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arXiv:2108.13663 · Physical Review Letters

Preparation and Readout of Multielectron High-Spin States in a Gate-Defined GaAs/AlGaAs Quantum Dot

Haruki Kiyama, K. Yoshimi, Takeo Kato, Takashi Nakajima, A. Oiwa, Seigo Tarucha

Published Aug 20, 2021• 4 citations
arXivHTMLPDF

Abstract

We report the preparation and readout of multielectron high-spin states, a three-electron quartet, and a four-electron quintet, in a gate-defined GaAs/AlGaAs single quantum dot using spin filtering by quantum Hall edge states coupled to the dot. The readout scheme consists of mapping from multielectron to two-electron spin states and a subsequent two-electron spin readout, thus obviating the need to resolve dense multielectron energy levels. Using this technique, we measure the relaxations of the high-spin states and find them to be an order of magnitude faster than those of low-spin states. Numerical calculations of spin relaxation rates using the exact diagonalization method agree with the experiment. The technique developed here offers a new tool for the study and application of high-spin states in quantum dots.

DOI
10.1103/physrevlett.127.086802

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We report the preparation and readout of multielectron high-spin states, a three-electron quartet, and a four-electron quintet, in a gate-defined GaAs/AlGaAs single quantum dot using spin filtering by quantum Hall edge states coupled to the dot. The readout scheme consists of mapping from multielectron to two-electron spin states and a subsequent two-electron spin readout, thus obviating the need to resolve dense multielectron energy levels. Using this technique, we measure the relaxations of the high-spin states and find them to be an order of magnitude faster than those of low-spin states. Numerical calculations of spin relaxation rates using the exact diagonalization method agree with the experiment. The technique developed here offers a new tool for the study and application of high-spin states in quantum dots.
We investigate the spin of a multielectron GaAs quantum dot in a sequence of nine charge occupancies, by exchange coupling the multielectron dot to a neighboring two-electron double quantum dot.For all nine occupancies, we make use of a leakage spectroscopy technique to reconstruct the spectrum of spin states in the vicinity of the interdot charge transition between a single-and a multielectron quantum dot.In the same regime we also perform time-resolved measurements of coherent exchange oscillations between the singleand multielectron quantum dot.With these measurements, we identify distinct characteristics of the multielectron spin state, depending on whether the dot's occupancy is even or odd.For three out of four even occupancies, we do not observe any exchange interaction with the single quantum dot, indicating a spin-0 ground state.For the one remaining even occupancy, we observe an exchange interaction that we associate with a spin-1 multielectron quantum dot ground state.For all five of the odd occupancies, we observe an exchange interaction associated with a spin-1=2 ground state.For three of these odd occupancies, we clearly demonstrate that the exchange interaction changes sign in the vicinity of the charge transition.For one of these, the exchange interaction is negative (i.e., triplet preferring) beyond the interdot charge transition, consistent with the observed spin-1 for the next (even) occupancy.Our experimental results are interpreted through the use of a Hubbard model involving two orbitals of the multielectron quantum dot.Allowing for the spin correlation energy (i.e., including a term favoring Hund's rules) and different tunnel coupling to different orbitals, we qualitatively reproduce the measured exchange profiles for all occupancies.
Spin qubits in semiconductor quantum circuits have been identified as a compelling platform for large scale quantum computing. Today an ongoing effort of the community is aiming at the scaling of such systems by filling larger and larger arrays. In this context the development of tuning and readout protocols allowing to reduce the inherent complexity of such systems seems to be a necessity.In the first part of my thesis, I demonstrate array partitioning of a linear triple quantum dot defined in a AlGaAs/GaAs heterostructure via a highly tunable interdot tunnel coupling. This allows us to operate independently smaller sections of the array which reduces the number of available charge states for the system and therefore reduces its complexity during the tuning.The second part of my thesis is focused on reducing the hardware overhead necessary to extract information during the readout of the spin qubits. In semiconductor spin qubits, high fidelity read-out is obtained through Pauli spin blockade in a double quantum dot with two electrons probed by a local electrometer. One electron is the qubit, the second is the ancilla and only one bit of information is extracted. To obtain the full two-bit information of the two-electron spin states, more quantum hardware is necessary (either dots, electrometers, or reservoirs).Based on an original protocol, I develop in this thesis a complete single-shot read-out procedure for two-electron spin states in a double quantum dot discriminating between S, T0, T+ and T-. It uses successively three distinct spin-to-charge conversion protocols interleaved with charge configuration measurements. The strategy is based on four novel experimental achievements:First, the control from the GHz to sub-Hz regime of the interdot tunnel coupling at the nanosecond timescale.Second, the separation of the spin-to-charge conversion and the charge read-out in the voltage gate space.Third, the suppression of spin relaxation process during the read-out procedure.Finally, the repetition of three distinct spin-to-charge conversion protocols.The read-out procedure is tested on various initialized populations and implemented in more complex spin manipulation sequences to assess its validity.
We use a one-electron quantum dot as a spectroscopic probe to study the spin properties of a gate-controlled multielectron GaAs quantum dot at the transition between odd and even occupation numbers. We observe that the multielectron ground-state transitions from spin-1/2-like to singletlike to tripletlike as we increase the detuning towards the next higher charge state. The sign reversal in the inferred exchange energy persists at zero magnetic field, and the exchange strength is tunable by gate voltages and in-plane magnetic fields. Complementing spin leakage spectroscopy data, the inspection of coherent multielectron spin exchange oscillations provides further evidence for the sign reversal and, inferentially, for the importance of nontrivial multielectron spin exchange correlations.
Single-electron transistors (SETs) are attractive candidates for spin qubits. An AlGaAs/GaAs SET consists of a confined two-dimensional droplet of electrons, called an artificial atom or quantum dot, coupled by tunnel barriers to two conducting leads. Controlling the voltages on the lithographic gates that define the quantum dot allows us to confine a single electron in the dot, as well as to adjust the tunnel barriers to the leads. By applying a magnetic field, we can split the spin-up and spin-down states of the electron by an energy |g|μBB; the goal is to utilize coherent superpositions of these spin states to construct a qubit. We will discuss our attempts to observe electron spin resonance (ESR) in this system by applying magnetic fields at microwave frequencies. Observation of ESR would demonstrate that we can manipulate a single spin and allow us to measure the decoherence time T2*.
Multielectron spin qubits are demonstrated, and performance examined by comparing coherent exchange oscillations in coupled single-electron and multielectron quantum dots, measured in the same device. Fast (>1 GHz) exchange oscillations with a quality factor Q∼15 are found for the multielectron case, compared to Q∼2 for the single-electron case, the latter consistent with experiments in the literature. A model of dephasing that includes voltage and hyperfine noise is developed that is in good agreement with both single- and multielectron data, though in both cases additional exchange-independent dephasing is needed to obtain quantitative agreement across a broad parameter range.