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IEEE Conference on Nanotechnology (IEEE NANO)

A circuit approach for implementing quantum memory

Hanwei Wang, I.-M. Tsai, Sy‐Yen Kuo

Published Apr 1, 2005• 0 citations
Publisher

Abstract

Decoherence is the process that a quantum particle loses its integrity by interacting with the surrounding environment. The problem of decoherence restricts the time available for implementing a given quantum computation process. This phenomenon has a major impact on the performance of quantum computers. In this paper, we propose a scheme to solve this problem. We use a circuit approach to protect our quantum bits (qubit) from decoherence. This results in a quantum memory that has many applications in quantum information science.

DOI
10.1109/nano.2004.1392339

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Decoherence is the process that a quantum particle loses its integrity by interacting with the surrounding environment. The problem of decoherence restricts the time available for implementing a given quantum computation process. This phenomenon has a major impact on the performance of quantum computers. In this paper, we propose a scheme to solve this problem. We use a circuit approach to protect our quantum bits (qubit) from decoherence. This results in a quantum memory that has many applications in quantum information science.
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Decoherence in quantum-computer memory due to the inevitable coupling to the external environment is examined. We make the assumption that all quantum bits (qubits) interact with the same environment rather than the assumption of separate environments for different qubits. It is found that the qubits decohere collectively. For some kinds of entangled input states, no decoherence occurs at all in the memory, even if the qubits are interacting with the environment. Based on this phenomenon, a scheme is proposed for reducing the collective decoherence. We also discuss possible implications of this decoherence model for quantum measurements.
If quantum bits (qubits) couple to the same environment, it has been found the qubits decohere coherently. An interesting result from this phenomenon is that, for a kind of input states, i.e., the coherence-preserving states, coherence of the qubits can be preserved perfectly in quantum memory. In this paper, we propose a feasible scheme to transform an arbitrary unknown input state to the corresponding coherence-preserving state. The transformed state undergoes no decoherence in the noisy memory and, after that, it can be transformed back into the original state with decoherence much reduced. This scheme involves the use of an analogy of the ideas of quantum teleportation.
Quantum technologies have shown immeasurable potential to effectively solve several information processing tasks such as prime number factorization, unstructured database search or complex macromolecule simulation. As a result of such capability to solve certain problems that are not classically tractable, quantum machines have the potential revolutionize the modern world via applications such as drug design, process optimization, unbreakable communications or machine learning. However, quantum information is prone to suffer from errors caused by the so-called decoherence, which describes the loss in coherence of quantum states associated to their interactions with the surrounding environment. This decoherence phenomenon is present in every quantum information task, be it transmission, processing or even storage of quantum information. Consequently, the protection of quantum information via quantum error correction codes (QECC) is of paramount importance to construct fully operational quantum computers. Understanding environmental decoherence processes and the way they are modeled is fundamental in order to construct effective error correction methods capable of protecting quantum information. In this thesis, the nature of decoherence is studied and mathematically modelled; and QECCs are designed and optimized so that they exhibit better error correction capabilities.
All physical systems that provide a good template for the implementation of qubits and quantum gates suffer from some degree of decoherence. The reason is simple: no physical system exists in complete isolation; if it did exist, it would be useless for quantum computing since you would not be able to interact with it! Any physical system, even when well protected, will eventually interact with its environment in a way that we cannot predict or completely control. As a result, coherence, which is essential for quantum information processing, is progressively lost. In the field of quantum computation, it was realized early on that strategies must be employed to mitigate not only imprecisions in the instrumentation but also errors associated to decoherence and relaxation due to interactions with the environment. These strategies, when employed at the quantum circuit level, are called quantum error correction (QEC).
A quantum memory interacts with its environment and loses information via decoherence as well as incoherence. A robust quantum control that prepares, preserves, and manipulates nonclassical correlations even in the presence of environmental influence is of paramount importance in quantum information processing. A well-known technique to suppress decoherence, namely Dynamical Decoupling (DD), consists of a sequence of rapid flips applied to the system in order to refocus the system-environment interactions. In this work, we integrate DD with quantum gates using optimal control techniques to realize robust quantum gates which offer protection against decoherence. To investigate the protection of non-classical correlation, we study the evolution of quantum discord in Grover's search algorithm implemented with dynamically protected gates. Using a two-qubit NMR system, we experimentally demonstrate a significant protection against decoherence and incoherence. We find better performances by phase alternating DD sequences with suitable spacings between the DD pulses. Interestingly, we also find that DD sequences based on π/2\pi/2π/2 pulses perform as well as or even better than those with π\piπ pulses in protecting the non-classical correlation. We also support the experimental results by analyzing the robustness of various DD schemes.