The size of silicon transistors used in microelectronic devices is shrinking to the level where quantum effects become important 1 . While this presents a significant challenge for the further scaling of microprocessors, it provides the potential for radical innovations in the form of spin-based quantum computers 2-4 and spintronic devices 5 . An electron spin in Si can represent a well-isolated quantum bit with long coherence times 6 because of the weak spin-orbit coupling 7 and the possibility to eliminate nuclear spins from the bulk crystal 8 . However, the control of single electrons in Si has proved challenging, and has so far hindered the observation and manipulation of a single spin. Here we report the first demonstration of single-shot, time-resolved readout of an electron spin in Si. This has been performed in a device consisting of implanted phosphorus donors 9 coupled to a metal-oxide-semiconductor single-electron transistor 10,11 -compatible with current microelectronic technology. We observed a spin lifetime approaching 1 second at magnetic fields below 2 T, and achieved spin readout fidelity better than 90%. High-fidelity single-shot spin readout in Si opens the path to the development of a new generation of quantum computing and spintronic devices, built using the most important material in the semiconductor industry.The projective, single-shot readout of a qubit is a crucial step in both circuit-based and measurement-based quantum computers 12 . For electron spins in solid state, this has only been achieved in GaAs/AlGaAs quantum dots coupled to charge detectors 13-15 . The spin readout was achieved utilizing spin-dependent tunnelling, in which the electron was displaced to a different location depending on its spin state. The charge detector, electrostatically coupled to the electron site, sensed whether the charge had been displaced, thereby determining the spin state. Here we apply a novel approach to charge sensing, where the detector is not only electrostatically coupled, but also tunnel-coupled to the electron site 11 , as shown in Fig. 1a. As a charge detector we employ here the silicon single-electron transistor 10 (SET), a nonlinear nanoelectronic device consisting of a small island of electrons tunnel-coupled to source and drain reservoirs, electrostatically induced beneath an insulating SiO 2 layer. A current can flow from source to drain only when the electrochemical potential of the island assumes specific values 16 , resulting in a characteristic pattern of sharp current peaks as a function of gate voltage (Fig. 1e). The shift in electrochemical potential arising from the tunnelling of a single electron from a nearby charge centre into the SET island is large enough to switch the current from zero to its maximum value. This tunnelling event becomes spin-dependent in the presence of a large magnetic field, when the spin-up state | ↑ has a higher energy than the spin-down state | ↓ , by an amount larger than the thermal and electromagnetic broadening of electron states in the SET isla...

A single atom is the prototypical quantum system, and a natural candidate for a quantum bit, or qubit--the elementary unit of a quantum computer. Atoms have been successfully used to store and process quantum information in electromagnetic traps, as well as in diamond through the use of the nitrogen-vacancy-centre point defect. Solid-state electrical devices possess great potential to scale up such demonstrations from few-qubit control to larger-scale quantum processors. Coherent control of spin qubits has been achieved in lithographically defined double quantum dots in both GaAs (refs 3-5) and Si (ref. 6). However, it is a formidable challenge to combine the electrical measurement capabilities of engineered nanostructures with the benefits inherent in atomic spin qubits. Here we demonstrate the coherent manipulation of an individual electron spin qubit bound to a phosphorus donor atom in natural silicon, measured electrically via single-shot read-out. We use electron spin resonance to drive Rabi oscillations, and a Hahn echo pulse sequence reveals a spin coherence time exceeding 200 µs. This time should be even longer in isotopically enriched (28)Si samples. Combined with a device architecture that is compatible with modern integrated circuit technology, the electron spin of a single phosphorus atom in silicon should be an excellent platform on which to build a scalable quantum computer.

