Transmon Architecture for Emission and Detection of Single Microwave Photons

The authors present a compact transmon emitter/detector (TED) architecture that functions as a dual-purpose single-photon source and detector with 95% inferred efficiency and rapid 4-microsecond operation, establishing a versatile interface for quantum communication, metrology, and fast qubit reset.

The Gist

Imagine you are trying to build a quantum internet, where different supercomputers (Quantum Processing Units, or QPUs) need to talk to each other. The problem is that these computers are incredibly fragile; if you try to connect them directly, the noise from the connection can destroy their delicate calculations.

This paper introduces a new "translator" device called the TED (Transmon Emitter/Detector). Think of the TED as a specialized, high-tech walkie-talkie that can both send and receive single packets of microwave energy (photons) without letting the noise back into the main computer.

Here is how it works, broken down into simple concepts:

1. The Architecture: A Three-Person Team

Inside the TED, there isn't just one component; there are three distinct "characters" working together, all made of superconducting circuits:

• The Data Keeper (Qd): This is the main memory of the quantum computer. It holds the information and needs to stay quiet and isolated.
• The Bridge (Qc): This is a middleman that connects the Data Keeper to the outside world.
• The Messenger (Qw): This character stands right at the door, ready to shout messages out into the "waveguide" (a cable that carries signals) or listen for incoming messages.

The Magic Trick: The Data Keeper and the Messenger are not directly connected. They are linked only through the Bridge. By wiggling a magnetic knob (flux) on the Bridge, the TED can make the Data Keeper and the Messenger talk to each other only when they want to. This keeps the Data Keeper safe from the noisy outside world 99% of the time.

2. The "Pitch and Catch" Game

The researchers built two of these TED devices to prove they work.

• The Sender (sTED): This device takes a single packet of energy (a photon) from its Data Keeper and "throws" it into a long coaxial cable (about a meter long).
• The Receiver (mTED): This device sits at the other end of the cable. It waits, listens, and if a photon arrives, it "catches" it.

To make sure the photon doesn't bounce back and cause trouble, they used a circulator. Think of a circulator like a one-way street or a roundabout that forces traffic to go only one way: from the Sender, to the Receiver, and then straight to a measurement tool, never back to the Sender.

3. How It Sends and Catches

• Sending (Emission): The Sender prepares a single photon. It then uses a precise "push" (a parametric drive) to transfer that photon from its internal memory to the Messenger, which immediately releases it into the cable. This whole process takes about 2 microseconds (two-millionths of a second).
• Catching (Detection): The Receiver is waiting in a specific state. When the photon arrives, it triggers a chain reaction. The Receiver absorbs the photon and changes its state permanently (it "latches"). This change is easy to detect, telling the computer, "Hey, a message arrived!" This also takes about 2 microseconds.

4. The Results: How Well Did It Work?

The team tested this system and found:

• Efficiency: When a photon was sent, the Receiver successfully caught it about 60% of the time.
• The Real Performance: After accounting for losses in the cable and the circulator, they calculated that the Receiver itself is actually 95% efficient. This means if a photon actually reaches the Receiver's door, it almost certainly gets caught.
• Speed: The entire cycle of resetting the device, sending the photon, and catching it takes about 4 microseconds. This is incredibly fast for quantum operations.

5. Why Is This Important?

The paper claims this architecture solves a major headache in quantum networking:

• Tunability: Unlike older designs that required the sender and receiver to be tuned to the exact same frequency (like two radios needing the exact same station), the TED can be tuned. The "Messenger" can change its frequency to match different partners, making it much easier to connect different types of quantum computers.
• Safety: It allows the main quantum computer to stay isolated and safe while still being able to talk to the outside world.
• Dual Use: The same device can act as a sender or a receiver, making it a flexible, "drop-in" tool for building quantum networks.

In short, the TED is a compact, fast, and safe interface that allows quantum computers to exchange single packets of information, paving the way for linking quantum processors together into a larger network.

Technical Summary

Technical Summary: Transmon Architecture for Emission and Detection of Single Microwave Photons

Problem and Motivation
Scaling quantum computers and realizing distributed sensor networks requires the ability to network physically separate quantum processing units (QPUs). Superconducting circuits, which interact strongly with one-dimensional waveguides, are well-suited for this task. However, existing Quantum Communication Interface (QCI) architectures face challenges regarding compactness, frequency tunability, and the ability to provide sufficient isolation to prevent QPU decoherence while enabling selective entanglement. Specifically, there is a need for a device that can function as both a single-photon source and a detector, operate with the strict frequency constraints of standard transmon QPUs, and interface with heterogeneous chips or microwave-to-optical transducers without requiring rigid frequency matching.

Methodology
The authors developed a compact Transmon Emitter/Detector (TED) superconducting circuit. The TED architecture utilizes a recently developed Double Transmon Coupler (DTC) to interface a long-lived "data" transmon ($Q_d$) with a waveguide-coupled transmon ($Q_w$). The DTC consists of two transmons ($Q_c$ and $Q_w$) linked by an inductive Josephson junction, with $Q_c$ acting as a coupling element between $Q_d$ and $Q_w$.

