Entanglement rate scales linearly with this new quantum interconnect

Jérémy Raskop and colleagues Labs Ltd, in collaboration with researchers from the Institute of Science and Technology Austria (ISTA), have demonstrated a quantum interconnect using a near-deterministic, robust photon–atom controlled-Z gate via a single atom trapped in a high-finesse cavity to mediate entanglement between modular processors. Unlike existing methods where entanglement rates scale quadratically with photon-delivery probability, this protocol achieves linear scaling over a wide range of parameters. This advancement removes a key bottleneck in modular quantum computing by allowing entanglement rates to increase with photon arrival probability.

Single-Atom Entanglement via Controlled-Z Gates

A new quantum interconnect uses a single atom to achieve a linear scaling of entanglement rate, a departure from the quadratic scaling that has limited previous designs. The system uses a near-deterministic, robust photon-atom controlled-Z gate, allowing entanglement to be established even with imperfect photon delivery. By removing the need for simultaneous photon arrival, the interconnect significantly improves performance over existing methods reliant on probabilistic Bell-state measurements.

The core of this interconnect is a single atom trapped within a high-finesse cavity, functioning as a memory element to facilitate entanglement distribution. Detecting and measuring a photon originating from one quantum processing unit immediately establishes entanglement between that unit and the atom. This entanglement is then preserved while the system awaits a photon from the second processing unit, effectively removing the stringent requirements for indistinguishable photons. The process concludes with readout of the atomic qubit, finalizing the entanglement between the two processors.

This approach allows for a wider range of parameters and reduces the potential for infidelity in the entanglement process. By providing nonlocal couplings between quantum processing units, this interconnect enables error-correcting codes to span multiple locally connected processors. The rate and fidelity of remote entanglement provided by this quantum interconnect are therefore critical system-level resources alongside local-gate speeds and fidelities within each quantum processing unit.

Asynchronous Photonic Interconnect for Modular QPUs

Detecting and measuring a photon from one processor heralds entanglement between that processor and the atom. This entanglement is preserved while the process repeats with the second processor until the second photon is detected and measured, finalizing the entanglement between the processors. As the entanglement is mediated by the atom, the photons from both processors do not need to be indistinguishable, removing a major source of infidelity.

Each processor prepares its communication qubit and outgoing photon in the Bell state + $\sqrt{2}$, and photons are timed to arrive at the atom in consecutive, nonoverlapping time slots. The researchers report that the system can function effectively even with time slots approximately 50 nanoseconds apart.

Linear Scaling of Entanglement Rate with Photon Probability

Conventional quantum interconnects face a fundamental limitation. The system’s design effectively removes a major source of error by eliminating the need for indistinguishable photons from each processor. The core of this advancement lies in the use of an atomic qubit memory, which preserves entanglement after the arrival of the first photon. This allows the second quantum processing unit to continue attempting photon delivery without a strict time constraint.

The probability of achieving a full success, both photons reaching the interconnect and generating heralding events, is traditionally limited by the square of the individual photon delivery probability. However, by introducing a waiting window of up to k additional attempts, the protocol significantly increases the likelihood of establishing entanglement. Analysis reveals that increasing the product of photon delivery probability and attempt rate, pR, provides a double benefit.

It not only raises the incoming herald rate but also increases the probability that a partner photon interacts before the stored entanglement link decoheres. The memory-assisted contribution to the entanglement rate grows quadratically with pR, while extending the coherence time, produces linear growth. This interplay between coherence and photon delivery is critical for maximizing performance. As the number of admissible attempts increases, the memory-assisted contribution grows, and the curves for different photon delivery probabilities converge.

At larger values of N, unheralded photon interactions limit the waiting-window length, causing the normalized entanglement rate to plateau below the theoretical unbounded-window limit. For context, the entanglement rate for traditional type-II fusion is Γ_bs =, where η is the probability of detecting a photon present at the interconnect input. This comparison emphasises the potential for substantial improvement offered by the memory-assisted approach.

Quantum Interconnects Enable Scalable Quantum Computation

This speed directly addresses a critical bottleneck in building modular quantum computers, where connecting individual processing units remains a significant challenge. The team’s design uses a single atom as an intermediary, mediating entanglement distribution between processors without requiring photons from each to be perfectly indistinguishable. This is particularly important for longer-distance connections, where photon loss is a major limiting factor. The researchers account for both the decay of entanglement with the first processor and decoherence caused by unintended photon interactions in their analysis, demonstrating robust performance under realistic conditions.

The core of the system is a cavity quantum electrodynamics (QED) setup, using a photon-atom controlled-Z gate to establish entanglement. Finalizing the entanglement between the processors requires reading out the atomic qubit, completing the connection.

The potential for integration with diverse quantum processing units is a significant advantage. The system’s design allows for relatively simple wavelength conversion, enabling connections between QPUs using different physical platforms, such as trapped ions, neutral atoms, and superconducting circuits. This flexibility is important for building hybrid quantum computers that leverage the strengths of each technology.

The team anticipates that this interconnect could facilitate the creation of more complex and powerful quantum systems by enabling transversal entangling gates between remote logical blocks and simplifying lattice-surgery routing. They derived fidelity-limited waiting windows for the stored entanglement link and the corresponding expected entanglement rate, accounting for coherence decay and unwanted photon interactions.

The design’s reliance on a single atom as a memory element is a minimalist approach to a complex problem, offering a potentially scalable solution for distributing entanglement between modular quantum processors. This interconnect’s architecture is set to support increasingly complex quantum computations.

Realistic Entanglement Rates and System-Level Resources

Deriving entanglement rates under realistic parameters reveals a significant advantage for this new interconnect. The nanosecond-scale of gate and read-out operations yields entanglement-rate gains orders of magnitude beyond those achievable with linear optics, directly addressing a key bottleneck in building modular quantum computers. The team’s analysis establishes a fidelity-limited waiting window for preserving entanglement, an essential parameter for practical implementation, and determined corresponding expected entanglement rates based on realistic coherence times.

This is particularly relevant as the selected waiting window remains finite even with extended coherence, preventing rates from reaching an unbounded limit and ensuring practical scalability. The team’s calculations demonstrate that the entanglement rate scales linearly with the photon-arrival probability over a wide parameter range, a departure from the quadratic scaling inherent in many existing protocols.

The analysis further details how the entanglement rate is influenced by the rate at which photons from each processor interact with the atom, denoted as pR. At low interaction rates, the waiting window is limited by the atom’s finite coherence time, resulting in quadratic growth of the entanglement rate.

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