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Register a new userA silicon quantum-dot-coupled nuclear spin qubit
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Single nuclear spins in the solid state have long been envisaged as a platform for quantum computing, due to their long coherence times and excellent controllability. Measurements can be performed via localised electrons, for example those in single atom dopants or crystal defects. However, establishing long-range interactions between multiple dopants or defects is challenging. Conversely, in lithographically-defined quantum dots, tuneable interdot electron tunnelling allows direct coupling of electron spin-based qubits in neighbouring dots. Moreover, compatibility with semiconductor fabrication techniques provides a compelling route to scaling to large numbers of qubits. Unfortunately, hyperfine interactions are typically too weak to address single nuclei. Here we show that for electrons in silicon metal-oxide-semiconductor quantum dots the hyperfine interaction is sufficient to initialise, read-out and control single silicon-29 nuclear spins, yielding a combination of the long coherence times of nuclear spins with the flexibility and scalability of quantum dot systems. We demonstrate high-fidelity projective readout and control of the nuclear spin qubit, as well as entanglement between the nuclear and electron spins. Crucially, we find that both the nuclear spin and electron spin retain their coherence while moving the electron between quantum dots, paving the way to long range nuclear-nuclear entanglement via electron shuttling. Our results establish nuclear spins in quantum dots as a powerful new resource for quantum processing.
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A two-qubit controlled-NOT (CNOT) gate, realized by a controlled-phase (C-phase) gate combined with single-qubit gates, has been experimentally implemented recently for quantum-dot spin qubits in isotopically enriched silicon, a promising solid-state system for practical quantum computation. In the experiments, the single-qubit gates have been demonstrated with fault-tolerant control-fidelity, but the infidelity of the two-qubit C-phase gate is, primarily due to the electrical noise, still higher than the required error threshold for fault-tolerant quantum computation (FTQC). Here, by taking the realistic system parameters and the experimental constraints on the control pulses into account, we construct experimentally realizable high-fidelity CNOT gates robust against electrical noise with the experimentally measured $1/f^{1.01}$ noise spectrum and also against the uncertainty in the interdot tunnel coupling amplitude. Our optimal CNOT gate has about two orders of magnitude improvement in gate infidelity over the ideal C-phase gate constructed without considering any noise effect. Furthermore, within the same control framework, high-fidelity and robust single-qubit gates can also be constructed, paving the way for large-scale FTQC.
We present a mechanism to protect quantum information stored in an ensemble of nuclear spins in a semiconductor quantum dot. When the dot is charged the nuclei interact with the spin of the excess electron through the hyperfine coupling. If this coupling is made off-resonant it leads to an energy gap between the collective storage states and all other states. We show that the energy gap protects the quantum memory from local spin-flip and spin-dephasing noise. Effects of non-perfect initial spin polarization and inhomogeneous hyperfine coupling are discussed.
The concept of quantum memory plays an incisive role in the quantum information theory. As confirmed by several recent rigorous mathematical studies, the quantum memory inmate in the bipartite system $rho_{AB}$ can reduce uncertainty about the part $B$, after measurements done on the part $A$. In the present work, we extend this concept to the systems with a spin-orbit coupling and introduce a notion of spin-orbit quantum memory. We self-consistently explore Uhlmann fidelity, pre and post measurement entanglement entropy and post measurement conditional quantum entropy of the system with spin-orbit coupling and show that measurement performed on the spin subsystem decreases the uncertainty of the orbital part. The uncovered effect enhances with the strength of the spin-orbit coupling. We explored the concept of macroscopic realism introduced by Leggett and Garg and observed that POVM measurements done on the system under the particular protocol are non-noninvasive. For the extended system, we performed the quantum Monte Carlo calculations and explored reshuffling of the electron densities due to the external electric field.
Semiconductor quantum dots are probably the preferred choice for interfacing anchored, matter spin qubits and flying photonic qubits. While full tomography of a flying qubit or light polarization is in general straightforward, matter spin tomography is a challenging and resource-consuming task. Here we present a novel all-optical method for conducting full tomography of quantum-dot-confined spins. Our method is applicable for electronic spin configurations such as the conduction-band electron, the valence-band hole, and for electron-hole pairs such as the bright and the dark exciton. We excite the spin qubit using short resonantly tuned, polarized optical pulse, which coherently converts the qubit to an excited qubit that decays by emitting a polarized single-photon. We perform the tomography by using two different orthogonal, linearly polarized excitations, followed by time-resolved measurements of the degree of circular polarization of the emitted light from the decaying excited qubit. We demonstrate our method on the dark exciton spin state with fidelity of 0.94, mainly limited by the accuracy of our polarization analyzers.
Fault-tolerant quantum operation is a key requirement for the development of quantum computing. This has been realized in various solid-state systems including isotopically purified silicon which provides a nuclear spin free environment for the qubits, but not in industry standard natural (unpurified) silicon. Here we demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet optimally designed for fast spin control. This optimized design allows us to achieve the optimum Rabi oscillation quality factor Q = 140 at a Rabi frequency of 10 MHz in the frequency range two orders of magnitude higher than that achieved in previous studies. This leads to a qubit fidelity of 99.6 %, which is the highest reported for natural silicon qubits and comparable to that obtained in isotopically purified silicon quantum-dot-based qubits. This result can inspire contributions from the industrial and quantum computing communities.
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