Session P25: Quantum Information Science in Atomic, Molecular, and Optical Physics

Cooperative effects for qubits in a semi-infinite one-dimensional transmission line

In a one-dimensional (1D) transmission line, it is possible to achieve nearly ideal spatial mode-matching for a microwave field. Taking advantage of this, experiments have demonstrated strong interaction between resonant propagating microwave photons and superconducting qubits in such a system. When the system contains more than one qubit, photon-mediated interactions between the qubits gives rise to many interesting phenomena. In this work, we embedded two transmon qubits, separated by wavelength distances, in a 1D transmission line terminated by a mirror. By tuning the qubits into resonance, we observe collective effects. In particular, both the relaxation rates and the Lamb shifts of the qubits are modified as a function of the inter-qubit and qubit-mirror distances.   

Quantum Zeno effect and the many-body entanglement transition

We introduce and explore a one-dimensional "hybrid" quantum circuit model consisting of both unitary gates and projective measurements. While the unitary gates are drawn from a random distribution and act uniformly in the circuit, the measurements are made at random positions and times throughout the system. By varying the measurement rate we can tune between the volume law entangled phase for the random unitary circuit model (no measurements) and a "quantum Zeno phase" where strong measurements suppress the entanglement growth to saturate in an area-law. Extensive numerical simulations of the quantum trajectories of the many-particle wavefunctions (exploiting Clifford circuitry to access systems up to 512 qubits) provide evidence for a stable "weak measurement phase" that exhibits volume-law entanglement entropy, with a coefficient decreasing with increasing measurement rate. We also present evidence for a novel continuous quantum dynamical phase transition between the "weak measurement phase" and the "quantum Zeno phase", driven by a competition between the entangling tendencies of unitary evolution and the disentangling tendencies of projective measurements.   

Tomography of the temporal-spectral state of subnatural-linewidth single photons from atomic ensembles

The temporal mode of a single photon is considered more and more important in quantum information processing. On one hand, qubits or multi-dimension qudits can be constructed in photonic temporal mode, which are advantageous in long-distance transportation compared to other degrees of freedom, and thus have important applications in photonic quantum processing. On the other hand, to achieve a maximum efficiency in quantum memory, the temporal mode of a single photon is important in the matter-light coupling. Here we utilize cavity-free homodyne detection scheme for reconstructing the temporal density matrix of the subnatural-linewidth single photons generated from a cold atomic cloud. The characterization of the pure temporal-spectral state of narrowband single photons paves the way for exploiting the temporal-spectral degree of freedom to develop photonic quantum information processing.   

Floquet-engineered quantum state preparation in a noisy qubit

Noise and decoherence caused by the environment are two major challenges in applying adiabatic protocols to quantum technologies. Counter-diabatic (CD) and fast-forward (FF) driving protocols, which are also known as ``shortcuts-to-adiabaticity," provide powerful alternatives by allowing one to change Hamiltonian parameters rapidly while still mimicking adiabatic dynamics. However, implementing exact CD or FF driving in complex systems is hard in general. In this work, we propose a new efficient and generic method of Floquet engineered FF protocols, which effectively realize CD driving. We report on its experimental realization in a single nitrogen-vacancy center in diamond. Our protocol achieves near unit fidelity with our target state for fast protocols when our system is driven periodically at high frequency. Moreover, we find experimentally that our Floquet-engineered protocols are less susceptible to decoherence than standard fast forward protocols and allow us to reach near perfect fidelity even in the presence of a strong external noise.   

Amplification without Population Inversion from a Strongly Driven Superconducting Qubit

Amplification of EM fields is often achieved by strongly driving a medium to induce population inversion such that a weak probe can be amplified through stimulated emission. Here we strongly couple a superconducting qubit to the field in a semi-infinite waveguide. When driving the qubit strongly on resonance such that a Mollow profile appears, we observe a few percent amplitude gain for a weak probe at frequencies in between the Mollow profile. This amplification is not due to population inversion, but instead results from a four-photon process that converts energy from the strong drive to the weak probe. We find excellent agreement between the experimental results and numerical simulations without any free fitting parameters.
  

