Entanglement Genesis by Ancilla-Based Parity Measurement in 2D Circuit QED

Abstract

We present an indirect two-qubit parity meter in planar circuit quantum electrodynamics, realized by discrete interaction with an ancilla and a subsequent projective ancilla measurement with a dedicated, dispersively coupled resonator. Quantum process tomography and successful entanglement by measurement demonstrate that the meter is intrinsically quantum nondemolition. Separate interaction and measurement steps allow the execution of subsequent data-qubit operations in parallel with ancilla measurement, offering time savings over continuous schemes. DOI: 10.1103/PhysRevLett.112.070502 PACS numbers: 03.67.Bg, 03.67.Lx, 42.50.Pq, 85.25.-j Controlling the entanglement between qubits is central to the development of every quantum computing architecture. Early efforts with superconducting quantum circuits relied on quantum interference for this purpose. Programmed sequences of one-and few-qubit gates fitting within qubit coherence times have allowed the generation of two-and three-qubit entanglement [1][2][3][4] and the implementation of elementary quantum algorithms [5][6][7][8][9] and games [10].Recently, focus has shifted toward generating and preserving entanglement by nondemolition measurement of multiqubit observables and their use in feedback loops as required for quantum error correction [11]. Of particular interest is the parity measurement [12][13][14] that discriminates between states in a multiqubit register with even or odd total excitation number. Parity measurement on four data qubits at the corners of every square tile on a lattice is needed to realize surface codes, offering the highest faulttolerance thresholds to date [15,16].A convenient approach to implementing a parity measurement is a two-step indirect scheme involving coherent interaction of the data qubits with an ancillary qubit and subsequent strong measurement of this ancilla. To date, indirect four-qubit parity measurements have been achieved only in trapped-ion systems [17]. In the solid state, parity measurement using an ancillary electron spin has been used to generate probabilistic entanglement between two nuclear spins in nitrogen-vacancy centers in diamond [18]. More recently, parity measurement of two transmon qubits using a dispersively coupled 3D cavity has been used in a digital feedback loop to generate entanglement deterministically [19]. An important next step is the realization of parity measurements in an architecture amenable to surface coding.In this Letter, we present an ancilla-based two-qubit parity measurement in a planar circuit QED (cQED) architecture [20]. Tomographic characterization shows that dephasing within even and odd parity subspaces is due to intrinsic qubit decoherence during interaction and measurement steps, making the parity meter intrinsically quantum nondemolition (QND). As a further demonstration of this nondemolition character, we generate entanglement by parity measurement on a maximal superposition state. Performing all tomographic data-qubit operations after the ancilla measurement, we achi...

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