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).

We describe quantum wires and point contacts fabricated in GaAs/AlxGa1−xAs heterostructures that are free of the disorder introduced by modulation doping and in which the electron density and the confining potential are separately adjustable by lithographically defined gates. We observe conductance plateaus quantized near even multiples of e2/h in 2 μm wires and up to 15 conductance steps in 5 μm wires at temperatures below 1 K. Near the conductance threshold the quantum point contact and the 2 μm wire both show additional structure below 2e2/h.

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices however requires atomic scale manipulation -an immense technological challenge. We demonstrate that it is possible to fabricate an atomically-precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a siliconbased quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth. (To appear in Phys. Rev. B Rapid Comm.) 03.67. Lx, 68.37.Ef, A quantum bit (or qubit) is a two level quantum system that is the building block of a quantum computer. To date the most advanced realisations of a quantum computer are qubit ion trap 1 and nuclear magnetic resonance 2-4 systems. However scaling these systems to large numbers of qubits will be difficult 5 , making solidstate architectures 6 , with their promise of scalability, important. In 1998 Kane proposed a novel solid state quantum computer design 7 using phosphorus 31 P nuclei (nuclear spin I = 1/2) as the qubits in isotopically-pure silicon 28 Si (I = 0). The device architecture is shown in Fig. 1a, with phosphorus qubits embedded in silicon approximately 20 nm apart. This separation allows the donor electron wavefunctions to overlap, whilst an insulating barrier isolates them from the surface control A and J gates. These A and J gates control the hyperfine interaction between the nuclear and electron spins and the coupling between adjacent donor electrons respectively. For a detailed description of the computer operation refer to Kane 7 . An alternative strategy using the electron spins of the phosphorus donors as qubits has also been proposed 8 .One of the major challenges of this design is to reliably fabricate an atomically-precise array of phosphorus nuclei in silicon -a feat that has yet to be achieved in a semiconductor system. Whilst a scanning tunnelling microscope (STM) tip has been used for atomic scale arrangement of metal atoms on metal surfaces 9 , rearrangement of individual atoms in a semiconductor system is not straightforward due to the strong covalent bonds involved. As a result, we have employed a hydrogen resist strategy outlined in Fig. 1b. Here the array is fabricated using a resist technology, much like in conventional lithography, where the resist is a layer of hydrogen atoms that terminate the silicon surface. An STM tip is used to selectively desorb individual hydrogen atoms, exposing the underlying silicon surface in the required array. STM induced hydrogen desorption has been developed and refined over the past ten years 10 and has been proposed 11 for the assembly of atomically-ordered device structure...

Zero length quantum wires (or point contacts) exhibit unexplained conductance structure close to 0.7 × 2e 2 /h in the absence of an applied magnetic field. We have studied the density-and temperature-dependent conductance of ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l=0 and 2µm, fabricated from structures free of the disorder associated with modulation doping. In a direct comparison we observe structure near 0.7 × 2e 2 /h for l=0 whereas the l = 2µm wires show structure evolving with increasing electron density to 0.5 × 2e 2 /h in zero magnetic field, the value expected for an ideal spin-split sub-band. Our results suggest the dominant mechanism through which electrons interact can be strongly affected by the length of the 1D region.73.61.-r, 73.23.Ad, 73.61.Ey III-V

We present a method for measuring single spins embedded in a solid by probing two-electron systems with a single-electron transistor ͑SET͒. Restrictions imposed by the Pauli principle on allowed two-electron states mean that the spin state of such systems has a profound impact on the orbital states ͑positions͒ of the electrons, a parameter which SET's are extremely well suited to measure. We focus on a particular system capable of being fabricated with current technology: a Te double donor in Si adjacent to a Si/SiO 2 interface and lying directly beneath the SET island electrode, and we outline a measurement strategy capable of resolving singleelectron and nuclear spins in this system. We discuss the limitations of the measurement imposed by spin scattering arising from fluctuations emanating from the SET and from lattice phonons. We conclude that measurement of single spins, a necessary requirement for several proposed quantum computer architectures, is feasible in Si using this strategy.

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 describe the conditional and unconditional dynamics of two coupled quantum dots when one dot is subjected to a measurement of its occupation number by coupling it to a third readout dot via the Coulomb interaction. The readout dot is coupled to source and drain leads under weak bias, and a tunnel current flows through a single bound state when energetically allowed. The occupation of the quantum dot near the readout dot shifts the bound state of the readout dot from a low conducting state to a high conducting state. The measurement is made by continuously monitoring the tunnel current through the readout dot. We show that there is a difference between the time scale for the measurement-induced decoherence between the localized states of the dots, and the time scale on which the system becomes localized due to the measurement.

Once the periodic properties of elements were unveiled, chemical bonds could be understood in terms of the valence of atoms. Ideally, this rationale would extend to quantum dots, often termed artificial atoms, and quantum computation could be performed by merely controlling the outer-shell electrons of dot-based qubits. Imperfections in the semiconductor material, including at the atomic scale, disrupt this analogy between atoms and quantum dots, so that real devices seldom display such a systematic many-electron arrangement. We demonstrate here an electrostaticallydefined quantum dot that is robust to disorder, revealing a well defined shell structure. We observe four shells (31 electrons) with multiplicities given by spin and valley degrees of freedom. We explore various fillings consisting of a single valence electron -namely 1, 5, 13 and 25 electrons -as potential qubits, and we identify fillings that yield a total spin-1 on the dot. An integrated micromagnet allows us to perform electrically-driven spin resonance (EDSR). Higher shell states are shown to be more susceptible to the driving field, leading to faster Rabi rotations of the qubit. We investigate the impact of orbital excitations of the p-and d-shell electrons on single qubits as a function of the dot deformation. This allows us to tune the dot excitation spectrum and exploit it for faster qubit control. Furthermore, hotspots arising from this tunable energy level structure provide a pathway towards fast spin initialisation. The observation of spin-1 states may be exploited in the future to study symmetry-protected topological states in antiferromagnetic spin chains and their application to quantum computing.Qubit architectures based on electron spins in gatedefined silicon quantum dots benefit from a high level of controllability, where single and multi-qubit coherent operations are realised solely with electrical and magnetic manipulation. Furthermore, their direct compatibility with silicon microelectronics fabrication offers unique scale-up opportunities 1 . However, fabrication reproducibility and disorder pose challenges for single electron quantum dots. Even when the single-electron regime is achievable, the last electron often is confined in a very small region, limiting the effectiveness of electrical control and interdot tunnel coupling. Many-electron quantum dots were proposed as a qubit platform decades ago 2 , with the potential of resilience to charge noise 3,4 and a more tunable tunnel coupling strength to other qubits 5 . In the multielectron regime, the operation of a quantum dot qubit is more sensitive to its shape. If it is axially symmetric, the orbital energy levels will be quasi-degenerate 6-8 , which is detrimental for quantum computing. On the contrary, if the quantum dot is very elongated, a regular shell structure will not form, and the valence electron will not operate as a simple spin-1/2 system 2,9 . FILLING S-, P-, D-AND F-ORBITALS IN A SILICON QUANTUM DOTThe scanning electron microscope (SEM) image in Fig. 1a shows a s...