Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits

Barends, R.; Kelly, J.; Megrant, A.; Sank, D.; Jeffrey, E.; Chen, Y.; Yin, Yi; Chiaro, B.; Mutus, J.; Neill, C.; O’Malley, P.; Roushan, P.; Wenner, J.; White, T.; Cleland, A. N.; Martinis, John M.

doi:10.1103/physrevlett.111.080502

Cited by 712 publications

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“…Variations in T 1 arise predominantly from the qubit interacting incoherently with weakly coupled two-level defects, as discussed in Ref. [1]. In this previous work we found that larger area (with longer and wider legs) Xmon qubits showed higher T 1 values as well as large, frequency-specific suppressions in the energy coherence: for certain frequencies the T 1 would decrease to values below 10 µs.…”

Section: Supplementary Information Device Fundamentals Fabricationmentioning

confidence: 50%

“…[1], with an important improvement: we have added crossovers to suppress stray microwave chip modes by tying the ground planes together with low impedance connections. Otherwise, the many control wires in our chip could lead to segmentation of the ground plane, and the appearance of parasitic slotline modes [2].…”

Section: Supplementary Information Device Fundamentals Fabricationmentioning

confidence: 99%

“…The Ramsey decay envelope is measured by phase tomography (see Ref. [1]) and fitted to the function exp[−t/T φ,1 − (t/T φ,2 ) 2 ]. Slow, Gaussian dephasing is captured in T φ,1 , and fast dephasing, from white noise as well as energy relaxation, is captured in T φ,2 .…”

Section: Supplementary Information Device Fundamentals Fabricationmentioning

confidence: 99%

“…1, we have designed a processor consisting of five Xmon qubits with nearestneighbour coupling, arranged in a linear array. The crossshaped qubit 14 offers a nodal approach to connectivity while maintaining a high level of coherence (see Supplementary Information for decoherence times). Here, the four legs of the cross allow for a natural segmentation of the design into coupling, control and readout.…”

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Superconducting quantum circuits at the surface code threshold for fault tolerance

Barends

Kelly

Megrant

et al. 2014

Nature

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A quantum computer can solve hard problems -such as prime factoring 1,2 , database searching 3,4 , and quantum simulation 5 -at the cost of needing to protect fragile quantum states from error. Quantum error correction 6 provides this protection, by distributing a logical state among many physical qubits via quantum entanglement. Superconductivity is an appealing platform, as it allows for constructing large quantum circuits, and is compatible with microfabrication. For superconducting qubits the surface code 7 is a natural choice for error correction, as it uses only nearest-neighbour coupling and rapidly-cycled entangling gates. The gate fidelity requirements are modest: The per-step fidelity threshold is only about 99%. Here, we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92% and a two-qubit gate fidelity up to 99.4%. This places Josephson quantum computing at the fault-tolerant threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit GreenbergerHorne-Zeilinger (GHZ) state 8,9 using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.The high fidelity performance we demonstrate here is achieved through a combination of highly coherent qubits, a straightforward interconnection architecture, and a novel implementation of the two-qubit controlled-phase (CZ) entangling gate. The CZ gate uses a fast but adiabatic frequency tuning of the qubits 10 , which is easily adjusted yet minimises decoherence and leakage from the computational basis [Martinis, J., et al., in preparation]. We note that previous demonstrations of two-qubit gates achieving better than 99% fidelity used fixed-frequency qubits: Systems based on nuclear magnetic resonance and ion traps have shown two-qubit gates with fidelities of 99.5% 11 and 99.3% 12 . Here, the tuneable nature of the qubits and their entangling gates provides, remarkably, both high fidelity and fast control.Superconducting integrated circuits give flexibility in building quantum systems due to the macroscopic nature of the electron condensate. As shown in Fig. 1, we have designed a processor consisting of five Xmon qubits with nearestneighbour coupling, arranged in a linear array. The crossshaped qubit 14 offers a nodal approach to connectivity while maintaining a high level of coherence (see Supplementary Information for decoherence times). Here, the four legs of the cross allow for a natural segmentation of the design into coupling, control and readout. We chose a modest inter-qubit capacitive coupling strength of g/2π = 30 MHz and use alternating qubit idle frequencies of 5.5 and 4.7 GHz, enabling a CZ gate in 40 ns when two qubits are brough...

