110 research outputs found
Quantum communication without alignment using multiple-qubit single-photon states
We propose a scheme for encoding logical qubits in a subspace protected
against collective rotations around the propagation axis using the polarization
and transverse spatial degrees of freedom of single photons. This encoding
allows for quantum key distribution without the need of a shared reference
frame. We present methods to generate entangled states of two logical qubits
using present day down-conversion sources and linear optics, and show that the
application of these entangled logical states to quantum information schemes
allows for alignment-free tests of Bell's inequalities, quantum dense coding
and quantum teleportation
Reliable quantum certification for photonic quantum technologies
A major roadblock for large-scale photonic quantum technologies is the lack
of practical reliable certification tools. We introduce an experimentally
friendly - yet mathematically rigorous - certification test for experimental
preparations of arbitrary m-mode pure Gaussian states, pure non-Gaussian states
generated by linear-optical circuits with n-boson Fock-basis states as inputs,
and states of these two classes subsequently post-selected with local
measurements on ancillary modes. The protocol is efficient in m and the inverse
post-selection success probability for all Gaussian states and all mentioned
non-Gaussian states with constant n. We follow the mindset of an untrusted
prover, who prepares the state, and a skeptic certifier, with classical
computing and single-mode homodyne-detection capabilities only. No assumptions
are made on the type of noise or capabilities of the prover. Our technique
exploits an extremality-based fidelity bound whose estimation relies on
non-Gaussian state nullifiers, which we introduce on the way as a byproduct
result. The certification of many-mode photonic networks, as those used for
photonic quantum simulations, boson samplers, and quantum metrology, is now
within reach.Comment: 8 pages + 20 pages appendix, 2 figures, results generalized to
scenarios with post-selection, presentation improve
High-fidelity ion-trap quantum computing with hyperfine clock states
We propose the implementation of a geometric-phase gate on
magnetic-field-insensitive qubits with -dependent forces for
trapped ion quantum computing. The force is exerted by two laser beams in a
Raman configuration. Qubit-state dependency is achieved by a small frequency
detuning from the virtually-excited state. Ion species with excited states of
long radiative lifetimes are used to reduce the chance of a spontaneous photon
emission to less than 10 per gate-run. This eliminates the main source
of gate infidelity of previous implementations. With this scheme it seems
possible to reach the fault tolerant threshold.Comment: 4 pages, 1 figur
Scaling laws for the decay of multiqubit entanglement
We investigate the decay of entanglement of generalized N-particle
Greenberger-Horne-Zeilinger (GHZ) states interacting with independent
reservoirs. Scaling laws for the decay of entanglement and for its finite-time
extinction (sudden death) are derived for different types of reservoirs. The
latter is found to increase with the number of particles. However, entanglement
becomes arbitrarily small, and therefore useless as a resource, much before it
completely disappears, around a time which is inversely proportional to the
number of particles. We also show that the decay of multi-particle GHZ states
can generate bound entangled states.Comment: Minor mistakes correcte
Fidelity Witnesses for Fermionic Quantum Simulations
The experimental interest and developments in quantum spin-1/2 chains has increased uninterruptedly over the past decade. In many instances, the target quantum simulation belongs to the broader class of noninteracting fermionic models, constituting an important benchmark. In spite of this class being analytically efficiently tractable, no direct certification tool has yet been reported for it. In fact, in experiments, certification has almost exclusively relied on notions of quantum state tomography scaling very unfavorably with the system size. Here, we develop experimentally friendly fidelity witnesses for all pure fermionic Gaussian target states. Their expectation value yields a tight lower bound to the fidelity and can be measured efficiently. We derive witnesses in full generality in the Majorana-fermion representation and apply them to experimentally relevant spin-1/2 chains. Among others, we show how to efficiently certify strongly out-of-equilibrium dynamics in critical Ising chains. At the heart of the measurement scheme is a variant of importance sampling specially tailored to overlaps between covariance matrices. The method is shown to be robust against finite experimental-state infidelities
Almost all quantum states have nonclassical correlations
Quantum discord quantifies non-classical correlations in a quantum system
including those not captured by entanglement. Thus, only states with zero
discord exhibit strictly classical correlations. We prove that these states are
negligible in the whole Hilbert space: typically a state picked out at random
has positive discord; and, given a state with zero discord, a generic
arbitrarily small perturbation drives it to a positive-discord state. These
results hold for any Hilbert-space dimension, and have direct implications on
quantum computation and on the foundations of the theory of open systems. In
addition, we provide a simple necessary criterion for zero quantum discord.
Finally, we show that, for almost all positive-discord states, an arbitrary
Markovian evolution cannot lead to a sudden, permanent vanishing of discord
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