Quantum computers have the potential to revolutionize aspects of modern society from fundamental science and medical research 1-3 to data analysis 4,5 . The successful demonstration of such a machine depends on the ability to perform high-fidelity control and measurement of individual qubits 6 -the building blocks of a quantum computer. Errors introduced by quantum operations and measurements can be mitigated by employing quantum error correction protocols 7 , provided that the probabilities of the

No abstract

Quantum computers are expected to outperform conventional computers for a range of important problems, from molecular simulation to search algorithms, once they can be scaled up to large numbers of quantum bits (qubits), typically millions [1][2][3]. For most solid-state qubit technologies, e.g. those using superconducting circuits or semiconductor spins, scaling poses a significant challenge as every additional qubit increases the heat generated, while the cooling power of dilution refrigerators is severely limited at their operating temperature below 100 mK [4][5][6]. Here we demonstrate operation of a scalable silicon quantum processor unit cell, comprising two qubits confined to quantum dots (QDs) at ∼1.5 Kelvin. We achieve this by isolating the QDs from the electron reservoir, initialising and reading the qubits solely via tunnelling of electrons between the two QDs [7-9]. We coherently control the qubits using electrically-driven spin resonance (EDSR) [10,11] in isotopically enriched silicon 28 Si [12], attaining single-qubit gate fidelities of 98.6% and coherence time T * 2 = 2 µs during 'hot' operation, comparable to those of spin qubits in natural silicon at millikelvin temperatures [13][14][15][16]. Furthermore, we show that the unit cell can be operated at magnetic fields as low as 0.1 T, corresponding to a qubit control frequency of 3.5 GHz, where the qubit energy is well below the thermal energy. The unit cell constitutes the core building block of a full-scale silicon quantum computer, and satisfies layout constraints required by error correction architectures [8,17]. Our work indicates that a spin-based quantum computer could be operated at elevated temperatures in a simple pumped 4 He system, offering orders of magnitude higher cooling power than dilution refrigerators, potentially enabling classical control electronics to be integrated with the qubit array [18,19].Electrostatically gated QDs in Si/SiGe or Si/SiO 2 heterostructures are prime candidates for spin-based quantum computing due to their long coherence times, high control fidelities, and industrial manufacturability [13,14,[20][21][22][23]. In large scale quantum processors the qubits will be arranged in either 1D chains [17] or 2D arrays [3] to enable quantum error correction schemes. For architectures relying on exchange coupling for twoqubit operation [15,16,24,25], the QDs are expected to be densely packed. Until now, two-qubit QD systems have been tunnel-coupled to a nearby charge reservoir that has typically been used for initialisation and readout using spin-to-charge conversion [26]. Here we demonstrate an isolated double QD system that requires no tunnel-coupled reservoir [7-9] to perform full two-qubit initialisation, control and readout -thus realising the elementary unit cell of a scalable quantum processor (see Figure 1h).

Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling. However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits. Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator. This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator. Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures. In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature. Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices. In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.

We focus on a recently experimentally realized scenario of normal-metal-insulator-superconductor tunnel junctions coupled to a superconducting resonator. We develop a first-principles theory to describe the effect of photon-assisted electron tunneling on the quantum state of the resonator. Our results are in very good quantitative agreement with the previous experiments on refrigeration and heating of the resonator using the photon-assisted tunneling, thus providing a stringent verification of the developed theory. Importantly, our results provide simple analytical estimates of the voltage-tunable coupling strength and temperature of the thermal reservoir formed by the photon-assisted tunneling. Consequently, they are used to introduce optimization principles for initialization of quantum devices using such a quantum-circuit refrigerator. Thanks to the first-principles nature of our approach, extension of the theory to the full spectrum of quantum electric devices seems plausible.

We have developed nanoscale double-gated field-effect-transistors for the study of electron states and transport properties of single deliberately implanted phosphorus donors. The devices provide a high-level of control of key parameters required for potential applications in nanoelectronics. For the donors, we resolve transitions corresponding to two charge states successively occupied by spin down and spin up electrons. The charging energies and the Lande g-factors are consistent with expectations for donors in gated nanostructures.