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Section: Supplementary Information Device Fundamentals Fabricationmentioning

confidence: 50%

Section: Supplementary Information Device Fundamentals Fabricationmentioning

confidence: 99%

Section: Supplementary Information Device Fundamentals Fabricationmentioning

confidence: 99%

mentioning

confidence: 99%

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Superconducting quantum circuits at the surface code threshold for fault tolerance

Barends

Kelly

Megrant

et al. 2014

Nature

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“…1b is a chain of nine qubits, which allows us to experimentally test both first-and second-order fault-tolerance. It consists of a superconducting aluminium film on a sapphire substrate, patterned into Xmon transmon qubits 20 with individual control and readout. The qubits are the cross-shaped devices; they are capacitively coupled to their nearest neighbours, controlled with microwave drive and frequency detuning pulses, and measured with a dispersive readout scheme.…”

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State preservation by repetitive error detection in a superconducting quantum circuit

Kelly

Barends

Fowler

et al. 2015

Nature

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Quantum computing becomes viable when a quantum state can be preserved from environmentally-induced error. If quantum bits (qubits) are sufficiently reliable, errors are sparse and quantum error correction (QEC) 1-6 is capable of identifying and correcting them. Adding more qubits improves the preservation by guaranteeing increasingly larger clusters of errors will not cause logical failure -a key requirement for large-scale systems. Using QEC to extend the qubit lifetime remains one of the outstanding experimental challenges in quantum computing. Here, we report the protection of classical states from environmental bit-flip errors and demonstrate the suppression of these errors with increasing system size. We use a linear array of nine qubits, which is a natural precursor of the twodimensional surface code QEC scheme 7 , and track errors as they occur by repeatedly performing projective quantum non-demolition (QND) parity measurements. Relative to a single physical qubit, we reduce the failure rate in retrieving an input state by a factor of 2.7 for five qubits and a factor of 8.5 for nine qubits after eight cycles. Additionally, we tomographically verify preservation of the non-classical Greenberger-Horne-Zeilinger (GHZ) state. The successful suppression of environmentally-induced errors strongly motivates further research into the many exciting challenges associated with building a large-scale superconducting quantum computer.The ability to withstand multiple errors during computation is a critical aspect of error correction. We define n-th order fault-tolerance to mean that any combination of n errors is tolerable. Previous experiments based on nuclear magnetic resonance 8,9 , ion traps 10 , and superconducting circuits [11][12][13] have demonstrated multi-qubit states that are first-order tolerant to one type of error. Recently, experiments with ion traps and superconducting circuits have shown the simultaneous detection of multiple types of errors 14,15 . The above hallmark experiments demonstrate error correction in a single round; however, quantum information must be preserved throughout computation using multiple error correction cycles. The basics of repeating cycles have been shown in ion traps 16 and superconducting circuits 17 . Until now, it has been an open challenge to combine these elements to make the information stored in a quantum system robust against errors which intrinsically arise from the environment.The key to detecting errors in quantum information is to perform QND parity measurements. In the surface code, this is done by arranging qubits in a chequerboard pattern -with data qubits corresponding to the white, and measure qubits to the black squares (see Fig. 1) -and using these ancilla measure qubits to repetitively perform parity measurements to detect bit-flip (X) and phase-flip (Ẑ) errors 7 . A square chequerboard with (4n + 1) 2 qubits is n-th order fault tolerant, meaning at least n+1 errors must occur to cause failure in preserving a state if fidelities are above a threshold. W...

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Quantum Computing

Lisenfeld

2016

Digital Encyclopedia of Applied Physics

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Coherent Josephson Qubit Suitable for Scalable Quantum Integrated Circuits

Cited by 712 publications

References 38 publications

Superconducting quantum circuits at the surface code threshold for fault tolerance

Superconducting quantum circuits at the surface code threshold for fault tolerance

State preservation by repetitive error detection in a superconducting quantum circuit

Quantum Computing

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