Tunable Spin-Qubit Coupling Mediated by a Multielectron Quantum Dot

Abstract

We present an approach for entangling electron spin qubits localized on spatially separated impurity atoms or quantum dots via a multi-electron, two-level quantum dot. The effective exchange interaction mediated by the dot can be understood as the simplest manifestation of RudermanKittel-Kasuya-Yosida exchange, and can be manipulated through gate voltage control of level splittings and tunneling amplitudes within the system. This provides both a high degree of tuneability and a means for realizing high-fidelity two-qubit gates between spatially separated spins, yielding an experimentally accessible method of coupling donor electron spins in silicon via a hybrid impurity-dot system.Single spins in solid-state systems represent versatile candidates for scalable quantum bits (qubits) in quantum information processing architectures [1][2][3][4][5][6]. In many proposals involving single-spin qubits localized on impurity atoms [2,7] and within quantum dots [1,8], two-qubit coupling schemes harness the advantages of tunnelingbased nearest-neighbor exchange interactions: exchange gates are rapid, tunable, and protected against multiple types of noise [9][10][11][12][13]. These features have been demonstrated for electron spins in quantum dots [14][15][16][17], while a similar demonstration for spins localized on impurity atoms such as phosphorus donors in silicon remains an outstanding experimental challenge [6]. Although the exchange interaction originates from the longrange Coulomb interaction, its strength typically decays exponentially with distance [8,18]. Long-range coupling via concatenation of multiple nearest-neighbor interactions is not ideal for coupling spatially separated electron spins, as it sets a low threshold error rate below which fault-tolerant quantum computing is feasible [19,20]. A mechanism for long-range coupling that simultaneously enables scalability and robustness against errors is therefore key to realizing practical spin-based quantum information processing devices.Approaches to implementing long-range interactions typically involve identifying a system that acts as a mediator of the interaction between the qubits, with proposed systems including optical cavities and microwave stripline resonators [21][22][23][24][25][26] [32,33], and multi-electron molecular cores [34]. Recently, long-range coupling of electrons located in the two outer quantum dots of a linear triple dot system has been demonstrated [35,36]. The effective exchange interaction in that system arises from electron cotunneling between the outer dots and exhibits the fourth-order dependence on tunneling amplitudes that is characteristic of superexchange [37], but suffers from a large virtual energy cost from the doubly occupied center dot states. In contrast, a many-electron quantum dot in the center can also couple distant spins via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, with low-energy intermediate states [38], but perhaps at the cost of low fidelity as impurityFermi sea correlations become hard to disentangle...

Negative Spin Exchange in a Multielectron Quantum Dot

“…In particular, using a large multielectron quantum dot as an exchange mediator has the potential to do both: provide fast spin interaction [30,31] and alleviate spatial control line crowding. To avoid entanglement with internal degrees of freedom of the mediator, recent theory [30,31] motivates the use of a multielectron quantum dot with a spinless ground state and a level spacing sufficiently large to suppress unwanted excitations by gate voltage pulses.In this Letter, we investigate a GaAs multielectron quantum dot and show that its spin properties make it suitable for use as a coherent spin mediator. The experiment involves a chain of three quantum dots that can be detuned relative to each other using top-gate voltage pulses.…”

“…Even though the exchange interaction is intrinsically short-ranged, its range can be increased by means of a quantum mediator [28,29]. In particular, using a large multielectron quantum dot as an exchange mediator has the potential to do both: provide fast spin interaction [30,31] and alleviate spatial control line crowding. To avoid entanglement with internal degrees of freedom of the mediator, recent theory [30,31] motivates the use of a multielectron quantum dot with a spinless ground state and a level spacing sufficiently large to suppress unwanted excitations by gate voltage pulses.…”

Self-Assembled Wigner Crystals as Mediators of Spin Currents and Quantum Information

“…Particularly we show how this setting can be used to transfer spin qubits between two quantum dots separated by µm scales, which is currently being actively considered as an important problem, with very few suggested solutions [24][25][26][27]. Additionally we suggest a feasible way of observing this phenomenon through pA scale currents, which, in turn, opens up a new option in low dissipation spintronics for a spin current without a charge current.…”

“…Compared to varied proposals for mediating between separated quantum dot spin qubits [24][25][26][27], the proposed mechanism can have various advantages. For example, it could have higher speed making it more resilient to decoherence, simpler fabrication compared to hybrid systems, and a greater spatial extension over few electron quantum dot mediators.…”

Fast long-distance control of spin qubits by photon-assisted cotunneling

“…Indeed, in contrast to standard electrostatic tuning of dot levels by gates [40], PACT allows one to tune to any virtual state [41][42][43], not just the lowest one. Using Floquet theory, we find that cotunneling amplitudes get boosted near such resonances.…”