arXiv highlights

 A randomized benchmarking suite for mid-circuit measurements

Mid-circuit measurements of ancilla qubits form a key component of near-term quantum algorithms including NISQ-TDA, future quantum error correction techniques, and quantum algorithms for fault-tolerant quantum computers. This preprint from IBM presents techniques for quantifying the performance and error rates of mid-circuit measurements which are now supported by IBM quantum processors.

 

Laser-guided lightning

 

Focused high-intensity laser pulses can ionize air molecules, creating channels of air with higher electrical conductivity which can be used to guide and induce lightning strikes. Peaceful applications include protecting airports and rockets from lightning strikes. No doubt there are also military applications of this kind of research.

 

Large-Scale Simulation of Quantum Computational Chemistry on a New Sunway Supercomputer

 

Quantum chemistry calculations using classical supercomputers. Exponential memory scaling is avoided by using density matrix embedding theory to eliminate irrelevant degrees of freedom and matrix product states to efficiently capture quantum correlations. Apparently this approach can handle chemical systems with up to 100 atoms and it may also be useful for benchmarking variational quantum algorithms run on near-term quantum processors.

 

The Quantum Approximate Optimization Algorithm performance with low entanglement and high circuit depth

 

"Our simulation with the lowest bond dimension D = 2—for which the fidelity is close to zero—obtains a solution to the classical problem providing the depth of the circuit is high enough p ≈ 20, even with a sub-optimal choice of parameters and single deterministic sampling of bitstrings...In conclusion, we observe that entanglement plays a minor role in finding the solution to the classical problems studied here for large-depth QAOA." More evidence that your "quantum" optimization algorithm can be efficiently run on an ordinary classical computer.

Computing Electronic Correlation Energies using Linear Depth Quantum Circuits

 

Finally, I am advertising our own recent work on incorporating electronic correlation effects into shallow quantum circuits suitable for near-term noisy quantum processors. 

Focking lasers, how do they work?

Fock lasers based on deep-strong coupling of light and matter

This summary is a little late because the preprint was posted when I was travelling last year.

This paper proposes a new kind of quantum light source using hybrid light-matter systems in the deep strong coupling regime. This is the regime in which the photonic mode with frequency ω (say, a microwave cavity) and a two-level system with transition frequency ω0 (such as a superconducting qubit) are coupled with a strength g >> ω, ω0. This hybrid system can act as a cavity with an effective nonlinearity of extremely high order, which gives rise to an N-photon blockade: the first N states can be populated, but the (N+1)th state cannot. When this hybrid system is coupled to an external pump it relaxes to a steady state in which the photon number distribution is strongly peaked at a Fock state with N = g^2 photons.

How does it work? The key is the spectrum of the hybrid system in the deep strong coupling regime. The low energy spectrum looks just like an ordinary quantum harmonic oscillator, with a uniform spacing ω between adjacent energy levels. But the eigenstates look very different, thanks to the photon-qubit coupling.

The ground state is twofold degenerate: the photonic mode is in a (Gaussian) coherent state displaced from the origin by +g (or -g), while the qubit's spin points in the -x (or +x) direction. Since g is large, these two states have a negligible overlap with each other; they are essentially independent.

 

 Two degenerate ladders of excited states are obtained by adding photons to each ground state. Each photon added increases the energy by ω (hence, the linear spectrum). The photonic wavefunction broadens and becomes more strongly oscillatory, just like excited states of the quantum harmonic oscillator. But each wavefunction's centre of mass is displaced by +g (or -g).

When the photon number reaches the critical value of N = g^2, the photonic parts of the two wavefunctions start to overlap and can therefore interfere. This means that the two ladders can no longer be treated independently; they become coupled. Interference between the -x and +x spin states of the qubit generates a nonzero z component of the spin. This gives an additional energy shift ω0 𝜎z (due to the qubit part of the Hamiltonian) that destroys the uniform level spacing. Because the large n harmonic oscillator wavefunctions oscillate rapidly, this energy shift is highly sensitive to n, making the spectrum nonlinear for n > g^2. Thus, only N quanta with energy ω can be resonantly coupled to the system before it tunes itself out of resonance. This is the mechanism behind the N-photon blockade.

