Quadrupolar Exchange-Only Spin Qubit

Abstract

We propose a quadrupolar exchange-only spin (QUEX) qubit that is highly robust against charge noise and nuclear spin dephasing, the dominant decoherence mechanisms in quantum dots. The qubit consists of four electrons trapped in three quantum dots, and operates in a decoherencefree subspace to mitigate dephasing due to nuclear spins. To reduce sensitivity to charge noise, the qubit can be completely operated at an extended charge noise sweet spot that is first-order insensitive to electrical fluctuations. Due to on-site exchange mediated by the Coulomb interaction, the qubit energy splitting is electrically controllable and can amount to several GHz even in the "off" configuration, making it compatible with conventional microwave cavities. arXiv:1807.10471v1 [cond-mat.mes-hall] 27 Jul 2018 H eff =     J 0,0J0,1J0,2 0

Resonant exchange operation in triple-quantum-dot qubits for spin–photon transduction

“…As g, A J , and κ are further optimized, other effects not considered here may also impact the fidelity, such as excitation into higher valley and orbital states during the transition between the TQD qubit and XRX states. While the calculations here have focused on transverse three-electron dipolar coupling, our methodology naturally extends to considering higher-order couplings, qubits at higher electron number, and longitudinal effects, each of which may be of interest for novel approaches to dot-photon coupling [46,47,48] and/or readout [42]. Numerical simulation of more realistic wave functions and device operation may offer new quantitative guidelines or operating insights for these applications as well.…”

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

“…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 Ref. [31] it was pointed out that one can implement the same qubit in a Si-based triple dot, where the on-site singlet-triplet splitting is typically set by the valley splitting, which can be 20-200 µeV. The drawback of this proposal is that (i ) the magnitude of the valley splitting is hard to control or predict in practice [2] and (ii ) uncontrollable phase differences between valley couplings on different dots can severely affect the exchange effects used to define and operate the qubit [32].…”

“…On a mean-field level, the effect of this interaction can be described by the Hamiltonian H hf = 1 2 gµ B i K i · σ i , with K i a random effective nuclear field acting on electron i, typically of the order or a few mT. In the device we propose in this paper, both qubit states are singlets and therefore the qubit splitting is not directly influenced by any intrinsic or external (gradient) of Zeeman fields acting on the electrons, thereby reducing the hyperfine-induced decoherence dramatically [30,31]. The dominating remaining effect of the nuclear fields is the coupling of the qubit states to nearby triplet states, which leads to random higher-order shifts of the qubit levels [30].…”