Quantum System Tracks Single Photons over 16 Metres

Scientists are increasingly exploring synthetic dimensions as a means to engineer and control quantum systems, but realising this potential at the single-photon level presents significant hurdles. Zheshu Xie from the Shenzhen Institute for Quantum Science and Engineering, Southern University of Science and Technology, Luojia Wang from the State Key Laboratory of Photonics and Communications, Shanghai Jiao Tong University, and Jiawei Qiu, also of the Shenzhen Institute for Quantum Science and Engineering, alongside colleagues from the International Quantum Academy and the Department of Physics at Southern University of Science and Technology, have now demonstrated quantum-state initialization and detection of single-photon evolutions within a synthetic frequency lattice. This was achieved through the integration of a superconducting circuit with a 16-metre aluminium coaxial cable, enabling the observation of quantum random walks, Bloch oscillations and nonadiabatic frequency conversion. The research establishes superconducting quantum circuits as a versatile platform for programmable Hamiltonians and extensible synthetic lattices with flexible single-photon control, potentially advancing the development of complex quantum simulations and information processing.

A 16-metre aluminum coaxial cable integrated with a superconducting qubit has enabled researchers to demonstrate a new level of control over single photons, paving the way for more complex quantum systems. By manipulating light within a specially designed circuit, they’ve created an artificial landscape for quantum particles to move and interact, offering a promising route towards building more powerful and adaptable quantum technologies.

Researchers have engineered a platform for simulating complex quantum systems using microwave photons travelling within a synthetic frequency lattice. This work overcomes longstanding challenges in extending synthetic dimension concepts into the single-photon quantum regime. Researchers integrated a superconducting qubit with an exceptionally long, 16-metre aluminum coaxial cable, effectively creating a controllable environment for quantum simulations.

The resulting system allows for the initialisation and detection of single-photon evolutions within this synthetic lattice, opening new avenues for exploring quantum phenomena. This achievement relies on a tunable superconducting quantum interference device (SQUID)-based modulator that synthesizes lattice couplings and artificial gauge fields. By precisely controlling these parameters, the team observed single-photon quantum random walks and Bloch oscillations, fundamental behaviours in lattice systems, alongside nonadiabatic, unidirectional frequency conversion achieved through rapid modulation of the lattice Hamiltonian.

Crucially, band-structure measurements were performed, confirming the properties of the synthetic lattice. The architecture’s flexibility extends beyond simple lattices; the connectivity can be readily reconfigured using multiple drive tones to construct higher-dimensional structures, circumventing the spatial limitations inherent in traditional experimental platforms.

By leveraging low-loss superconducting cables and precise control mechanisms, this research establishes superconducting quantum circuits as a versatile platform for programmable Hamiltonians and extensible synthetic lattices with unprecedented single-photon control. The experimental setup connects a superconducting qubit chip to the 16-metre cable via a tunable coupler, with the cable’s opposite end terminating at a modulator chip.

This long cable supports a series of evenly spaced standing-wave modes, creating the discrete lattice sites essential for synthetic dimension simulations. The small free spectral range, 7.33MHz, afforded by the cable’s length allows access to over 30 adjacent modes within a narrow frequency window, providing ample lattice sites for complex simulations. The cable itself exhibits high coherence, with a T1 relaxation time of approximately 29.1 microseconds and a T2 dephasing time of around 57.9 microseconds, ensuring the fidelity of quantum dynamics within the synthetic lattice.

Long-coherence coaxial cable supports qubit-mediated synthetic lattice exploration

A superconducting qubit integrated with a 16-metre aluminum coaxial cable and a tunable SQUID-based modulator forms the core of this work, enabling the observation of single-photon quantum dynamics within a synthetic frequency lattice. The cable supports a series of evenly spaced standing-wave modes with a free spectral range of 7.33MHz, allowing access to over 30 adjacent modes within a 300MHz bandwidth.

This relatively small free spectral range, a direct consequence of the cable’s length, provides a substantial number of lattice sites for simulating complex models. Crucially, the cable demonstrates high coherence times of 29.1 microseconds for T1 and 57.9 microseconds for T2, establishing its suitability for investigating quantum dynamics. The system’s ability to address individual modes is confirmed by observing vacuum-Rabi oscillations between the qubit and the cable modes with a Jaynes-Cummings coupling strength of 0.36MHz.

