Search

Larger arrays of electron spin qubits require radical improvements in fabrication and device uniformity. Here we demonstrate excellent qubit device uniformity and tunability from 300K down to mK temperatures. This is achieved, for the first time, by integrating an overlapping polycrystalline silicon-based gate stack in an 'all-Silicon' and lithographically flexible 300mm flow. Low-disorder Si/SiO$_2$ is proved by a 10K Hall mobility of $1.5 \cdot 10^4$ $cm^2$/Vs. Well-controlled sensors with low charge noise (3.6 $\mu$eV/$\sqrt{\mathrm{Hz}}$ at 1 Hz) are used for charge sensing down to the last electron. We demonstrate excellent and reproducible interdot coupling control over nearly 2 decades (2-100 GHz). We show spin manipulation and single-shot spin readout, extracting a valley splitting energy of around 150 $\mu$eV. These low-disorder, uniform qubit devices and 300mm fab integration pave the way for fast scale-up to large quantum processors.

Building quantum computers requires not only a large number of qubits with high fidelity and low variability, but also a large amount of analog and digital components to drive the qubits. Larger arrays of solid-state qubits with high fidelity and low variability require improvements in fabrication processes and array layout design co-optimized with the underlying hardware technology. Here we outline progress on 300mm fabrication of qubit devices and on classical CMOS components to enable the quantum system. We describe work on superconducting qubits and spin qubits in Si, both types of devices fabricated on 300mm experimental platforms and discuss challenges related to variability. Massive electrical characterization is key over wide temperature range is key to enabling system upscaling for QC.

We report on a flexible 300 mm process that optimally combines optical and electron beam lithography to fabricate silicon spin qubits. It enables on-the-fly layout design modifications while allowing devices with either n- or p-type ohmic implants, a pitch smaller than 100 nm, and uniform critical dimensions down to 30 nm with a standard deviation ~ 1.6 nm. Various n- and p-type qubits are characterized in a dilution refrigerator at temperatures ~ 10 mK. Electrical measurements demonstrate well-defined quantum dots, tunable tunnel couplings, and coherent spin control, which are essential requirements for the implementation of a large-scale quantum processor., 2020 IEEE International Electron Devices Meeting (IEDM), December 12-18, 2020

We propose a solid scalable design and operation methodology for 1-D spin qubit arrays with nanomagnets. Each qubit in the array is independently addressable, with single and 2-qubit quantum gate fidelities that exceed 99% in the presence of realistic noise and small fabrication errors. Proposals to extend the array beyond 1-dimension are further discussed.

We summarize the design parameters and modeling techniques for silicon quantum dot qubit devices. A general overview on the operation of the devices - including various methods of qubit readout, control, and interaction - is provided with relevant parameters. With these blocks forming the backbone of silicon quantum computation, the paper provides a guideline to aid and accelerate the design and optimization of silicon qubit devices.

Presently, one of the most ambitious technological goals is the development of devices working under the laws of quantum mechanics. One prominent target is the quantum computer, which would allow the processing of information at quantum level for purposes not achievable with even the most powerful computer resources. The large-scale implementation of quantum information would be a game changer for current technology, because it would allow unprecedented parallelised computation and secure encryption based on the principles of quantum superposition and entanglement. Currently, there are several physical platforms racing to achieve the level of performance required for the quantum hardware to step into the realm of practical quantum information applications. Several materials have been proposed to fulfil this task, ranging from quantum dots, Bose-Einstein condensates, spin impurities, superconducting circuits, molecules, amongst others. Magnetic molecules are among the list of promising building blocks, due to (i) their intrinsic monodispersity, (ii) discrete energy levels (iii) the possibility of chemical quantum state engineering, and (iv) their multilevel characteristics, leading to the so called Qudits (d > 2), amongst others. Herein we review how a molecular multilevel nuclear spin qubit (or qudit, where d = 4), known as TbPc2, gathers all the necessary requirements to perform as a molecular hardware platform with a first generation of molecular devices enabling even quantum algorithm operations., Chem. Soc. Rev., 2017, Advance Article

The dynamics of a quantum system in a dissipative environment presents a rich and still largely unexplored phenomenology that is relevant for the control of quantum devices. The simplest problem can be modeled as a two-level system—such as a spin—in contact with a thermal bath. Here, we present experiments in which we monitor the spin reversal in a TbPc2 single-molecule spin transistor, and we show that the application of microwave pulses can cause the spin to flip between its two lowest-lying states ( | ↑ ⟩ and | ↓ ⟩) at a rate that increases with increasing duration and power of the pulses. This process is neither resonant nor coherent with the microwave pulses. Moreover, an asymmetry appears in the | ↓ ⟩→ | ↑ ⟩ and | ↑ ⟩→ | ↓ ⟩ transition probabilities, suggesting that the process occurs out of equilibrium. We explain the experimental results and provide an estimate of the local temperature increase induced by the microwave pulses by means of a model that takes into account the energy exchange between the single Tb3+ electron spin and the local environment.The dynamics of a quantum system in a dissipative environment presents a rich and still largely unexplored phenomenology that is relevant for the control of quantum devices. The simplest problem can be modeled as a two-level system—such as a spin—in contact with a thermal bath. Here, we present experiments in which we monitor the spin reversal in a TbPc2 single-molecule spin transistor, and we show that the application of microwave pulses can cause the spin to flip between its two lowest-lying states ( | ↑ ⟩ and | ↓ ⟩) at a rate that increases with increasing duration and power of the pulses. This process is neither resonant nor coherent with the microwave pulses. Moreover, an asymmetry appears in the | ↓ ⟩→ | ↑ ⟩ and | ↑ ⟩→ | ↓ ⟩ transition probabilities, suggesting that the process occurs out of equilibrium. We explain the experimental results and provide an estimate of the local temperature increase induced by the microwave pulses by means of a model that takes into account the energy exchange...

