Extending the lifetime of a quantum bit with error correction in superconducting circuits
Jul 20, 2016
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Abstract: (Springer)
A quantum-error-correction system is demonstrated in which natural errors due to energy loss are suppressed by encoding a logical state as a superposition of Schrödinger-cat states, which results in the system reaching the ‘break-even’ point, at which the lifetime of a qubit exceeds the lifetime of the constituents of the system. Although quantum error correction is a crucial ingredient when building a fully-fledged quantum computer, experimental demonstrations of this concept have so far been incomplete proof-of-principles. Here, Nissim Ofek et al. take another important step towards practical quantum error correction. Encoding a logical state in superpositions of Schrödinger-cat states, the authors demonstrate that they can extend the lifetime of a superconducting qubit beyond the lifetime of any of its constituents. Although the scheme is not fully fault-tolerant, it demonstrates all of the crucial ingredients of quantum error correction. Because the scheme is hardware efficient, it might become the method of choice when correcting errors in superconducting qubits. Quantum error correction (QEC) can overcome the errors experienced by qubits1 and is therefore an essential component of a future quantum computer. To implement QEC, a qubit is redundantly encoded in a higher-dimensional space using quantum states with carefully tailored symmetry properties. Projective measurements of these parity-type observables provide error syndrome information, with which errors can be corrected via simple operations2. The ‘break-even’ point of QEC—at which the lifetime of a qubit exceeds the lifetime of the constituents of the system—has so far remained out of reach3. Although previous works have demonstrated elements of QEC4,5,6,7,8,9,10,11,12,13,14,15,16, they primarily illustrate the signatures or scaling properties of QEC codes rather than test the capacity of the system to preserve a qubit over time. Here we demonstrate a QEC system that reaches the break-even point by suppressing the natural errors due to energy loss for a qubit logically encoded in superpositions of Schrödinger-cat states17 of a superconducting resonator18,19,20,21. We implement a full QEC protocol by using real-time feedback to encode, monitor naturally occurring errors, decode and correct. As measured by full process tomography, without any post-selection, the corrected qubit lifetime is 320 microseconds, which is longer than the lifetime of any of the parts of the system: 20 times longer than the lifetime of the transmon, about 2.2 times longer than the lifetime of an uncorrected logical encoding and about 1.1 longer than the lifetime of the best physical qubit (the |0〉f and |1〉f Fock states of the resonator). Our results illustrate the benefit of using hardware-efficient qubit encodings rather than traditional QEC schemes. Furthermore, they advance the field of experimental error correction from confirming basic concepts to exploring the metrics that drive system performance and the challenges in realizing a fault-tolerant system.- Quantum information
- Qubits
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