We propose entangled (M+1)-mode quantum states as a multiuser quantum channel for continuous-variable communication. Arbitrary quantum states can be sent via this channel simultaneously to M remote and separated locations with equal minimum excess noise in each output mode. For a set of coherent-state inputs, the channel realizes optimum symmetric 1-to-M cloning at a distance (``telecloning''). It also provides the optimal cloning of coherent states without the need of amplifying the state of interest. The generation of the multiuser quantum channel requires no more than two 10\log_{10}[(\sqrt{M}-1)/(\sqrt{M}+1)] dB squeezed states and M beam splitters.
Quantum telecloning, a pivotal multiuser quantum communication protocol in the realm of quantum information science, facilitates the copy of a quantum state across M distinct locations through teleportation technique. In the continuous-variable regime, the implementation of quantum telecloning necessitates the distribution of multipartite entanglement among the sender and M receiver parties. Following this, the sender carries out optic-electro conversion and transmits information via classical channel to M spatially separated receivers simultaneously. To successfully reconstruct the input state, electro-optic conversion needs to be employed by each receiver. However, due to these conversions, the bandwidth of the optical mode in this process is largely constrained. In this Letter, we present an all-optical version of the 1→2 continuous-variable quantum telecloning scheme, wherein both optic-electro and electro-optic conversions are replaced by optical components. Our scheme allows the two receivers to achieve input state reconstruction solely by utilizing beam splitters, significantly simplifying its complexity. We experimentally demonstrate all-optical 1→2 quantum telecloning of coherent state and achieve the fidelities of 58.6%±1.0% and 58.6%±1.1% for two clones, exceeding the corresponding classical limits (51.9%±0.5% and 51.9%±0.6%). Our results establish a platform for constructing a flexible all-optical multiuser quantum network and promote the field of all-optical quantum information processing.
We introduce and describe in detail a different quantum-information protocol: quantum telecloning of optical images or spatially multimode states of light. This protocol is a natural generalization of a continuous-variable quantum telecloning of single-mode states, investigated earlier theoretically and recently realized experimentally. Our protocol is an example of a multiuser parallel quantum channel. In order to realize this scheme one needs a multimode multipartite entanglement between the original and the target systems. We calculate the fidelity of quantum parallel telecloning in dependence on the spatiotemporal scales of our scheme and on the number of pixels. The fidelity is superior to the best classical fidelity that can be achieved without this kind of entanglement.
We first classify the quantum clone into irreversible and reversible types from the perspective of quantum-information distribution. We propose a scheme of continuous-variable reversible telecloning, which broadcast the information of an unknown state without loss from a sender to several spatially separated receivers exploiting multipartite entanglement as quantum channels. In this scheme, the quantum information of an unknown state is distributed into
M optimal clones and $M\ensuremath{-}1$ anticlones using
2M-partite entanglement. For the perfect quantum-information distribution that is optimal cloning,
2M-partite entanglement is required to be a maximum two-party entanglement.
We address the generation, propagation, and application of multipartite continuous variable entanglement in a noisy environment. In particular, we focus our attention on the multimode entangled states achievable by second-order nonlinear crystals---i.e., coherent states of the
SU(m,1) group---which provide a generalization of the twin-beam state of a bipartite system. The full inseparability in the ideal case is shown, whereas thresholds for separability are given for the tripartite case in the presence of noise. We find that entanglement of tripartite states is robust against thermal noise, both in the generation process and during propagation. We then consider coherent states of
SU(m,1) as a resource for multipartite distribution of quantum information and analyze a specific protocol for telecloning, proving its optimality in the case of symmetric cloning of pure Gaussian states. We show that the proposed protocol also provides the first example of a completely asymmetric $1\ensuremath{\rightarrow}m$ telecloning and derive explicitly the optimal relation among the different fidelities of the
m clones. The effect of noise in the various stages of the protocol is taken into account, and the fidelities of the clones are analytically obtained as a function of the noise parameters. In turn, this permits the optimization of the telecloning protocol, including its adaptive modifications to the noisy environment. In the optimized scheme the clones' fidelity remains maximal even in the presence of losses (in the absence of thermal noise), for propagation times that diverge as the number of modes increases. In the optimization procedure the prominent role played by the location of the entanglement source is analyzed in details. Our results indicate that, when only losses are present, telecloning is a more effective way to distribute quantum information than direct transmission followed by local cloning.
We consider the quantum cloning of continuous variable entangled states. This is achieved by introducing two symmetric entanglement cloning machines (or
e-cloners): a local
e-cloner and a global
e-cloner; where we look at the preservation of entanglement in the clones under the condition that the fidelity of the clones is maximized. These cloning machines are implemented using simple linear optical elements such as beam splitters and homodyne detection along with squeeze gates. We show that the global
e-cloner out-performs the local
e-cloner both in terms of the fidelity of the cloned states as well as the strength of the entanglement of the clones. There is a minimum strength of entanglement (3 dB using the inseparability criterion) of the input state of the global
e-cloner that is required to preserve the entanglement in the clones.