180 research outputs found
Entanglement Verification in Quantum Networks with Tampered Nodes
In this paper, we consider the problem of entanglement verification across
the quantum memories of any two nodes of a quantum network. Its solution can be
a means for detecting (albeit not preventing) the presence of intruders that
have taken full control of a node, either to make a denial-of-service attack or
to reprogram the node. Looking for strategies that only require local
operations and classical communication (LOCC), we propose two entanglement
verification protocols characterized by increasing robustness and efficiency.Comment: 14 pages, 7 figure
Superadiabatic driving of a three-level quantum system
We study superadiabatic quantum control of a three-level quantum system whose
energy spectrum exhibits multiple avoided crossings. In particular, we
investigate the possibility of treating the full control task in terms of
independent two-level Landau-Zener problems. We first show that the time
profiles of the elements of the full control Hamiltonian are characterized by
peaks centered around the crossing times. These peaks decay algebraically for
large times. In principle, such a power-law scaling invalidates the hypothesis
of perfect separability. Nonetheless, we address the problem from a pragmatic
point of view by studying the fidelity obtained through separate control as a
function of the intercrossing separation. This procedure may be a good approach
to achieve approximate adiabatic driving of a specific instantaneous eigenstate
in realistic implementations.Comment: 11 pages, 7 figure
Digital quantum simulators in a scalable architecture of hybrid spin-photon qubits
Resolving quantum many-body problems represents one of the greatest
challenges in physics and physical chemistry, due to the prohibitively large
computational resources that would be required by using classical computers. A
solution has been foreseen by directly simulating the time evolution through
sequences of quantum gates applied to arrays of qubits, i.e. by implementing a
digital quantum simulator. Superconducting circuits and resonators are emerging
as an extremely-promising platform for quantum computation architectures, but a
digital quantum simulator proposal that is straightforwardly scalable,
universal, and realizable with state-of-the-art technology is presently
lacking. Here we propose a viable scheme to implement a universal quantum
simulator with hybrid spin-photon qubits in an array of superconducting
resonators, which is intrinsically scalable and allows for local control. As
representative examples we consider the transverse-field Ising model, a spin-1
Hamiltonian, and the two-dimensional Hubbard model; for these, we numerically
simulate the scheme by including the main sources of decoherence. In addition,
we show how to circumvent the potentially harmful effects of inhomogeneous
broadening of the spin systems
Quantum computers as universal quantum simulators: state-of-art and perspectives
The past few years have witnessed the concrete and fast spreading of quantum
technologies for practical computation and simulation. In particular, quantum
computing platforms based on either trapped ions or superconducting qubits have
become available for simulations and benchmarking, with up to few tens of
qubits that can be reliably initialized, controlled, and measured. The present
review aims at giving a comprehensive outlook on the state of art capabilities
offered from these near-term noisy devices as universal quantum simulators,
i.e. programmable quantum computers potentially able to calculate the time
evolution of many physical models. First, we give a pedagogic overview on the
basic theoretical background pertaining digital quantum simulations, with a
focus on hardware-dependent mapping of spin-type Hamiltonians into the
corresponding quantum circuit model as a key initial step towards simulating
more complex models. Then, we review the main experimental achievements
obtained in the last decade regarding the digital quantum simulation of such
spin models, mostly employing the two leading quantum architectures. We compare
their performances and outline future challenges, also in view of prospective
hybrid technologies, towards the ultimate goal of reaching the long sought
quantum advantage for the simulation of complex many body models in the
physical sciences.Comment: 27 pages, 12 figures. Pre-submission manuscript, see Journal
Reference for the final versio
Fault-Tolerant Computing with Single Qudit Encoding
We present a general approach for the Fault Tolerant implementation of
stabilizer codes with a logical qubit encoded into a single multi-level qudit,
preventing the explosion of resources of multi-qubit codes. The proposed scheme
allows for correction and universal quantum computation. We demonstrate its
effectiveness by simulations on molecular spin qudits, finding an almost
exponential suppression of logical errors with the qudit size. The resulting
performance on a small qudit is remarkable when compared to qubit codes using
thousands of units
Constructing Clock-Transition-Based Two-Qubit Gates from Dimers of Molecular Nanomagnets
A good qubit must have a coherence time long enough for gate operations to be
performed. Avoided level crossings allow for clock transitions in which
coherence is enhanced by the insensitivity of the transition to fluctuations in
external fields. Because of this insensitivity, it is not obvious how to
effectively couple qubits together while retaining clock-transition behavior.
Here we present a scheme for using a heterodimer of two coupled molecular
nanomagnets, each with a clock transition at zero magnetic field, in which all
of the gate operations needed to implement one- and two-qubit gates can be
implemented with pulsed radio-frequency radiation. We show that given realistic
coupling strengths between the nanomagnets in the dimer, good gate fidelities
(99.4\%) can be achieved. We identify the primary sources of error in
implementing gates and discuss how these may be mitigated, and investigate the
range of coherence times necessary for such a system to be a viable platform
for implementing quantum computing protocols.Comment: Version accepted by Phys. Rev. Research. Fig. 1 has minor
modifications. References adde
Chiral-Induced Spin Selectivity in Photo-Induced Electron Transfer: investigating charge and spin dynamics in a master equation framework
Investigating the role of chiral-induced spin selectivity in the generation of spin correlated radical pairs in a photoexcited donor–chiral bridge–acceptor system is fundamental to exploit it in quantum technologies. This requires a minimal master equation description of both charge separation and recombination through a chiral bridge. To achieve this without adding complexity and entering in the microscopic origin of the phenomenon, we investigate the implications of spin-polarizing reaction operators to the master equation. The explicit inclusion of coherent evolution yields non-trivial behaviors in the charge and spin dynamics of the system. Finally, we apply this master equation to a setup comprising a molecular qubit attached to the donor–bridge–acceptor molecule, enabling qubit initialization, control, and read-out. Promising results are found by simulating this sequence of operations assuming realistic parameters and achievable experimental conditions
Correction: Quantum error correction with molecular spin qudits
Correction for 'Quantum error correction with molecular spin qudits' by Mario Chizzini et al., Phys. Chem. Chem. Phys., 2022, https://doi.org/10.1039/D2CP01228F
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