9,276 research outputs found
Classically-Controlled Quantum Computation
Quantum computations usually take place under the control of the classical
world. We introduce a Classically-controlled Quantum Turing Machine (CQTM)
which is a Turing Machine (TM) with a quantum tape for acting on quantum data,
and a classical transition function for a formalized classical control. In
CQTM, unitary transformations and measurements are allowed. We show that any
classical TM is simulated by a CQTM without loss of efficiency. The gap between
classical and quantum computations, already pointed out in the framework of
measurement-based quantum computation is confirmed. To appreciate the
similarity of programming classical TM and CQTM, examples are given.Comment: 20 page
Causal Quantum Theory and the Collapse Locality Loophole
Causal quantum theory is an umbrella term for ordinary quantum theory
modified by two hypotheses: state vector reduction is a well-defined process,
and strict local causality applies. The first of these holds in some versions
of Copenhagen quantum theory and need not necessarily imply practically
testable deviations from ordinary quantum theory. The second implies that
measurement events which are spacelike separated have no non-local
correlations. To test this prediction, which sharply differs from standard
quantum theory, requires a precise theory of state vector reduction.
Formally speaking, any precise version of causal quantum theory defines a
local hidden variable theory. However, causal quantum theory is most naturally
seen as a variant of standard quantum theory. For that reason it seems a more
serious rival to standard quantum theory than local hidden variable models
relying on the locality or detector efficiency loopholes.
Some plausible versions of causal quantum theory are not refuted by any Bell
experiments to date, nor is it obvious that they are inconsistent with other
experiments. They evade refutation via a neglected loophole in Bell experiments
-- the {\it collapse locality loophole} -- which exists because of the possible
time lag between a particle entering a measuring device and a collapse taking
place. Fairly definitive tests of causal versus standard quantum theory could
be made by observing entangled particles separated by light
seconds.Comment: Discussion expanded; typos corrected; references adde
A Taxonomy of Views about Time in Buddhist and Western Philosophy
We find the claim that time is not real in both western and eastern philosophical traditions. In what follows I will call the view that time does not exist temporal error theory. Temporal error theory was made famous in western analytic philosophy in the early 1900s by John McTaggart (1908) and, in much the same tradition, temporal error theory was subsequently defended by Gödel (1949). The idea that time is not real, however, stretches back much further than that. It is common to hear it said that according to Buddhist philosophy (as though that were a monolithic view) time is illusory. While it is not true that, in general, either contemporary or ancient Buddhist scholars have thought time to be illusory, there are certainly some schools of Buddhist thought, such as that of traditional Dzogchen practitioners, according to which there is no time. This paper is an attempt to set out a taxonomy of different views about what it takes for there to be time and, alongside that, a taxonomy of views about whether there is time or not, and if there is time what it is like
Quantum Bit Commitment with a Composite Evidence
Entanglement-based attacks, which are subtle and powerful, are usually
believed to render quantum bit commitment insecure. We point out that the no-go
argument leading to this view implicitly assumes the evidence-of-commitment to
be a monolithic quantum system. We argue that more general evidence structures,
allowing for a composite, hybrid (classical-quantum) evidence, conduce to
improved security. In particular, we present and prove the security of the
following protocol: Bob sends Alice an anonymous state. She inscribes her
commitment by measuring part of it in the + (for ) or (for
) basis. She then communicates to him the (classical) measurement outcome
and the part-measured anonymous state interpolated into other, randomly
prepared qubits as her evidence-of-commitment.Comment: 6 pages, minor changes, journal reference adde
Perfect Computational Equivalence between Quantum Turing Machines and Finitely Generated Uniform Quantum Circuit Families
In order to establish the computational equivalence between quantum Turing
machines (QTMs) and quantum circuit families (QCFs) using Yao's quantum circuit
simulation of QTMs, we previously introduced the class of uniform QCFs based on
an infinite set of elementary gates, which has been shown to be computationally
equivalent to the polynomial-time QTMs (with appropriate restriction of
amplitudes) up to bounded error simulation. This result implies that the
complexity class BQP introduced by Bernstein and Vazirani for QTMs equals its
counterpart for uniform QCFs. However, the complexity classes ZQP and EQP for
QTMs do not appear to equal their counterparts for uniform QCFs. In this paper,
we introduce a subclass of uniform QCFs, the finitely generated uniform QCFs,
based on finite number of elementary gates and show that the class of finitely
generated uniform QCFs is perfectly equivalent to the class of polynomial-time
QTMs; they can exactly simulate each other. This naturally implies that BQP as
well as ZQP and EQP equal the corresponding complexity classes of the finitely
generated uniform QCFs.Comment: 11page
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