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Bulletin of the American Physical Society

Energy relaxation time of an rf SQUID flux qubit

Shaoxiong Li, Wei Qiu, Matthew H. Matheny, Zhongyuan Zhou, Siyuan Han

Published Mar 24, 2005• 1 citation
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The dissipation-induced relaxation (T_1) time of a macroscopic quantum system - a \{lambda}-type three-level rf SQUID flux qubit weakly coupled to control and readout circuitry (CRC) - is investigated via time-domain measurement. The measured interwell relaxation time of the qubit's first excited state, T_1=3.45+/-0.06 \{mu}s, corresponds to an effective damping resistance of the flux qubit R=1.6+/-0.1 M\{omega} which is much lower than the intrinsic quasiparticle resistance of the Josephson tunnel junction. An analysis of the system shows that although the CRC is very weakly coupled to the qubit it is the primary source of damping. This type of damping can be significantly reduced by the use of more sophisticated circuit design to allow coherent manipulation of qubit states.

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Measurement of the Energy Relaxation Time in rf SQUID Flux Qubits

Wei Qiu

It is well known that that superconducting qubits based on Josephson junctions have the advantages of scalability and the qubit states are easy to prepare and control. In addition, demonstration of Rabi oscillations in various superconducting circuits shows that superconducting qubits are promising for scalable quantum information processing. However, despite flexibility of design and fabrication, easier to scale up, and fast gate speed, superconducting qubits usually have much shorter decoherence time than trapped ions, NMR etc. due to the relatively strong interactions between qubits and environment. Recent experiments show that low frequency flux noise is the dominant mechanism of decoherence in superconducting flux qubits. However, despite extensive effort the origin of flux noise is still not well understood. The goal of this work is to identify the source and characterize the property of flux noisein rf SQUID flux qubit through various spectroscopy and time-resolved measurements. Our result show that one can determine all circuit parameters needed for reconstructing qubit Hamiltonian with high accuracy via measurement of macroscopic resonant tunneling (MRT) and photon assisted tunneling (PAT) and that the amount of flux noise in rf SQUID qubits scales linearly with self inductance of the qubits. In addition, we have investigated the dynamics of a three-level flux qubit in incoherent regime. The result demonstrates that treating a multi-level physical qubit, such as the superconducting flux qubit, as an ideal two-level quantum system may be inadequate under certain circumstances.

•Dec 17, 2007•0 citations

Measurements of Decoherence in rf SQUID Qubits

D. A. Bennett, Luigi Longobardi, Vijay Patel +2 more

Bulletin of the American Physical Society•Mar 11, 2008•0 citations
The dissipation-induced relaxation (T_1) time of a macroscopic quantum system - a \{lambda}-type three-level rf SQUID flux qubit weakly coupled to control and readout circuitry (CRC) - is investigated via time-domain measurement. The measured interwell relaxation time of the qubit's first excited state, T_1=3.45+/-0.06 \{mu}s, corresponds to an effective damping resistance of the flux qubit R=1.6+/-0.1 M\{omega} which is much lower than the intrinsic quasiparticle resistance of the Josephson tunnel junction. An analysis of the system shows that although the CRC is very weakly coupled to the qubit it is the primary source of damping. This type of damping can be significantly reduced by the use of more sophisticated circuit design to allow coherent manipulation of qubit states.
It is well known that that superconducting qubits based on Josephson junctions have the advantages of scalability and the qubit states are easy to prepare and control. In addition, demonstration of Rabi oscillations in various superconducting circuits shows that superconducting qubits are promising for scalable quantum information processing. However, despite flexibility of design and fabrication, easier to scale up, and fast gate speed, superconducting qubits usually have much shorter decoherence time than trapped ions, NMR etc. due to the relatively strong interactions between qubits and environment. Recent experiments show that low frequency flux noise is the dominant mechanism of decoherence in superconducting flux qubits. However, despite extensive effort the origin of flux noise is still not well understood. The goal of this work is to identify the source and characterize the property of flux noisein rf SQUID flux qubit through various spectroscopy and time-resolved measurements. Our result show that one can determine all circuit parameters needed for reconstructing qubit Hamiltonian with high accuracy via measurement of macroscopic resonant tunneling (MRT) and photon assisted tunneling (PAT) and that the amount of flux noise in rf SQUID qubits scales linearly with self inductance of the qubits. In addition, we have investigated the dynamics of a three-level flux qubit in incoherent regime. The result demonstrates that treating a multi-level physical qubit, such as the superconducting flux qubit, as an ideal two-level quantum system may be inadequate under certain circumstances.