Generation of Non-Classical Microwave Photon States in an Inductor-Capacitor Resonator Coupled to a Superconducting Flux Qubit


Kosuke Kakuyanagi, Shiro Saito, Hayato Nakano, and Kouichi Semba
Physical Science Laboratory

 To realize quantum memories for quantum computation, we need to establish quantum information transfer and the generation of a photon number state (especially a one-photon state). However it is difficult to generate a photon number state because the method, which excites an LC resonator by using a resonant microwave, forms a classical coherent state. So, we attempt to generate a non-classical photon state by transferring a photon from a superconducting qubit to an LC resonator.
 The LC resonator, which is constructed from a line inductor and a capacitor, surrounds a superconducting flux qubit (Fig. 1). The magnetic coupling energy between the flux qubit and the LC resonator is 360 MHz. A large coupling energy is one of the advantages of using a superconducting qubit. In this LC resonator and flux qubit system, we shift the external magnetic field non-adiabatically, after realizing the excited qubit state. By tuning the resonant condition of the LC resonator and qubit, we generate a photon oscillation, which is called a vacuum Rabi oscillation. Time domain control of this quantum oscillation makes it possible to transfer a photon from a qubit to an LC resonator. By repeating this process, we can transfer photons one by one. The ideal photon state achieved using this method becomes the photon number state. Period time of quantum oscillation of this system depends on the photon number, so we can obtain information about the photon distribution of the LC resonator by analyzing the time domain oscillation.
 Figure 2 shows the photon distribution results when we transfer one and two photons to the LC resonator. Energy relaxation and the imperfection of the control pulses mean that this photon distribution is different from the pure photon number state. However, we succeeded in generating a non-classical photon distribution, which is clearly different from a classical Poisson distribution [1]. This is the result of using the LC resonator’s quantum characteristics, so we can expect the development of quantum memories by using high-Q resonators.

[1] K. Kakuyanagi et al., Appl. Phys. Express 3 (2010) 103101.

Fig. 1. Sample design of LC resonator and superconducting
flux qubit.
Fig. 2. Corresponding Poisson distributions and results
of photon distributions of LC resonator when we
transfer one and two photons.

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