Purification of Quantum States through Zeno-like Measurements

湯浅 一哉 氏
(産業技術総合研究所 情報セキュリティ研究センター)

3月9日(金) 15時00分

The dynamics of a quantum system is described by a Schroedinger equation given an initial condition, but the preparation of the initial state is not discussed usually. The ideas of quantum information make use of entanglements, but it is not a trivial problem how to prepare such highly quantum states. Coherent superposition of quantum states (a pure state) is easily destroyed in laboratory by environmental perturbation, and the state preparation is an important issue to be tackled.

It is known that a quantum system is disturbed when it is measured. An interesting manifestation of the effect is the quantum Zeno effect: if an unstable quantum system is frequently measured to check whether it still remains in its initial state, the decay of the unstable state is slowed down and eventually halted in the limit of infinitely-frequent measurement.

In this seminar, I will show another interesting effect of the repeated measurement. A series of measurements (Zeno-like measurement) on a quantum system indirectly affects another quantum system in interaction with the former and drives the latter from an arbitrary (mixed) state into a pure state. It works as a method of purifying quantum states. It has a variety of applications like initialization of multiple qubits and entanglement purification, required for quantum computation. An extension of the scheme enables us to establish entanglement between spatially separated qubits, required for quantum communication protocols.

References:
[1] H. Nakazato, T. Takazawa, and K. Yuasa, Phys. Rev. Lett. 90, 060401 (2003).
[2] H. Nakazato, M. Unoki, and K. Yuasa, Phys. Rev. A 70, 012303 (2004).
[3] G. Compagno et al., Phys. Rev. A 70, 052316 (2004).
[4] K. Yuasa and H. Nakazato, Prog. Theor. Phys. 114, 523 (2005).