The project focuses on providing an understanding of the interaction between a microscopic system (the electron spins) and a macroscopic system (the dot environment, control or measurement) using quantum theory. Concepts such as stochasticity arise out of the evolution of the combined system, as demonstrated by measured results of control and by experimental observation of decoherence and its reduction by active optical control. The research is at the frontier of applied quantum physics. It involves collaborators in the physics of semiconductor nanostructures, in high-precision coherent optical control and spectroscopy of quantum dots, and in many-body theory and light-matter interaction.
This grant supports the experimental and theoretical study of the basic physics underlying the laser manipulation of the electron spin in a quantum dot and transmission of quantum information to another dot via a single photon. The use of ultrafast lasers and single photon emission provides a much broader bandwidth for dot manipulation and information transmission than microwave. The power of the spin qubit comes from the superposition of its two states but the degree of quantum perfection of this superposition, known as "coherence", is fragile against the environment and the side effects of quantum operations. Nuclear spin fluctuations are the ultimate cause of electron spin decoherence after the removal of other causes of decoherence. A laser can add a second electron to make the qubit electron spin inert and a hole whose spin causes a small net alignment of the nuclear spins and reduces their fluctuations. A hole is the absence of an electron in the semiconductor ground state, in exact analogy with the antiparticle of the electron. Our theory to interpret the experiments showed the cause of the nuclear spin calming to be the existence of two collective nuclear states, normal and quiescent, and a nonlinear phenomenon driven feedback loop composed of the hole acting on the nuclear spins which in turn disturb the qubit state which is pumped by the laser to generate the hole (and the inert electron spin pair). The new ingredient is the hole-nuclear spin interaction, for without the hole spin, the process is fundamentally different. A microscopic theory was constructed to show that the control of the nuclear spins to narrow their fluctuations by optical pumping of the single electron is a quantum feedback loop from the electron to the nuclear spins via the interaction between the excited electron and the nuclei and back via the interaction between the ground state electron and the nuclei. It shows bistability of the nuclei with the narrow fluctuation nuclear state having more quantum nature than the thermal state. A hundredfold increase in the electron spin decoherence time is established by using two laser beams to form a superposition of the two electron spin states and using one of them or a third beam to measure the resultant state. The measurement (~microsecond) is limited by the signal to noise. The effect lasts longer than a 1200 msec, more than adequate for the requirements of a robust quantum computation. We also pursued the demonstration of a simple quantum gate in a single quantum dot. The two qubit state was defined by the clockwise or the counterclockwise spin rotation about a fixed magnetic field. A spin state about an axis of a fixed orientation away from the magnetic field axis is a superposition of these two qubit states. We demonstrated an operation of the qubit called a phase gate, setting the stage for demonstrating a two qubits, logic operation: a controlled-phase gate, fundamental to universal computation. This also prepares the quantum dot state leading to a deterministic entanglement between a photon and an electron spin (proposed by Sham and co-workers). We also made progress on our work on entangling two dots at a distance using measurement based entanglement. Success on this project will enable us to connect single or clusters of dots. The first step in this work is to demonstrate entanglement between the quantum dot spin and the photon. Intellectual Merit: The discovery of the use of laser light to disable the background nuclear spins’ ability to destroy the quantum coherence of the electron spin qubit is an important fundamental contribution towards quantum information processing by the optical control of the solid state quantum system. This method makes use of the slow recovery rate of the nuclear spins to thermal equilibrium to enable fast quantum operations of the qubits with a long quantum coherence period without continuing control of the nuclear background. This serves as a paradigm, in the Kuhn sense, of a general approach of separation of control of the environment from control of information. This is a markedly different approach from the current schemes that use extra dynamical control of the qubits to decouple them from the environmental influence. The byproduct of the control of the nuclear spins is the emerging quantum nature of the mesoscopic system of nuclear spins raising the possibility of information processing in an analog rather than in the digital mode. Broader Impact: In technology, the gates of a quantum computer may also serve as a trigger for metrology and control of larger devices. In education, both co-PIs were actively involved in developing new dimensions to graduate level courses in quantum physics, expected to be needed for the new generations of quantum scientists and technologists. In addition Steel at Michigan has been active in the NSF supported Imes-Moore program in Applied Physics.
