Photonic graph states are central resources for measurement-based quantum computing and a range of quantum communication and sensing protocols. However, most existing photon sources deliver single photons with low probabilities of surviving to detection, so attempts to assemble many-photon graph states tend to produce fragile states with missing photons. Efforts to identify these missing photons by direct detection destroy the quantum state and prevent simply refilling the lost positions.
The Illinois team, led by associate professor of physics Elizabeth Goldschmidt and professor of electrical and computer engineering Eric Chitambar, approached the problem by asking what could be achieved with realistic quantum emitters and detectors rather than idealized components. They recognized that for many useful applications, it is acceptable to destructively measure photons during the state-generation process. That insight allowed them to intentionally incorporate destructive measurements into their protocol instead of treating them as an unavoidable limitation.
In their work, the researchers introduce the concept of virtual graph states to separate the abstract structure of the entangled state from the actual stream of photons in the lab. Rather than trying to build a full graph state in physical photons and then check if it survived, they add a photon to the virtual graph only after it has been successfully detected. This emit-then-add procedure means that failed emission or collection events are simply discarded, and the entangled structure is updated only when a photon is known to have arrived.
With this approach, the main constraint on how large and complex a photonic graph state can become shifts away from the optical loss rate. Instead, performance is limited by the coherence time of the spin qubits that act as quantum emitters and mediate correlations between photons. Many leading emitter platforms, such as trapped ions and neutral atoms, offer long-lived spin coherence, making them well suited to the virtual graph framework even if their photon collection efficiencies are modest.
The authors emphasize that their scheme is fully general in scenarios where non-destructive photon measurements are available, because in that case photons can be incorporated into graph states without being lost during detection. While such measurements remain beyond current experimental capabilities, the team outlines a broad family of protocols that are compatible with destructive measurements. These protocols retain the key advantages of photonic graph states while operating within near-term hardware limits.
To illustrate the practicality of their ideas, the researchers propose a specific implementation for secure two-party computation using small photonic graph states generated repeatedly. In this setting, the emit-then-add method supports quantum correlations distributed between parties even when the underlying photons do not coexist in time. The mediating spin qubits carry the memory of earlier emissions, allowing multi-photon entanglement to persist across different emission events.
Graduate students Max Gold and Jianlong Lin, co-lead authors on the study, highlight the counterintuitive character of these correlations. They note that the protocol builds entanglement between photons that never exist simultaneously, linked only through quantum interactions with the emitter system. Although the resulting state is described as a single graph of many qubits, not all of those qubits are present at once in the laboratory.
The team points out that many existing experimental platforms worldwide could, in principle, implement their protocol with standard equipment. The method is compatible with emitter-based systems that traditionally suffer from low photon collection efficiency, including trapped ions and neutral atoms. A successful demonstration would rank among the few realizations of photonic graph states tailored for concrete, practical applications rather than purely foundational tests.
Goldschmidt and colleagues are now split between experimental and theoretical follow-ups to the work. In the laboratory, Lin is focusing on the early experimental steps required to bring the emit-then-add scheme into operation on real hardware. On the theory side, Gold is exploring additional applications of virtual graph states beyond the initial secure computation example, looking for other quantum information tasks that can benefit from the new protocol.
The researchers argue that defining protocols around realistic device constraints is essential for near-term progress in quantum technologies. They contrast their approach with earlier studies that often assume idealized, lossless components when designing photonic graph state generators. By grounding their protocol in what can be achieved with current emitters and detectors, they hope to encourage broader efforts to align quantum information schemes with actual hardware performance.
Research Report: Heralded photonic graph states with inefficient quantum emitters
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