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Optical Fiber Communication Conference

Environmental Noise on a Qubit Through Entanglement with a Quantum Circuit

Lin Tian, Seth Lloyd, Terry P. Orlando

Published Jan 1, 2001• 0 citations
Publisher

Abstract

An external quantum system that interacts with a qubit transmits its environmental noise to the qubit and induces qubit decoherence and relaxation. We present a general method to derive the noise spectrum of this composite environment for arbitrary quantum circuits. The method is applied to the measurement induced noise of the superconducting persistent current qubit.

DOI
10.1364/icqi.2001.pb23

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An external quantum system that interacts with a qubit transmits its environmental noise to the qubit and induces qubit decoherence and relaxation. We present a general method to derive the noise spectrum of this composite environment for arbitrary quantum circuits. The method is applied to the measurement induced noise of the superconducting persistent current qubit.
During measurement, information is transferred from the measured quantum system to the detector via their coupling. The same coupling that extracts information from the quantum system transmits noise from the detector's environment to the system as well. In this paper, we derive the limit to the measurement efficiency of a superconducting persistent-current qubit measured by a dc superconducting quantum interference device (SQUID) and calculate the noise transmitted to the qubit from the environment of the dc SQUID. The method, with simple linear circuitry correspondence, can be applied to calculate the noise transferred to a quantum system from the environment of an arbitrary external system that interacts with it. The relaxation and decoherence induced by this noise are also estimated.
Abstract Superconducting qubits are currently a leading platform for developing a scalable quantum computer. However, random and uncontrollable noises from the environment during the design and measurement of qubits lead to limitations in qubit coherence time and gate fidelity, which is a major challenge in current state of the art for superconducting quantum computing. To advance superconducting qubits technologies, it is essential to understand and mitigate environmentally-induced errors. This requires modeling superconducting qubits as open quantum systems coupled to their surroundings. The present study aims to provide useful open quantum system approaches to analyze and quantify the interaction between superconducting qubits and their environments. We provide an accessible introduction to open quantum system formalism for newcomers to the field. For experts, we discuss recently developed methods for analyzing superconducting qubit dynamics under realistic noises. We outline how these techniques provide quantitative insights into decoherence mechanisms and how they can guide design improvements to enhance qubit coherence time. This self-contained review of open quantum system approaches can be used to model, understand, and improve superconducting qubit performance in the presence of unavoidable environmental noises.
We consider a qubit interacting with its environment and continuously monitored by a detector represented by a point contact. Bloch-type equations describing the entire system of the qubit, the environment, and the detector are derived. Using these equations we evaluate the detector current and its noise spectrum in terms of the decoherence and relaxation rates of the qubit. Simple expressions are obtained that show how these quantities can be accurately measured. We demonstrate that due to interaction with the environment, the measurement can never localize a qubit even for infinite decoherence rate.
Electrical engineers and physicists are naturally very interested in noise in circuits, amplifiers and detectors. With the advent of quantum computation and other high frequency electronics operating at low temperatures, we have entered a regime where quantum noise and quantum-limited detectors are important. Here we describe the general concept of a two-level system as a quantum spectrum analyzer and apply it to a simple superconducting qubit, the Cooper-pair box. We then discuss the coupling of a Cooper-pair box to its electromagnetic environment, whose noise leads to a finite polarization and excited-state lifetime of the qubit. Finally, we describe a theoretical technique for treating a qubit coupled to a measurement system, which allows one to calculate the full quantum noise of the measurement device. We present results for such a calculation for the case of a normal SET.
We address the use of entangled qubits as quantum probes to characterize the noise induced by complex environments. In particular, we show that a joint measurement on entangled probes can improve estimation of the correlation time for a broad class of environmental noises compared to sequential strategies involving single-qubit preparation. The enhancement appears when the noise is faster than a threshold value, a regime which may always be achieved by tuning the coupling between the quantum probe and the environment inducing the noise. Our scheme exploits time-dependent sensitivity of quantum systems to decoherence and does not require dynamical control on the probes. We derive the optimal interaction time and the optimal probe preparation, showing that it corresponds to multiqubit Greenberger-Horne-Zeilinger states when entanglement is useful. We also show the robustness of the scheme against depolarization or dephasing of the probe, and discuss simple measurements approaching optimal precision.
Decoherence is one of the most important obstacles that must be overcome in quantum information processing. It depends on the qubit-environment coupling strength, but also on the spectral composition of the noise generated by the environment. If the spectral density is known, fighting the effect of decoherence can be made more effective. Applying sequences of inversion pulses to the qubit system, we developed a method for dynamical decoupling noise spectroscopy. We generate effective filter functions that probe the environmental spectral density without requiring assumptions about its shape. Comparing different pulse sequences, we recover the complete spectral density function and distinguish different contributions to the overall decoherence.