qc

Classical communication systems based on optics and optoelectronics changed our society. On the other hand, when the intensity of few-photon signals is reduced, we unavoidably enter the realm of quantum physics. Quantum communication, namely the art of transferring quantum states from one place to another, has arrived at a crucial stage of developing a commercial and technological impact on our society. This is witnessed by the first quantum cryptosystems available on the market. At the same time, quantum communication is a fascinating field: it combines concepts and techniques from basic quantum physics and information science to optical engineering.

Quantum communication channels use quantum systems to transfer classical or quantum information. In the first case, classical bits are encoded by means of quantum states. In the latter, one may want to distribute entanglement between two or more communicating parties or to transfer an unknown quantum state between different units of a quantum computer. The fundamental quantities characterizing a quantum channel are the classical and the quantum channel capacities, that are defined as the maximum number of bits/qubits that can be reliably transmitted per channel use .

Quantum channels are the natural theoretical framework to investigate both quantum communication and computation in a noisy environment. In the first case, information is transmitted in space, in the latter in time. In both cases, noise can have relevant low frequency components, which traduce themselves in memory effects. That is, consecutive uses of a channel can be correlated. Memory effects may be important, for instance, in quantum communication protocols realized by means of photons travelling across fibers with birefringence fluctuating with characteristic time scales longer than the separation between successive light pulses. Moreover, solid state implementations of quantum hardware show a characteristic low-frequency noise.

Dephasing channel

In spite of their physical relevance, quantum channels with memory are very hard to analyze and their classical and quantum capacities rarely known. Indeed, due to the peculiar features of quantum entanglement, one has to solve a sequence of optimization problems of increasing size (Hilbert space dimension) as the number of channel uses grows, and then take the limit of infinite number of uses. We have computed [1] the quantum capacity of a Markov chain dephasing channel with memory and shown that it is greater than in the memoryless case. Furthermore, based on theoretical arguments and numerical simulations, we have conjectured that memory-induced quantum capacity enhancement takes place also when the dephasing environment is modelled by a bosonic bath. We plan to further explore the connection between quantum channels with memory and the statistical mechanics of interacting many-body systems. This may open fruitful new possibilities for the study of information transmission across quantum channels with memory.

We have considered the transfer of quantum information down a single-mode quantum transmission line. Such quantum channel is modeled as a damped harmonic oscillator, the interaction between the information carriers -a train of n qubits- and the oscillator being of the Jaynes-Cummings kind. The oscillator acts as a local environment, coupled to a memoryless reservoir damping both its phases and populations, which mimics any cooling process resetting the oscillator to its ground state. Memory effects appear if the state of the oscillator is not reset after each channel use. The model is visualized by a qubit-micromaser system, the qubit train being a stream of two-level Rydberg atoms injected at low rate into the cavity; it also describes the dynamics of a quantum memory, which may be implemented by coupling n superconducting qubits to a microstrip cavity, in a circuit-QED architecture.
We have shown [2] that the quantum information transmission worsens with increasing transmission frequency due to the increase of memory effects. However, the decrease is found to be only moderate, so that the quantum transmission rate increases with increasing transmission frequency. Therefore, operating the memory channel at high transmission frequency, thus accepting prima facie deleterious memory effects, will be more beneficial than using low frequency. These results are relevant also for the secure transmission of classical information, that is, for cryptographic purposes.