A Two Qubit Logic Gate in Silicon (1411.5760v1)

Abstract: Quantum computation requires qubits that can be coupled and realized in a scalable manner, together with universal and high-fidelity one- and two-qubit logic gates \cite{DiVincenzo2000, Loss1998}. Strong effort across several fields have led to an impressive array of qubit realizations, including trapped ions \cite{Brown2011}, superconducting circuits \cite{Barends2014}, single photons\cite{Kok2007}, single defects or atoms in diamond \cite{Waldherr2014, Dolde2014} and silicon \cite{Muhonen2014}, and semiconductor quantum dots \cite{Veldhorst2014}, all with single qubit fidelities exceeding the stringent thresholds required for fault-tolerant quantum computing \cite{Fowler2012}. Despite this, high-fidelity two-qubit gates in the solid-state that can be manufactured using standard lithographic techniques have so far been limited to superconducting qubits \cite{Barends2014}, as semiconductor systems have suffered from difficulties in coupling qubits and dephasing \cite{Nowack2011, Brunner2011, Shulman2012}. Here, we show that these issues can be eliminated altogether using single spins in isotopically enriched silicon\cite{Itoh2014} by demonstrating single- and two-qubit operations in a quantum dot system using the exchange interaction, as envisaged in the original Loss-DiVincenzo proposal \cite{Loss1998}. We realize CNOT gates via either controlled rotation (CROT) or controlled phase (CZ) operations combined with single-qubit operations. Direct gate-voltage control provides single-qubit addressability, together with a switchable exchange interaction that is employed in the two-qubit CZ gate. The speed of the two-qubit CZ operations is controlled electrically via the detuning energy and we find that over 100 two-qubit gates can be performed within a two-qubit coherence time of 8 \textmu s, thereby satisfying the criteria required for scalable quantum computation.

• The paper introduces a method for implementing a high-fidelity two-qubit gate in silicon using quantum dot exchange interactions.
• It employs isotopically enriched 28Si to suppress decoherence and enables over 25 CZ oscillations within an 8µs coherence window.
• The approach aligns with CMOS manufacturing processes, paving the way for scalable and fault-tolerant quantum computing architectures.

A Two Qubit Logic Gate in Silicon

The paper "A Two Qubit Logic Gate in Silicon" presents advancements in the realization of a two-qubit logic gate utilizing quantum dot systems in silicon. This work addresses critical challenges in quantum computing related to the scalability, fidelity, and control of qubit operations, providing insights into methods for implementing high-fidelity two-qubit gates in a solid-state environment compatible with standard lithographic techniques.

Overview and Methodology

The primary focus of the paper is the implementation of a Controlled-NOT (CNOT) gate using silicon-based quantum dots. Silicon is chosen due to its minimal nuclear spin background, which significantly enhances coherence times, thus overcoming previous limitations experienced in other semiconductor materials like GaAs, which suffer from dephasing due to natural nuclear spins. The authors demonstrate a two-qubit system wherein single and two-qubit gate operations are realized using the exchange interaction between two electron spins confined in quantum dots.

A critical component of the experimental setup is the use of isotopically enriched $^{28}$Si epilayers, which contain a reduced presence of $^{29}$Si, thereby further minimizing decoherence. This platform allows for individual qubit control via gate-voltage adjustments that tune the qubit resonance frequency, and qubit coupling using the exchange interaction, which can be turned on and off by electrical gating. The employed techniques permit the realization of a CZ gate, combining it with single-qubit rotations to accomplish a universal set of quantum gates necessary for scalable quantum computation.

Numerical Results and Claims

The paper claims over 100 CZ operations can be executed within the two-qubit coherence time of 8 \textmu s, which represents a significant step towards meeting the criteria required for scalable quantum computing. The demonstration of over 25 CZ oscillations is noted, with coherence times being measured and optimized to achieve an operation frequency of approximately 3.14 MHz, indicating potential gate fidelities above 99%.

Implications and Future Directions

This research not only illustrates the feasibility of silicon-based quantum dots for building scalable quantum computers but also highlights the compatibility of the fabrication process with existing semiconductor manufacturing technologies. From a theoretical standpoint, it solidifies the utility of quantum dot systems in implementing fault-tolerant quantum operations, potentially reducing the complexity of quantum error correction.

Future advancements could focus on refining the exchange coupling to minimize sensitivity to electrical noise further or developing integrated structures that support simultaneous initializations and measurements of multiple qubits. Additionally, expanding this approach to larger qubit networks could address some challenges of large-scale quantum processors.

The work paves the way for significant advancements in silicon-based quantum computing, suggesting that leveraging existing CMOS technologies could accelerate the development of practical quantum systems.