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...

Genome signatures in metagenomic datasets

Genome signatures are used to identify and cluster sequences de novo from an acid biofilm microbial community metagenomic dataset, revealing information about the low-abundance community members.

Quantum memories capable of storing and retrieving coherent information for extended times at room temperature would enable a host of new technologies. Electron and nuclear spin qubits using shallow neutral donors in semiconductors have been studied extensively but are limited to low temperatures (≲10 kelvin); however, the nuclear spins of ionized donors have the potential for high-temperature operation. We used optical methods and dynamical decoupling to realize this potential for an ensemble of phosphorous-31 donors in isotopically purified silicon-28 and observed a room-temperature coherence time of over 39 minutes. We further showed that a coherent spin superposition can be cycled from 4.2 kelvin to room temperature and back, and we report a cryogenic coherence time of 3 hours in the same system.