Approximation of quantum control correction scheme using deep neural networks

We study the functional relationship between quantum control pulses in the idealized case and the pulses in the presence of an unwanted drift. We show that a class of artificial neural networks called LSTM is able to model this functional relationship with high efficiency, and hence the correction scheme required to counterbalance the effect of the drift. Our solution allows studying the mapping from quantum control pulses to system dynamics and then analysing the robustness of the latter against local variations in the control profile.Comment: 6 pages, 3 figures, Python code available upon request. arXiv admin note: text overlap with arXiv:1803.0516

Initializing an unmodulated spin chain to operate as a high quality quantum data-bus

We study the quality of state and entanglement transmission through quantum channels described by spin chains varying both the system parameters and the initial state of the channel. We consider a vast class of one-dimensional many-body models which contains some of the most relevant experimental realizations of quantum data-buses. In particular, we consider spin-1/2 XY and XXZ model with open boundary conditions. Our results show a significant difference between free-fermionic (non-interacting) systems (XY) and interacting ones (XXZ), where in the former case initialization can be exploited for improving the entanglement distribution, while in the latter case it also determines the quality of state transmission. In fact, we find that in non interacting systems the exchange with fermions in the initial state of the chain always has a destructive effect, and we prove that it can be completely removed in the isotropic XX model by initializing the chain in a ferromagnetic state. On the other hand, in interacting systems constructive effects can arise by scattering between hopping fermions and a proper initialization procedure. Remarkably our results are the first example in which state and entanglement transmission show maxima at different points as the interactions and initializations of spin chain channels are varied.Comment: 10 pages, 7 figure

Investigation of the Effects of Chirped RZ Signals in Reducing the Transmission Impairments in R-SOA-Based Bidirectional PONs

Distributed and concentrated reflections represent the two main limitations in reflective-semiconductor optical amplifier (R-SOA)-based passive optical networks (PONs). In this paper, we experimentally discuss how the use of chirped signals in centralized light seeding bidirectional PON can increase the resilience of the system against those two types of reflections. An experimental comparison of the performance of a highly chirped return to zero (RZ) modulation format and the nonreturn to zero is given. Error-free operation is achieved down to 10 dB of signal to crosstalk ratio in presence of distributed reflection, when the upstream signal is highly chirped RZ signal. The same chirped modulation leads to a tolerance of more than dB network return loss due to concentrated reflections. Finally, we assess also the system feasibility of a R-SOA-based full-duplex PON where both the upstream and the downstream are modulated signals

Optimal dynamics for quantum-state and entanglement transfer through homogeneous quantum wires

It is shown that effective quantum-state and entanglement transfer can be obtained by inducing a coherent dynamics in quantum wires with homogeneous intrawire interactions. This goal is accomplished by tuning the coupling between the wire endpoints and the two qubits there attached, to an optimal value. A general procedure to determine such value is devised, and scaling laws between the optimal coupling and the length of the wire are found. The procedure is implemented in the case of a wire consisting of a spin-1/2 XY chain: results for the time dependence of the quantities which characterize quantum-state and entanglement transfer are found of extremely good quality and almost independent of the wire length. The present approach does not require `ad hoc' engineering of the intrawire interactions nor a specific initial pulse shaping, and can be applied to a vast class of quantum channels.Comment: 5 pages, 5 figure

System feasibility of using stimulated Brillouin scattering in self coherent detection schemes

We demonstrate the first self-coherent detection of 10 Gbit/s BPSK signals based on narrow-band amplification of the optical carrier by means of Stimulated Brillouin effect in a common fiber. We found that this technique is very effective only if it is combined with proper line coding and high-pass electrical filtering at the receiver. In this case we obtain OSNR-performance close to the ideal coherent receiver. (C) 2010 Optical Society of Americ

Supervised learning of time-independent Hamiltonians for gate design

We present a general framework to tackle the problem of finding time-independent dynamics generating target unitary evolutions. We show that this problem is equivalently stated as a set of conditions over the spectrum of the time-independent gate generator, thus translating the task into an inverse eigenvalue problem. We illustrate our methodology by identifying suitable time-independent generators implementing Toffoli and Fredkin gates without the need for ancillae or effective evolutions. We show how the same conditions can be used to solve the problem numerically, via supervised learning techniques. In turn, this allows us to solve problems that are not amenable, in general, to direct analytical solution, providing at the same time a high degree of flexibility over the types of gate-design problems that can be approached. As a significant example, we find generators for the Toffoli gate using only diagonal pairwise interactions, which are easier to implement in some experimental architectures. To showcase the flexibility of the supervised learning approach, we give an example of a non-trivial four-qubit gate that is implementable using only diagonal, pairwise interactions

Long quantum channels for high-quality entanglement transfer

High-quality quantum-state and entanglement transfer can be achieved in an unmodulated spin bus operating in the ballistic regime, which occurs when the endpoint qubits A and B are coupled to the chain by an exchange interaction $j_0$ comparable with the intrachain exchange. Indeed, the transition amplitude characterizing the transfer quality exhibits a maximum for a finite optimal value $j_0^{opt}(N)$, where $N$ is the channel length. We show that $j_0^{opt}(N)$ scales as $N^{-1/6}$ for large $N$ and that it ensures a high-quality entanglement transfer even in the limit of arbitrarily long channels, almost independently of the channel initialization. For instance, the average quantum-state transmission fidelity exceeds 90% for any chain length. We emphasize that, taking the reverse point of view, should $j_0$ be experimentally constrained, high-quality transfer can still be obtained by adjusting the channel length to its optimal value.Comment: 12 pages, 9 figure