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arXiv:quant-ph/9810032 · Kluwer Academic Publishers eBooks

Just Two Nonorthogonal Quantum States

Christopher A. Fuchs

Published Dec 19, 2005• 30 citations
arXivHTMLPDF

Abstract

From the perspective of quantum information theory, a system so simple as one restricted to just two nonorthogonal states can be surprisingly rich in physics. In this paper, we explore the extent of this statement through a review of three topics: (1) “nonlocality without entanglement” as exhibited in binary quantum communication channels, (2) the tradeoff between information gain and state disturbance for two prescribed states, and (3) the quantitative clonability of those states. Each topic in its own way quantifies the extent to which two states are “quantum” with respect to each other, i.e., the extent to which the two together violate some classical precept. It is suggested that even toy examples such as these hold some promise for shedding light on the foundations of quantum theory.

DOI
10.1007/0-306-47097-7_2

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From the perspective of quantum information theory, a system so simple as one restricted to just two nonorthogonal states can be surprisingly rich in physics. In this paper, we explore the extent of this statement through a review of three topics: (1) “nonlocality without entanglement” as exhibited in binary quantum communication channels, (2) the tradeoff between information gain and state disturbance for two prescribed states, and (3) the quantitative clonability of those states. Each topic in its own way quantifies the extent to which two states are “quantum” with respect to each other, i.e., the extent to which the two together violate some classical precept. It is suggested that even toy examples such as these hold some promise for shedding light on the foundations of quantum theory.
When an observer wants to identify a quantum state, which is known to be one of a given set of nonorthogonal states, the act of observation causes a disturbance to that state. We investigate the trade-off between the information gain and that disturbance. This issue has important applications in quantum cryptography. The optimal detection method, for a given tolerated disturbance, is explicitly found in the case of two equiprobable nonorthogonal pure states. \textcopyright{} 1996 The American Physical Society.
The engine that powers quantum cryptography is the principle that there are no physical means for gathering information about the identity of a quantum system's state (when it is known to be prepared in one of a set of nonorthogonal states) without disturbing the system in a statistically detectable way. This situation is often mistakenly described as a consequence of the ``Heisenberg uncertainty principle.'' A more accurate account is that it is a unique feature of quantum phenomena that rests ultimately on the Hilbert space structure of the theory along with the fact that time evolutions for isolated systems are unitary. In this paper I explore several aspects of the ``information / disturbance principle'' in an attempt to make it firmly quantitative for both pure and mixed states. The final section briefly explores the extent to which such a principle can be taken as a foundation for unitary dynamics rather than as a consequence.
The surprising aspects of quantum information are due to two distinctly non-classical features of the quantum world: first, different quantum states need not be orthogonal and, second, quantum states may be entangled. Non-orthogonality leads to the blurring of classical distinctions. On the other hand, entanglement leads via non-locality to teleportation and other ``entanglement-assisted'' forms of communication that go beyond what is classically possible. In this article we attempt to understand these new possibilities via an analysis of the significance of entanglement for the basic physical concepts of a ``particle'' and a ``localized physical system''. Classical particles can be individuated on the basis of qualitative differences in their sets of properties. But in entangled states the ``particle labels'' of the quantum formalism usually do not pick out such sets of individuating particle properties. It is sometimes nevertheless possible to think in terms of individual particles, which may be localized; but we argue that in general the structure of quantum mechanics is at odds with such a particle interpretation. This finally leads us to the conclusion that quantum mechanics is best seen as not belonging to the category of space-time theories, in which physical quantities are functions on space-time points. The resulting picture of the quantum world is relevant for our understanding of the way in which quantum theory is non-local, and it sheds light on the novel aspects of quantum information.
The engine that powers quantum cryptography is the principle that there are no physical means for gathering information about the identity of a quantum system's state (when it is known to be prepared in one of a set of nonorthogonal states) without disturbing the system in a statistically detectable way. This situation is often mistakenly described as a consequence of the “Heisenberg uncertainty principle.” A more accurate account is that it is a unique feature of quantum phenomena that rests ultimately on the Hilbert space structure of the theory along with the fact that time evolutions for isolated systems are unitary. In this paper we shall explore several aspects of the information-disturbance principle in an attempt to make it firmly quantitative and flesh out its significance for quantum theory as a whole.
The engine that powers quantum cryptography is the principle that there are no physical means for gathering information about the identity of a quantum system's state (when it is known to be prepared in one of a set of nonorthogonal states) without disturbing the system in a statistically detectable way. This situation is often mistakenly described as a consequence of the ``Heisenberg uncertainty principle.'' A more accurate account is that it is a unique feature of quantum phenomena that rests ultimately on the Hilbert space structure of the theory along with the fact that time evolutions for isolated systems are unitary. In this paper we shall explore several aspects of the information--disturbance principle in an attempt to make it firmly quantitative and flesh out its significance for quantum theory as a whole.
If a quantum channel or process cannot be described by any measure-and-prepare scheme, we may say the channel is in the quantum domain (QD) since it can transmit quantum correlations. The concept of the QD clarifies the role of the quantum channel in quantum-information theory based on the local operation and classical communication (LOCC) paradigm: The quantum channel is useful only if it cannot be simulated by LOCC. We construct a simple scheme to verify that a given physical process or channel is in the QD by using two nonorthogonal states. We also consider its application for experiments such as the transmission or storage of quantum optical coherent states, single-photon polarization states, and squeezed vacuum states.