Introduction to the Jaynes-Cummings-Hubbard Model: Three-Site System¶

Authors: Vanshaj Bindal (Cardiff University)

Introduction¶

The Jaynes-Cummings-Hubbard Model (JCHM) represents a fundamental quantum system at the intersection of quantum optics and condensed matter physics. It describes an array of optical cavities, each containing a two-level atom (or other two-level system), with photons able to tunnel between neighboring cavities.

Theoretical Background¶

The JCHM combines two important quantum models:

1. Jaynes-Cummings Model: Describes the interaction between a single two-level atom and a quantized electromagnetic field in a cavity. This model captures the fundamental light-matter interaction in cavity quantum electrodynamics (QED).

2. Bose-Hubbard Model: Describes bosonic particles (in our case, photons) hopping between sites in a lattice, with on-site interactions. This model is central to describing quantum phase transitions in many-body systems.

When these models are combined, the JCHM exhibits rich physics including quantum phase transitions between Mott insulator-like states (where photons are localized in cavities) and superfluid-like states (where photons are delocalized across the lattice).

Mathematical Description¶

For a three-site system, the Hamiltonian can be written as:

$$H = \omega_c \sum_{i=1}^3 a_i^\dagger a_i + \frac{\omega_a}{2} \sum_{i=1}^3 \sigma_i^z + g \sum_{i=1}^3 (a_i^\dagger \sigma_i^- + a_i \sigma_i^+) - J \sum_{i=1}^2 (a_i^\dagger a_{i+1} + a_{i+1}^\dagger a_i)$$

where:

• $\omega_c$ is the cavity frequency
• $\omega_a$ is the atomic transition frequency
• $g$ is the atom-cavity coupling strength
• $J$ is the hopping strength between adjacent cavities
• $a_i^\dagger$ and $a_i$ are the creation and annihilation operators for photons in cavity $i$
• $\sigma_i^z$, $\sigma_i^+$ and $\sigma_i^-$ are the Pauli and ladder operators for atom $i$

The Hamiltonian consists of four terms:

1. Cavity energy terms: $\omega_c \sum_{i=1}^3 a_i^\dagger a_i$
2. Atom energy terms: $\frac{\omega_a}{2} \sum_{i=1}^3 \sigma_i^z$
3. Atom-cavity interaction terms: $g \sum_{i=1}^3 (a_i^\dagger \sigma_i^- + a_i \sigma_i^+)$
4. Photon hopping terms: $-J \sum_{i=1}^2 (a_i^\dagger a_{i+1} + a_{i+1}^\dagger a_i)$

Package import¶

First, we'll import the necessary libraries. QuTiP (Quantum Toolbox in Python) provides the quantum operators and solvers we need.

Constructing the Three-Site JCHM Hamiltonian¶

We'll start by defining the system parameters. These determine the physical properties of our quantum system:

• N: The number of Fock states per cavity, limiting the maximum number of photons
• omega_c and omega_a: The frequencies of the cavity and atom (in resonance when equal)
• g: The coupling strength between each atom and its cavity
• J: The hopping strength between adjacent cavities, controlling how easily photons tunnel

Creating the Hamiltonian¶

Now, we'll create a function to construct the full Hamiltonian for our three-site JCHM. This requires carefully constructing tensor products of operators to represent the combined Hilbert space of all three cavities and atoms.

The key challenge is to properly represent operators that act on specific subsystems within the total system. We use tensor products with identity operators to extend local operators to the full Hilbert space.

Now, let's create the Hamiltonian for our system. The dimension of the resulting Hilbert space will be quite large, even with just three sites, because the total dimension scales as $(N \times 2)^3$.

Exploring the Ground State¶

The ground state of a quantum system is its lowest energy state. For the JCHM, the nature of the ground state changes as we vary the parameters, particularly the ratio of hopping strength J to coupling strength g.

In the strong coupling regime (g >> J), the ground state resembles a Mott insulator state where photons are strongly bound to atoms, forming polaritons. In the strong hopping regime (J >> g), the ground state becomes more superfluid-like, with photons delocalized across cavities.

Let's examine the ground state of our system to understand its fundamental properties:

Time Evolution and Dynamics¶

One of the most interesting aspects of the JCHM is the dynamics of photons and atomic excitations. By solving the time-dependent Schrödinger equation, we can observe how photons propagate through the cavity chain and interact with atoms.

We'll create an initial state where the first cavity has one photon and all atoms are in their ground state. This represents a localized excitation that will evolve over time.

Theoretical Background on Dynamics¶

The time evolution of a quantum state is governed by the Schrödinger equation:

$$i\hbar\frac{d|\psi(t)\rangle}{dt} = H|\psi(t)\rangle$$

In QuTiP, we use the mesolve function to numerically solve this equation. For a closed quantum system without dissipation, the dynamics will be purely coherent, showing quantum oscillations as photons hop between cavities and interact with atoms. We usually use sesolve for closed quantum system, but here mesolve will delegate it to sesolve.

