Ultrahigh quality cavities

Recently, two groups reported their progresses in manipulating ultrahigh Q cavities.

Ivan S. Grudinin et al. demonstrated a record quality factor of (6.3 pm 0.8) times 10^10 in crystalline whispering gallery mode resonator, corresponding to cavity ring-down time of tau approx 36 {mu}s. They found for a 100 {mu}m LiNbO3 cavity, a single photon would shift the cavity resonance by as much as 6 Hz. Such shift could be detected with optical techniques. This allows the quantum nondemoliion measurement for the number of photons in a cavity. I hope the shift can be enlarged in future. If the shift is large enough, this resonator may be used as a device to entangle photon qubits.

S. Kuhr et al. built a Fabry-Perot supeconducting resonator with quality factor Q = 4.2 times 10^10 and finesse 4.6time 10^9. The demping time T_c of the cavity is as long as 130 ms at resonante frequency 51 GHz and temperature 0.8 K. In previous experiments, T_c was limited to 1 ms. The field damping time of their cavity is 100 times longer than previous ones. I think this microwave F-P resonator is very powerful for realizing quantum information processes. The coupling strength g between atom and cavity is about 310 kHz, which is 4 orders larger than cavity decay rate. As I discussed before, realizing a entangling gate in this system only requires time of 1.7/g, and realizing a nearly perfect controlled-Z gate only needs 3.4/g.. Therefore, the effects of cavity damping can be neglected. Besides, for Rydberg atoms with quantum number 51 and 50, the spontaneous emission rate is of the order 10^2 which is 3 orders lower than time needed in quantum processes. Therefore, a almost perfect quantum logic gate (with fidelity larger than 0.99) may be realized in this cavity.



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