Fast control of qubit coherence with phase shift method

Hirotaka Tanaka, Tatsuya Kutsuzawa, Shiro Saito
Physical Science Research Laboratory

 A superconductor ring with Josephson junctions behaves as quantum 2-level system and is expected to be a candidate for a quantum bit (qubit), which is a key ingredient of quantum computer( Fig. 1). We controlled and measured the quantum coherence of this ring using two sequential microwave pulses with phase shifted. This ring has two different states of opposite directions of current flowing in clockwise and anti-clockwise and shows the quantum superposition of these states. The size of the ring is about 10μm, which is much larger than that of atoms or molecules but it still shows quantum nature of 2-level system. We experimentally confirmed that the ring shows quantum superposition between |0> and |1> states. The quantum superposition in a macroscopic scale object is sometimes called Schrodinger’s cat.
 We utilized phase shift method, which uses simultaneous phase-pulse modulation of microwave pulses. Two sequential microwave pulses are applied to the ring with frequency of 11.4 GHz and duration of 5 ns. The phase of the pulses is continuously shifted. The first pulse makes the state of the ring superposition of |0> and |1>. (The point on the equator in the sphere in Fig. 2) The state of the ring has its phase coherence depending on the phase of the first pulse. Then second pulse transforms the state of the ring depending on the phase of the second pulse. (The point on the equator, north and south pole in the sphere in Fig. 2)
 We experimentally showed the fast and effective quantum coherent control with the phase shift method[1]. It can be applied also to Hadamard gate, which is an important gate operation in quantum computation. In addition, the phase shift method allows us to choose optimum pulse sequences in a 2-qubit system for fast and effective operations.
[1] T. Kutsuzawa, et al., Appl. Phys. Lett. 87, (2005): (accepted). ArXiv : cond-mat/0501592.
Fig.1 Scanning electron microscope picture of superconductor qubit. Inner squared loop is a qubit and outer is a DC-SQUID(Superconducting quantum interference device) for readout.   Fig. 2 Qubit state measurement with two pulses applied. The state is oscillating by the relative phase of the microwave pulses.

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