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Fault-tolerant quantum computation requires qubit measurements to be both high fidelity and fast to ensure that idling qubits do not generate more errors during the measurement of ancilla qubits than can be corrected. Towards this goal, we demonstrate single-shot readout of semiconductor spin qubits with 97% fidelity in 1.5 μs. In particular, we show that we can engineer donor-based single-electron transistors (SETs) in silicon with atomic precision to measure single spins much faster than the spin decoherence times in isotopically purified silicon (270 μs). By designing the SET to have a large capacitive coupling between the SET and target charge, we can optimally operate in the "strong-response" regime to ensure maximal signal contrast. We demonstrate single-charge detection with a signal-to-noise ratio (SNR) of 12.7 at 10 MHz bandwidth, corresponding to a SET charge sensitivity (integration time for SNR ¼ 2) of 2.5 ns. We present a theory of the shot-noise sensitivity limit for the strong-response regime which predicts that the present sensitivity is about one order of magnitude above the shot-noise limit. By reducing cold amplification noise to reach the shot-noise limit, it should be theoretically possible to achieve high-fidelity, single-shot readout of an electron spin in silicon with a total readout time of approximately 36 ns.

Substitutional donor atoms in silicon are promising qubits for quantum computation with extremely long relaxation and dephasing times demonstrated. One of the critical challenges of scaling these systems is determining inter-donor distances to achieve controllable wavefunction overlap while at the same time performing high fidelity spin readout on each qubit. Here we achieve such a device by means of scanning tunnelling microscopy lithography. We measure anti-correlated spin states between two donor-based spin qubits in silicon separated by 16 ± 1 nm. By utilising an asymmetric system with two phosphorus donors at one qubit site and one on the other (2P−1P), we demonstrate that the exchange interaction can be turned on and off via electrical control of two in-plane phosphorus doped detuning gates. We determine the tunnel coupling between the 2P−1P system to be 200 MHz and provide a roadmap for the observation of two-electron coherent exchange oscillations.

100s 21 a) Value calculated using the noise spectrum (Equation (1)); b) A transition is observed between α = 1 and α = 2; c) Device implanted with P donors; d) Spin-orbit qubit; e) Singlet-triplet qubit formed between Si-MOS quantum dot and a P donor.

In this work we perform direct single-shot readout of the singlet-triplet states in exchange coupled electrons confined to precision-placed donor atoms in silicon. Our method takes advantage of the large energy splitting given by the Pauli-spin blockaded (2,0) triplet states, from which we can achieve a singleshot readout fidelity of 98.4 AE 0.2%. We measure the triplet-minus relaxation time to be of the order 3 s at 2.5 T and observe its predicted decrease as a function of magnetic field, reaching 0.5 s at 1 T. DOI: 10.1103/PhysRevLett.119.046802 An increased ability to control and manipulate quantum systems is driving the field of quantum computation forward [1][2][3][4]. The spin of a single electron in the solid state has long been utilized in this context [5][6][7][8][9][10][11], providing a superbly clean quantum system with two orthogonal quantum states that can be measured with over 99% fidelity [12]. As a natural next step, the coupling of two electrons at separate sites has been studied in gate-defined quantum dots [5,13,14], as well as in donor systems [15][16][17]. In addition to being the eigenstates for two coupled spins, the singlet-triplet (ST) states of two electrons can form a qubit subspace, and have previously been utilized for quantum information processing [6,[18][19][20][21][22]. Unlike in gate-defined quantum dots, donor systems do not require electrodes to confine electrons. The resulting decrease in physical complexity makes donor nanodevices very appealing for scaling up to many electron sites [15].In the (1,1) charge configuration the ST states are eigenstates if the exchange coupling is greater than any difference in Zeeman energy between the two spins. The singlet and three triplet states are split only by the Zeeman energy in the cases of jT þ i ¼ j↑↑i and jT − i ¼ j↓↓i, and an exchange energy, J, for the singlet jSi ¼ ðj↑↓i − j↓↑iÞ= ffiffi ffi 2 p and jT 0 i ¼ ðj↑↓i þ j↓↑iÞ= ffiffi ffi 2 p states. However, in the (2,0) configuration all triplet states split from the singlet jSð2; 0Þi by a larger exchange interaction, Δ ST , measured in previous works to be > 5 meV for donors [23]. The triplet states are therefore blocked from tunneling from the ð1; 1Þ → ð2; 0Þ charge configuration, known as Pauli spin blockade.Typically, direct ST readout is performed by charge discrimination between the (1,1) and (2,0) states below the ST energy splitting Δ ST . However, this relies on the charge sensor having a large enough differential capacitive coupling to each dot to discriminate between the two charge states. This is not possible in some architectures due to symmetry constraints, in particular, for donors it is advantageous for multiple donor sites to be coupled equally to a charge sensor for independent readout and/or loading. The tightly confined electron wave function at each donor site therefore necessitates that they are equidistant from the charge sensor. As a consequence ð1; 1Þ ↔ ð2; 0Þ charge transfer signals are often too small to detect directly in this architecture.Until now s...

Quantum dot quantum computing architectures rely on systems in which inversion symmetry is broken, and spin-orbit coupling is present, causing even single-spin qubits to be susceptible to charge noise. We derive an effective Hamiltonian for the combined action of noise and spin-orbit coupling on a single-spin qubit, identify the mechanisms behind dephasing, and estimate the free induction decay dephasing times T * 2 for common materials such as Si and GaAs. Dephasing is driven by noise matrix elements that cause relative fluctuations between orbital levels, which are dominated by screened whole charge defects and unscreened dipole defects in the substrate. Dephasing times T * 2 differ markedly between materials, and can be enhanced by increasing gate fields, choosing materials with weak spin-orbit, making dots narrower, or using accumulation dots. arXiv:1408.4123v2 [cond-mat.mes-hall]

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