Quantum reading capacity

Abstract

The readout of a classical memory can be modelled as a problem of quantum channel discrimination, where a decoder retrieves information by distinguishing the different quantum channels encoded in each cell of the memory [S. Pirandola, Phys. Rev. Lett. 106, 090504 (2011)]. In the case of optical memories, such as CDs and DVDs, this discrimination involves lossy bosonic channels and can be remarkably boosted by the use of nonclassical light (quantum reading). Here we generalize these concepts by extending the model of memory from single-cell to multi-cell encoding. In general, information is stored in a block of cells by using a channel-codeword, i.e., a sequence of channels chosen according to a classical code. Correspondingly, the readout of data is realized by a process of "parallel" channel discrimination, where the entire block of cells is probed simultaneously and decoded via an optimal collective measurement. In the limit of an infinite block we define the quantum reading capacity of the memory, quantifying the maximum number of readable bits per cell. This notion of capacity is nontrivial when we suitably constrain the physical resources of the decoder. For optical memories (encoding bosonic channels), such a constraint is energetic and corresponds to fixing the mean total number of photons per cell. In this case, we are able to prove a separation between the quantum reading capacity and the maximum information rate achievable by classical transmitters, i.e., arbitrary classical mixtures of coherent states. In fact, we can easily construct nonclassical transmitters that are able to outperform any classical transmitter, thus showing that the advantages of quantum reading persist in the optimal multi-cell scenario.

Measures and applications of quantum correlations

“…An analogous analysis applies to establish the operational role played by the Hellinger discord of response Q U He A in quantum reading of classical digital memories [274,275], as demonstrated in [197] in a continuous variable setting restricted to Gaussian states and operations. The authors of [197] further provided an operational interpretation for the trace distance based Gaussian discord of response in terms of (one minus) the maximum error probability in quantum reading of a classical memory.…”

Cryptographic Aspects of Quantum Reading

“…It has been recently proven that quantum entanglement can provide an advantage in the readout of classical data from optical memories, especially in the low-energy regime, i.e., when a few photons are irradiated over the memory cells. This approach is known as quantum reading [11] (see also follow-up papers [12][13][14][15][16][17][18][19][20][21][22][23]), a notable application of quantum channel discrimination to a practical task as the memory readout. From this point of view, another well-known protocol is quantum illumination, which aims at improving target detection [25][26][27][28][29][30][31], and has been recently extended to its most natural domain, the microwaves [32].…”

Microwave Quantum Illumination

“…Because of this feature, QI is perhaps one of the most surprising protocols for quantum sensing. Together with quantum reading [10][11][12][13], it represents a practical example of quantum channel discrimination [3], in which entanglement is beneficial for a technologically driven information task.…”