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Photon echo is one of the basic tools for studying the coherent nonlinear
interaction of light pulses with inhomogeneously broadened resonant atomic
media. We generalize the McCall-Hahn area theorem to the formation of photon
echo in the optically dense media and we find the analytic solutions for the
pulse areas of the photon echo signals arising after the two-pulse excitation.
The solutions allowed us for the first time to reveal the picture and mechanism

Leakage errors take qubits out of the computational subspace and will
accumulate if not addressed. A leaked qubit will reduce the effectiveness of
quantum error correction protocols due to the cost of implementing leakage
reduction circuits and the harm caused by interacting leaked states with qubit
states. Ion trap qubits driven by Raman gates have a natural choice between
qubits encoded in magnetically insensitive hyperfine states that can leak and
qubits encoded in magnetically sensitive Zeeman states of the electron spin

We study the identification capacity of classical-quantum channels
("cq-channels"), under channel uncertainty and privacy constraints. To be
precise, we consider first compound memoryless cq-channels and determine their
identification capacity; then we add an eavesdropper, considering compound
memoryless wiretap cqq-channels, and determine their secret identification
capacity. In the first case (without privacy), we find the identification
capacity always equal to the transmission capacity. In the second case, we find

Hybrid quantum systems have the potential of mitigating current challenges in
developing a scalable quantum computer. Of particular interest is the
hybridization between atomic and superconducting qubits. We demonstrate a novel
experimental setup for transferring and trapping ultracold atoms inside a
millikelvin cryogenic environment, where interactions between atomic and
superconducting qubits can be established, paving the way for hybrid quantum
systems. $^{87}\text{Rb}$ atoms are prepared in a conventional magneto-optical

We perform quantum logic spectroscopy with a $^{27}$Al$^{+}$/$^{40}$Ca$^{+}$
mixed ion crystal in a linear Paul trap for a measurement of the
$(3s^{2})\,^{1}\mathrm{S}_{0} \leftrightarrow \, (3s3p)\,^{3}\mathrm{P}_{1},
F=7/2$ intercombination transition in $^{27}$Al$^{+}$. Towards this end, Ramsey
spectroscopy is used for probing the transition in $^{27}$Al$^{+}$ and the
$(4s^{2})\,\mathrm{S}_{1/2} \leftrightarrow \, (4s3d)\,\mathrm{D}_{5/2}$ clock
transition in $^{40}$Ca$^{+}$ in interleaved measurements. By using the

The use of spatial quantum superpositions of electron states in a gated vdW
heterostructure as a charge qubit is presented. We theoretically demonstrate
the concept for the ZrSe$_2$/SnSe$_2$ vdW heterostructure using rigorous ab
initio calculations. In the proposed scheme, the quantum state is prepared by
applying a vertical electric field, is manipulated by short field pulses, and
is measured via electric currents. The qubit is robust, operational at high

It is common, when dealing with quantum processes involving a subsystem of a
much larger composite closed system, to treat them as effectively memory-less
(Markovian). While open systems theory tells us that non-Markovian processes
should be the norm, the ubiquity of Markovian processes is undeniable. Here,
without resorting to the Born-Markov assumption of weak coupling or making any
approximations, we formally prove that processes are close to Markovian ones,

It has become increasingly common for high-school students to see media
reports on the importance of quantum mechanics in the development of
next-generation industries such as drug development and secure communication,
but few of them have been exposed to fundamental quantum mechanical concepts in
a meaningful classroom activity. In order to bridge this gap, we design and
test a low-cost 20-minute demonstration of the Bell test, which is used in
several entanglement-based quantum key distribution protocols. The

Machine learning can help us in solving problems in the context big data
analysis and classification, as well as in playing complex games such as Go.
But can it also be used to find novel protocols and algorithms for applications
such as large-scale quantum communication? Here we show that machine learning
can be used to identify central quantum protocols, including teleportation,
entanglement purification and the quantum repeater. These schemes are of
importance in long-distance quantum communication, and their discovery has

We propose a classical-quantum hybrid algorithm for machine learning on
near-term quantum processors, which we call quantum circuit learning. A quantum
circuit driven by our framework learns a given task by tuning parameters
implemented on it. The iterative optimization of the parameters allows us to
circumvent the high-depth circuit. Theoretical investigation shows that a
quantum circuit can approximate nonlinear functions, which is further confirmed
by numerical simulations. Hybridizing a low-depth quantum circuit and a