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Nuclear spins are highly coherent quantum objects. In large ensembles, their control and detection via magnetic resonance is widely exploited, e.g. in chemistry, medicine, materials science and mining. Nuclear spins also featured in early ideas [1] and demonstrations [2] of quantum information processing. Scaling up these ideas requires controlling individual nuclei, which can be detected when coupled to an electron [3, 4, 5]. However, the need to address the nuclei via oscillating magnetic fields complicates their integration in multispin nanoscale devices, because the field cannot be localized or screened. Control via electric fields would resolve this problem, but previous methods [6, 7, 8] relied upon transducing electric signals into magnetic fields via the electron-nuclear hyperfine interaction, which severely affects the nuclear coherence. Here we demonstrate the coherent quantum control of a single antimony (spin-7/2) nucleus, using localized electric fields produced within a silicon nanoelectronic device. The method exploits an idea first proposed in 1961 [9] but never realized experimentally with a single nucleus. Our results are quantitatively supported by a microscopic theoretical model that reveals how the purely electrical modulation of the nuclear electric quadrupole interaction, in the presence of lat- † To whom correspondence should be addressed;

We analyze the electron spin relaxation rate 1/T1 of individual ion-implanted 31 P donors, in a large set of metal-oxide-semiconductor (MOS) silicon nanoscale devices, with the aim of identifying spin relaxation mechanisms peculiar to the environment of the spins. The measurements are conducted at low temperatures (T ≈ 100 mK), as a function of external magnetic field B0 and donor electrochemical potential µ D . We observe a magnetic field dependence of the form 1/T1 ∝ B 5 0 for B0 3 T, corresponding to the phonon-induced relaxation typical of donors in the bulk. However, the relaxation rate varies by up to two orders of magnitude between different devices. We attribute these differences to variations in lattice strain at the location of the donor. For B0 3 T, the relaxation rate changes to 1/T1 ∝ B0 for two devices. This is consistent with relaxation induced by evanescent-wave Johnson noise created by the metal structures fabricated above the donors. At such low fields, where T1 > 1 s, we also observe and quantify the spurious increase of 1/T1 when the electrochemical potential of the spin excited state |↑ comes in proximity to empty states in the charge reservoir, leading to spin-dependent tunneling that resets the spin to |↓ . These results give precious insights into the microscopic phenomena that affect spin relaxation in MOS nanoscale devices, and provide strategies for engineering spin qubits with improved spin lifetimes. arXiv:1812.06644v2 [cond-mat.mes-hall]

Interconnecting well-functioning, scalable stationary qubits and photonic qubits could substantially advance quantum communication applications and serve to link future quantum processors. Here, we present two protocols for transferring the state of a photonic qubit to a single-spin and to a two-spin qubit hosted in gate-defined quantum dots (GDQD). Both protocols are based on using a localized exciton as intermediary between the photonic and the spin qubit. We use effective Hamiltonian models to describe the hybrid systems formed by the the exciton and the GDQDs and apply simple but realistic noise models to analyze the viability of the proposed protocols. Using realistic parameters, we find that the protocols can be completed with a success probability ranging between 85-97 %. I. INTODUCTIONSemiconductor quantum-dot devices have demonstrated considerable potential for quantum information applications. A prominent example are gate-defined quantum dots (GDQD), i.e. quantum dots realized in semiconductor heterostructures in which individual electrons are confined by an electrostatic trapping potential. Spin qubits based on GDQD in GaAs/Al x Ga x−1 As heterostructures have demonstrated all key requirements for quantum information processing, such as qubit initialization, readout, 1,2 coherent control 3,4 with high fidelity 5,6 and two-qubit gates. 7,8 Moreover, thanks to their similarity to the transistors used in modern computer chips, these top-down fabricated quantum dots have good prospects for realizing large scale quantum processing nodes. However, unlike self-assembled quantum dots, where excellent optical control and information transfer has been demonstrated, [9][10][11] GDQDs pose a number of challenges when it comes to couple them coherently with light. The problems come from the lack of exciton confinement: while the electron states are confined, the hole states are not. Since in the creation of an exciton the spin of the photo-excited electron is always entangled with the one of the hole, discarding the hole-spin inevitably leads to decoherence of the electron spin. This limits considerably the possibility of optically controlling and manipulating spins in GDQDs, and it hinders their applicability in quantum communications.Despite these difficulties, first steps towards the goal of coherently coupling photons and electron spins in GDQDs have already been made, by trapping and detecting photo-generated carriers in GDQDs, 12 and by proving transfer of angular momentum between photons and electrons. 13 Much of this effort is motivated by the fact that robust spin-photon entanglement is a key requirement for quantum repeaters for long-distance quantum communications, 14 as well as for distributed quantum computing, where different computing nodes based on GDQD are connected by optical channels. 15 One strategy to avoid entanglement between the spins of the electron and the hole is to use g-factor engineering to obtain a much smaller g-factor for the electrons than for holes. [16][17][18] Here we propose a ...

