Session R26: Cavity QED and Quantum Optics

Strong Coupling of Microwave Photons to Antiferromagnetic Fluctuations in an Organic Magnet

We present experiments on the coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium radicals and a superconducting microwave resonator in a circuit quantum electrodynamics (circuit QED) architecture [1]. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multispin modes in organic crystals are suitable for circuit QED, offering a platform for their coherent manipulation. They also utilize the circuit QED architecture as a way to probe spin correlations at low temperature.

[1] Mergenthaler, M. et al., Phys. Rev. Lett. 119, 147701 (2017).   

Superconducting Mm-wave Photonic Crystal Cavities for Rydberg Cavity Quantum Electrodynamics.

I will describe progress towards a hybrid experimental system for engineering strong interactions between single optical and mm-wave photons using Rydberg atoms as an interface. Entanglement between photons with 100 gigahertz and optical frequencies creates a new platform to access exotic photonic quantum states as well as powerful new techniques in quantum computing and simulation at 1K. I will present recent experimental developments including trapping and cooling atoms in a cryogenic Magneto Optical Trap, measuring high-Q superconducting cavities at 100 GHz and coupling atoms to an optical cavity inside our custom designed and home-made cryostat. I will discuss in detail our use of photonic crystals as a new design for fabricated 100-gigahertz superconducting resonators.
  

Population inversion and entanglement dynamics of two coupled qubits in cavity QED

The great interest in realizing quantum information processing systems in the last few decades sparked intense efforts and led to a significant progress in engineering new quantum systems that are considered as very promising candidates for the task. These developed systems (such as semiconducting quantum dots and superconducting circuits) in addition to natural atomic systems (such as Rydberg atoms and trapped atoms, ions and molecules) enjoy a strong coupling with a similar type of system or even with a different type (when implemented in a hybrid system). Their dynamics and mutual interaction can be controlled when exposed to quantized radiation fileds in cavity (circuit) QED. We study the dynamics of a pair of such coupled quantum systems interacting with a radiation field at resonance (zero detuning) and out of reasonance with a focus on their population inversion and entanglement dynamics.   

Superradiance phase transition in the presence of parameter fluctuations

One of the remarkable phenomena in the study of light-matter
interaction under strong-coupling conditions is the possibility of a
phase transition to a superradiant phase, where the atoms and light
become strongly correlated even in the ground state. This phase
transition has generally been studied under the assumption of
identical atoms. We theoretically analyze the effect of parameter
fluctuations on the superradiance phase transition in a cavity quantum
electrodynamics setup with many emitters (or qubits) coupled to a single cavity
[1]. We include parameter fluctuations qubit gaps,
bias points, and qubit-cavity coupling strengths. We find that the
phase transition should occur in this case, although it manifests
itself differently from the case with no fluctuations. We
also find that fluctuations in the qubit gaps and qubit-cavity
coupling strengths do not make it more difficult to reach
the transition point. Fluctuations in the bias points, however,
increase the coupling strength required to reach the quantum phase
transition point and enter the superradiant phase. Similarly, these
fluctuations lower the critical temperature for the thermal phase
transition. [1] S. Ashhab and K. Semba, Phys. Rev. A 95, 053833 (2017).   

Waveguide Quantum Electrodynamics in Squeezed Vacuum

We study the dynamics of a general multi-emitter system coupled to the squeezed vacuum reservoir and derive a master equation for this system based on the Weisskopf-Wigner approximation. In this theory, we include the effect of positions of the squeezing sources which is usually neglected in the previous studies. We apply this theory to a quasi-one-dimensional waveguide case where the squeezing in one dimension is experimentally achievable. We show that while dipole-dipole interaction induced by ordinary vacuum depends on the emitter separation, the two-photon process due to the squeezed vacuum depends on the positions of the emitters with respect to the squeezing sources. The dephasing rate, decay rate and the resonance fluorescence of the waveguide-QED in the squeezed vacuum can be controllable by changing the positions of emitters. Furthermore, we demonstrate that the stationary maximum entangled NOON state for identical emitters can be reached with arbitrary initial state which is shown to be impossible in the previous theory.
  

