Qubit

Résumé

En informatique quantique, un qubit ou qu-bit (quantum + bit ; prononcé ), parfois écrit qbit, est un système quantique à deux niveaux, qui représente la plus petite unité de stockage d'information quantique. Ces deux niveaux, notés \left|0 \right\rangle et \left|1 \right\rangle selon le formalisme de Dirac, représentent chacun un état de base du qubit et en font donc l'analogue quantique du bit. Grâce à la propriété de superposition quantique, un qubit stocke une information qualitativement différente de celle d'un bit. D'un point de vue quantitatif, un qbit peut être dans une infinité d'états, qui sont autant de probabilités différentes d'obtenir \left|0 \right\rangle ou \left|1 \right\rangle, mais il se réduit à un seul bit d'information au moment de sa mesure. Le concept de qubit, tout en étant discuté dès les années 1980, fut formalisé par Benjamin Schumacher en 1995. Définition Superposition d'états Un

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https://fr.wikipedia.org/wiki/Qubit

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Fully Tunable Longitudinal Spin-Photon Interactions in Si and Ge Quantum Dots

Pasquale Scarlino

Spin qubits in silicon and germanium quantum dots are promising platforms for quantum computing, but entangling spin qubits over micrometer distances remains a critical challenge. Current prototypical architectures maximize transversal interactions between qubits and microwave resonators, where the spin state is flipped by nearly resonant photons. However, these interactions cause backaction on the qubit that yields unavoidable residual qubit-qubit couplings and significantly affects the gate fidelity. Strikingly, residual couplings vanish when spin-photon interactions are longitudinal and photons couple to the phase of the qubit. We show that large and tunable spin-photon interactions emerge naturally in state-of-the-art hole spin qubits and that they change from transversal to longitudinal depending on the magnetic field direction. We propose ways to electrically control and measure these interactions, as well as realistic protocols to implement fast high-fidelity two-qubit entangling gates. These protocols work also at high temperatures, paving the way toward the implementation of large-scale quantum processors.

AMER PHYSICAL SOC2022

Chargement

Quantum computing is one of the great scientific challenges of the 21st century. Small-scalesystems today promise to surpass classical computers in the coming years and to enable thesolution of classically intractable computational tasks in the fields of quantum chemistry,optimization, cryptography and more.In contrast to classical computers, quantum computers based on superconducting quantumbits (qubits) can to date not be linked over long distance in a network to improve their computingcapacity, since devices, which preserve the quantumstate when it is transferred from onemachine to another, are not available. Several approaches are being pursued to realize such acomponent, one of themost promising to date makes use of an intermediary, micromechanicalelement that enables quantum coherent conversion between the information presentin the quantum computer and an optical fiber, without compromising the quantum natureof the information, via optomechanical interaction. This approach could allow fiber-opticquantum networks between separate quantum computers based on superconducting qubitsin the future.In this work a platformfor such a microwave-to-optic link was developed based on the piezoelectricmaterial gallium phosphide. This III-V semiconductor offers not only a piezoelectriccoupling between the electric field of a microwave circuit and a mechanicalmode, but also awide optical bandgap E_g = 2.26eV which reduces nonlinear optical absorption in the deviceand a large refractive index n(1550nm) = 3.01 which allows strong optical confinement atnear-infrared wavelengths.Importantly and in contrast to other approaches with gallium phosphide, an epitaxiallygrown, single crystal thin film of the material is integrated directly on a silicon wafer withpre-structured niobium electrodes by direct wafer-bonding. This opens up the possibility ofintegrating the device design presented here directly with superconducting qubits fabricatedwith this material system.A microwave-to-optical transducer design was simulated and fabricated in the galliumphosphideon-silicon platform. The device was found to exhibit large vacuum optomechanical couplingrates g0/2 pi ~ 290kHz and a high intrinsic optical quality factor Q >10^5 while at the same timepermitting electromechanical coupling to a microwave electrode. Coherent microwave-toopticaltransductionwas shown at room temperature for this device and the electromechanicalcoupling rate could be extracted from a model derived by input-output theory.The electromechanical coupling between the electro-optomechanical device and a superconductingqubit was estimated to be g/2 pi = O(200kHz) which indicates that strong couplingbetween the here presented device and a superconducting transmon qubit is achievable.In addition, superconducting microwave cavities with high quality factor at single photonenergy Q ~ 5x10^5 were fabricated and measured to verify that fabrication process of themicrowave-to-optical transducer is compatible with high-quality superconducting microwavecircuits.

EPFL2021

Chargement

Coherent spin–photon coupling using a resonant exchange qubit

Pasquale Scarlino

Electron spins hold great promise for quantum computation because of their long coherence times. Long-distance coherent coupling of spins is a crucial step towards quantum information processing with spin qubits. One approach to realizing interactions between distant spin qubits is to use photons as carriers of quantum information. Here we demonstrate strong coupling between single microwave photons in a niobium titanium nitride high-impedance resonator and a three-electron spin qubit (also known as a resonant exchange qubit) in a gallium arsenide device consisting of three quantum dots. We observe the vacuum Rabi mode splitting of the resonance of the resonator, which is a signature of strong coupling; specifically, we observe a coherent coupling strength of about 31 megahertz and a qubit decoherence rate of about 20 megahertz. We can tune the decoherence electrostatically to obtain a minimal decoherence rate of around 10 megahertz for a coupling strength of around 23 megahertz. We directly measure the dependence of the qubit–photon coupling strength on the tunable electric dipole moment of the qubit using the ‘AC Stark’ effect. Our demonstration of strong qubit–photon coupling for a three-electron spin qubit is an important step towards coherent long-distance coupling of spin qubits.

2018

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EPFL2021