[Paper Review] Spin Manipulation of Free 2-Dimensional Electrons in Si/SiGe Quantum Wells

Alexei M. Tyryshkin, S. A. Lyon+2 more|arXiv (Cornell University)|Apr 11, 2003

Quantum and electron transport phenomena1 references3 citations

TL;DR

This paper demonstrates coherent spin manipulation of free 2D electrons in Si/SiGe quantum wells using resonant microwave pulses and a magnetic field in a pulsed electron paramagnetic resonance setup. The key result is a spin coherence time of approximately 3 microseconds, enabling at least 100 quantum operations before decoherence, marking a critical step toward semiconductor-based spin quantum computing with practical constraints on system design.

ABSTRACT

An important requirement for a physical embodiment of a quantum computer is that arbitrary single-qubit operations can be performed. In the case of spin-qubits, this means that arbitrary spin rotations must be possible. Here we demonstrate spin rotations of an ensemble of free 2-dimensional electrons confined to a silicon quantum well embedded in a silicon-germanium alloy host. The spins are manipulated by resonant microwave pulses and an applied magnetic field in a pulsed electron paramagnetic resonance spectrometer. From the pulsed measurements we deduce a spin coherence time in this system of about 3 microsec, allowing at least 100 elementary operations before decoherence destroys the spin state. These measurements represent an important step towards the realization of quantum computation using electron spins in semiconductors, but at the same time establish some constraints on the design of such a system.

Motivation & Objective

• To achieve coherent control of electron spins in a 2D electron gas within Si/SiGe quantum wells for quantum computing applications.
• To assess the feasibility of using free 2D electrons in Si-based heterostructures as robust qubits.
• To measure spin coherence times and evaluate the number of possible quantum operations before decoherence.
• To identify design constraints for scalable spin-qubit architectures based on experimental limitations.

Proposed method

• Employed a pulsed electron paramagnetic resonance (EPR) spectrometer to apply resonant microwave pulses and a static magnetic field.
• Confined free 2D electrons in a silicon quantum well embedded in a SiGe alloy matrix.
• Used microwave pulses to induce Rabi oscillations, enabling controlled spin rotations.
• Measured spin relaxation and dephasing times via time-resolved EPR techniques.
• Analyzed the system's coherence by observing Rabi oscillations and fitting decay envelopes.
• Determined spin coherence time (T2) from the decay of coherent spin oscillations in the time domain.

Experimental results

Research questions

• RQ1 Can arbitrary single-qubit operations be performed on electron spins in Si/SiGe quantum wells using microwave pulses?
• RQ2 What is the spin coherence time (T2) of free 2D electrons in this system, and how many quantum operations can be performed before decoherence?
• RQ3 What are the dominant decoherence mechanisms limiting spin control in Si/SiGe heterostructures?
• RQ4 How does the spin coherence time compare to theoretical limits and other semiconductor spin systems?
• RQ5 What practical constraints does this system impose on the design of scalable spin-based quantum computers?

Key findings

• The spin coherence time (T2) for free 2D electrons in Si/SiGe quantum wells was measured to be approximately 3 microseconds.
• This coherence time supports at least 100 elementary quantum operations before decoherence destroys the qubit state.
• Rabi oscillations were observed, confirming the ability to perform coherent spin rotations using resonant microwave pulses.
• The system exhibits sufficient coherence for implementing arbitrary single-qubit operations required in quantum computation.
• The results indicate that Si/SiGe heterostructures are viable candidates for spin qubits, though decoherence mechanisms impose design constraints.
• The measured T2 is limited by spin dephasing, suggesting that further material optimization is needed to extend coherence times.