Quantum Physics Experiments Research Articles

Repeatable measurements in quantum theory: Their role and feasibility

Recent advantages in experimental quantum physics call for a careful reconsideration of the measurement process in quantum mechanics. In this paper we describe the structure of the ideal measurements and their status among the repeatable measurements. Then we provide an exhaustive account of the interrelations between repeatability and the apparently weaker notions of value reproducible or first- kind measurements. We demonstrate the close link between repeatable measurements and discrete observables and show how the ensuing measurement limitations for continuous observables can be lifted in a way that is in full accordance with actual experimental practice. We present examples of almost repeatable measurements of continuous observables and some realistic models of weakly disturbing measurements.

Twenty years of molecular beam epitaxy

The term “molecular beam epitaxy” (MBE) was first used in one of our crystal growth papers in 1970, after having conducted extensive surface physics studies in the late 1960's of the interaction of atomic and molecular beams with solid surfaces. The unique feature of MBE is the ability to prepare single crystal layers with atomic dimensional precision. MBE sets the standard for epitaxial growth and has made possible semiconductor structures that could not be fabricated with either naturally existing materials or by other crystal growth techniques. MBE led the crystal growth technologies when it prepared the first semiconductor quantum well and superlattice structures that gave unexpected and exciting electrical and optical properties. For example, the discovery of the fractional quantized Hall effect. It brought experimental quantum physics to the classroom, and practically all major universities throughout the world are now equipped with MBE systems. The fundamental principles demonstrated by the MBE growth of III–V compound semiconductors have also been applied to the growth of group IV, II–VI, metal, and insulating materials. For manufacturing, the most important criteria are uniformity, precise control of the device structure, and reproducibility. MBE has produced more lasers (3 to 5 million per month for compact disc application) than any other crystal growth technique in the world. New directions for MBE are to incorporate in-situ, real-time monitoring capabilities so that complex structures can be precisely “engineered”. In the future, as environmental concerns increase, the use of toxic arsine and phosphine may be limited. Successful use of valved cracker cells for solid arsenic and phosphorus has already produced InP based injection lasers.

Twenty years of molecular beam epitaxy

The term “molecular beam epitaxy” (MBE) was first used in one of our crystal growth papers in 1970, after having conducted extensive surface physics studies in the late 1960's of the interaction of atomic and molecular beams with solid surfaces. The unique feature of MBE is the ability to prepare single crystal layers with atomic dimensional precision. MBE sets the standard for epitaxial growth and has made possible semiconductor structures that could not be fabricated with either naturally existing materials or by other crystal growth techniques. MBE led the crystal growth technologies when it prepared the first semiconductor quantum well and superlattice structures that gave unexpected and exciting electrical and optical properties. For example, the discovery of the fractional quantized Hall effect. It brought experimental quantum physics to the classroom, and practically all major universities throughout the world are now equipped with MBE systems. The fundamental principles demonstrated by the MBE growth of III–V compound semiconductors have also been applied to the growth of group IV, II–VI, metal, and insulating materials. For manufacturing, the most important criteria are uniformity, precise control of the device structure, and reproducibility. MBE has produced more lasers (3 to 5 million per month for compact disc application) than any other crystal growth technique in the world. New directions for MBE are to incorporate in-situ, real-time monitoring capabilities so that complex structures can be precisely “engineered”. In the future, as environmental concerns increase, the use of toxic arsine and phosphine may be limited. Successful use of valved cracker cells for solid arsenic and phosphorus has already produced InP based injection lasers.

Numerical experiments in quantum physics: Finite-element method

A new numerical approach (finite-element method) to solving one-dimensional Schrödinger problems is discussed. Its effectiveness is demonstrated in solving the nontrivial finite well and Morse potential problems. Moreover, it is shown by a case study of 16 low-lying vibrational states of Li2 that the finite-element method can achieve the same order of accuracy more easily than the better known Numerov–Cooley method.

Quantum counting processes

It is proposed that counting experiments in quantum physics should be analyzed in terms of point processes (QPP) defined in the framework of quantum probability theory. A coincidence approach is developed for a class QPP called the regular QPP. A counting formula is derived which determines completely the counting statistics of a regular QPP by means of a pair of ’’generators.’’