Short answer: An erasure qubit is designed so that its most common errors can be detected and flagged, turning them into erasures, which are much easier to correct than errors at unknown locations. The dual-rail cavity qubit does this by encoding a qubit in a single microwave photon shared between two superconducting cavities, so photon loss shows up as a detectable event. A 2023 proposal and two 2024 experiments report this approach, including detection of over 99% of cavity decay events as erasures. These are small laboratory demonstrations, not a full error-correcting computer.
Why erasures help
The 2024 Nature Physics paper explains that a critical challenge in scalable quantum systems is correcting the accumulation of errors while performing operations and measurements. It says systems where dominant errors can be detected and converted into erasures have relaxed requirements for quantum error correction. A 2023 PNAS paper adds that erasure errors are in general much easier to correct.
The dual-rail cavity qubit
The 2023 PNAS paper introduces the circuit quantum electrodynamics dual-rail qubit. The physical qubit is encoded in the single-photon subspace of two superconducting microwave cavities, with one photon either in the first cavity or in the second. The logical zero and one are these two states. Encoding in a space with odd joint photon number parity lets the dominant errors, photon loss, be detected and converted to erasures. The authors say a circuit-QED version offers capabilities beyond linear optics. With just one additional transmon ancilla per dual-rail qubit, they describe how to perform a universal gate set including state preparation, logical readout and parametrizable single- and two-qubit gates. First-order hardware errors in the cavities and transmon can be detected and converted to erasures in all operations, leaving background Pauli errors that are orders of magnitude smaller. They conclude that the qubit has a favourable hierarchy of error rates and is expected to perform well below the relevant error correction thresholds with today's coherence times. That is a projection from the paper's analysis.
Experiments
The 2024 Nature Physics paper demonstrates a projective logical measurement of a dual-rail cavity qubit with integrated erasure detection and measures the idling errors. The authors measure logical state preparation and measurement errors at the 0.01% level and detect over 99% of cavity decay events as erasures. They find decay errors occur with a probability of about 0.2% per microsecond, phase errors occur 6 times less frequently, and bit flips at least 150 times less frequently. They describe this as confirming the expected error hierarchy necessary to concatenate dual-rail cavity qubits into a highly efficient erasure code.
A 2024 Physical Review Letters paper implements a dual-rail qubit in a compact double-post superconducting cavity. It notes that qubits with predominantly erasure errors have distinctive advantages for error correction and fault-tolerant computing, and that dual-rail encodings have been proposed using coupled transmons or cavities. Using an auxiliary transmon, the authors perform erasure detection on the dual-rail subspace and characterize the code space with a new method of joint Wigner tomography. They report a strong hierarchy of error rates, which together with the compact, hardware-efficient architecture they say holds promise for error correction schemes with enhanced thresholds and improved scaling.
What this does not tell you
These experiments involve single dual-rail qubits and their measurement. The papers describe concatenating them into an erasure code as a next step, and do not report a working error-corrected computation. The error rates quoted are for specific devices.
FAQ
What is an erasure error?
An error whose location is known, which makes it much easier to correct.
How does dual-rail encoding work?
A single microwave photon in one of two cavities encodes the qubit, so photon loss can be detected.
What was demonstrated?
Over 99% of cavity decay events detected as erasures, with state preparation and measurement errors at the 0.01% level.
Is this a finished error-correcting machine?
No. The papers describe single-qubit experiments and plans to concatenate them.
Why does the error hierarchy matter?
Detectable decay errors dominate, while phase errors and bit flips are much rarer, which suits an erasure code.
All field notes