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Optimization problems in disciplines such as machine learning are commonly
solved with iterative methods. Gradient descent algorithms find local minima by
moving along the direction of steepest descent while Newton's method takes into
account curvature information and thereby often improves convergence. Here, we
develop quantum versions of these iterative optimization algorithms and apply
them to polynomial optimization with a unit norm constraint. In each step,

The resonance fluorescence of a four-level atom in J = 1/2 to J = 1/2
transition driven by two coherent fields is studied. We find that the
incoherent fluorescence spectrum shows a direct indication of vacuum-induced
coherence in the atomic system. We show that such coherence manifests itself
via an enhancement or suppression of the spectral peaks in the $\pi$-polarized
fluorescence. The effect of the relative phase of the driving fields on the
spectral features is also investigated. We show that phase-dependent

We introduce a protocol capable of generating a general measurement operator
for a mechanical resonator. The technique requires a qubit-resonator
interaction and uses a coherent pulse to drive qubit transitions. This is
followed by projective measurement of the qubit's energy, constraining the
resonator in a state that depends on the pulse shape. The freedom to choose a
pulse shape for the coherent drive enables an arbitrary position-basis
measurement operator. Using this measurement operator, we outline a two pulse

Quantum computers, which take advantage of the superposition and entanglement
of physical states, could outperform their classical counterparts in solving
problems with technological impact, such as factoring large numbers and
searching databases. A quantum processor executes algorithms by applying a
programmable sequence of gates to an initialized state of qubits, which
coherently evolves into a final state containing the result of the computation.
Although quantum processors with a few qubits have been demonstrated on

We propose the concept of machine learning configuration interaction (MLCI)
whereby an artificial neural network is trained on-the-fly to predict important
new configurations in an iterative selected configuration interaction
procedure. We demonstrate that the neural network can discriminate between
important and unimportant configurations, that it has not been trained on, much
better than by chance. MLCI is then used to find compact wavefunctions for
carbon monoxide at both stretched and equilibrium geometries. We also consider

Valid transformations between quantum states are necessarily described by
completely positive maps, instead of just positive maps. Positive but not
completely positive maps such as the transposition map cannot be implemented
due to the existence of entanglement in composite quantum systems, but there
are classes of states for which the positivity is guaranteed, e.g., states not
correlated to other systems. In this paper, we introduce the concept of N-copy

The color excitations of interacting fermions carrying an $SU(3)$ color and
$U(N_f)$ flavor index in one spatial dimension are studied in the framework of
a perturbed $SU(3)_{N_f}$ Wess-Zumino-Novikov-Witten model. Using Bethe ansatz
methods the low energy quasi-particles are found to be massive solitons forming
$SU(3)$ quark and antiquark multiplets. In addition to the color index the
solitons carry an internal degree of freedom with non-integer quantum
dimension. These zero modes are identified as non-Abelian anyons satisfying

We show that a pulsed stimulus can be used to generate many-body quantum
coherences in light-matter systems of general size. Specifically, we calculate
the exact real-time evolution of a driven, generic out-of-equilibrium system
comprising an arbitrary number N qubits coupled to a global boson field. A
novel form of dynamically-driven quantum coherence emerges for general N and
without having to access the empirically challenging strong-coupling regime.
Its properties depend on the speed of the changes in the stimulus.

In free-space quantum key distribution (QKD) between moving parties, e.g.,
free-space QKD via satellite, the reference frame rotation and fluctuation
degrades the performance of QKD. Reference-frame-independent QKD (RFI-QKD)
provides a simple but efficient way to overcome this problem. While there has
been a number of theoretical and experimental studies on RFI-QKD, the
experimental verification of the robustness of RFI-QKD over other QKD protocols
under the reference frame rotation and fluctuation is still missing. Here, we

We study perfect state transfer in Kendon's model of discrete quantum walks.
In particular, we give a characterization of perfect state transfer purely in
terms of the graph spectra, and construct an infinite family of $4$-regular
circulant graphs that admit perfect state transfer. Prior to our work, the only
known infinite families of examples were variants of cycles and diamond chains.