Phase Transition Signatures¶

One of the most interesting aspects of the JCHM is its quantum phase transition from a Mott insulator-like phase to a superfluid-like phase as the hopping strength increases.

Theoretical Background on Phase Transitions¶

In the thermodynamic limit (infinite system size), the JCHM undergoes a quantum phase transition at a critical ratio of J/g:

1. Mott insulator phase (J < J_critical): Photons are localized within cavities, strongly coupled to their respective atoms, forming polaritons.

2. Superfluid phase (J > J_critical): Photons are delocalized across the lattice, with long-range coherence.

While our three-site system is too small to exhibit a true phase transition, we can observe precursors or signatures of this transition by examining order parameters.

Order Parameters¶

To characterize the different phases, we'll use two order parameters:

1. Photon number fluctuations (Δn): Measures the variance in photon number. This increases in the superfluid phase due to greater uncertainty in photon locations.

2. Cavity field amplitude (⟨a⟩): Acts as an "order parameter" that becomes non-zero in the superfluid phase.

Now let's calculate these order parameters across a range of hopping strengths to see how they change as we approach and cross the phase transition:

For a finite-size system (especially a small 3-site system), the ground state preserves certain symmetries that cause the expectation value of the field operator (⟨a⟩) to be exactly zero, even across the phase transition. The photon number fluctuations (blue circles) still increase as expected, showing a precursor to the phase transition.

In larger systems (let's say extremely large), ⟨a⟩ would become non-zero in the superfluid phase, serving as a true order parameter. But in our small system, we need to look at other quantities (like photon number fluctuations) to see signatures of the transition.

Photon Propagation in the Chain¶

To better visualize how photons propagate through our three-site chain, we'll create a color map showing the photon number in each cavity over time. This provides a clear picture of the wave-like nature of photon propagation in the JCHM.

Theoretical Background on Photon Propagation¶

In the JCHM, photons don't simply move from one cavity to the next in a classical way. Instead, they show quantum mechanical wave-like behavior, with interference effects and probability amplitudes spreading across the lattice. The propagation pattern depends on both the hopping strength J and the cavity-atom coupling strength g.

Effect of Detuning¶

So far, we've kept the cavity and atomic frequencies in resonance (ωc = ωa). Now, let's explore what happens when we introduce detuning between them. Detuning is defined as Δ = ωa - ωc, and it significantly affects the dynamics of the system.

Theoretical Background on Detuning¶

When a cavity and atom are detuned, their energy levels don't match exactly. This affects how efficiently they can exchange energy. In the JCHM, detuning can:

1. Modify the effective coupling between cavities and atoms
2. Change the speed of photon transfer through the chain
3. Alter the probability of exciting atoms

Large detuning generally reduces the effective coupling between cavities and atoms, making the system behave more like uncoupled cavities with nearly free photons.

Varying Coupling Strength¶

The coupling strength g between atoms and cavities is another crucial parameter that affects the dynamics of the JCHM. Let's explore how different values of g change the photon transfer behavior.

Theoretical Background on Coupling Strength¶

The atom-cavity coupling strength g determines how strongly photons interact with atoms:

1. Weak coupling (small g): Photons interact weakly with atoms and can hop more freely between cavities
2. Strong coupling (large g): Photons strongly interact with atoms, forming polaritons that are partly photon and partly atomic excitation
3. Ultrastrong coupling (g ≈ ωc): Leads to the breakdown of the rotating wave approximation (not modeled in our simulation)

As g increases relative to J, the system moves closer to the Mott insulator phase, where photons are more strongly bound to their respective cavities.

Conclusion¶

In this tutorial, we've explored the Jaynes-Cummings-Hubbard Model using a three-site system implemented in QuTiP. This model bridges quantum optics and condensed matter physics, offering insights into light-matter interactions in coupled cavity systems.

Key Physical Insights:¶

1. Ground State Properties: The ground state of the JCHM shows the formation of polaritons, with high atomic excitation and low photon numbers.

2. Quantum Dynamics: Photons propagate through the cavity chain in a wave-like manner, showing quantum interference effects rather than classical particle-like behavior.

3. Phase Transition Signatures: Even in our small system, we observe precursors of the Mott insulator to superfluid phase transition as hopping strength increases.

4. Parametric Dependencies: The system's behavior is highly sensitive to detuning and coupling strength, with these parameters controlling the balance between localization and delocalization of excitations.

The JCHM has important applications in quantum simulation, quantum optics, and quantum information processing. It serves as a building block for understanding more complex quantum optical lattice systems and could potentially be implemented in various experimental platforms including superconducting circuits, photonic crystals, and cold atoms in optical lattices.

Software version¶