• Circuit Design: The data transmon $Q_d$ is capacitively coupled to the coupling transmon $Q_c$. The coupling transmon $Q_c$ and the waveguide transmon $Q_w$ are inductively coupled. An external flux line modulates the DTC, allowing for parametric control of the interaction between $Q_d$ and $Q_w$ while keeping $Q_c$ far off-resonance.
• Operation Principle: The TED operates by modulating the DTC flux at the difference frequency between $Q_d$ and $Q_w$. This creates a parametric exchange interaction that hybridizes the states of $Q_d$ and $Q_w$.
  • Emission: To emit a photon, the system is initialized, and a parametric drive hybridizes $Q_d$ with $Q_w$, allowing the excitation to decay into the waveguide.
  • Detection: To detect a photon, the system is prepared in an excited state. An incoming photon triggers a transition, and a parametric drive drives the system to a higher excited state which then irreversibly relaxes to the ground state, "latching" the detector.
• Experimental Setup: Two TED devices (a source TED, sTED, and a measurement TED, mTED) with nominally identical parameters were fabricated. They were connected via approximately one meter of coaxial cable, a directional coupler, and a circulator. The circulator enforced a continuum of modes, routing photons from the sTED to the mTED while directing reflected fields to the measurement chain.
• Theoretical Modeling: The system was modeled using the SLH (Scattering-Lindblad-Hamiltonian) formalism to describe the network of localized components (TEDs and circulator) interacting via itinerant bosonic modes. Coherent backscatter measurements were used to calibrate the system and extract parameters such as relaxation rates and thermal occupation.

Key Contributions

1. Dual-Functionality Architecture: The paper demonstrates a single circuit architecture capable of acting as both a single-photon source and a single-photon detector, highlighting the flexibility of the DTC-based design.
2. Frequency Tunability and Isolation: The TED decouples the resonance frequency of the data qubit ($Q_d$) from the waveguide-coupled transmon ($Q_w$). $Q_w$ is tunable over several hundred MHz via flux bias, relaxing the stringent frequency matching requirements for entanglement distribution between heterogeneous devices, while $Q_d$ remains static and compatible with standard QPU frequency constraints.
3. Efficient Detection Scheme: The authors introduced and characterized a photon detection scheme tailored to the TED. This scheme relies on a transition-selective process where an incident photon triggers a parametric drive that latches the detector to the ground state.
4. Comprehensive Characterization: The work includes a detailed analysis of coherent reflective scattering (backscatter) to extract system parameters and a theoretical model of the composite system using SLH theory.

Results

• Device Performance: The mTED was characterized via coherent backscatter, yielding a resonance frequency of $\omega_{wm}/2\pi = 5.65811(1)$ GHz and a relaxation rate $\gamma_m = 11.2(1) \times 10^6$ s$^{-1}$. The thermal occupation of the waveguide was measured at $n_{th} = 0.015(3)$.
• Source Operation: The sTED successfully emitted Fock state photons. The emission process was controlled by a shaped parametric drive, releasing the photon into the waveguide with a narrow frequency profile. The reset and emission processes each required approximately 2 $\mu$s.
• Detection Operation: The mTED demonstrated high-efficiency detection. The protocol involved preparing the mTED in an excited state and applying a parametric drive to facilitate the latching mechanism upon photon arrival.
• End-to-End Demonstration: In the "pitch-and-detect" configuration, the sTED emitted a single microwave Fock state which was subsequently detected by the mTED.
  • The system detected 60% of the emitted Fock state photons.
  • Based on the measured baseline fidelity and accounting for cable losses (estimated at 35%), the authors infer a 95% detection efficiency at the input of the measurement TED.
  • The total protocol duration (reset + emission/detection) was approximately 4 $\mu$s.
  • The detection probability was found to be largely independent of the photon's arrival time within the 10 $\mu$s detection window.
• Dark Counts: The system exhibited an 8% dark count rate (reduced to 16% with a longer 10 $\mu$s window), consistent with the $T_1$ relaxation of the data qubit.

Significance
The paper claims that the TED architecture represents a new use case for the Double Transmon Coupler (DTC), offering a compact, drop-in, tunable, and transition-selective link between a coherent data transmon and a waveguide. The authors assert that circuits like the TED will play a vital role in quantum information processing by:

• Facilitating unconditional fast reset.
• Enabling microwave photon metrology.
• Serving as nascent Quantum Communication Interfaces (QCIs).

These QCIs are described as mediators for entanglement distribution between QPUs within a cryostat or as interfaces for microwave-to-optical transducers for long-range quantum networking. The work emphasizes that the TED's ability to decouple the data qubit frequency from the waveguide interface frequency is a critical advantage for interoperability in scalable quantum networks.