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62

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## Publications

Publications (62)

We model and experimentally demonstrate the full time-dependent counting statistics of photons emitted by a single nitrogen-vacancy (N-V) center in diamond under nonresonant laser excitation and resonant microwave control. A generalization of the quantum jump formalism for the seven electronic states involved in the fast intrinsic dynamics of a N-V...

We propose quantum neural networks that include multi-qubit interactions in the neural potential leading to a reduction of the network depth without losing approximative power. We show that the presence of multi-qubit potentials in the quantum perceptrons enables more efficient information processing tasks such as XOR gate implementation and prime...

The use of nitrogen-vacancy centers in diamond as a non-invasive platform for hyperpolarizing nuclear spins in molecular samples is a promising area of research with the potential to enhance the sensitivity of nuclear magnetic resonance experiments. Transferring NV polarization out of the diamond structure has been achieved on nanoscale targets usi...

We present an invariant-based quantum control scheme leading to a highly monochromatic ion beam from a Paul trap. Our protocol is implementable by supplying the segmented electrodes in the trap with voltages of the order of volts. This mitigates the impact of fluctuations in previous designs and leads to a low-dispersion beam of ions. Moreover, our...

We present a unitary quantum control scheme that produces a highly monochromatic ion beam from a Paul trap. Our protocol is implementable by supplying the segmented electrodes with voltages of the order of Volts, which mitigates the impact of fluctuating voltages in previous designs and leads to a low-dispersion beam of ions. Moreover, our proposal...

We propose to optimally control the harmonic potential of a levitated nanoparticle to quantum delocalize its center-of-mass motional state to a length scale orders of magnitude larger than the quantum zero-point motion. Using a bang-bang control of the harmonic potential, including the possibility of inverting it, the initial ground-state-cooled le...

We propose quantum neural networks that include multi-qubit interactions in the neural potential leading to a reduction of the network depth without losing approximative power. We show that the presence of multi-qubit potentials in the quantum perceptrons enables more efficient information processing tasks such as XOR gate implementation and prime...

The quantum perceptron is a fundamental building block for quantum machine learning. This is a multidisciplinary field that incorporates abilities of quantum computing, such as state superposition and entanglement, to classical machine learning schemes. Motivated by the techniques of shortcuts to adiabaticity, we propose a speed-up quantum perceptr...

The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling-gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of s...

Two-dimensional (2D) systems with time-dependent controls admit a quadratic Hamiltonian modeling near potential minima. Independent, dynamical normal modes facilitate inverse Hamiltonian engineering to control the system dynamics, but some systems are not separable into independent modes by a point transformation. For these “coupled systems” 2D inv...

In this work we analyze the implementation of a control-phase gate through the resonance between the ∣11⟩ and ∣20⟩ states of two statically coupled transmons. We find that there are many different controls for the transmon frequency that implement the same gate with fidelities around 99.8% (T1=T2=17μs) and 99.99% (T1=T2=300μs) within a time that ap...

We propose a new protocol to implement ultra-fast two-qubit phase gates with trapped ions using spin-dependent kicks induced by resonant transitions. By only optimizing the allocation of the arrival times in a pulse train sequence the gate is implemented in times faster than the trapping oscillation period T < 2 π / ω . Such gates allow us to incre...

Two-dimensional systems with time-dependent controls admit a quadratic Hamiltonian modelling near potential minima. Independent, dynamical normal modes facilitate inverse Hamiltonian engineering to control the system dynamics, but some systems are not separable into independent modes by a point transformation. For these "coupled systems" 2D invaria...

The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of s...

We propose a new protocol to implement ultra-fast two-qubit phase gates with trapped ions using spin-dependent kicks induced by resonant transitions. By only optimizing the allocation of the arrival times in a pulse train sequence the gate is implemented in times faster than the trapping oscillation period $T<2\pi/\omega$. Such gates allow us to in...

We propose employing a quantum heat engine as a sensitive probe for thermal baths. In particular, we study a single-atom Otto engine operating in an open thermodynamic cycle. Owing to its cyclic nature, the engine is capable of translating small temperature differences between two baths into a macroscopic oscillation in a flywheel. We present analy...

The quantum perceptron is a fundamental building block in the area of quantum machine learning. This is a multidisciplinary field that incorporates properties of quantum computing, such as state superposition and entanglement, to classical machine learning schemes. Motivated by the techniques of shortcuts to adiabaticity, we propose a speed-up quan...

In this work we analyze the implementation of a control-phase gate through the resonance between the $|11\rangle$ and $|20\rangle$ states of two statically coupled transmons. We find that there are many different controls for the transmon frequency that implement the same gate with fidelities around $99.8\%$ ($T_1=15$ ns) and $99.97\%$ ($T_1=100$ n...

Shortcuts to adiabaticity (STA) are fast routes to the final results of slow, adiabatic changes of the controlling parameters of a system. The shortcuts are designed by a set of analytical and numerical methods suitable for different systems and conditions. A motivation to apply STA methods to quantum systems is to manipulate them on timescales sho...

Shortcuts to adiabaticity (STA) are fast routes to the final results of slow, adiabatic changes of the controlling parameters of a system. The shortcuts are designed by a set of analytical and numerical methods suitable for different systems and conditions. A motivation to apply STA methods to quantum systems is to manipulate them in timescales sho...

We demonstrate that it is possible to implement a quantum perceptron with a sigmoid activation function as an efficient, reversible many-body unitary operation. When inserted in a neural network, the perceptron's response is parameterized by the potential exerted by other neurons. We prove that such a quantum neural network is a universal approxima...

We develop energy efficient, continuous microwave schemes to couple electron and nuclear spins, using phase or amplitude modulation to bridge their frequency difference. These controls have promising applications in biological systems, where microwave power should be limited, as well as in situations with high Larmor frequencies due to large magnet...

We propose speeding up a single ion heat pump based on a tapered ion trap. If a trapped ion is excited in an oscillatory motion axially the radial degrees of freedom are cyclically expanded and compressed such that heat can be pumped between two reservoirs coupled to the ion at the turning points of oscillation. Through the use of invariant-based i...

We propose speeding up a single ion heat pump based on a tapered ion trap. If a trapped ion is excited in an oscillatory motion axially the radial degrees of freedom are cyclically expanded and compressed such that heat can be pumped between two reservoirs coupled to the ion at the turning points of oscillation. Through the use of invariant-based i...

We develop energy efficient, continuous microwave schemes to couple electron and nuclear spins, using phase or amplitude modulation to bridge their frequency difference. These controls have promising applications in biological systems, where microwave power should be limited, as well as in situations with high Larmor frequencies due to large magnet...

A systematic approach to design robust control protocols against the influence of different types of noise is introduced. We present control schemes which protect the decay of the populations avoiding dissipation in the adiabatic and non-adiabatic regimes and minimize the effect of dephasing. The effectiveness of the protocols is demonstrated in tw...

We study the effect of action noise on state-to-state control protocols. Action noise creates dephasing in the instantaneous eigenbasis of the Hamiltonian and hampers the fidelity of the final state with respect to the target state. We find that for shorter protocols the noise more strongly influences the dynamics and degrades fidelity. We suggest...

By applying invariant-based inverse engineering in the small-oscillations regime, we design the time dependence of the control parameters of an overhead crane (trolley displacement and rope length), to transport a load between two positions at different heights with minimal final energy excitation for arbitrary initial conditions. The analogies bet...

Shortcuts to adiabaticity let a system reach the results of a slow adiabatic process in a shorter time by implementing specific protocols for the time-dependent control parameters. We propose to quantify the "energy cost" of the shortcut by the energy consumption of the system enlarged by including the control device. A mechanical model where the d...

We analyze different decoherence processes in a system coupled to a bath. Apart from the well known standard dephasing mechanism which is temperature dependent an alternative mechanism is presented, the spin-swap dephasing which does not need initial bath activation and is temperature independent. We show that for dipole interaction in the weak cou...

We analyze different decoherence processes in a system coupled to a bath.
Apart from the well known standard dephasing mechanism which is temperature
dependent an alternative mechanism is presented, the spin-swap dephasing which
does not need initial bath activation and is temperature independent. We show
that for dipolar interaction the separation...

