to exchange information as, in particular, it is not straightforward to convert information between physically different qubits. Another approach described in seminal papers by Bose 5,6 suggested the use of spin chains as quantum channels for short-or mid-range communication, showing that, by means of the magnetic interaction between the spins that compose the chain, the transfer of information arises naturally as the system evolves dynamically, without the requirement for any external control. Transferring quantum information between distant qubits through spin chains would be highly desirable. However, this procedure often requires the repeated application of swapping gates and is, consequently, very demanding experimentally. Moreover, the features that characterize the transmission, such as teleportation, fidelity, transfer quality and speed, depend on the properties of the underlying quantum system 2,3 .Other developments have explored the conditions for longdistance entanglement without the need to perform operations and measurements. Among the many possible spin configurations it seems that antiferromagnetic spin arrays characterized by spin gaps above the ground state can exhibit true entanglement over long distances 7 . Furthermore, the entanglement is a slowly decreasing function of distance, allowing robust teleportation across finite di...

Gapped quantum criticality gains long-time quantum correlations

“…Hence the energy gap could block the propagation of decoherence along the environment and consequently reduces its effect on the central spin. These results highlight the current outlook of using quantum spin chains as entanglers or quantum channels in quantum information devises [47,48]. Besides, quantum gapped criticality may have potential applications in quantum computations.…”

Robust quantum entanglement generation and generation-plus-storage protocols with spin chains

“…However, all these approaches exploit photons to create and communicate entanglement and, when short distances are involved, the decoherence and losses introduced by having to inter-convert the quantum information between the different physical realisations and flying qubits can lead to inefficiencies and thus low data transfer rates. While optical devices and systems are widely regarded as the most applicable candidates for long range quantum communication, spin chains have acquired significant interest within the field of quantum information processing as a means of efficiently transferring information over short distances [14][15][16], and for creating and/or distributing entanglement [17][18][19][20]. Such model devices can be experimentally implemented for any ensemble of two-level systems where it is possible to engineer the couplings between the systems (sites).…”

System Size Dependence of Entanglement in Quantum One-Dimensional Models

“…Simultaneously, there is also unabated interest in quantum entanglement of spin qubits, see for example [3][4][5][6]. Therefore it seems that simple formulae which describe the entanglement in the case of states accessible in the lab may be very useful and allow experimentalists to develop their experimental intuition.…”