Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Fault-tolerant quantum computing requires many qubits with long lifetimes and accurate quantum gate operations. However, external noise limits the computing time and hampers accurate quantum gate operations. Quantum error correction (QEC) codes may extend such limits, but imperfect gate operations during QEC cause errors, which could cancel out QEC. We used density matrix simulations to examine the performance of QEC codes with five qubits. In current quantum devices, less than ten qubits are needed to conduct sufficient gate operations within their lifetime so that it is feasible to implement QEC codes. We analyzed the maximum tolerable error rate and error correction effect of individual QEC codes according to the qubit arrangement and gate accuracy. Assuming a 0.1% gate error probability, a logical <jats:inline-formula> <jats:tex-math> <?CDATA $| 1\rangle $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">∣</mml:mo> <mml:mn>1</mml:mn> <mml:mo stretchy="false">〉</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="qstab5887ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> state encoded by a five-qubit QEC code is expected to have a 0.25 higher fidelity than its physical counterpart.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Future universal quantum computers solving problems of practical relevance are expected to require at least 10<jats:sup>6</jats:sup> qubits, which is a massive scale-up from the present numbers of less than 50 qubits operated together. Out of the different types of qubits, solid state qubits are considered to be viable candidates for this scale-up, but interfacing to and controlling such a large number of qubits is a complex challenge that has not been solved yet. One possibility to address this challenge is to use qubit control circuits located close to the qubits at cryogenic temperatures. In this work we evaluate the feasibility of this idea, taking as a reference the physical requirements of a two-electron spin qubit and the specifications of a standard 65 nm complementary metal-oxide-semiconductor process. Using principles and flows from electrical systems engineering we provide realistic estimates of the footprint and of the power consumption of a complete control-circuit architecture. Our results show that with further research it is possible to provide scalable electrical control in the vicinity of the qubit, with our concept.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantum transduction, the process of converting quantum signals from one form of energy to another, is an important area of quantum science and technology. The present perspective article reviews quantum transduction between microwave and optical photons, an area that has recently seen a lot of activity and progress because of its relevance for connecting superconducting quantum processors over long distances, among other applications. Our review covers the leading approaches to achieving such transduction, with an emphasis on those based on atomic ensembles, opto-electro-mechanics, and electro-optics. We briefly discuss relevant metrics from the point of view of different applications, as well as challenges for the future.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We introduce and describe the multiconfigurational time-depenent Hartree for indistinguishable particles (MCTDH-X) software, which is hosted, documented, and distributed at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://ultracold.org" xlink:type="simple">http://ultracold.org</jats:ext-link>. This powerful tool allows the investigation of ground state properties and dynamics of interacting quantum many-body systems in different spatial dimensions. The MCTDH-X software is a set of programs and scripts to compute, analyze, and visualize solutions for the time-dependent and time-independent many-body Schrödinger equation for indistinguishable quantum particles. As the MCTDH-X software represents a general solver for the Schrödinger equation, it is applicable to a wide range of problems in the fields of atomic, optical, molecular physics, and condensed matter systems. In particular, it can be used to study light–matter interactions, correlated dynamics of electrons in the solid state as well as some aspects related to quantum information and computing. The MCTDH-X software solves a set of nonlinear coupled working equations based on the application of the time-dependent variational principle to the Schrödinger equation. These equations are obtained by using an ansatz for the many-body wavefunction that is a expansion in a set of time-dependent, fully symmetrized bosonic (X = B) or fully anti-symmetrized fermionic (X = F) many-body basis states. It is the time-dependence of the basis set that enables MCTDH-X to deal with quantum dynamics at a superior accuracy as compared to, for instance, exact diagonalization approaches with a static basis, where the number of basis states necessary to capture the dynamics of the wavefunction typically grows rapidly with time. Herein, we give an introduction to the MCTDH-X software via an easy-to-follow tutorial with a focus on accessibility. The illustrated exemplary problems are hosted at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://ultracold.org/tutorial" xlink:type="simple">http://ultracold.org/tutorial</jats:ext-link> and consider the physics of a few interacting bosons or fermions in a double-well potential. We explore computationally the position-space and momentum-space density, the one-body reduced density matrix, Glauber correlation functions, phases, (dynamical) phase transitions, and the imaging of the quantum systems in single-shot images. Although a few particles in a double well potential represent a minimal model system, we are able to demonstrate a rich variety of phenomena with it. We use the double well to illustrate the fermionization of bosonic particles, the crystallization of fermionic particles, characteristics of the superfluid and Mott-insulator quantum phases in Hubbard models, and even dynamical phase transitions. We provide a complete set of input files and scripts to redo all computations in this paper at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="http://ultracold.org/data/tutorial_input_files.zip" xlink:type="simple">http://ultracold.org/data/tutorial_input_files.zip</jats:ext-link>, accompanied by tutorial videos at <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://tinyurl.com/tjx35sq" xlink:type="simple">https://tinyurl.com/tjx35sq</jats:ext-link>. Our tutorial should guide the potential users to apply the MCTDH-X software also to more complex systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We develop general tools to characterise and efficiently compute relevant observables of multimode <jats:italic>N</jats:italic>-photon states generated in nonlinear decays in one-dimensional waveguides. We then consider optical interferometry in a Mach–Zender interferometer where a <jats:italic>d</jats:italic>-mode photonic state enters in each arm of the interferometer. We derive a simple expression for the quantum Fisher information in terms of the average photon number in each mode, and show that it can be saturated by number-resolved photon measurements that do not distinguish between the different <jats:italic>d</jats:italic> modes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report dispersive coupling of an optically trapped nanoparticle to the field of a Fabry–Perot cavity in high vacuum. We demonstrate nanometer-level control in positioning the particle with respect to the cavity field, which allows access to linear, quadratic, and tertiary optomechanical interactions in the resolved sideband regime. We determine all relevant coupling rates of the system, i.e. mechanical and optical losses as well as optomechanical interaction, and obtain a quantum cooperativity of <jats:italic>C</jats:italic> <jats:sub>Q</jats:sub> = 0.01. Based on the presented performance, the regime of strong cooperativity (<jats:italic>C</jats:italic> <jats:sub>Q</jats:sub> &gt; 1) is clearly within reach by further decreasing the mode volume of the cavity.</jats:p>