Demonstration of an All-Microwave Controlled-Phase Gate Between Far Detuned Qubits

Demonstration of an All-Microwave Controlled-Phase Gate between Far Detuned Qubits S. Krinner,1, ∗ P. Kurpiers,1, † B. Royer,2, 3 P. Magnard,1 I. Tsitsilin,1, 4 J.-C. Besse,1 A. Remm,1 A. Blais,2, 5 and A. Wallraff1, ‡ 1Department of Physics, ETH Zurich, 8093 Zurich, Switzerland 2Institut Quantique and Départment de Physique, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada 3Department of Physics, Yale University, New Haven, Connecticut 06520, USA 4Russian Quantum Center, National University of Science and Technology MISIS, Moscow 119049, Russia 5Canadian Institute for Advanced Research, Toronto, Canada (Dated: June 19, 2020) A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit addressability, qubit-qubit coupling strength, circuit complexity and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and benefit from high coherence but suffer from frequency crowding and limited addressability in multi-qubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transversely coupled transmon qubits which are far detuned compared to the qubit anharmonicity. The gate is activated by applying a single, strong microwave tone to one of the qubits, inducing a coupling between the two-qubit |f, gi and |g, ei states, with |gi, |ei, and |fi denoting the lowest energy states of a transmon qubit. Interleaved randomized benchmarking yields a gate fidelity of 97.5 ± 0.3% at a gate duration of 126 ns, with the dominant error source being decoherence. We model the gate in presence of the strong drive field using Floquet theory and find good agreement with our data. