513 research outputs found
Quantum memories with zero-energy Majorana modes and experimental constraints
In this work we address the problem of realizing a reliable quantum memory
based on zero-energy Majorana modes in the presence of experimental constraints
on the operations aimed at recovering the information. In particular, we
characterize the best recovery operation acting only on the zero-energy
Majorana modes and the memory fidelity that can be therewith achieved. In order
to understand the effect of such restriction, we discuss two examples of noise
models acting on the topological system and compare the amount of information
that can be recovered by accessing either the whole system, or the zero-modes
only, with particular attention to the scaling with the size of the system and
the energy gap. We explicitly discuss the case of a thermal bosonic environment
inducing a parity-preserving Markovian dynamics in which the introduced memory
fidelity decays exponentially in time, independent from system size, thus
showing the impossibility to retrieve the information by acting on the
zero-modes only. We argue, however, that even in the presence of experimental
limitations, the Hamiltonian gap is still beneficial to the storage of
information.Comment: 18 pages, 7 figures. Updated to published versio
Multi-Phase Hadamard receivers for classical communication on lossy bosonic channels
A scheme for transferring classical information over a lossy bosonic channel
is proposed by generalizing the proposal presented in Phys. Rev. Lett. 106,
240502 (2011) by Guha. It employs codewords formed by products of coherent
states of fixed mean photon number with multiple phases which, through a
passive unitary transformation, reduce to a Pulse-Position Modulation code with
multiple pulse phases. The maximum information rate achievable with optimal,
yet difficult to implement, detection schemes is computed and shown to saturate
the classical capacity of the channel in the low energy regime. An easy to
implement receiver based on a conditional Dolinar detection scheme is also
proposed finding that, while suboptimal, it allows for improvements in an
intermediate photon-number regime with respect to previous proposals.Comment: final version: minor changes; 8+3 pages and 5 figure
The capacity of coherent-state adaptive decoders with interferometry and single-mode detectors
A class of Adaptive Decoders (AD's) for coherent-state sequences is studied,
including in particular the most common technology for optical-signal
processing, e.g., interferometers, coherent displacements and photon-counting
detectors. More generally we consider AD's comprising adaptive procedures based
on passive multi-mode Gaussian unitaries and arbitrary single-mode destructive
measurements. For classical communication on quantum phase-insensitive Gaussian
channels with a coherent-state encoding, we show that the AD's optimal
information transmission rate is not greater than that of a single-mode
decoder. Our result also implies that the ultimate classical capacity of
quantum phase-insensitive Gaussian channels is unlikely to be achieved with the
considered class of AD's.Comment: v3: final version; 6 pages; 2 figure
Coherent-state discrimination via non-heralded probabilistic amplification
A scheme for the detection of low-intensity optical coherent signals was
studied which uses a probabilistic amplifier operated in the non-heralded
version, as the underlying non-linear operation to improve the detection
efficiency. This approach allows us to improve the statistics by keeping track
of all possible outcomes of the amplification stage (including failures). When
compared with an optimized Kennedy receiver, the resulting discrimination
success probability we obtain presents a gain up to ~1.85% and it approaches
the Helstrom bound appreciably faster than the Dolinar receiver, when employed
in an adaptive strategy. We also notice that the advantages obtained can be
ultimately associated with the fact that, in the high gain limit, the
non-heralded version of the probabilistic amplifier induces a partial dephasing
which preserves quantum coherence among low energy eigenvectors while removing
it elsewhere. A proposal to realize such transformation based on an optical
cavity implementation is presented.Comment: Final version: 6 pages and 4 figure
Optimal quantum state discrimination via nested binary measurements
A method to compute the optimal success probability of discrimination of N
arbitrary quantum states is presented, based on the decomposition of any
N-outcome measurement into sequences of nested two-outcome ones. In this way
the optimization of the measurement operators can be carried out in successive
steps, optimizing first the binary measurements at the deepest nesting level
and then moving on to those at higher levels. We obtain an analytical
expression for the maximum success probability after the first optimization
step and examine its form for the specific case of N=3,4 states of a qubit. In
this case, at variance with previous proposals, we are able to provide a
