Although silicon is a promising material for quantum computation, the degeneracy of the conduction band minima (valleys) must be lifted with a splitting sufficient to ensure the formation of well-defined and long-lived spin qubits. Here we demonstrate that valley separation can be accurately tuned via electrostatic gate control in a metal-oxidesemiconductor quantum dot, providing splittings spanning 0.3-0.8 meV. The splitting varies linearly with applied electric field, with a ratio in agreement with atomistic tight-binding predictions. We demonstrate single-shot spin read-out and measure the spin relaxation for different valley configurations and dot occupancies, finding one-electron lifetimes exceeding 2 s. Spin relaxation occurs via phonon emission due to spin-orbit coupling between the valley states, a process not previously anticipated for silicon quantum dots. An analytical theory describes the magnetic field dependence of the relaxation rate, including the presence of a dramatic rate enhancement (or hot-spot) when Zeeman and valley splittings coincide.

Electron spins in silicon quantum dots provide a promising route towards realising the large number of coupled qubits required for a useful quantum processor [1][2][3][4][5][6][7][8]. At present, the requisite single-shot spin qubit measurements are performed using on-chip charge sensors, capacitively coupled to the quantum dots. However, as the number of qubits is increased, this approach becomes impractical due to the footprint and complexity of the charge sensors, combined with the required proximity to the quantum dots [5]. Alternatively, the spin state can be measured directly by detecting the complex impedance of spin-dependent electron tunnelling between quantum dots [9][10][11]. This can be achieved using radio-frequency reflectometry on a single gate electrode defining the quantum dot itself [11][12][13][14][15], significantly reducing gate count and architectural complexity, but thus far it has not been possible to achieve single-shot spin readout using this technique. Here, we detect single electron tunnelling in a double quantum dot and demonstrate that gate-based sensing can be used to read out the electron spin state in a single shot, with an average readout fidelity of 73%. The result demonstrates a key step towards the readout of many spin qubits in parallel, using a compact gate design that will be needed for a large-scale semiconductor quantum processor.

Nanoscale single-electron pumps can be used to generate accurate currents, and can potentially serve to realize a new standard of electrical current based on elementary charge. Here, we use a silicon-based quantum dot with tunable tunnel barriers as an accurate source of quantized current. The charge transfer accuracy of our pump can be dramatically enhanced by controlling the electrostatic confinement of the dot using purposely engineered gate electrodes. Improvements in the operational robustness, as well as suppression of non-adiabatic transitions that reduce pumping accuracy, are achieved via small adjustments of the gate voltages. We can produce an output current in excess of 80 pA with experimentally determined relative uncertainty below 50 parts per million.As early as one and a half centuries ago, J. C. Maxwell envisaged the need for a system of standards based on phenomena at the atomic scale and directly related to invariant constants of nature. 1 However, Maxwell could not anticipate that, in order to harness the behaviour of the world at the nanometer scale, a completely new physical interpretation was needed, namely, quantum mechanics. At first, the laws of quantum mechanics seemed to reveal fundamental limits to the accuracy of physical measurements. Concepts like the Heisenberg uncertainty principle, which imposes intrinsic fluctuations on the values of non-commuting observables, and the wavefunction collapse, responsible for the randomization of a system configuration after performing a measurement, appeared to be at odds with the requirement of deterministic consistency that is paramount for metrological purposes. Nevertheless, quantum-based systems are today acknowledged as the most stable and reliable metrological tools, as they can be strongly intertwined with fundamental constants. Exquisitely quantum-mechanical phenomena such as the ac Josephson effect 2 and the quantum Hall effect 3 have paved the way towards new and more reliable reference standards for the units of voltage and resistance, respectively.Major efforts are currently ongoing to re-define the unit of electrical current, the ampere (A), in terms of the elementary charge, e, by means of quantum technologies 4,5 . A practical implementation of this standard may be the electron pump, a device in which a quantum phenomenon, namely tunnelling, and classical Coulomb repulsion, are combined to control the transfer of an integer number of elementary charges. This device ideally generates a quantized output current, I P = nef , where n is an integer and f is the frequency of an external periodic drive. Several enabling technologies have already been developed including metal/oxide tunnel barrier devices 6,7 , normal-metal/superconductor turnstiles 8,9 , graphene double quantum dots 10 , donor-based pumps 11-13 , silicon-based quantum dot pumps 14-18 and GaAs-based quantum dot pumps [19][20][21][22][23][24][25][26][27] . To date, the latter scheme provides the lowest uncertainty of 1.2 parts per million (ppm) yielding current in excess o...

Franson's Bell experiment with energy-time entanglement [Phys. Rev. Lett. 62, 2205(1989 does not rule out all local hidden variable models. This defect can be exploited to compromise the security of Bell inequality-based quantum cryptography. We introduce a novel Bell experiment using genuine energy-time entanglement, based on a novel interferometer, which rules out all local hidden variable models. The scheme is feasible with actual technology.PACS numbers: 03.65. Ud, 03.65.Ta, 03.67.Mn, 42.50.Xa Two particles exhibit "energy-time entanglement" when they are emitted at the same time in an energyconserving process and the essential uncertainty in the time of emission makes undistinguishable two alternative paths that the particles can take. Despite this fundamental deficiency, and despite that this defect can be exploited to create a Trojan horse attack in Bell inequality-based quantum cryptography [6], Franson-type experiments have been extensively used for Bell tests and Bell inequality-based quantum cryptography [7], have become standard in quantum optics [8,9], and an extended belief is that "the results of experiments with the Franson experiment violate Bell's inequalities" [9]. This is particularly surprising, given that recent research has emphasized the fundamental role of a (loophole-free) violation of the Bell inequalities in proving the device-independent security of key distribution protocols [10], and in detecting entanglement [11].Polarization entanglement can be transformed into energy-time entanglement [12]. However, to our knowledge, there is no single experiment showing a violation of the Bell-CHSH inequality using genuine energy-time entanglement (or "time-bin entanglement" [13]) that cannot be simulated by a LHV model. By "genuine" we mean not obtained by transforming a previous form of entanglement, but created because the essential uncertainty in the time of emission makes two alternative paths undistinguishable.Because of the above reasons, a single experiment using energy-time entanglement able to rule out all possible LHV models is of particular interest. The aim of this Letter is to describe such an experiment by means of a novel interferometric scheme. The main purpose of the new scheme is not to compete with existing interferometers used for quantum communication in terms of practical usability, but to fix a fundamental defect common to all of them.We will first describe the Franson Bell-CHSH experiment. Then, we will introduce a LHV model reproducing any conceivable violation of the Bell-CHSH inequality. The model underlines why a Franson-type experiment does not and cannot be used to violate local realism. Then, we will introduce a new two-photon energy-time Bell-CHSH experiment that avoids these problems and can be used for a conclusive Bell test.The Franson Bell-CHSH experiment.-The setup of a Franson Bell-CHSH experiment is in Fig. 1. The source emits two photons, photon 1 to the left and photon 2 to the right. Each of them is fed into an unbalanced interferometer. BS i are be...