We introduce a solid-state qubit in which exchange interactions among confined electrons provide both the static longitudinal field and the oscillatory transverse field, allowing rapid and full qubit control via rf gate-voltage pulses. We demonstrate two-axis control at a detuning sweet spot, where leakage due to hyperfine coupling is suppressed by the large exchange gap. A π/2-gate time of 2.5 ns and a coherence time of 19 μs, using multipulse echo, are also demonstrated. Model calculations that include effects of hyperfine noise are in excellent quantitative agreement with experiment.

Quantum-dot spin qubits characteristically use oscillating magnetic or electric fields, or quasi-static Zeeman field gradients, to realize full qubit control. For the case of three confined electrons, exchange interaction between two pairs allows qubit rotation around two axes, hence full control, using only electrostatic gates. Here, we report initialization, full control, and single-shot readout of a three-electron exchange-driven spin qubit. Control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in less than 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements and non-orthogonal control axes.

We investigate the scaling of coherence time T(2) with the number of π pulses n(π) in a singlet-triplet spin qubit using Carr-Purcell-Meiboom-Gill (CPMG) and concatenated dynamical decoupling (CDD) pulse sequences. For an even numbers of CPMG pulses, we find a power law T(2) is proportional to (n(π))(γ(e)), with γ(e)=0.72±0.01, essentially independent of the envelope function used to extract T(2). From this surprisingly robust value, a power-law model of the noise spectrum of the environment, S(ω)~ω(-β), yields β=γ(e)/(1-γ(e))=2.6±0.1. Model values for T(2)(n(π)) using β=2.6 for CPMG with both even and odd n(π) up to 32 and CDD orders 3 through 6 compare very well with the experiment.

We present a modulated microwave approach for quantum computing with qubits comprising three spins in a triple quantum dot. This approach includes single-and two-qubit gates that are protected against low-frequency electrical noise, due to an operating point with a narrowband response to high frequency electric fields. Furthermore, existing double quantum dot advances, including robust preparation and measurement via spin-to-charge conversion, are immediately applicable to the new qubit. Finally, the electric dipole terms implicit in the high frequency coupling enable strong coupling with superconducting microwave resonators, leading to more robust two-qubit gates. PACS numbers:Spins in quantum dots as an architecture for quantum information processing require some combination of electric and magnetic field control at the nanometer scale [1]. While the intrinsic coherence properties of the spins can be remarkable, the need for such control inevitably couples the qubit degree of freedom to low-frequency electric or magnetic noise [2][3][4][5][6][7][8]. Approaches that mitigate this coupling, via dynamical decoupling or composite pulse sequences, all require rapid 'pulsed gate' control either for individual qubits or for two-qubit gates, which in turn requires wide bandwidths for the control electronics. While this has led to a variety of advances in the field, paradoxically it also leads to the use of quantum bits as sensors, rather than as protected devices [9].Instead, we suggest that the use of so-called exchangeonly qubits [10,11], comprising three spins in a triple quantum dot [12][13][14][15][16][17] and implemented experimentally [18][19][20][21], provide an opportunity for protection against low-frequency control noise in analogy to advances in superconducting devices [22]. In particular, by having exchange couplings always on, a regime with no low-frequency field response and a narrowband, resonant response becomes accessible. We denote this the resonant exchange (RX) qubit, and refer the reader to the concurrent Ref. [23] for an experimental demonstration of these ideas. Furthermore, our approach has a protected two-qubit interaction via exchange [24,25] or via resonant dipole-dipole interactions. As coupling between qubits relies on electric fields rather than tunneling, devices could be implemented in a wide variety of potential materials such as two dimensional electron gas and nanowire depletion dots. Finally, we show that the dipolar nature of the RX qubit also enables strong coupling with high quality factor microwave cavities.The few electron regime of interest for our triple dot system we describe by the Hubbard model [15](1) where U is the individual dot charging energy, U c is the cross-charging energy, V i is the local potential set by applied gate voltages on dot i, t ij is the tunneling between dots i and j, and c † iσ is the creation operator for an electron on dot i with spin σ. For simplicity, we assume a linear array and set t 12 = t l , t 23 = t r , t 13 = 0, and have defined tunneling...

Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported, with times down to 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point contact sensor, and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced screening and smaller characteristic energy needed to change transmission. PACS numbers:Experiments on few-electron quantum dots [1], including spin qubits, have benefitted in recent years from the use of proximal charge sensing, a technique that allows the number and arrangement of charges confined in nanostructures to be measured via changes in conductance of a nearby sensor to which the device of interest is capacitively coupled [2,3]. Quantum point contacts (QPCs) have been widely used as charge sensors, allowing, for instance, high-fidelity single-shot readout of spin qubits via spin-to-charge conversion [4,5]. Single electron transistors (SETs) based on metallic tunnel junctions, and gate defined sensor quantum dots (SQD), conceptually equivalent to SETs, have also been widely used as proximal sensors, and provide similar sensitivity and bandwidth [6][7][8][9]. As a typical application, measuring the state of a spin qubit via spin-to-charge conversion involves determining whether two electrons in a double quantum dot are in the (1, 1) or the (0, 2) charge configuration, where (left, right) denotes occupancies in the double dot [ Fig. 1(a)], on time scales faster than the spin relaxation time [5].In this Communication, we demonstrate the use of a sensor quantum dot for fast charge and two-electron spinstate measurement in a GaAs double quantum dot, biased near the (1,1)-(0,2) charge transition. We compare the performance of the SQD to conventional quantum point contact (QPC) sensors for dc and radio-frequency (rf) measurement. We find experimentally that the SQD is up to 30 times more sensitive, and provides roughly three times the signal to noise ratio (SNR) of a comparable QPC sensor for detecting the charge arrangement and spin state of a double quantum dot. Numerical simulations, also presented, give results consistent with experiment and elucidate the role of screening in determining the sensitivity of these proximal charge sensors.Double quantum dots with integrated sensors are defined by Ti/Au depletion gates on a GaAs/Al 0.3 Ga 0.7 As heterostructure with a two-dimensional electron gas (density 2 × 10 15 m −2 , mobility 20 m 2 /Vs) 100 nm be-low the surface. The charge state of the double quantum dot is controlled by gate voltages V L , V R [see Fig. 1(a)]. Three gates next to the right dot form the SQD, which is operated in the multi-electron Coulomb blockade (CB) regime, with center gate voltage V D setting the SQD energy. A single gate next to ...