[Paper Review] Fast spin exchange between two distant quantum dots

Filip K. Malinowski, Frederico Martins+9 more|arXiv (Cornell University)|Aug 29, 2018

Quantum and electron transport phenomena32 references17 citations

TL;DR

This paper demonstrates fast, voltage-controlled spin exchange between two distant quantum dots mediated by a large multielectron quantum dot containing 50–100 electrons. By tuning gate voltages to control electron tunneling and exchange coupling, the authors achieve gigahertz-scale two-qubit interaction rates over micrometer distances, with tunable interaction sign and robustness to charge noise, enabling scalable spin qubit architectures.

ABSTRACT

The Heisenberg exchange interaction between neighboring quantum dots allows precise voltage control over spin dynamics, due to the ability to precisely control the overlap of orbital wavefunctions by gate electrodes. This allows the study of fundamental electronic phenomena and finds applications in quantum information processing. Although spin-based quantum circuits based on short-range exchange interactions are possible, the development of scalable, longer-range coupling schemes constitutes a critical challenge within the spin-qubit community. Approaches based on capacitative coupling and cavity-mediated interactions effectively couple spin qubits to the charge degree of freedom, making them susceptible to electrically-induced decoherence. The alternative is to extend the range of the Heisenberg exchange interaction by means of a quantum mediator. Here, we show that a multielectron quantum dot with 50-100 electrons serves as an excellent mediator, preserving speed and coherence of the resulting spin-spin coupling while providing several functionalities that are of practical importance. These include speed (mediated two-qubit rates up to several gigahertz), distance (of order of a micrometer), voltage control, possibility of sweet spot operation (reducing susceptibility to charge noise), and reversal of the interaction sign (useful for dynamical decoupling from noise).

Motivation & Objective

• Address the challenge of long-range spin-spin coupling in scalable spin-qubit quantum computing architectures.
• Overcome limitations of capacitive or cavity-mediated coupling, which are prone to charge noise and decoherence.
• Develop a robust, voltage-controllable, and fast spin exchange mechanism between distant quantum dots using a multielectron mediator.
• Enable dynamic control of interaction sign and operation at charge noise sweet spots for improved coherence.
• Demonstrate experimentally feasible, high-speed spin exchange in a scalable device architecture using standard gate-defined quantum dots.

Proposed method

• Use a linear array of five quantum dots defined in a GaAs two-dimensional electron gas via electrostatic gate electrodes.
• Employ a central multielectron dot (50–100 electrons) as a quantum mediator to enable spin exchange between two outer double quantum dots.
• Apply sub-microsecond voltage pulses to gate electrodes to control electron localization and induce exchange coupling via virtual tunneling through the mediator dot.
• Implement a sequence of steps: initialize double dots in singlet states, separate electrons to create reference spins, apply a pulse to induce exchange coupling $ J $, and measure spin correlations via spin-to-charge conversion.
• Use a phenomenological model for finite rise time of gate voltage pulses to simulate realistic oscillation patterns and correct for experimental distortions.
• Analyze the effective exchange interaction $ J_{\text{eff}} \approx \frac{2t_1^2}{\Delta_S} - \frac{t_2^2}{\Delta_T} $, where $ t_1, t_2 $ are tunneling matrix elements and $ \Delta_S, \Delta_T $ are energy cost terms for singlet and triplet states.

Experimental results

Research questions

• RQ1 Can a large multielectron quantum dot mediate fast and coherent spin exchange between two distant quantum dots?
• RQ2 How does the exchange coupling strength and sign depend on gate voltage tuning and electron occupation in the mediator dot?
• RQ3 To what extent do finite rise time effects in gate voltage pulses distort the observed spin oscillation patterns?
• RQ4 Can the interaction be tuned to be positive (singlet-favoring) or negative (triplet-favoring), and what are the implications for dynamical decoupling?
• RQ5 Is the mediated exchange interaction robust against charge noise, and can it be operated at a charge noise sweet spot?

Key findings

• The mediated two-qubit exchange interaction reaches rates up to several gigahertz, enabling fast quantum gate operations.
• Spin exchange is achieved over a distance of approximately one micrometer, significantly extending the range of coherent spin coupling.
• The effective exchange interaction $ J_{\text{eff}} $ can be tuned to be positive or negative by adjusting gate voltages, enabling dynamic decoupling from noise.
• A maximum in $ J_{\text{eff}} $ occurs near the charge transition between (1, 2N+1, 0)/(0, 2N+1, 1) and (0, 2N+2, 0), producing characteristic chevron patterns in oscillation data.
• Finite rise time of gate voltage pulses distorts the oscillation fringes, particularly at short interaction times (e.g., 2 ns), and is captured by a phenomenological model with $ \tau_0 = 0.8 $ ns.
• The system exhibits robustness to charge noise due to operation near a charge noise sweet spot, and the interaction remains coherent even at large inter-dot distances.