Experimental Demonstration of Entanglement-Enhanced Classical Communication over a Quantum Channel with Correlated Noise

Abstract

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 motivat...

Quantum channels with memory

“…These results have recently been extended to some bosonic Gaussian channels [12,13]. Such an effect has been demonstrated experimentally for optical fiber channels with fluctuating birefringence, in which consecutive light pulses undergo strongly correlated polarization transformation [14,15]. (Whether such examples exist in the memoryless setting is still an open question, and presently considered one of the most eminent open problems of quantum information theory, with wide implications for other problems in the field [16,17].…”

Suppressing correlated noise in signals transmitted over the Gaussian memory channels using a2N-port splitter and phase flips

“…However, a predetermined condition required: the magnitude of the correlated noise should be known for structuring the transmissivity of beam splitters. Realistically, the magnitude of the correlated noise may not be accurately grasped when considering the randomly varying of the external environments [16]. This limits the execution of encodingdecoding procedures.…”

“…The covariance matrixes have forms [19]: V b = I 2N ×2N /2, V e = (T + 1/2)I 2N ×2N and V m1 = I 2×2 /2. It is worth noting that all these modes are independent of each other, so we have C = 0 and D = 0 for the off-diagonal terms in (16).…”

Classical capacity of Gaussian thermal memory channels

“…Realistic communication lines however, if used at high rates (larger than the relaxation time of the environment), may exhibit memory effects in which the output states are influenced by the previous input signals [12][13][14][15]. In other words, the noise introduced by the channel instead of being independent and identically distributed can be correlated with the previous input states preventing one from express-ing the input-output mapping of n successive channel uses as a simple tensor product Φ ⊗n and hence from using Eq.…”