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International Journal of Advanced Research

PROTECTION OF A SINGLE QUBIT QUANTUM STATE FROM DECOHERENCE.

L NavyaS, N Shaji.

Published Jun 30, 2017• 0 citations
Publisher

Abstract

When a quantum system interacts with the environment its superposition or coherence can be lost. Thus there will be decoherence. Decoherence causes loss of information. For effective information processing
DOI
10.21474/ijar01/4377

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Decoherence of quantum signals is one of the main challenges of modern quantum information systems and technologies. This phenomenon is defined as the degradation of the coherent state of a quantum system as a result of environmental influences, which directly affects the information reliability of qubits, as well as the preservation of oscillations and superposition. Decoherence is a natural process of a quantum system when the system interacts with a thermally active or other quantum-mechanically active environment. This effect is important both in theoretical research and in practical quantum devices, including the development of quantum telecommunications, quantum computing, and high-precision quantum sensors. In cryogenic environments, where the temperature approaches zero Kelvin, the reduction of thermal excitations ensures the long-term preservation of the quantum signal. Under such conditions, zero-point fluctuations remain the only source of decoherence, which significantly increases the coherence of qubits and the possibility of performing complex quantum operations. At relatively high temperatures, bosonic and color spectrum excitations significantly increase the decoherence rate, which leads to rapid degradation of the quantum signal and a rapid transition of the system state towards the classical paradigm.
A quantum system is always in interaction with its environment. This interaction inevitably alters the state of the quantum system, which causes loss of information encoded in the system. The system under consideration is not a closed system any more when interaction with the outside world is in action. We formulate the theory of an open quantum system in this chapter by regarding the combined system of the quantum system and its environment as a closed system and subsequently trace out the environmental degrees of freedom. Let ρS and ρE be the density matrices of the system and the environment, respectively. Even when the initial state is an uncorrelated state ρS ⊗ ρE , the system-environment interaction entangles the total system so that the total state develops to an inseparable entangled state in general. Decoherence is a process in which environment causes various changes in the quantum system, which manifests itself as undesirable noise.
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).
We present a theoretical discussion of various aspects of the dynamics of information in quantum systems of qubits. We begin by reviewing ideas about entanglement and information in systems of qubits, different models for realistic qubits coupled to an external environment, how this causes the loss of information stored in qubits, and some practical designs for qubits that have been created. Then we study how the state of many-body systems can be decomposed in terms of its different subsystems and derived a hierarchy of equations describing the motion of these different parts. We show that in a qubit system, the central objects in this hierarchy are correlators between different components of different qubits. Systems where the environment coupled to a central qubit is a "spin bath'' of environmental qubits are then considered in detail and we find that information lost by the central qubit is transferred via a cascade to higher and higher order correlations with the environment. We discuses this process in the realistic example of an "Fe₈'' magnetic molecule qubit. Finally we discuss simple models of the dynamics of many entangled qubits.
The interaction of a quantum system with its environment causes decoherence, setting a fundamental limit on its suitability for quantum information processing. However, we show that if the system consists of coupled parts with different internal energy scales then the interaction of one part with a thermal bath need not lead to loss of coherence from the other. Remarkably, we find that the protected part can remain coherent for longer when the coupling to the bath becomes stronger or the temperature is raised. Our theory will enable the design of decoherence-resistant hybrid quantum computers.
We show that, contrary to common prejudice, a measurement of an open quantum system can reduce its decoherence rate. We demonstrate this in an example of indirect measurement of a qubit, where information regarding its state is hidden in the environment. This information is extracted by a distant device, coupled with the environment only. We also show that the reduction of decoherence generated by this device is accompanied by diminution of the environmental noise in the vicinity of the qubit. An interpretation of these results in terms of quantum interference on large scales is presented.