Construct Anyons from Photons in a One-Dimensional Array of Dynamically-Modulated Resonators

The exploration of anyon physics is of great importance, where the capability for achieving anyon states points to possibilities for achieving non-trivial many-body photon states that are potentially interesting for quantum information processing. We create anyons out of photons in a photonic structure composed of resonators with Kerr nonlinearity undergoing modulation. With the proper choice of the modulation profile, the Hamiltonian of the photonic system is mapped into the Hamiltonian corresponding to a system of the one-dimensional anyons. We further couple the resonator to external waveguides. This open system enables us to directly conduct anyon interference experiments through a photon transport measurement. Also, it allows us to selectively probe anyon density distributions inside resonators, such as the ground state and the excited states of the anyon system. Our work opens an avenue of exploring fundamental physics using photonic structures and provides the platform that can be useful for demonstrating a wide range of anyon physics effects.   

Quantum control of phononic modes via Rabi-sideband resonance

Mechanical resonators usually have many modes with high Q factor and provide a good candidate for multimode quantum memory. However, these modes are usually harmonic; in order to manipulate quantum information, one needs to couple the modes to some nonlinear element such as a qubit. In this work, we study a system that consists of a qubit coupled linearly to a set of phononic modes whose frequencies are comparable to that of the qubit. We consider the coupling strength is smaller than the free spectral range of phonon modes but much stronger than their decay rates which is currently achievable. We show that by Rabi pumping the qubit, one can selectively bring the "dressed" qubit into resonance with one of the phonon modes even though they are off-resonant without pump. The driving-induced coupling allows state swap between the phonon and dressed qubit or simultaneously excites or de-excites them. We also show that by pumping the qubit with two tones whose frequency difference (or sum) equals to that of two phonon modes, one can generate a beamsplitter (or two-mode squeezing) interaction between two phonon modes. The study applies not only to phonon modes but also to multiple microwave cavities or multiple modes in a single cavity coupled to a common superconducting qubit.   

Non-perturbative Dynamical Casimir Effect in Optomechanical Systems: Vacuum Casimir-Rabi Splittings

By rapidly changing a boundary condition for the electromagnetic field, e.g., by moving a mirror very quickly, fluctuations in the quantum vacuum can be converted into real photons. This is known as the dynamical Casimir effect (DCE), the first experimental demonstration of which was realized only a few years ago, using superconducting circuits. In most theoretical studies treating the DCE, the trajectory of the mirror is classical. We use a fully quantum-mechanical description of both the cavity field and the oscillating mirror in optomechanical setup. We do not linearize the dynamics, nor do we adopt any parametric or perturbative approximation. We show that the resonant generation of photons from the vacuum is determined by a ladder of mirror-field Rabi splittings, and that vacuum Casimir-Rabi oscillations can occur. We also study the case where the mirror is coherently driven. We find that, for strong optomechanical coupling, a resonant production of photons out from the vacuum can be observed even for mechanical frequencies below the cavity frequency. Since high mechanical frequencies, which are hard to achieve experimentally, were thought to be imperative for realizing the DCE, this result removes a major obstacle for its experimental observation in optomechanical setup.   

Tailoring Casimir Forces and Torques Through Geometry and Optical Response

The confinement of quantum electromagnetic fluctuations between two macroscopic objects results in a force, i.e. the Casimir force, which draws the two objects closer together to minimize the energy of the system. This force depends on both the geometry and the optical properties of the materials involved. Here we present measurements of the Casimir force between two spheres – a geometry that has previously eluded measurement due to experimental difficulties. Further, a Casimir torque is thought to exist between two optically anisotropic materials. Here we will also describe how careful choice of dielectrics can enhance the predicted Casimir torque, making it within reach of current experiments.   

