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July 5, 2006

What’s in noise?

Experimental evidence confirms that two fundamentally different sources ofnoise in quantum computers share the same origin

Figure 1: Micrograph of the Josephson charge qubit with readout circuit (SET is a single electron transistor).

Amongst the promises of quantum computing is the expectation to perform certain computations much faster than any conventional computer could. However, one of the problems that plague quantum computers is the lifetime of quantum states that are used for the mathematical algorithms, the qubits.

Detrimental to the lifetime of those qubits is noise, tiny fluctuations influencing the quantum states of the devices. “Once the origin of noise is identified, the noise can be reduced. This will help to capitalize on the advantage of solid-state qubits to build chips for quantum computers,”

explains Oleg Astafiev from RIKEN’s Frontier Research System in Wako. Joint with the NEC Fundamental and Environmental Research Laboratories in Tsukuba, the group has been long

studying the noise in a particular set of qubits—those formed by the interface of two superconductors, known as the Josephson junction (Fig. 1).

At those junctions, two types of noise occur. One originates from the fluctuations of electronic charge in the device. The intensity of this noise decreases with frequency as 1/ƒ. The second type of noise is associated with the quantum state itself and scales with ƒ. In a previous work of the same research group, indications suggested that these two types of noise might have a common origin. This result was unexpected and led to intensive theoretical studies. “But it was difficult to convince theoreticians that the two noises are really coupled,” says Astafiev. The aim was therefore to conclusively prove this hypothesis.

The evidence for this coupling lies in the temperature-dependence of the 1/ƒ noise, which the research team demonstrated experimentally. These experiments are not as straightforward as one might expect and several precautions have to be taken to correct for various effects that influence the measurements at different temperatures. However, reporting in the journal Physical Review Letters1, the group presents firm evidence that these two noises have indeed a common origin.

The pressing question now is to uncover the reason behind this intriguing coupling. It will be no easy task. “We are planning to continue the experiments that can clarify the origin of the noise,” says Astafiev. “I think it is hard to imagine that just one or a set of experiments will provide a definite answer. Maybe this can be achieved through close collaboration of experimentalists and theoreticians.”

References

  1. Astafiev, O., Pashkin, Yu. A., Nakamura, Y., Yamamoto, T. & Tsai, J.S. Temperature square dependence of the low frequency 1/ƒ charge noise in the Josephson junction qubits. Phys. Rev. Lett. 96, 137001 (2006). || (Link)