Sped-up protocols (shortcuts to adiabaticity) that drive a system quickly to
the same populations than a slow adiabatic process may involve Hamiltonian
terms difficult to realize in practice. We use the dynamical symmetry of the
Hamiltonian to find, by means of Lie transforms, alternative Hamiltonians that
achieve the same goals without the problem...

We use the dynamical algebra of a quantum system and its dynamical invariants
to inverse engineer feasible Hamiltonians for implementing shortcuts to
adiabaticity. These are speeded up processes that end up with the same
populations than slow, adiabatic ones. As application examples we design
families of shortcut Hamiltonians that drive two and a t...

Sending multiple messages on qubits encoded in different vibrational modes of cold atoms or ions along a transmission waveguide requires us to merge first and then separate the modes at input and output ends. Similarly, different qubits can be stored in the modes of a trap and be separated later. We design the fast splitting of a harmonic trap into...

We design fast trajectories of a trap to transport two ions using a
shortcut-to-adiabaticity technique based on invariants. The effects of
anharmonicity are analyzed first perturbatively, with an approximate, single
relative-motion mode, description. Then we use classical calculations and full
quantum calculations. This allows to identify discrete...

We extend a recent method to shortcut the adiabatic following to internal
bosonic Josephson junctions in which the control parameter is the linear
coupling between the modes. The approach is based on the mapping between the
two-site Bose-Hubbard Hamiltonian and a 1D effective Schr\"odinger-like
equation, valid in the large $N$ (number of particles)...

A reciprocating quantum refrigerator is analyzed with the intention to study the limitations imposed by external noise. In particular we focus on the behavior of the refrigerator when it approaches the absolute zero. The cooling cycle is based on the Otto cycle with a working medium constituted by an ensemble of noninteracting harmonic oscillators....

Quantum backflow is a classically forbidden effect consisting of a negative flux for states with negligible negative-momentum components. It has never been observed experimentally so far. We derive a general relation that connects backflow with a critical value of the particle density, paving the way for the detection of backflow by a density measu...

When attempting to split coherent cold atom clouds or a Bose-Einstein
condensate (BEC) by bifurcation of the trap into a double well, slow adiabatic
following is unstable with respect to any slight asymmetry, and the wave
"collapses" to the lower well, whereas a generic fast chopping splits the wave
but it also excites it. Shortcuts to adiabaticity...

Quantum adiabatic processes—that keep constant the populations in the instantaneous eigenbasis of a time-dependent Hamiltonian—are very useful to prepare and manipulate states, but take typically a long time. This is often problematic because decoherence and noise may spoil the desired final state, or because some applications require many repetiti...

A Schr\"odinger equation may be transformed by unitary operators into
dynamical equations in different interaction pictures which share with it a
common physical frame, i.e., the same underlying interactions, processes and
dynamics. In contrast to this standard scenario, other relations are also
possible, such as a common interaction-picture dynami...

DOI:https://doi.org/10.1103/PhysRevA.86.019901

We describe methods for fast production of highly coherent-spin-squeezed
many-body states in bosonic Josephson junctions (BJJs). We start from the known
mapping of the two-site Bose-Hubbard (BH) Hamiltonian to that of a single
effective particle evolving according to a Schr\"odinger-like equation in Fock
space. Since, for repulsive interactions, th...

The "fast-forward" approach by Masuda and Nakamura generates driving
potentials to accelerate slow quantum adiabatic dynamics. First we present a
streamlined version of the formalism that produces the main results in a few
steps. Then we show the connection between this approach and inverse
engineering based on Lewis-Riesenfeld invariants. We ident...

We study fast expansions of cold atoms in a three-dimensional Gaussian-beam
optical trap. Three different methods to avoid final motional excitation are
compared: inverse engineering using Lewis-Riesenfeld invariants, which provides
the best overall performance, a bang-bang approach, and a fast adiabatic
approach. We analyze the excitation effect o...

We propose an inverse method to accelerate without final excitation the
adiabatic transport of a Bose Einstein condensate. The method, applicable to
arbitrary potential traps, is based on a partial extension of the
Lewis-Riesenfeld invariants, and provides transport protocols that satisfy
exactly the no-excitation conditions without constraints or...

We study a scaling and coordinate transformation to physically simulate quantum three-body collinear chemical reactions of the type A+BC → AB+C by the motion of a single ultracold atom or a weakly interacting Bose–Einstein condensate on an L-shaped waveguide. We determine its feasibility with current technology and its limitations. As an example we...

Adiabatic processes driven by non-Hermitian, time-dependent Hamiltonians may
be sped up by generalizing inverse engineering techniques based on Berry's
transitionless driving algorithm or on dynamical invariants. We work out the
basic theory and examples described by two-level Hamiltonians: the acceleration
of rapid adiabatic passage with a decayin...

We design optimal harmonic-trap trajectories to transport cold atoms without
final excitation, combining an inverse engineering techniqe based on
Lewis-Riesenfeld invariants with optimal control theory. Since actual traps are
not really harmonic, we keep the relative displacement between the center of
mass and the trap center bounded. Under this co...

We review different ways to accelerate adiabatic processes in cold atom physics and atomic state preparation. Trap expansions or contractions and atomic transport may be accelerated by an invariant-based inverse engineering approach. Berry's inverse engineering method is also applied to produce fast versions of adiabatic passage methods.

Different methods have been recently put forward and implemented experimentally to inverse engineer the time-dependent Hamiltonian of a quantum system and accelerate slow adiabatic processes via nonadiabatic shortcuts. In the “transitionless quantum driving” proposed by Berry, shortcut Hamiltonians are designed so that the system follows exactly, i...

We study the scaling and coordinate transformation to physically simulate
quantum three-body collinear chemical reactions of the type A+BC $\rightarrow$
AB+C by the motion of single ultracold atoms or a weakly interacting
Bose-Einstein condensate on an $L$-shaped waveguide. As an example we show that
the parameters to model the reaction F+HH $\to$...

We use the dynamical invariants associated with the Hamiltonian of an atom in a one dimensional moving trap to inverse engineer the trap motion and perform fast atomic transport without final vibrational heating. The atom is driven nonadiabatically through a shortcut to the result of adiabatic, slow trap motion. For harmonic potentials this only re...

Different ways to accelerate adiabatic processes in cold atom physics and atomic state preparation are reviewed. The invariant-based inverse engineering approach is applied to trap expansions and contractions, and to atomic transport. Berry’s Hamiltonian is applied to produce fast versions of adiabatic passage methods.

Different ways to accelerate adiabatic processes in cold atom physics and atomic state preparation are reviewed. The invariant-based inverse engineering approach is applied to trap expansions and contractions, and to atomic transport. Berry's Hamiltonian is applied to produce fast versions of adiabatic passage methods.

We review the post-exponential decay regime in quantum mechanics, with special attention to potential scattering. The mathematical and physical reasons for this regime are discussed using several techniques. In particular, the possible effects of measurement or interaction with the environment are analyzed. Both general and particular results are p...

Diffraction in time (DIT) is a fundamental phenomenon in quantum dynamics due
to time-dependent obstacles and slits. It is formally analogous to diffraction
of light, and is expected to play an increasing role to design coherent matter
wave sources, as in the atom laser, to analyze time-of-flight information and
emission from ultrafast pulsed excit...

We study the dynamics of neutral cold atoms in an L-shaped crossed-beam optical waveguide formed by two perpendicular red-detuned lasers of different intensities and a blue-detuned laser at the corner. The motion in one sense is optimized, and the motion in the other sense may be suppressed even if it is energetically allowed. Quantum and classical...

Post-exponential decay of the probability density of a quantum particle
leaving a trap can be reproduced accurately, except for interference
oscillations at the transition to the post-exponential regime, by means of an
ensemble of classical particles emitted with constant probability per unit time
and the same half-life as the quantum system. The e...

We study the elusive transition from exponential to post-exponential
(algebraic) decay of the probability density of a quantum particle emitted by
an exponentially decaying source, in one dimension. The main finding is that
the probability density at the transition time, and thus its observability,
increases with the distance of the detector from t...