Analysis of Cavity-QED-Based Single Photon Generation toward Scalable Quantum Information Processing

Deterministic generation of single photons, which is a key ingredient to realize optical quantum information processing, has been experimentally demonstrated by making use of cavity QED systems with various settings so far. To combine such single photon generators with other elementary components such as quantum gates, it is important to understand the characteristics of the output photons and to choose the optimal parameters by considering the scalability, which have not been sufficiently discussed yet. Here, to tackle the above problem, we analyze two single photon generation schemes. We consider a scheme which relies on stimulated Raman adiabatic passage (STIRAP) in a three-level system, and we compare this with a scheme which relies on the Purcell effect in a two-level system. We numerically simulate the process with a wide range of parameters, where the adiabatic condition is not always satisfied. We also derive a concise analytic expression for the photon loss probability in the STIRAP case. Finally, as a practical example, we show that nanofiber cavities¹ have the capability to generate photons with high probability even with overwhelmingly large cavity internal losses compared to free-space Fabry-Perot cavities.
¹S. Kato and T. Aoki, Phys. Rev. Lett. 115, 093603 (2015).   

Fundamental Limits to Single-Photon Detection Determined by Quantum Coherence and Backaction

Single-photon detectors have achieved impressive performance, and have led to a number of new scientific discoveries and technological applications. Existing models of photodetectors are semiclassical in that the field-matter interaction is treated perturbatively and time-separated from physical processes in the absorbing matter. An open question is whether a fully quantum detector, whereby the optical field, the optical absorption, and the amplification are considered as one quantum system, could have improved performance. In this talk we describe a theoretical model of such photodetectors and show simulations that reveal the critical role played by quantum coherence and amplification backaction in dictating the performance. We show that coherence and backaction lead to tradeoffs between detector metrics, and also determine optimal system designs through control of the quantum-classical interface. Importantly, we establish the design parameters that result in a perfect photodetector with 100% efficiency, no dark counts, and minimal jitter, thus paving the route for next generation detectors.   

Vacuum modes in Homodyne detection

Shot noise in homodyne detection is generally attributed to vacuum fluctuations. Changing the boundary conditions of the electromagnetic field around a detector changes the vacuum modes, and should affect the noise in homodyne detection. We analyze this thought experiment using Glauber's photodetection theory. It is shown that perfect absorbers of the field would not respond to any change in vacuum modes. However, real detectors always scatter some of the light incident on them. We use a generalized detector model to incorporate this emission in the photodetection process, and show that such detectors will detect any change in vacuum modes.. The approach connects photodetection theory with atomic resonance fluorescence techniques, potentially allowing to build detectors sensitive to the quantum vacuum.   

Scattering of Coherent Pulses from Quantum-Optical Systems

We present a new calculation tool and framework for characterizing the scattering of photons by energy-nonconserving Hamiltonians into unidirectional (chiral) waveguides, for example, with coherent pulsed excitation. The temporal waveguide modes are a natural basis for characterizing scattering in quantum optics, and afford a powerful technique based on a coarse discretization of time. This overcomes limitations imposed by singularities in the waveguide-system coupling. Moreover, the integrated discretized equations can be faithfully converted to a continuous-time result by taking the appropriate limit. This approach provides a complete solution to the scattered photon field in the waveguide, and can also be used to track system-waveguide entanglement during evolution. Our method is most applicable when the number of photons scattered is known to be small, i.e. for a single-photon or photon-pair source. We illustrate two examples: analytic solutions for short laser pulses scattering off a two-level system and numerically exact solutions for short laser pulses scattering off a spontaneous parametric downconversion (SPDC) or spontaneous four-wave mixing (SFWM) source.   

Tuning the quantum entanglement of three qubits in a nonstationary cavity

We investigated the tunable quantum entanglement and the probabilities of excitations in a system of three qubits in a nonstationary cavity. The entanglement between the qubits arises because of the dynamical Lamb effect, a novel quantum phenomenon caused by nonadiabatic fast change of the boundary conditions of the cavity. The transition amplitudes and the probabilities of excitation of qubits due to the dynamical Lamb effect were evaluated. Furthermore, we introduced the conditional concurrence and the conditional residual tangle for each fixed number of created photons in the cavity as measures of the pairwise or three-way dynamical quantum entanglement of the qubits. A prescription on how to increase the values of those quantities by controlling the frequency of the cavity photons was found. A physical realization of the system using superconducting qubits coupled to a coplanar waveguide terminated by a superconducting quantum interference device is proposed.