Qudits, with their state space of dimension d > 2, open fascinating experimental prospects. The quantum properties of their states provide new potentialities for quantum information, quantum contextuality, expressions of geometric phases, facets of quantum entanglement and many other foundational aspects of the quantum world, which are unapproachable with qubits. We here experimentally investigate the quantum dynamics of a qudit (d = 4) that consists of a single 3/2 nuclear spin embedded in a molecular magnet transistor geometry, coherently driven by a microwave electric field. We propose and implement three protocols based on a generalization of the Ramsey interferometry to a multilevel system. First, the standard Ramsey interference is used to measure the accumulation of geometric phases. Then, two distinct transitions of the nuclear spin are addressed to measure the phase of an iSWAP quantum gate. Finally, through a succession of two Hadamard gates, the coherence time of a 3-state superposition is measured., 20 pages, 5 figures

We monitor the Landau-Zener dynamics of a single-ion magnet in a spin-transistor geometry. For increasing field-sweep rates, the spin reversal probability shows increasing deviations from that of a closed system. In the low-conductance limit, such deviations are shown to result from a dephasing process. In particular, the observed behaviors are succesfully simulated by means of an adiabatic master equation, with time averaged dephasing (Lindblad) operators. The time average is tentatively interpeted in terms of the finite time resolution of the continuous measurement., To appear in Physical Review Letters

International audience; We present a new way of continuously reading-out the state of a single electronic spin. Our detection scheme is based on an exchange interaction between the electronic spin and a nearby read-out quantum dot. The coupling between the two systems results in a spin-dependent conductance through the read-out dot and establishes an all electrical and non-destructive single spin detection. With conductance variations up to 4% and read-out fidelities greater than 99.5%, this method represents an alternative to systems where spin to charge conversion cannot be implemented. Using a semi-classical approach, we present an asymetric exchange coupling model in good agreement with our experimental results.

Quantum algorithms use the principles of quantum mechanics, as for example quantum superposition, in order to solve particular problems outperforming standard computation. They are developed for cryptography, searching, optimisation, simulation and solving large systems of linear equations. Here, we implement Grover's quantum algorithm, proposed to find an element in an unsorted list, using a single nuclear 3/2-spin carried by a Single Molecular Magnet (SMM) transistor. The coherent manipulation of this multi-level qudit is achieved by means of electric fields only. Grover's search algorithm was implemented by constructing a quantum database via a multi-level Hadamard gate. The Grover sequence then allows us to select each state. The presented method is of universal character and can be implemented in any multi-level quantum system with non-equal spaced energy levels, opening the way to novel quantum search algorithms., Comment: 5 pages, 4 figures

Finding reliable methods to exploit molecular degrees of freedom represents an intriguing problem involving the control of new mechanisms at the nanoscale and several technological challenges. Here, we report a novel approach to address a single molecular spin embedded in an electronic circuit. Our devices make use of molecules with well-defined magnetic anisotropy (TbPc2) embedded in nanogapped electrodes obtained by electroburning graphene layers. Such devices work as molecular spin transistors allowing the detection of the Tb spin flip during the sweep of an external magnetic field. The spin readout is made by the molecular quantum dot that, in turns, is driven by an auxiliary gate voltage. In the general context of (spin-)electronics, these results demonstrate that: (1) molecular quantum dots can be used as ultra-sensitive detectors for spin flip detection and (2) the use of graphene electrodes as a platform to contact organometallic molecules is a viable route to design more complex nanoarchitectures.

Several technological issues have to be faced to realize devices working at the single molecule level. One of the main challenges consists of defining methods to fabricate electrodes to make contact with single molecules. Here, we report the realization of novel spintronic devices made of a TbPc2 single molecule embedded between two nanometer-separated graphene electrodes, obtained by feedback-controlled electroburning. We demonstrate that this approach allows the realisation of devices working at low temperature. With these, we were able to characterize the magnetic exchange coupling between the electronic spin of the Tb3+ magnetic core and the current passing through the molecular system in the Coulomb blockade regime, thus showing that the use of graphene is a promising way forward in addressing single molecules.