Multiphoton pumping between two microwaves mediated by transmon qubit

In this work, we analyze the interference patterns, observed in the multiphoton absorption spectrum of a transmon qubit, when it is driven by a pair of microwaves, tuned near and far off the qubit resonance. The appearance of such structures in the spectrum is explained as various photon contributions from the two microwave drives. We show that each of these manifolds demonstrates single or multiphoton pumping from one microwave to another. A many-mode Floquet formalism, with longitudinal coupling, is used to simulate the observed
quantum interference. An intuitive graph theoretic approach is introduced to derive the effective Hamiltonian that elucidate main features of the Floquet results. The analytical solutions also illustrate how controllability is achievable for desired single- or multiphoton pumping processes in a wide frequency range. The proposed experimental design and the theoretical approach can be extended to tunable qubit circuits or solid state semiconductors, in general, which offer full controllability of the energy gaps.   

Periodically Pulsed Quantum Light from a Superconducting Qubit Ensemble

Nonclassical light sources are extremely important in the fields of quantum information, quantum communication, and photonic quantum technologies. We study a driven inhomogeneous ensemble of superconducting qubits coupled to a microwave resonator. The constructive rephasing of spins in the frequency comb is predicted to result in a periodic pulse train of quantum light (g₂(t,0)<1) which corresponds to the collective transfer of excitations from the qubit ensemble to the resonator [1]. Such periodic nonclassical pulses are interesting for a temporal synchronization in quantum memory protocols. We will present results of our ongoing experiments on periodic quantum light generation in the system of five transmon qubits capacitively coupled to a coplanar waveguide resonator.

[1] Himadri Shekhar Dhar, Matthias Zens, Dmitry O. Krimer, and Stefan Rotter. Variational renormalization group for dissipative spin-cavity systems: Periodic pulses of nonclassical photons from mesoscopic spin ensembles. Phys. Rev. Lett., 121:133601, Sep 2018.   

Nano-strings swing circuit QED

In nano-electromechanics, quantum mechanical phenomena can be studied in the literal sense. For example, the coupling of a nanomechanical element to a superconducting resonator allows to cool the mechanical mode to its ground state and to squeeze its motion. Replacing the linear microwave resonator with a nonlinear one enables the preparation of more complex non-classical mechanical states.
In this presentation, we discuss various opto-mechanical coupling schemes in nano-electromechanical circuits employing nonlinear elements. In this context, Josephson junctions in superconducting circuit environments are the obvious choice. In particular, we envisage the scenario of a mechanically compliant tensile string embedded in a resonator including Josephson junctions, e.g. a transmon qubit or a flux-tunable resonator. We discuss realistically achievable opto-mechanical coupling strengths for these circuit layouts. In addition, we set this in context with the limitations imposed by such circuits, in particular in view of the photon numbers. Such hybrid systems open new perspectives in the field of optomechanics ranging from sensing applications to the preparation of quantum states.   

Engineered Reservoirs for Thermalization of Many-Body Quantum Systems

Engineering genuine thermal states in quantum analog simulation platforms are addressed by a technique based on a many-body spin Hamiltonians coupled to driven, dissipative ancilla spins. A Born-Markov master equation describing the dynamics of a many-body system coupled to fast-relaxing, driven ancilla qubits is developed, and if the ancilla energies are periodically modulated and swept across the system energy spectrum, with a carefully chosen hierarchy of timescales, one can effectively thermalize a many-body system. Analytical proofs and numerical investigations are used to validate the requirements of thermalization and demonstrate the true thermal state is an approximate dynamical fixed point.   

A phononic network of spin qubits for quantum computing

We describe a general architecture for networking solid-state qubits through
vibrational motion. The architecture makes use of alternating phononic
crystal waveguides to facilitate communication between distant qubits in a
scalable manner. We discuss a specific realization of the architecture
using Nitrogen vacancy centers in diamond, and show that state-of-the-art
devices operating at 500 mK can implement two-qubit universal quantum gates
with a fidelity exceeding 0.99.   

