We propose a feasible and scalable quantum-dot-based implementation of a singlet-only spin qubit which is to leading order intrinsically insensitive to random effective magnetic fields set up by fluctuating nuclear spins in the host semiconductor. Our proposal thus removes an important obstacle for further improvement of spin qubits hosted in high-quality III-V semiconductors such as GaAs. We show how the resulting qubit could be initialized, manipulated, and read out by electrical means only, in a way very similar to a triple-dot exchange-only spin qubit. Due to the intrinsic elimination of the effective nuclear fields from the qubit Hamiltonian, we find an improvement of the dephasing time T * 2 of several orders of magnitude as compared to similar existing spin qubits.Spin qubits in semiconductor quantum dots are one of the more promising scalable qubit implementations put forward so far [1]. The original proposal almost two decades ago [2] was rapidly followed by early experimental successes, including demonstration of the principles of qubit initialization, manipulation, and read-out [3, 4]. At the same time, two main challenges for further progress were identified: (i) Single-qubit manipulation requires highly localized oscillating magnetic fields, which are very hard to realize in practice. (ii) All high-quality III-V semiconductors (such as GaAs) consist of atoms carrying non-zero nuclear spin, and the fluctuating ensemble of nuclear spins in each quantum dot couples to the spin of localized electrons through hyperfine interaction. This coupling causes spin relaxation [5] and yields random effective local magnetic fields acting on the electron spins, which present an important source of qubit decoherence [6, 7]. Most of the work in the field of semiconductor spin qubits in the past decades has been aimed at overcoming these two challenges.One proposed way to overcome the requirement of oscillating magnetic fields is to use a material with relatively strong spin-orbit interaction (such as InAs), in which coherent spin rotations could be achieved by the application of oscillating electric fields [8][9][10]. A drawback is that spin-orbit interaction contributes to qubit relaxation [11] and also interferes with the spin-to-charge conversion commonly used for qubit initialization and read-out [12]. Another approach is to encode the qubit in a multi -electron spin state, which enables qubit control through (gate-tunable) exchange interactions [13]: Using two-electron spin states in a double quantum dot, one can define a qubit in the unpolarized singlet-triplet (S-T 0 ) subspace, which allows for electrical control of qubit rotations along one axis of the Bloch sphere [14,15]; and recently it was realized that with one more quantum dot (and electron) one can use two three-electron spin states to define a qubit that has two of such control axes [16]. The resulting triple-dot exchange-only (XO) qubit can thus be fully operated by electrical means only [16][17][18]. The downside of using exchange-operated spin qu...

We devise a method to shortcut the adiabatic evolution of a spin-1 Bose gas with an external magnetic field as the control parameter. An initial many-body state with almost all bosons populating the Zeeman sublevel m = 0, is evolved to a final state very close to a macroscopic spin-singlet condensate, a fragmented state with three macroscopically occupied Zeeman states. The shortcut protocol, obtained by an approximate mapping to a harmonic oscillator Hamiltonian, is compared to linear and exponential variations of the control parameter. We find a dramatic speedup of the dynamics when using the shortcut protocol.

Exchange-only spin qubits hosted in 28 Si-based triple quantum dots do not suffer from decoherence caused by randomly fluctuating nuclear-spin ensembles and can be relatively robust against electrical noise when operated at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage out of the qubit subspace, and dephasing due to residual effects of charge noise, the latter two of which are the focus of this work. We investigate spin-orbit-mediated leakage rates to the three-spin ground state accompanied by virtual (i) tunneling, (ii) orbital excitation, and (iii) valley excitation of an electron. We find different power-law dependencies on the applied magnetic field B for the three mechanisms as well as for the two leakage rates, ranging from ∝ B 5 to ∝ B 11 , and identify the sweet spot as a point of minimal leakage. We also revisit the role of electrical noise at the sweet spot, and show that it causes a decay of coherent qubit oscillations that follows a power law ∝ 1/t (as opposed to the more common exponential decay) and introduces a π/2 phase shift.

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We propose an implementation of a singlet-only spin qubit in a GaAs-based triple quantum dot with a (1,4,1) charge occupation. In the central multi-electron dot, the interplay between Coulomb interaction and an out-of-plane magnetic field creates an energy spectrum with a tunable singlettriplet splitting, which can be exploited to create a six-particle singlet-only qubit with a qubit splitting that can straightforwardly be tuned over tens of µeV by adjusting the external magnetic field. We confirm the full exchange-based electric control of the qubit and demonstrate its superior coherence properties due to its singlet-only nature. arXiv:1911.08345v1 [cond-mat.mes-hall]