82 research outputs found
Quantum gate learning in engineered qubit networks: Toffoli gate with always-on interactions
We put forward a strategy to encode a quantum operation into the unmodulated
dynamics of a quantum network without the need of external control pulses,
measurements or active feedback. Our optimization scheme, inspired by
supervised machine learning, consists in engineering the pairwise couplings
between the network qubits so that the target quantum operation is encoded in
the natural reduced dynamics of a network section. The efficacy of the proposed
scheme is demonstrated by the finding of uncontrolled four-qubit networks that
implement either the Toffoli gate, the Fredkin gate, or remote logic
operations. The proposed Toffoli gate is stable against imperfections, has a
high-fidelity for fault tolerant quantum computation, and is fast, being based
on the non-equilibrium dynamics.Comment: 8 pages, 3 figure
Quantum Gates Between Distant Qubits via Spin-Independent Scattering
We show how the spin independent scattering of two initially distant qubits,
say, in distinct traps or in remote sites of a lattice, can be used to
implement an entangling quantum gate between them. The scattering takes place
under 1D confinement for which we consider two different scenarios: a 1D
wave-guide and a tight-binding lattice. We consider models with contact-like
interaction between two fermionic or two bosonic particles. A qubit is encoded
in two distinct spins (or other internal) states of each particle. Our scheme
enables the implementation of a gate between two qubits which are initially too
far to interact directly, and provides an alternative to photonic mediators for
the scaling of quantum computers. Fundamentally, an interesting feature is that
"identical particles" (e.g., two atoms of the same species) and the 1D
confinement, are both necessary for the action of the gate. Finally, we discuss
the feasibility of our scheme, the degree of control required to initialize the
wave-packets momenta, and show how the quality of the gate is affected by
momentum distributions and initial distance. In a lattice, the control of
quasi-momenta is naturally provided by few local edge impurities in the lattice
potential.Comment: 10 pages, 7 figures. This article supersedes arXiv:1106.2329.
Accepted in Quantu
Machine Learning Assisted Many-Body Entanglement Measurement
Entanglement not only plays a crucial role in quantum technologies, but is
key to our understanding of quantum correlations in many-body systems. However,
in an experiment, the only way of measuring entanglement in a generic mixed
state is through reconstructive quantum tomography, requiring an exponential
number of measurements in the system size. Here, we propose a machine learning
assisted scheme to measure the entanglement between arbitrary subsystems of
size and , with measurements, and without
any prior knowledge of the state. The method exploits a neural network to learn
the unknown, non-linear function relating certain measurable moments and the
logarithmic negativity. Our procedure will allow entanglement measurements in a
wide variety of systems, including strongly interacting many body systems in
both equilibrium and non-equilibrium regimes.Comment: 16 pages, 10 figures, including appendi
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