10 research outputs found
Experimental demonstration of a BDCZ quantum repeater node
Quantum communication is a method that offers efficient and secure ways for
the exchange of information in a network. Large-scale quantum communication (of
the order of 100 km) has been achieved; however, serious problems occur beyond
this distance scale, mainly due to inevitable photon loss in the transmission
channel. Quantum communication eventually fails when the probability of a dark
count in the photon detectors becomes comparable to the probability that a
photon is correctly detected. To overcome this problem, Briegel, D\"{u}r, Cirac
and Zoller (BDCZ) introduced the concept of quantum repeaters, combining
entanglement swapping and quantum memory to efficiently extend the achievable
distances. Although entanglement swapping has been experimentally demonstrated,
the implementation of BDCZ quantum repeaters has proved challenging owing to
the difficulty of integrating a quantum memory. Here we realize entanglement
swapping with storage and retrieval of light, a building block of the BDCZ
quantum repeater. We follow a scheme that incorporates the strategy of BDCZ
with atomic quantum memories. Two atomic ensembles, each originally entangled
with a single emitted photon, are projected into an entangled state by
performing a joint Bell state measurement on the two single photons after they
have passed through a 300-m fibre-based communication channel. The entanglement
is stored in the atomic ensembles and later verified by converting the atomic
excitations into photons. Our method is intrinsically phase insensitive and
establishes the essential element needed to realize quantum repeaters with
stationary atomic qubits as quantum memories and flying photonic qubits as
quantum messengers.Comment: 5 pages, 4 figure
Towards experimental entanglement connection with atomic ensembles in the single excitation regime
We present a protocol for performing entanglement connection between pairs of
atomic ensembles in the single excitation regime. Two pairs are prepared in an
asynchronous fashion and then connected via a Bell measurement. The resulting
state of the two remaining ensembles is mapped to photonic modes and a reduced
density matrix is then reconstructed. Our observations confirm for the first
time the creation of coherence between atomic systems that never interacted, a
first step towards entanglement connection, a critical requirement for quantum
networking and long distance quantum communications
Memory-built-in quantum teleportation with photonic and atomic qubits
The combination of quantum teleportation and quantum memory of photonic
qubits is essential for future implementations of large-scale quantum
communication and measurement-based quantum computation. Both steps have been
achieved separately in many proof-of-principle experiments, but the
demonstration of memory-built-in teleportation of photonic qubits remains an
experimental challenge. Here, we demonstrate teleportation between photonic
(flying) and atomic (stationary) qubits. In our experiment, an unknown
polarization state of a single photon is teleported over 7 m onto a remote
atomic qubit that also serves as a quantum memory. The teleported state can be
stored and successfully read out for up to 8 micro-second. Besides being of
fundamental interest, teleportation between photonic and atomic qubits with the
direct inclusion of a readable quantum memory represents a step towards an
efficient and scalable quantum network.Comment: 19 pages 3 figures 1 tabl
Experimental Quantum Teleportation of a Two-Qubit Composite System
Quantum teleportation, a way to transfer the state of a quantum system from
one location to another, is central to quantum communication and plays an
important role in a number of quantum computation protocols. Previous
experimental demonstrations have been implemented with photonic or ionic
qubits. Very recently long-distance teleportation and open-destination
teleportation have also been realized. Until now, previous experiments have
only been able to teleport single qubits. However, since teleportation of
single qubits is insufficient for a large-scale realization of quantum
communication and computation2-5, teleportation of a composite system
containing two or more qubits has been seen as a long-standing goal in quantum
information science. Here, we present the experimental realization of quantum
teleportation of a two-qubit composite system. In the experiment, we develop
and exploit a six-photon interferometer to teleport an arbitrary polarization
state of two photons. The observed teleportation fidelities for different
initial states are all well beyond the state estimation limit of 0.40 for a
two-qubit system. Not only does our six-photon interferometer provide an
important step towards teleportation of a complex system, it will also enable
future experimental investigations on a number of fundamental quantum
communication and computation protocols such as multi-stage realization of
quantum-relay, fault-tolerant quantum computation, universal quantum
error-correction and one-way quantum computation.Comment: 16pages, 4 figure
A millisecond quantum memory for scalable quantum networks
Scalable quantum information processing critically depends on the capability
of storage of a quantum state. In particular, a long-lived storable and
retrievable quantum memory for single excitations is of crucial importance to
the atomic-ensemble-based long-distance quantum communication. Although atomic
memories for classical lights and continuous variables have been demonstrated
with milliseconds storage time, there is no equal advance in the development of
quantum memory for single excitations, where only around 10 s storage time
was achieved. Here we report our experimental investigations on extending the
storage time of quantum memory for single excitations. We isolate and identify
distinct mechanisms for the decoherence of spin wave (SW) in atomic ensemble
quantum memories. By exploiting the magnetic field insensitive state, ``clock
state", and generating a long-wavelength SW to suppress the dephasing, we
succeed in extending the storage time of the quantum memory to 1 ms. Our result
represents a substantial progress towards long-distance quantum communication
and enables a realistic avenue for large-scale quantum information processing.Comment: 11pages, 4 figures, submitted for publicatio