Switching Rydberg interactions by three orders of magnitude using a terahertz field

This paper demonstrates that a pulsed terahertz field can rapidly switch the strength of interactions between Rydberg atoms by three orders of magnitude, overcoming the limitations of microwave fields and offering significant advantages for applications in quantum computing and Rydberg quantum optics.

The Gist

Imagine you have a group of atoms acting like tiny, super-sensitive magnets. In the world of quantum computing, scientists use these atoms in a special, high-energy state called a "Rydberg state." When atoms are in this state, they become huge and start interacting with each other strongly, almost like magnets snapping together. This interaction is the secret sauce for building quantum computers, but usually, once you turn the interaction "on," it's hard to turn it "off" or change its strength quickly.

Think of it like trying to control the volume on a radio. Most methods let you only turn the volume up or down a little bit, or they only work on stations that are very close to each other.

The Big Breakthrough
This paper describes a new trick: using a burst of Terahertz (THz) light (a type of invisible energy wave that sits between microwaves and infrared light) to act like a giant, instant volume knob.

The researchers showed they could use this THz pulse to switch the strength of the interaction between these atoms by 1,000 times (three orders of magnitude) in a flash. It's like going from a whisper to a shout instantly, or from a gentle breeze to a hurricane, just by flipping a switch.

How They Did It: The "Light Switch" Analogy
To understand how they achieved this, imagine the atoms are like people standing in a line.

• The Old Way (Microwaves): Usually, scientists use microwaves to talk to these atoms. But microwaves are like a small key that only fits locks on the same floor of a building. They can only move atoms to nearby energy levels, which don't change how strongly they interact with each other very much.
• The New Way (Terahertz): The Terahertz field is like a super-key that can open doors to different floors of the building. It can jump atoms to energy levels that are very different from where they started. Some of these new levels make the atoms interact weakly (like strangers passing in the hall), while others make them interact incredibly strongly (like best friends hugging).

By using a short, nanosecond-long pulse of this Terahertz light, the team could instantly jump the atoms from a "weak interaction" state to a "super-strong interaction" state and back again.

The Experiment: Storing Light in a Bottle
To prove this worked, they didn't just watch the atoms; they tried to store a message (a photon of light) inside them.

1. The Setup: They trapped a cloud of ultra-cold Rubidium atoms.
2. The Storage: They used lasers to turn a flash of light into a "frozen" wave of energy inside the atoms (like putting a message in a bottle).
3. The Switch: While the message was stored, they hit the atoms with their Terahertz pulse.
   • If they switched the atoms to a state with weak interaction, the message came out clearly, just as they put it in.
   • If they switched them to a state with strong interaction, the atoms started "arguing" with each other (interacting so strongly that they messed up the message). The message came out garbled or disappeared.

This proved they could effectively "turn off" the ability to store the message by switching the interaction strength, and then "turn it back on" just as quickly.

Why This Matters (According to the Paper)
The authors say this ability to rapidly switch interaction strength is a game-changer for:

• Reading Quantum Bits: It helps in checking the status of quantum information (single-qubit readout).
• Finding States: It makes it easier to detect specific quantum states.
• Quantum Annealing: This is a method for solving complex optimization problems, where being able to turn interactions on and off helps the computer find the best answer faster.
• Quantum Optics: It allows scientists to separate how light moves from how atoms interact, giving them more control over light itself.

The Technical Challenge
The paper also notes that Terahertz light is notoriously difficult to work with. It's too high-energy for standard electronic detectors but too low-energy for the sensors used for visible light. It's like trying to catch a ghost with a net made of the wrong material. The team had to build a custom setup using special lenses, mirrors, and a powerful source to generate these short, precise pulses, effectively creating a "Terahertz flashlight" that could be turned on and off in billionths of a second.

In short, they built a new, powerful tool that lets scientists control how atoms talk to each other with unprecedented speed and range, opening the door to more flexible and powerful quantum technologies.

Technical Summary

Technical Summary: Switching Rydberg Interactions by Three Orders of Magnitude using a Terahertz Field

Problem Statement
While Rydberg atoms offer a powerful platform for quantum computing and optics due to their enhanced interactions, controlling the strength of these interactions remains a challenge. Current methods often rely on microwave (mw) fields to drive transitions between Rydberg states. However, mw fields are limited to transitions between states with similar principal quantum numbers ($n$), restricting the range of accessible interaction strengths. Furthermore, the technical difficulties in generating and detecting high-power, tunable terahertz (THz) fields have prevented their widespread use in Rydberg physics, despite the fact that THz frequencies (0.1–10 THz) can couple states with large differences in principal quantum number ($\Delta n$). This limitation restricts the ability to rapidly switch interaction strengths, a capability desirable for quantum annealing, state detection, and decoupling light propagation from interactions in quantum optics.