These oscillations, used to characterise the qubit-cable interaction, reveal a full swap time between the qubit and a single cable mode. Single-photon quantum random walks and Bloch oscillations were directly observed within the synthetic lattice, demonstrating the platform’s capacity to simulate fundamental quantum phenomena. Furthermore, nonadiabatic, unidirectional frequency conversion was achieved through rapid temporal modulation of the lattice Hamiltonian.

This modulation, combined with the introduction of long-range inter-mode couplings, allows for the engineering of effective gauge fluxes. These fluxes effectively “fold” the frequency dimension, enabling the construction of higher-dimensional models in the single-photon regime. Band-structure measurements corroborate the successful implementation of these synthetic dimensions and confirm the precise control over the system’s Hamiltonian. The lattice connectivity is readily reconfigurable using multiple drive tones, expanding the versatility of the platform for exploring diverse quantum simulations.

Coaxial Cable Implementation of a Millihertz Free Spectral Range Lattice

A 16-metre aluminum coaxial cable serves as the central component in this work, forming the basis of a synthetic frequency lattice for quantum simulations. This cable connects a superconducting qubit chip to a tunable SQUID-based modulator, establishing a platform for exploring single-photon quantum dynamics. Multiple short, parallel wirebonds minimise impedance mismatch and provide galvanic connections between the cable and both the qubit and modulator chips.

The extended length of the cable enables a small free spectral range (FSR) of 7.33MHz, allowing access to over 30 adjacent modes within a few hundred megahertz. This synthetic lattice is constructed by exploiting the evenly spaced standing-wave modes supported by the cable, with angular frequencies defined as ωm = ω0 + mΩfsr, where ‘m’ represents the mode index and ω0 is the base frequency.

A transmon qubit functions both as a single-photon source and a detector for these mode states, operating near a frequency of 4.32GHz. Coupling between the qubit and the cable modes is achieved using a tunable coupler, allowing for precise control over the interaction strength. The research leverages the cable’s relatively high coherence, with a T1 relaxation time of approximately 29.1μs and a T2 dephasing time of around 57.9μs, to investigate quantum dynamics within the synthetic frequency lattice.

This approach overcomes limitations inherent in photonic systems, such as photon loss and weak interactions, by utilising low-loss superconducting cables and operating in the single-photon quantum regime. The ability to abruptly change the Hamiltonian in time and introduce long-range inter-mode couplings further expands the capabilities of this platform, enabling the construction of higher-dimensional models.

Synthetic dimensions realised via a scalable superconducting coaxial circuit

The persistent challenge of building complex and controllable quantum systems has long hinged on the ability to engineer interactions between qubits with precision. This work represents a significant step forward, not simply because it demonstrates quantum random walks and Bloch oscillations within a synthetic frequency lattice, but because it achieves this using a surprisingly simple and scalable architecture, a superconducting circuit coupled to a conventional coaxial cable.

For years, researchers have struggled to translate the elegant theoretical promise of synthetic dimensions into tangible hardware, often requiring intricate fabrication or exotic materials. This approach sidesteps those hurdles. The beauty of this platform lies in its programmability. By reconfiguring the lattice connectivity with multiple drive tones, the researchers unlock the potential for creating higher-dimensional structures and, crucially, for exploring more complex quantum phenomena.

This isn’t just about simulating known physics; it’s about opening up new avenues for designing quantum algorithms and materials with tailored properties. While the current demonstration is limited to single-qubit evolution, the extensibility of the design suggests a clear pathway towards multi-qubit systems and more sophisticated control schemes. However, maintaining coherence in these extended circuits remains a key obstacle.

Losses within the 16-metre cable will inevitably limit the complexity of the lattices that can be reliably probed. Furthermore, scaling up to many qubits will demand careful consideration of cross-talk and control signal fidelity. Future work will likely focus on mitigating these decoherence effects, perhaps through improved materials or error correction protocols, and on integrating this synthetic dimension approach with other qubit technologies to leverage their respective strengths. The long-term vision extends beyond simulation, potentially offering a novel route to building robust and adaptable quantum processors.