Donor spins in silicon have achieved record values of coherence times and single-qubit gate fidelities. The next stage of development involves demonstrating high-fidelity two-qubit logic gates, where the most natural coupling is the exchange interaction. To aid the efficient design of scalable donor-based quantum processors, we model the two-electron wave function using a full configuration interaction method within a multi-valley effective mass theory. We exploit the high computational efficiency of our code to investigate the exchange interaction, valley population, and electron densities for two phosphorus donors in a wide range of lattice positions, orientations, and as a function of applied electric fields. The outcomes are visualized with interactive images where donor positions can be swept while watching the valley and orbital components evolve accordingly. Our results provide a physically intuitive and quantitatively accurate understanding of the placement and tuning criteria necessary to achieve high-fidelity two-qubit gates with donors in silicon.

The spins of atoms and atom-like systems are among the most coherent objects in which to store quantum information. However, the need to address them using oscillating magnetic fields hinders their integration with quantum electronic devices. Here, we circumvent this hurdle by operating a single-atom “flip-flop” qubit in silicon, where quantum information is encoded in the electron-nuclear states of a phosphorus donor. The qubit is controlled using local electric fields at microwave frequencies, produced within a metal-oxide-semiconductor device. The electrical drive is mediated by the modulation of the electron-nuclear hyperfine coupling, a method that can be extended to many other atomic and molecular systems and to the hyperpolarization of nuclear spin ensembles. These results pave the way to the construction of solid-state quantum processors where dense arrays of atoms can be controlled using only local electric fields.

Mechanical strain plays a key role in the physics and operation of nanoscale semiconductor systems, including quantum dots and single-dopant devices. Here, we describe the design of a nanoelectronic device, where a single nuclear spin is coherently controlled via nuclear acoustic resonance (NAR) through the local application of dynamical strain. The strain drives spin transitions by modulating the nuclear quadrupole interaction. We adopt an AlN piezoelectric actuator compatible with standard silicon metal–oxide–semiconductor processing and optimize the device layout to maximize the NAR drive. We predict NAR Rabi frequencies of order 200 Hz for a single 123Sb nucleus in a wide region of the device. Spin transitions driven directly by electric fields are suppressed in the center of the device, allowing the observation of pure NAR. Using electric field gradient-elastic tensors calculated by the density-functional theory, we extend our predictions to other high-spin group-V donors in silicon and to the isoelectronic 73Ge atom.

The spins of atoms and atom-like systems are among the most coherent objects in which to store quantum information. However, the need to address them using oscillating magnetic fields hinders their integration with quantum electronic devices. Here we circumvent this hurdle by operating a single-atom 'flip-flop' qubit in silicon, where quantum information is encoded in the electronnuclear states of a phosphorus donor. The qubit is controlled using local electric fields at microwave frequencies, produced within a metal-oxide-semiconductor device. The electrical drive is mediated by the modulation of the electron-nuclear hyperfine coupling, a method that can be extended to many other atomic and molecular systems. These results pave the way to the construction of solidstate quantum processors where dense arrays of atoms can be controlled using only local electric fields.