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arXiv:1611.04945 · npj Quantum Information

Extending the coherence of a quantum dot hybrid qubit

Brandur Thorgrimsson, Dohun Kim, Yuan‐Chi Yang, Luke W. Smith, C. B. Simmons, Daniel R. Ward, Ryan H. Foote, J. Corrigan, D. E. Savage, M. G. Lagally, Mark G. Friesen, S. N. Coppersmith, M. A. Eriksson

Published Aug 21, 2017• 88 citations
arXivHTMLPDF

Abstract

Abstract Identifying and ameliorating dominant sources of decoherence are important steps in understanding and improving quantum systems. Here, we show that the free induction decay time ( $$T_2^*$$ T2* ) and the Rabi decay rate (ΓRabi) of the quantum dot hybrid qubit can be increased by more than an order of magnitude by appropriate tuning of the qubit parameters and operating points. By operating in the spin-like regime of this qubit, and choosing parameters that increase the qubit’s resilience to charge noise (which we show is presently the limiting noise source for this qubit), we achieve a Ramsey decay time $$T_2^*$$ T2* of 177 ns and a Rabi decay time 1/ΓRabi exceeding 1 μs. We find that the slowest ΓRabi is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.

DOI
10.1038/s41534-017-0034-2

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Abstract Identifying and ameliorating dominant sources of decoherence are important steps in understanding and improving quantum systems. Here, we show that the free induction decay time ( T2∗T_2^*T2∗​ T2* ) and the Rabi decay rate (ΓRabi) of the quantum dot hybrid qubit can be increased by more than an order of magnitude by appropriate tuning of the qubit parameters and operating points. By operating in the spin-like regime of this qubit, and choosing parameters that increase the qubit’s resilience to charge noise (which we show is presently the limiting noise source for this qubit), we achieve a Ramsey decay time T2∗T_2^*T2∗​ T2* of 177 ns and a Rabi decay time 1/ΓRabi exceeding 1 μs. We find that the slowest ΓRabi is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.
Identifying dominant sources of decoherence is an important step in understanding and improving quantum systems. Here we show that the free induction decay time (T2∗T_{2}^{*}T2∗​) and the Rabi decay rate (ΓRabi\Gamma_{\mathrm{Rabi}}ΓRabi​) of the quantum dot hybrid qubit are limited by charge noise for a large range of detunings. We show that by tuning the parameters of the qubit, and by operating the qubit at larger detunings, the coherence times can be increased by more than an order of magnitude. We achieve a Ramsey decay time T2∗T_{2}^{*}T2∗​ of 127 ns127~\mathrm{ns}127 ns and a Rabi decay time, 1/ΓRabi1/\Gamma_{\mathrm{Rabi}}1/ΓRabi​, exceeding 1 μs1~\mathrm{\mu s}1 μs. We show that the slowest ΓRabi\Gamma_{\mathrm{Rabi}}ΓRabi​ is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.
Abstract The unavoidable effect of the environmental noise due to nuclear spins and charge traps is included in the study of the hybrid qubit dynamics. Hybrid qubit owes its name to the advantageous combination of manipulation speed of a charge qubit with the longevity of a spin qubit. It consists of three electrons confined through external gate voltages in a double quantum dot and deserves special interest in quantum computation applications due to its advantages in terms of fabrication, control, and only electrical manipulation. The fluctuations of the global magnetic field do not affect the dynamics of the hybrid qubit that is defined in a decoherence‐free subspace. The main sources of decoherence come from local magnetic fluctuations and charge fluctuations. This work is based on the hypothesis that gate voltages are fixed and kept constant while qubit evolves in time. Coherence time of the hybrid qubit is extracted when model parameters take values achievable experimentally in the nuclear‐free isotope 28 Si, natural Si, and GaAs hosts.
Fast quantum oscillations of a charge qubit in a double quantum dot fabricated in a Si/SiGe heterostructure are demonstrated and characterized experimentally. The measured inhomogeneous dephasing time T2∗{T}_{2}^{*}T2∗​ ranges from 127 ps to 2.1 ns; it depends substantially on how the energy difference of the two qubit states varies with external voltages, consistent with a decoherence process that is dominated by detuning noise (charge noise that changes the asymmetry of the qubit's double-well potential). In the regime with the shortest T2∗{T}_{2}^{*}T2∗​, applying a charge-echo pulse sequence increases the measured inhomogeneous decoherence time from 127 to 760 ps, demonstrating that low-frequency noise processes are an important dephasing mechanism.
Many quantum dot qubits operate in regimes where the energy splittings between qubit states are large and phonons can be the dominant source of decoherence. The recently proposed charge quadrupole qubit, based on one electron in a triple quantum dot, employs a highly symmetric charge distribution to suppress the influence of long-wavelength charge noise. To study the effects of phonons on the charge quadrupole qubit, we consider Larmor and Ramsey pulse sequences to identify favorable operating parameters. We show that phonon-induced decoherence increases with the qubit frequency, in contrast to the effects of charge noise. We also show that there is an optimum value of the tunnel coupling of the qubit at which the decohering effects of phonons and charge noise are small enough to be consistent with single qubit gate fidelities > 99.99%.
Maturing hybrid circuit quantum electrodynamics architectures composed of semiconductor quantum dots and on-chip microwave resonators promise an effective approach for quantum nondemolition readouts in semiconductor systems. Here, we study quantum coherence in a semiconductor charge qubit coupled with a high-impedance resonator. The qubit is controlled by a periodic pulse sequence, and a dispersive measurement is performed by measuring the time-averaged resonator response. From the Rabi oscillation and Ramsey fringe measurements, the coherence time of the charge qubit is determined as T2, Rabi ≈2.7 ns and T2, Ramsey ≈3.5 ns. Compared with the time-resolved (pulsed) measurement, this time-averaged dispersive measurement process eliminates the limitation caused by the resonator response time, enabling a high repetition rate of the pulse sequence in the experiments.
Abstract We analyze in detail a procedure of entangling of two singlet–triplet (S– T0T_{0}T0​ T 0 ) qubits operated in a regime when energy associated with the magnetic field gradient, ΔBz\Delta B_{z}ΔBz​ Δ B z , is an order of magnitude smaller than the exchange energy, J, between singlet and triplet states (Shulman et al. in Science 336:202, 2012). We have studied theoretically a single S– T0T_{0}T0​ T 0 qubit in free induction decay and spin echo experiments. We have obtained analytical expressions for the time dependence of components of its Bloch vector for quasistatic fluctuations of ΔBz\Delta B_{z}ΔBz​ Δ B z and quasistatic or dynamical 1/fβ1/f^{\beta }1/fβ 1 / f β -type fluctuations of J. We have then considered the impact of fluctuations of these parameters on the efficiency of the entangling procedure which uses an Ising-type coupling between two S– T0T_{0}T0​ T 0 qubits. In particular, we have obtained an analytical expression for evolution of two qubits affected by 1/fβ1/f^{\beta }1/fβ 1 / f β -type fluctuations of J. This expression indicates the maximal level of entanglement that can be generated by performing the entangling procedure. Our results deliver also an evidence that in the above-mentioned experiment S– T0T_{0}T0​ T 0 qubits were affected by uncorrelated 1/fβ1/f^{\beta }1/fβ 1 / f β charge noises.