These plots are based on g = 5. State-of-the-art experiments can reached g=2. Hopefully further improvements in superconducting quantum circuits will allow us to push g to even higher values.

arXiv highlights

Some arXiv preprints that caught my attention over the past week:

Fock lasers based on deep-strong coupling of light and matter

Researchers at MIT predict that the deep strong coupling regime achievable in state-of-the-art superconducting quantum circuits can be used to design novel light sources of N photon Fock states with N ~100. The mechanism is a suppression of stimulated emission via a many-body photon blockade. Large N Fock states are a key ingredient in certain quantum sensing and superresolution schemes, but are extremely hard to make using conventional quantum light sources. This proposal focuses on the generation of microwave photons. A related preprint considers the extension to optical frequencies.

Eigenvalue topology of non-Hermitian band structures in two and three dimensions.

This is a theory follow-up to the authors recent publication demonstrating knotting and braiding of non-Hermitian energy bands. In 2D systems the braid group describes non-contractible loops in the Brillouin zone which can either encircle non-Hermitian degeneracy points or wrap around the whole Brillouin zone. In 3D it is the non-Hermitian degeneracies themselves that form curves that can be knotted or braided. The group theory used to classify general N-band non-Hermitian systems seems rather heavy at first glance.

Anderson localization of a Rydberg electron.

Rydberg atoms host highly excited electrons residing in orbitals with large principal quantum numbers n. Thanks to spherical symmetry, each n corresponds to n^2 degenerate orbital angular momentum modes, forming a large set of available modes. Using now-sophisticated optical tweezer technology it the authors propose placing strong scatterers in the vicinity of the Rydberg orbitals, inducing coupling between them. By controlling the effective inter-orbital coupling and energies by the scatterer positions' the authors propose the realization of the Anderson tight binding model describing localization of electrons in disordered systems. This is a neat idea as it means a single Rydberg atom may be used as a toolbox for exploring various fundamental models from condensed matter physics.

Superconducting optomechanics in topological lattices.

Topological lattice designs are now attracting growing experimental interest in optomechanics, following seminal theoretical proposals several years ago. Here 1D Su-Schrieffer-Heeger lattices and 2D honeycomb lattices are implemented. Optomechanics is a promising setting for exploring both nonlinear and quantum topological phenomena in lattices.

Finally, some self-promotion:

Nonlinear signatures of Floquet band topology.

We finally finished the follow-up to our PRL published earlier this year on measuring Chern numbers using nonlinear modulational instability. The approach also works for periodically-driven Floquet systems, at least for identifying Chern insulator phases. The detection of anomalous Floquet phases is a little more tricky.

 

Lasing in a Mobius strip microcavity

 

A neat paper was just published in Physical Review Letters and highlighted as an Editor's Suggestion: Möbius Strip Microlasers: A Testbed for Non-Euclidean Photonics

 

In conventional microring lasers based on whispering gallery modes each lasing mode can be described semiclassically as a light ray that is reflected off the boundary between the outer edge of the high refractive index ring and the ambient medium. The ray returns to the same position after completing one cycle of the ring, forming a periodic orbit. Rays with a large angle of incidence with the outer edge will undergo total internal reflection and thus have extremely low losses. Thus, the rays follow the outer edge closely.

What happens if we add a twist to the ring to form a Mobius strip? Do the whispering gallery lasing modes survive?

 

No. The boundary of a Mobius strip forms a single closed curve; following the (concave) outer edge for one cycle you will end up at the (convex) inner edge, which cannot support whispering gallery modes.

 

The simplest periodic orbits supported by the Mobius strip involve alternating reflections off the inner and outer edges. The low angle of incidence makes these orbits highly lossy and unlikely to lase.

 

The authors fabricated Mobius strip cavities and measured their lasing spectra. The spacing between the peaks was used to obtain the optical path length of the lasing modes, revealing that they are neither whispering gallery modes nor the alternating reflection periodic orbits. Instead, they reveal a novel class of periodic orbit unique to three-dimensional Mobius strips, in which the ray only reflects off the outer edge (ensuring low losses), but also crosses the interior of the strip to avoid ending up on the inner edge.

The experimental observations are corroborated with numerical solutions of the Helmholtz equation and semiclassical computation of the periodic orbits. Future work will consider the full vectorial Maxwell's equations to unveil the role of polarization in this peculiar class of twisted cavities.