Quantum computation requires qubits that can be coupled and realized in a scalable manner, together with universal and high-fidelity one-and two-qubit logic gates [1, 2]. Strong effort across several fields have led to an impressive array of qubit realizations, including trapped ions [4], superconducting circuits [5], single photons [3], single defects or atoms in diamond [6,7] and silicon [8], and semiconductor quantum dots [4], all with single qubit fidelities exceeding the stringent thresholds required for fault-tolerant quantum computing [10]. Despite this, high-fidelity two-qubit gates in the solid-state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits [5], as semiconductor systems have suffered from difficulties in coupling qubits and dephasing [11][12][13]. Here, we show that these issues can be eliminated altogether using single spins in isotopically enriched silicon [14] by demonstrating single-and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the original Loss-DiVincenzo proposal [2]. We realize CNOT gates via either controlled rotation (CROT) or controlled phase (CZ) operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is employed in the two-qubit CZ gate. The speed of the two-qubit CZ operations is controlled electrically via the detuning energy and we find that over 100 two-qubit gates can be performed within a two-qubit coherence time of 8 µs, thereby satisfying the criteria required for scalable quantum computation.Quantum dots have high potential as a qubit platform [2]. Large arrays can be conveniently realized using conventional lithographic approaches, while reading, initializing, controlling and coupling can be done purely by electrical means. Early research focussed mainly on III-V semiconductor compounds such as GaAs, resulting in single spin qubits [15], singlet-triplet qubits [16] and exchange only qubits [17], which can be coupled capacitively [11] or via the exchange interaction [12,13]. While these approaches demonstrate the potential of quantum dot qubits, strong dephasing due to the nuclear spin background have limited the quality of the quantum operations. A strong improvement in coherence times has been observed by defining the quantum dots in silicon [18,19], which can be isotopically purified [14], such that quantum dots with single spin fidelities above the threshold of surface codes [10] can be realized [4].A scalable approach towards quantum computation ideally requires that the coupling between qubits can be turned on and off [1], so that single and two-qubit operations can be selectively chosen. Here, we demonstrate this by realizing a CZ gate, which is commonly used in superconducting qubits [5] and has been theoretically discussed for quantum dot systems [20]. This two-qubit gate, together with single-qubit gates provides all of the necessary opera...

Universal quantum computation will require qubit technology based on a scalable platform, together with quantum error correction protocols that place strict limits on the maximum infidelities for one-and two-qubit gate operations 1,2 . While a variety of qubit systems have shown high fidelities at the one-qubit level 3-9 , superconductor technologies have been the only solidstate qubits manufactured via standard lithographic techniques which have demonstrated twoqubit fidelities near the fault-tolerant threshold 5 . Silicon-based quantum dot qubits are also amenable to large-scale manufacture and can achieve high single-qubit gate fidelities (exceeding 99.9 %) using isotopically enriched silicon 10-12 . However, while two-qubit gates have been demonstrated in silicon 13-15 , it has not yet been possible to rigorously assess their fidelities using randomized benchmarking, since this requires sequences of significant numbers of qubit operations ( 20) to be completed with non-vanishing fidelity. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80 % to 89 %, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7 % and average Controlled-ROT (CROT) fidelity of 98.0 %. These fidelities are found to be limited by the relatively slow gate times employed here compared with the decoherence times T * 2 of the qubits. Silicon qubit designs employing fast gate operations based on high Rabi frequencies 16-18 , together with advanced pulsing techniques 19 , should therefore enable significantly higher fidelities in the near future.Silicon provides an ideal environment for spin qubits thanks to its compatibility with industrial manufacturing technologies and the near-perfect nuclear-spin vacuum that isotopically enriched 28 Si provides 10,11 . Qubits can be encoded directly on the spins of individual nuclei, donor-bound electrons, or electrons confined in gatedefined quantum dots, or they can be encoded in subspaces provided by two or more spins 12 . Electrostatic gate electrodes allow initialization, readout 23 and, in some cases, manipulation of qubits 24 to be implemented with local electrical pulses. For qubits encoded on single spins, one-qubit gates can be driven using an AC magnetic field to perform electron spin resonance (ESR) directly 8,25 , through an AC electric field produced by a gate electrode combined with the magnetic field gradient from an on-chip micro-magnet 16,17,26 , or with an AC electric field acting on the spin-orbit field 27-29 . In enriched 28 Si devices such one-qubit gates have attained fidelities of 99.9 % or above 18,30,31 .Two-qubit gates, required to complete the universal gate set, are commonly implemented in spin systems as the √ SW AP 24,32 , the C-Phase 13,14 or the CROT 13,15 . While the √ SW AP and the C-Phase gates require fast temporal control of the exchange interaction J, accurately synchronized with spin resonance pulses, the CROT can also be implemented wit...

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).