Topic Name: Electrically Measuring Method for a Quantum State of a Semiconductor Artificial Molecule : Applications in quantum information processing are expected
Research persons: Yoshihisa Yamamoto,Hideo Kosaka,Hiroshi Imamura
Location: Tsukuba, Japan
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 two-spin state is singlet or triplet. With the
conventional method, the judgment is made according to whether the Pauli spin
blockade phenomenon s 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
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 spin-orbit
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 singlet-triplet 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.
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 solid-state quantum devices. Through these efforts,
we hope to contribute to establishing the technical foundation for the future
quantum information society.
About the research programme:
The above research was sponsored by the following research fund program.
Program name: Core Research for Evolutional Science and Technology (CREST)
Research area: Creation of New Technology Aiming for the Realization of
Quantum Information Processing Systems
(Research supervisor: Yoshihisa Yamamoto (Professor) of Principles of
Informatics Research Division, National Institute of Informatics, the
Research Organization of Information and Systems, and of Applied Physics and
Electrical Engineering, Stanford University)
Research project: Quantum media conversion from a photon qubit to an
Research director: Hideo Kosaka (Associate Professor), Tohoku University
Research partner: Hiroshi Imamura (Senior Research Scientist), AIST
Research period: October 2004 - March 2010
JST is committed to establishing technological foundations in quantum
information processing that can lead to revolutionary information and
communication technology. Within the above research area, interdisciplinary
research on nanotechnology, spintronics, photonics, and quantum information
processing is being conducted with the aim of converting media from photon
qubits to electron-spin qubits.
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