34,966 research outputs found
Just Two Nonorthogonal Quantum States
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.Comment: 7 pages, no figures, requires myplenumQPH.st
Quantum Bayesianism at the Perimeter
The author summarizes the Quantum Bayesian viewpoint of quantum mechanics,
developed originally by C. M. Caves, R. Schack, and himself. It is a view
crucially dependent upon the tools of quantum information theory. Work at the
Perimeter Institute for Theoretical Physics continues the development and is
focused on the hard technical problem of a finding a good representation of
quantum mechanics purely in terms of probabilities, without amplitudes or
Hilbert-space operators. The best candidate representation involves a
mysterious entity called a symmetric informationally complete quantum
measurement. Contemplation of it gives a way of thinking of the Born Rule as an
addition to the rules of probability theory, applicable when one gambles on the
consequences of interactions with physical systems. The article ends by
outlining some directions for future work.Comment: 6 pages, 2 figures; an abridged version of this will appear in
Physics in Canada
Interview with a Quantum Bayesian
This paper collects into one place (most of) my answers to the questions
Maximilian Schlosshauer posed in his interview volume, "Elegance and Enigma:
The Quantum Interviews" (Springer, Frontiers Collection, 2011).Comment: 22 page
Copenhagen Interpretation Delenda Est?
This is a slightly extended version of a review of Adam Becker's book What is
Real? The Unfinished Quest for the Meaning of Quantum Physics, to appear in
American Journal of Physics. The key addition is the reference list.Comment: 6 page
Quantum Mechanics as Quantum Information (and only a little more)
In this paper, I try once again to cause some good-natured trouble. The issue
remains, when will we ever stop burdening the taxpayer with conferences devoted
to the quantum foundations? The suspicion is expressed that no end will be in
sight until a means is found to reduce quantum theory to two or three
statements of crisp physical (rather than abstract, axiomatic) significance. In
this regard, no tool appears better calibrated for a direct assault than
quantum information theory. Far from a strained application of the latest fad
to a time-honored problem, this method holds promise precisely because a large
part--but not all--of the structure of quantum theory has always concerned
information. It is just that the physics community needs reminding.
This paper, though taking quant-ph/0106166 as its core, corrects one mistake
and offers several observations beyond the previous version. In particular, I
identify one element of quantum mechanics that I would not label a subjective
term in the theory--it is the integer parameter D traditionally ascribed to a
quantum system via its Hilbert-space dimension.Comment: 59 pages, 5 figures, 140 equations, one simple ide
On Participatory Realism
In the Philosophical Investigations, Ludwig Wittgenstein wrote, " 'I' is not
the name of a person, nor 'here' of a place, .... But they are connected with
names. ... [And] it is characteristic of physics not to use these words." This
statement expresses the dominant way of thinking in physics: Physics is about
the impersonal laws of nature; the "I" never makes an appearance in it. Since
the advent of quantum theory, however, there has always been a nagging pressure
to insert a first-person perspective into the heart of physics. In incarnations
of lesser or greater strength, one may consider the "Copenhagen" views of Bohr,
Heisenberg, and Pauli, the observer-participator view of John Wheeler, the
informational interpretation of Anton Zeilinger and Caslav Brukner, the
relational interpretation of Carlo Rovelli, and, most radically, the QBism of
N. David Mermin, Ruediger Schack, and the present author, as acceding to the
pressure. These views have lately been termed "participatory realism" to
emphasize that rather than relinquishing the idea of reality (as they are often
accused of), they are saying that reality is more than any third-person
perspective can capture. Thus, far from instances of instrumentalism or
antirealism, these views of quantum theory should be regarded as attempts to
make a deep statement about the nature of reality. This paper explicates the
idea for the case of QBism. As well, it highlights the influence of John
Wheeler's "law without law" on QBism's formulation.Comment: 23 pages, to appear in "Information & Interaction: Eddington,
Wheeler, and the Limits of Knowledge", edited by Ian T. Durham and Dean
Rickles; v3 corrects word omissions from the Wheeler notebook page
Commanding Heights? : The Strength and Fragility of Business Power in Global Politics
International relations urgently needs theoretical development that takes into the account the power of non-state actors, in particular business. The present paper aims to prepare the ground for such theoretical development by analysing the political power of business along three dimensions: its instrumental, structural, and discursive power. The paper discusses the extent to which business's political power has grown in each of the dimensions. While the analysis indicates that the political power of business in general and corporate actors in particular has increased in certain areas, it also highlights limits and challenges to this power. Specifically, the paper argues that the `commanding heights' to which business allegedly has climbed are built on shaky ground. This, in turn, implies the need for any IR theory that takes non-state actors into account to develop and include a new concept of vulnerability of political power
Notwithstanding Bohr, the Reasons for QBism
Without Niels Bohr, QBism would be nothing. But QBism is not Bohr. This paper
attempts to show that, despite a popular misconception, QBism is no minor tweak
to Bohr's interpretation of quantum mechanics. It is something quite distinct.
Along the way, we lay out three tenets of QBism in some detail: 1) The Born
Rule---the foundation of what quantum theory means for QBism---is a normative
statement. It is about the decision-making behavior any individual agent should
strive for; it is not a descriptive "law of nature" in the usual sense. 2) All
probabilities, including all quantum probabilities, are so subjective they
never tell nature what to do. This includes probability-1 assignments. Quantum
states thus have no "ontic hold" on the world. 3) Quantum measurement outcomes
just are personal experiences for the agent gambling upon them. Particularly,
quantum measurement outcomes are not, to paraphrase Bohr, instances of
"irreversible amplification in devices whose design is communicable in common
language suitably refined by the terminology of classical physics." Finally, an
explicit comparison is given between QBism and Bohr with regard to three
subjects: a) The issue of the "detached observer" as it arose in a debate
between Pauli and Bohr, b) Bohr's reply to Einstein, Podolsky, and Rosen, and
c) Bohr's mature notion of "quantum phenomena." At the end, we discuss how
Bohr's notion of phenomena may have something to offer the philosophy of
William James: A physics from which to further develop his vision of the
world---call it an ontology if you will---in which "new being comes in local
spots and patches."Comment: 44 pages, 1 figur
Information Gain vs. State Disturbance in Quantum Theory
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.Comment: 5 pages LaTeX, Extended Abstract for PhysComp96, requires
pac96sty.te
Quantum Foundations in the Light of Quantum Information
This paper reports three almost trivial theorems that nevertheless appear to
have significant import for quantum foundations studies. 1) A Gleason-like
derivation of the quantum probability law, but based on the positive
operator-valued measures as the basic notion of measurement (see also Busch,
quant-ph/9909073). Of note, this theorem also works for 2-dimensional vector
spaces and for vector spaces over the rational numbers, where the standard
Gleason theorem fails. 2) A way of rewriting the quantum collapse rule so that
it looks almost precisely identical to Bayes rule for updating probabilities in
classical probability theory. And 3) a derivation of the tensor-product rule
for combining quantum systems (and with it the very notion of quantum
entanglement) from Gleason-like considerations for local measurements on
bipartite systems along with classical communication.Comment: 45 pages, 2 figures, a very slightly different version of this paper
will appear in "Proceedings of the NATO Advanced Research Workshop on
Decoherence and its Implications in Quantum Computation and Information
Transfer," edited by A. Goni
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