Exchange-only singlet-only spin qubit

Abstract

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...

Transport-induced suppression of nuclear field fluctuations in multi-quantum-dot systems

“…Due to the way the dynamics of P M depend on P Σ [see Eq. (38)] one expects that the longtime stable polarization of P M is not at zero but at a small positive value; a careful look at Fig. 8(d,iii) shows that this is indeed the case in our simulations.…”

“…II D for the double dot. We started with initial polarizations P LR (0) = 0.001, P M (0) = 0.002, and 37) and (38). In both plots we also included the (same) vector field (dPM /dt, dPLR/dt), represented by the black arrows.…”

“…However, silicon comes with the complication of the extra valley degree of freedom [2], which is hard to control [33][34][35] and provides an extra channel for leakage and dephasing [36,37]. (ii) It is possible to encode the qubit in a four-electron singlet-only subspace [38][39][40], which makes it intrinsically insensitive to the fluctuating nuclear fields. This, however, presents significant complications for device design and tuning.…”

Highly tunable exchange-only singlet-only qubit in a GaAs triple quantum dot

“…There have been several proposals put forward to increase the coherence time of quantum-dot-based XO qubits while retaining their conceptual simplicity and ease of manipulation. Of special interest are (i ) proposals to suppress the effects of charge noise and electronphonon interaction, via a symmetric operation of the qubit or operating at a sweet spot (SS) [16,[25][26][27], and (ii ) proposals to reduce magnetic noise or suppress its effects, either by isotope purification or by constructing decoherence-free qubit subspaces [8,[28][29][30][31].…”

“…In the exchange-only singlet-only (XOSO) spin qubit proposed in Ref. [30], the leading effects of magnetic noise are suppressed by encoding the qubit states in a fourelectron singlet-only subspace, while electric noise can be mitigated by operating the system symmetrically at a SS. However, the exceptionally long coherence time of the qubit comes at the cost of an increase in device complexity (a quadruple quantum dot in a T-geometry) and the proposal suffers from the common problem with XO qubits of having a relatively small qubit splitting.…”