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Quantum algorithms can deliver asymptotic speedups over their classical
counterparts. However, there are few cases where a substantial quantum speedup
has been worked out in detail for reasonably-sized problems, when compared with
the best classical algorithms and taking into account realistic hardware
parameters and overheads for fault-tolerance. All known examples of such
speedups correspond to problems related to simulation of quantum systems and
cryptography. Here we apply general-purpose quantum algorithms for solving

Coupled parametric oscillators were recently employed as simulators of
artificial Ising networks, with the potential to solve computationally hard
minimization problems. We demonstrate a new dynamical regime within the
simplest network - two coupled parametric oscillators, where the oscillators
never reach a steady state, but show persistent, full-scale, coherent beats,
whose frequency reflects the coupling properties and strength. We present a
detailed theoretical and experimental study and show that this new dynamical

The existence of a spectral gap above the ground state has far-reaching
consequences for the low-energy physics of a quantum many-body system. A recent
work of Movassagh [R. Movassagh, PRL 119 (2017), 220504] shows that a spatially
random local quantum Hamiltonian is generically gapless. Here we observe that a
gap is more common for translation-invariant quantum spin chains, more
specifically, that these are gapped with a positive probability if the
interaction is of small rank. This is in line with a previous analysis of the

An optimal single-photon source should deterministically deliver one and only
one photon at a time, with no trade-off between the source's efficiency and the
photon indistinguishability. However, all reported solid-state sources of
indistinguishable single photons had to rely on polarization filtering which
reduced the efficiency by 50%, which fundamentally limited the scaling of
photonic quantum technologies. Here, we overcome this final long-standing
challenge by coherently driving quantum dots deterministically coupled to

Coupling a quantum many-body system to an external environment dramatically
changes its dynamics and offers novel possibilities not found in closed
systems. Of special interest are the properties of the steady state of such
open quantum many-body systems, as well as the relaxation dynamics towards the
steady state. However, new computational tools are required to simulate open
quantum many-body systems, as methods developed for closed systems cannot be
readily applied. We review several approaches to simulate open many-body

Author(s): Changhun Oh, Changhyoup Lee, Leonardo Banchi, Su-Yong Lee, Carsten Rockstuhl, and Hyunseok Jeong
Quantum fidelity is a measure to quantify the closeness between two quantum states. In an operational sense, it is defined as the minimal overlap between the probability distributions of measurement outcomes and the minimum is taken over all possible positive-operator valued measures (POVMs). Quantu...
[Phys. Rev. A 100, 012323] Published Tue Jul 16, 2019

Author(s): Zhen-Tao Zhang, Feng Mei, Xiang-Guo Meng, Bao-Long Liang, and Zhen-Shan Yang
The braiding of two non-Abelian Majorana modes is important for realizing topological quantum computation. It can be achieved through tuning the coupling between the two Majorana modes to be exchanged and two ancillary Majorana modes. However, this coupling also makes the braiding subject to environ...
[Phys. Rev. A 100, 012324] Published Tue Jul 16, 2019

Author(s): Shi-Yang Shen, Ming-Wei Dai, Xue-Tao Zheng, Qi-Yao Sun, Guang-Can Guo, and Zheng-Fu Han
An experiment evaluating continuous-variable quantum key distribution (CV-QKD) in an urban environment free-space channel has been accomplished using a single homodyne detector. This is based on Gaussian modulation with coherent states in the polarization degree of freedom. We achieved a QKD distanc...
[Phys. Rev. A 100, 012325] Published Tue Jul 16, 2019

Author(s): Cheng Ma, Zhiling Wang, Yukai Wu, Zenghui Bao, Yipu Song, Hongyi Zhang, and Luming Duan
Understanding the spin relaxation in superconducting quantum circuit and solid-state spin hybrid systems is of great importance especially for quantum storage purposes. We have studied the longitudinal relaxation for electron spins of substitutional nitrogen (P1) centers in a hybrid quantum device c...
[Phys. Rev. A 100, 012322] Published Tue Jul 16, 2019

Quantum simulation of chemical systems is one of the most promising near-term
applications of quantum computers. The variational quantum eigensolver, a
leading algorithm for molecular simulations on quantum hardware, has a serious
limitation in that it typically relies on a pre-selected wavefunction ansatz
that results in approximate wavefunctions and energies. Here we present an
arbitrarily accurate variational algorithm that instead of fixing an ansatz

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