compact expression for the success probability of any set of states, whose
numerical optimization is straightforward; the results thus obtained highlight
some lesser-known features of the discrimination problem.Comment: v2: added references to previous works closely related to Sec. II;
8+3 pages; 3 figure
A Perturbative Approach to Continuous-Time Quantum Error Correction
We present a novel discussion of the continuous-time quantum error correction
introduced by Paz and Zurek in 1998 [Paz and Zurek, Proc. R. Soc. A 454, 355
(1998)]. We study the general Lindbladian which describes the effects of both
noise and error correction in the weak-noise (or strong-correction) regime
through a perturbative expansion. We use this tool to derive quantitative
aspects of the continuous-time dynamics both in general and through two
illustrative examples: the 3-qubit and the 5-qubit stabilizer codes, which can
be independently solved by analytical and numerical methods and then used as
benchmarks for the perturbative approach. The perturbatively accessible time
frame features a short initial transient in which error correction is
ineffective, followed by a slow decay of the information content consistent
with the known facts about discrete-time error correction in the limit of fast
operations. This behavior is explained in the two case studies through a
geometric description of the continuous transformation of the state space
induced by the combined action of noise and error correction.Comment: 14 pages, 10 figure
Narrow Bounds for the Quantum Capacity of Thermal Attenuators
Thermal attenuator channels model the decoherence of quantum systems
interacting with a thermal bath, e.g., a two-level system subject to thermal
noise and an electromagnetic signal travelling through a fiber or in
free-space. Hence determining the quantum capacity of these channels is an
outstanding open problem for quantum computation and communication. Here we
derive several upper bounds on the quantum capacity of qubit and bosonic
thermal attenuators. We introduce an extended version of such channels which is
degradable and hence has a single-letter quantum capacity, bounding that of the
original thermal attenuators. Another bound for bosonic attenuators is given by
the bottleneck inequality applied to a particular channel decomposition. With
respect to previously known bounds we report better results in a broad range of
attenuation and noise: we can now approximate the quantum capacity up to a
negligible uncertainty for most practical applications, e.g., for low thermal
noise.Comment: v4: corrected typo in Eq. 40; final version, minor corrections; 8+3
pages, 4 figure
Asymptotically-deterministic robust preparation of maximally entangled bosonic states
We introduce a theoretical scheme to prepare a pure Bell singlet state of two
bosonic qubits, in a way that is robust under the action of arbitrary local
noise. Focusing on a photonic platform, the proposed procedure employs passive
optical devices and a polarization-insensitive, non-absorbing, parity check
detector in an iterative process which achieves determinism asymptotically with
the number of repetitions. Distributing the photons over two distinct spatial
modes, we further show that the elements of the related basis composed of
maximally entangled states can be divided in two groups according to an
equivalence based on passive optical transformations. We demonstrate that the
parity check detector can be used to connect the two sets of states. We thus
conclude that the proposed protocol can be employed to prepare any pure state
of two bosons which are maximally entangled in either the internal degree of
freedom (Bell states) or the spatial mode (NOON states).Comment: 5 pages, 3 figure
Robust engineering of maximally entangled states by identical particle interferometry
We propose a procedure for the robust preparation of maximally entangled
states of identical fermionic qubits, studying the role played by particle
statistics in the process. The protocol exploits externally activated noisy
channels to reset the system to a known state. The subsequent interference
effects generated at a beam splitter result in a mixture of maximally entangled
Bell states and NOON states. We also discuss how every maximally entangled
state of two fermionic qubits distributed over two spatial modes can be
obtained from one another by fermionic passive optical transformations. Using a
pseudospin-insensitive, non-absorbing, parity check detector, the proposed
technique is thus shown to deterministically prepare any arbitrary maximally
entangled state of two identical fermions. These results extend recent findings
related to bosonic qubits. Finally, we analyze the performance of the protocol
for both bosons and fermions when the externally activated noisy channels are
not used and the two qubits undergo standard types of noise. The results supply
further insights towards viable strategies for noise-protected entanglement
exploitable in quantum-enhanced technologies.Comment: 9 pages, 6 figure
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