Posted: August 11, 2009  
Measuring the quantum state of a semiconductor artificial molecule 

(Nanowerk News) Under a targeted basic research program of the Japan Science and Technology Agency (JST), Hiroshi Imamura (Senior Research Scientist) of the National Institute of Advanced Industrial Science and Technology (AIST), and Nobuhiko Yokoshi (Researcher) of JST have developed a new method for electrically measuring a quantum superposition spin state of two electrons captured in a gallium arsenide (GaAs) based semiconductor artificial molecule (double quantum dots).  
In addition to its electrical property, "electric charge", an electron has a magnetic property referred to as "electron spin". A new approach called "spintronics", which attempts to exploit this magnetic property for electronics applications, is attracting considerable attention. Above all, it is expected that the electron spins captured in artificial semiconductor atoms or molecules can be used as the basis for future quantum information processing devices.  
This group established the theoretical foundation for a method to electrically measure the quantum superposition of the two spin states (singlet and triplet) of two electrons captured in a double quantum dot. With the conventional methods, only the probability of singlet and triplet spin states could be measured. Since the new method can also detect the quantummechanical relative phase of the two states, the twoelectron spin states are fully measured. This method is valuable not only in the context of basic physics but also as a means for confirming the initialization of the quantum states and reading out the computed results that are necessary to develop a quantum information processing device. Therefore, this method is expected to contribute to the development of quantum information processing devices based on semiconductor artificial molecules.  
The research was conducted in collaboration with Hideo Kosaka (Associate Professor) of the Research Institute of Electrical Communication (RIEC), Tohoku University.  
Background and history of research  
As electronic commerce increases in popularity and information security management at work and at home becomes more critical, there is a growing need to improve encryption technology for open networks. Research on improving encryption technology has focused on quantum information processing, which exploits the property of quantum mechanics. In quantum mechanics, the world looks different from what we usually see. Quantum superposition is one example. While each element of the world is described by either 0 or 1 with the ordinary digital system, quantum mechanics says that each element can be both 0 and 1 at the same time. This unique behavior, namely superposition state, is broken when the state is "tapped" (information is retrieved). Therefore, the use of quantum states provides an ultimate security.  
Many candidates such as photons and electrons have been examined as a mechanism for representing a qubit, the smallest element in quantum information processing. Among them, electrons are the most natural choice considering their existence in semiconductor devices at the heart of communication devices and computers, and the fact that these semiconductor devices are intrinsically versatile with high integration. Especially, the idea of using the two spins of electrons as a qubit has advantages in the degree of integration and computational efficiency. Hence, this method is widely studied. There is an urgent need to bring this electron spin qubit technology to commercial application for the future of the advanced information society.  
AIST and RIEC, together with the Sendai National College of Technology, have been studying quantum media conversion between photons and electron spins. The group has succeeded in coherently reading and writing the electron spin states in a semiconductor by using light. This research attempts to develop a measuring method that can electrically read out any arbitrary quantum states transferred from two photons to the two electrons captured in a double quantum dot.  
When each quantum dot contains an electron, quantum information is read out by determining if the twospin state is singlet or triplet. With the conventional method, the judgment is made according to whether the Pauli spin blockade phenomenon is observed. Such conventional methods, however, can only detect the probability of singlet and triplet spin states, which is only one aspect of the quantum superposition states.  
Details of research  
In this research, a new measuring method is proposed. The method adiabatically controls the voltage of electrodes placed on the two quantum dots (L and R) as well as between the two dots, and then counts the number of electrons in the L and R quantum dots. Note that the voltage change to gates L and R corresponds to controlling the electrostatic energy of the two quantum dots, and the voltage change to gate B corresponds to controlling the probability of quantum tunneling. The group has performed a theoretical analysis on this process using a new quantum tunneling model that takes into account electron spinorbit coupling in semiconductor material.  
After a sequence of gate operations, the number of electrons in the two quantum dots is measured, and ensemble averages are computed. As a result, it is shown that the difference in the number of L and R electrons oscillates as a function of the singlettriplet relative phase in quantum superposition. This means that both the probability of singlet and triplet spin states and the relative phase of the two states are measured simultaneously.  
Fig. 1: Bloch sphere of a spin qubit.  
Any quantum superposition of the singlet (S) and triplet (T0) spin states can be represented as a point on the Bloch sphere (Fig. 1). While conventional methods only provide longitude, the newly proposed method can identify the coordinates (both longitude and latitude) on the sphere, which gives the entire picture of the quantum state. In this respect, the new measuring method is the breakthrough. It is valuable not only in the context of basic physics but also as a means for confirming the initialization of the quantum states and reading out the computed results that are necessary to develop a quantum information processing device. Therefore, this method will greatly contribute to the development of semiconductor artificial molecules based quantum information processing device.  
Future work  
In order to commercially apply electron spin qubits, it is necessary to improve the quality of materials such as GaAs and achieve further theoretical development to add and extend quantum functions. We will continue to put our efforts into designing new solidstate quantum devices. Through these efforts, we hope to contribute to establishing the technical foundation for the future quantum information society. 
Source: AIST 
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