Toward solid-state quantum information

Circuit quantum electrodynamics

With superconducting qubit called "transmon" placed inside a Nb cavity, one produces a large qubit-photon interaction and may observe interesting quantum electrodynamical effects. Figure shows how qubit energy level changes with magnetic field(near vertical lines) and how the state affect that of cavity photons. The cavity frequency(horizontal line) is 5.14GHz while the qubit level is observable from 4GHz to 7GHz. The interaction between qubit and photon produces an "anti-crossing" structure when the two energies are in resonance.

Qubit readout in dispersive regime

Circuit QED is practical for qubit readout when the qubit is off-resonance to the resonator. The coupling between the resonator and qubit creates the dispersive shift to the frequency of the resonator that can be easily seen from power dependent spectroscopy. When the probe power becomes large, the qubit is saturated so the spectroscopy shows the bare resonator frequency. By using 2-tone measurement, in which the qubit is excited by an additional microwave, one can find the qubit frequency.

Quantum coherency of the qubits

We recently perform the manipulation and readout experiments on transmon qubits. Rabi oscillations ensures the coherence of the qubit and possible quantum gates.

Gating of the qubit

From Rabi oscillation, we can determine the π/2-pulse and π-pulse. The π-pulse can control the qubit becoming excited. After a certain waiting time Tw, one can observe the qubit relaxing to the ground state and determine the qubit relaxation time, T1. On the other hand, by implementing 2 sequential π/2-pulses separated by Tw, one can observe the Ramsey fringes when varying control frequency. The amplitude of Ramsey fringes as a function of Tw provides an estimation of dephasing time, T2.

Radio-frequency single-electron transisitors

With the implement of the radio-frequency SET (rf-SET), people have so far demonstrated a 10MHz bandwidth detection, in combination with a charge resolution on the order of 10⁻⁵ e/(Hz)^1/2. The basic concept of rf-SET is to embed the SET into a tank circuit, which serves as a resonance circuit. For the loss of tank circuit can be modulated by the effective resistance of the SET, the reflecting rf signal can be used for detecting the on-off state of the SET. We successfully built a rf-SET system and obtained a charge sensitivity of about 10^-3. We also studied charge sensitivities of a rf-SET by using amplitude (AD) and phase-shift detection (PSD) of the reflected RF signals. For AD, the highest sensitivity was obtained by using a carrier frequency nearby resonant frequency, whereas the highest sensitivity of PSD was obtained with a frequency slightly off the resonance. Our study on SET bias voltage revealed that both reflection amplitude and phase-shift varied in response to the (differential) conductance of SET.

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