Methodology
The authors present an experimental demonstration using a pulsed terahertz field to coherently drive transitions between Rydberg states in an ultra-cold $^{87}$Rb ensemble.

• Experimental Setup: Atoms are cooled in a 3D magneto-optical trap (MOT) and loaded into an optical dipole trap. Photons are stored as collective Rydberg excitations (Rydberg polaritons) via electromagnetically induced transparency (EIT) using probe and coupling lasers.
• THz Source: A terahertz field is generated using an amplifier multiplier chain (AMC) seeded by a microwave source. This setup allows for the generation of nanosecond-duration THz pulses (centered at 0.6 THz) by pulsing the microwave seed rather than the AMC itself, achieving rise times comparable to the seed.
• Transition Selection: The experiment targets transitions from a storage state $|35S_{1/2}\rangle$ to two distinct $P$-states: $|32P_{3/2}\rangle$ and $|38P_{3/2}\rangle$.
  • The $|32P_{3/2}\rangle$ state has a weak interaction strength ($C_6 \approx -0.29$ GHz $\mu$m$^6$).
  • The $|38P_{3/2}\rangle$ state is near a Förster resonance, resulting in a strong interaction strength ($C_6 \approx -307.5$ GHz $\mu$m$^6$).
• Measurement: The strength of interactions is probed by storing photons, applying a THz pulse to drive the transition to the target Rydberg state, waiting for a specific duration ($t_{wait}$), and then retrieving the photons. The retrieval efficiency and photon counts are measured as a function of incident photon numbers.

Key Results

• Interaction Switching: The authors successfully demonstrated the ability to switch the interaction strength between Rydberg atoms by three orders of magnitude. By driving the system to the $|38P_{3/2}\rangle$ state, the interaction strength was increased from $\sim 0.2$ GHz $\mu$m$^6$ to $\sim 300$ GHz $\mu$m$^6$.
• Interaction-Induced Dephasing: When the THz field drove the system to the strongly interacting $|38P_{3/2}\rangle$ state, the retrieved photon signal exhibited saturation at low incident photon numbers ($\langle N_p \rangle \approx 2$). This saturation is attributed to interaction-induced dephasing of multiple excitations, effectively "cleaning up" the polaritonic mode to a single excitation. In contrast, driving to the weakly interacting $|32P_{3/2}\rangle$ state resulted in a linear retrieval efficiency with no such saturation.
• Coherent Control: The study confirmed coherent Rabi oscillations on the THz transition ($|35S_{1/2}\rangle \leftrightarrow |38P_{3/2}\rangle$). For low photon numbers, coherent oscillations were observed. For higher photon numbers, the amplitude of the oscillations was suppressed after the first cycle due to the strong interactions in the $|38P_{3/2}\rangle$ state, consistent with a Monte-Carlo dephasing model.
• Pulse Characteristics: The system successfully generated coherent, nanosecond THz pulses, overcoming previous technical hurdles regarding the creation and detection of pulsed THz radiation in atomic physics.

Significance and Claims
The paper claims that the ability to rapidly switch Rydberg interaction strengths by orders of magnitude using THz fields offers distinct advantages for several quantum technologies:

• Quantum Computing and Readout: The technique enables new state-detection schemes and single-qubit readout sequences, particularly beneficial at lower principal quantum numbers where external field perturbations are less severe and optical transition dipole moments are larger.
• Quantum Optics: The ability to decouple light propagation from interactions is highlighted as a specific benefit for Rydberg quantum optics.
• Sensing: The high sensitivity of the THz transitions to the electric field (due to large dipole moments) suggests potential for these transitions to serve as sensitive detectors for pulsed THz radiation.
• Technical Feasibility: The work demonstrates that it is feasible to construct a setup that combines THz optics with optical beams to apply coherent THz pulses to an atomic cloud, unlocking a range of transitions inaccessible to microwave fields.

The authors maintain a modest tone regarding future implications, focusing on the demonstrated feasibility of the method and its immediate utility for interaction switching, rather than proposing specific untested applications.