Quantum phase investigation of a single nuclear spin in a magnetic molecule

More than a hundred years ago, physicists used thought experiments to debate the controversial point of quantum theory while undoing the experimental constraints. Since then, advances in experimental techniques offer physicists the opportunity to study systems worthy of those imagined by the founders of the quantum theory. During the presentation, we will investigate the quantum dynamic of one of these toy model systems, namely a single 3/2 nuclear spin.
We will first see how to measure a single electronic [1] and nuclear spin using a molecular magnet transistor. Then we will couple the nuclear spin to a microwave electric field [2] and measure the coherent manipulation of the 3 nuclear spin transitions, thus demonstrating a full controlled of a 4-level quantum system, a qudit [3]. With their state space of dimension d>2, Qudits open fascinating experimental prospects. In this presentation I will show implementation of protocols based on a generalization of the Ramsey interferometry to a multi-level system to measure the accumulation of geometric phases and of quantum gate phase [4].

[1] Godfrin C. et al. ACS Nano 11, 3984 (2017)
[2] Thiele S. et al. Science 344, 1135 (2014)
[3] Godfrin C. et al. Phys. Rev. Lett. 119, 187702 (2017)
[4] Godfrin C. et al. npj Quant. inf. 4, 53 (2018)
  

Practical Implementations of Photonic Quantum Walks on Graphs

Discrete-time quantum walks have become a subject of great interest because of their potential applications for quantum information processing tasks. Quantum walk-based algorithms provide significant speedup over classical algorithms in a range of tasks including search algorithms, solving the element distinctness problem, and evaluating NAND trees. Up to this point, work on quantum walk-based algorithms has been largely theoretical, as discrete-time quantum walk systems are difficult to implement beyond a few time steps. Here we demonstrate a step forward in producing practical experimental realization of optical quantum walks by means of directionally-unbiased linear optical multiports, which have recently been experimentally demonstrated in the form of tabletop setups. These multiports or other related devices, such as reversible optical tritters, can be considered as additional fundamental blocks for quantum walk applications, and can serve as scattering vertices for practical implementations of optical graph systems.   

Double-slit Interference as a Lossy Beam-splitter

A post-selected unitary description of optical interferometers makes them a good machine for quantum information processing and computation. However, slit diffraction/interference lacks a unitary description. We present a classical post-selected unitary description of slit diffraction, bringing it at par with other interferometries.
The solution of Helmholtz equation in three dimensions with N sources are projected on two-dimensional surfaces called slices, all parallel to each other. A post-selected unitary description of slit-diffraction is achieved by representing diffraction as a map between slices as they pass through slits and projecting the slices on N detectors to obtain an N × N transfer matrix.
Using such a formalism, we show that a double-slit with post-selection taking into account the losses in the diffraction process, is a beam-splitter. With the use of FDTD simulations, the application of this formalism is demonstrated for near-field and far-field cases. We also show that such a framework can be used to get a post-selected unitary description of a diffraction grating.
The classical treatment sets the stage for future research on diffraction-based quantum interferometry which involves quantizing the fields and the construction of sophisticated interferometers.   

Nearly optimal quantum control: an analytical approach

We propose nearly optimal control strategies for changing the states of a quantum system. We argue that quantum control optimization can be studied analytically within some protocol families that depend on a small set of parameters for optimization. This optimization strategy can be preferred in practice because it is physically transparent and does not lead to combinatorial complexity in multistate problems. As a demonstration, we design optimized control protocols that achieve switching between orthogonal states of a naturally biased quantum two-level system.   

Limitations on the Use of the Heisenberg Picture in Quantum Optics

The Heisenberg picture is often used to analyze the performance of optical components, such as a beam splitter or an optical parametric amplifier. We consider a sequence of two or more unitary transformations and show that the Heisenberg operator produced by the first transformation cannot be used as the input to the second transformation [1]. As a result, an inappropriate use of the Heisenberg picture can give misleading or incorrect conclusions. The experimental consequences of this will be illustrated using several examples from quantum optics.

1. J.D. Franson and R.A. Brewster, submitted to Phys. Rev. Lett.