2,571 research outputs found
Quantum simulation of partially distinguishable boson sampling
Boson Sampling is the problem of sampling from the same output probability
distribution as a collection of indistinguishable single photons input into a
linear interferometer. It has been shown that, subject to certain computational
complexity conjectures, in general the problem is difficult to solve
classically, motivating optical experiments aimed at demonstrating quantum
computational "supremacy". There are a number of challenges faced by such
experiments, including the generation of indistinguishable single photons. We
provide a quantum circuit that simulates bosonic sampling with arbitrarily
distinguishable particles. This makes clear how distinguishabililty leads to
decoherence in the standard quantum circuit model, allowing insight to be
gained. At the heart of the circuit is the quantum Schur transform, which
follows from a representation theoretic approach to the physics of
distinguishable particles in first quantisation. The techniques are quite
general and have application beyond boson sampling.Comment: 25 pages, 4 figures, 2 algorithms, comments welcom
Randomized benchmarking in measurement-based quantum computing
Randomized benchmarking is routinely used as an efficient method for
characterizing the performance of sets of elementary logic gates in small
quantum devices. In the measurement-based model of quantum computation, logic
gates are implemented via single-site measurements on a fixed universal
resource state. Here we adapt the randomized benchmarking protocol for a single
qubit to a linear cluster state computation, which provides partial, yet
efficient characterization of the noise associated with the target gate set.
Applying randomized benchmarking to measurement-based quantum computation
exhibits an interesting interplay between the inherent randomness associated
with logic gates in the measurement-based model and the random gate sequences
used in benchmarking. We consider two different approaches: the first makes use
of the standard single-qubit Clifford group, while the second uses recently
introduced (non-Clifford) measurement-based 2-designs, which harness inherent
randomness to implement gate sequences.Comment: 10 pages, 4 figures, comments welcome; v2 published versio
Generating entanglement with linear optics
Entanglement is the basic building block of linear optical quantum
computation, and as such understanding how to generate it in detail is of great
importance for optical architectures. We prove that Bell states cannot be
generated using only 3 photons in the dual-rail encoding, and give strong
numerical evidence for the optimality of the existing 4 photon schemes. In a
setup with a single photon in each input mode, we find a fundamental limit on
the possible entanglement between a single mode Alice and arbitrary Bob. We
investigate and compare other setups aimed at characterizing entanglement in
settings more general than dual-rail encoding. The results draw attention to
the trade-off between the entanglement a state has and the probability of
postselecting that state, which can give surprising constant bounds on
entanglement even with increasing numbers of photons.Comment: 13 pages, 10 figures, 1 table, comments welcom
Phase-random states: ensembles of states with fixed amplitudes and uniformly distributed phases in a fixed basis
Motivated by studies of typical properties of quantum states in statistical
mechanics, we introduce phase-random states, an ensemble of pure states with
fixed amplitudes and uniformly distributed phases in a fixed basis. We first
show that canonical states typically appear in subsystems of phase-random
states. We then investigate the simulatability of phase-random states, which is
directly related to that of time evolution in closed systems, by studying their
entanglement properties. We find that starting from a separable state, time
evolutions under Hamiltonians composed of only separable eigenstates generate
extremely high entanglement and are difficult to simulate with matrix product
states. We also show that random quantum circuits consisting of only two-qubit
diagonal unitaries can generate an ensemble with the same average entanglement
as phase-random states.Comment: Revised, 12 pages, 4 figur
A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New Zealand
We document magnetostratigraphies for three river sections (Turakina, Rangitikei, Wanganui) in Wanganui Basin and interpret them as corresponding to the Upper Gilbert, the Gauss and lower Matuyama Chrons of the Geomagnetic Polarity Timescale, in agreement with foraminiferal biostratigraphic datums. The Gauss-Gilbert transition (3.58 Ma) is located in both the Turakina and Wanganui River sections, while the Gauss-Matuyama transition (2.58 Ma) is located in all three sections, as are the lower and upper boundaries of the Mammoth (3.33–3.22 Ma) and Kaena (3.11–3.04 Ma) Subchrons. Our interpretations are based in part on the re-analysis of existing datasets and in part on the acquisition and analysis of new data, particularly for the Wanganui River section. The palaeomagnetic dates of these six horizons provide the only numerical age control for a thick (up to 2000 m) mudstone succession (Tangahoe Mudstone) that accumulated chiefly in upper bathyal and outer neritic palaeoenvironments. In the Wanganui River section the mean sediment accumulation rate is estimated to have been about 1.8 m/k.y., in the Turakina section it was about 1.5 m/k.y., and in the Rangitikei section, the mean rate from the beginning of the Mammoth Subchron to the Hautawa Shellbed was about 1.1 m/k.y. The high rates may be associated with the progradation of slope clinoforms northward through the basin. This new palaeomagnetic timescale allows revised correlations to be made between cyclothems in the Rangitikei River section and the global Oxygen Isotope Stages (OIS) as represented in Ocean Drilling Program (ODP) Site 846. The 16 depositional sequences between the end of the Mammoth Subchron and the Gauss-Matuyama Boundary are correlated with OIS MG2 to 100. The cyclothems average 39 k.y. in duration in our age model, which is close to the 41 k.y. duration of the orbital obliquity cycles. We support the arguments advanced recently in defence of the need for local New Zealand stages as a means of classifying New Zealand sedimentary successions, and strongly oppose the proposal to move stage boundaries to selected geomagnetic polarity transitions. The primary magnetisation of New Zealand mudstone is frequently overprinted with secondary components of diagenetic origin, and hence it is often difficult to obtain reliable magnetostratigraphic records. We suggest specific approaches, analytical methods, and criteria to help ensure robustness and coherency in the palaeomagnetic identification of chron boundaries in typical New Zealand Cenozoic mudstone successions
Time-reversal frameness and superselection
We show that appropriate superpositions of motional states are a reference
frame resource that enables breaking of time -reversal superselection so that
two parties lacking knowledge about the other's direction of time can still
communicate. We identify the time-reversal reference frame resource states and
determine the corresponding frameness monotone, which connects time-reversal
frameness to entanglement. In contradistinction to other studies of reference
frame quantum resources, this is the first analysis that involves an
antiunitary rather than unitary representation.Comment: 10 p
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