Quantum computing | EPFL Graph Search

A quantum computer is a computer that exploits quantum mechanical phenomena. At small scales, physical matter exhibits properties of both particles and waves, and quantum computing leverages this behavior, specifically quantum superposition and entanglement, using specialized hardware that supports the preparation and manipulation of quantum states. Classical physics cannot explain the operation of these quantum devices, and a scalable quantum computer could perform some calculations exponentially faster than any modern "classical" computer. In particular, a large-scale quantum computer could break widely used encryption schemes and aid physicists in performing physical simulations; however, the current state of the art is largely experimental and impractical, with several obstacles to useful applications. The basic unit of information in quantum computing is the qubit, similar to the bit in traditional digital electronics. Unlike a classical bit, a qubit can exist in a superpositio

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

Microwave-to-Optical Transduction with Gallium Phosphide Electro-Optomechanical Devices

Simon Benjamin Klaus Hönl

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

Gate fidelity and coherence of an electron spin in an Si/SiGe quantum dot with micromagnet

Pasquale Scarlino

The gate fidelity and the coherence time of a quantum bit (qubit) are important benchmarks for quantum computation. We construct a qubit using a single electron spin in an Si/SiGe quantum dot and control it electrically via an artificial spin-orbit field from a micromagnet. We measure an average single-qubit gate fidelity of ∼99% using randomized benchmarking, which is consistent with dephasing from the slowly evolving nuclear spins in the substrate. The coherence time measured using dynamical decoupling extends up to ∼400 μs for 128 decoupling pulses, with no sign of saturation. We find evidence that the coherence time is limited by noise in the 10-kHz to 1-MHz range, possibly because charge noise affects the spin via the micromagnet gradient. This work shows that an electron spin in an Si/SiGe quantum dot is a good candidate for quantum information processing as well as for a quantum memory, even without isotopic purification.

2016

Microwave-to-Optical Transduction with Gallium Phosphide Electro-Optomechanical Devices

Simon Benjamin Klaus Hönl

EPFL2021