We present an experiment demonstrating entanglement-enhanced classical communication capacity of a quantum channel with correlated noise. The channel is modelled by a fiber optic link exhibiting random birefringence that fluctuates on a time scale much longer than the temporal separation between consecutive uses of the channel. In this setting, introducing entanglement between two photons travelling down the fiber allows one to encode reliably up to one bit of information into their joint polarization degree of freedom. When no quantum correlations between two separate uses of the channel are allowed, this capacity is reduced by a factor of more than three. We demonstrated this effect using a fiber-coupled source of entagled photon pairs based on spontaneous parametric down-conversion, and a linear-optics Bell state measurement. PACS numbers: 03.67. Hk, 03.65.Yz, 42.50.Dv, 89.70.+c Quantum mechanics admits existence of correlations that cannot be explained simply as statistical uncertainty in assigning definite realistic properties locally to each of the subsystems. This feature of quantum mechanics was quantified first in the form of Bell's inequalities that discriminate it against theories based on the assumption of local realism [1] . In recent years, this specifically quantum form of correlations, known commonly as entanglement, is being exploited to develop novel modes of information processing that offer advantages not available through the classical approach [2] . The well-known examples include solving certain computation tasks via collective unitary operations on coherent registers of quantum particles [3], or distributing a cryptographic key with the security verified by the detection of entanglement [4] . Entanglement plays also a non-trivial role in a number of scenarios for sending both classical information and quantum states over noisy quantum channels [5] . In this Letter, we demonstrate experimentally how entanglement can be used to enhance classical communication over a noisy channel. Our experiment follows recent theoretical studies [6, 7, 8 ] that analyzed classical communication over quantum channels in which the noise affecting consecutive uses is correlated. It is then natural to consider two types of input ensembles used for communication. The first one is restricted only to introducing classical correlations between separate uses of the channel, and it can be prepared by adjusting individually the quantum state of each particle sent through the channel in a single use. The second, completely general input ensemble includes entangled states of many particles that are subsequently sent through the channel one-byone. Theoretical analysis of model situations showed that indeed the use of entangled input ensemble can substantially enhance the classical capacity, and our work provides a proofof-principle experimental verification of this prediction. Our experiment implements the idea described in Ref. [8] which we will now briefly review. The model of a noisy quantum channel analyzed there was motivated by fluctuating birefringence of a standard optical fiber, which scrambles the polarization of an input light pulse to a completely mixed state. However, the characteristic time scale of birefringence fluctuations is usually much longer than the temporal separation between consecutive light pulses. The fact that neighboring light pulses undergo a random but nearly identical polarization transformation opens up the possibility of encoding information in the polarization degree of freedom of the transmitted light. Let us consider a simple scenario, when each of the two pulses contains exactly one photon, which is the minimum amount of light required to trigger a definite reponse on an ideal, noise-free photon counting detector. If the sender does not have technical capabilities to generate entangled photon pairs, all she can do is to assign a definite polarization to each of the photons. In this setting, the optimal encoding of information turns out to be a pair of photons with polarizations that are either identical or mutually orthogonal, for example |ր ւր ւ and |ր ւտ ց . (The diagonal polarization basis introduced here will be natural in the description of our experiment.) However, after collective polarization scrambling these two states cannot be discriminated unambiguously, and the highest achievable transfer rate is log 2 (5/4) ≈ 0.322 bits per two photons [8] . The advantage of employing entangled states in the above scenario becomes obvious when we recall that the singlet polarization state of two photons |Ψ − = (|ր ւտ ց − |տ ցր ւ )/ √ 2 remains invariant under correlated depolarization [9] , and therefore it can be discriminated unambiguously against the orthogonal triplet subspace. This fact allows one to encode faithfully one full bit of information into the polarization state of two photons by sending either the singlet state or one of the triplet states, which represents more than a three-fold improvement over the separable case. A detailed calculation based on the Holevo bound shows that this is the optimal capacity attainable with two-photon entangled states [8] . The measurement required on the receiver side to maximize the capacity in both the entangled and the separable cases has
In REF , entangled photon pairs are used to improve the capacity of the communication system through using the correlation behavior to encode more number of bits, using optical fiber channel.
Experimental Demonstration of Entanglement-Enhanced Classical Communication over a Quantum Channel with Correlated Noise
Phys. Rev. Lett. 92, 257901 (2004)
Physics Medicine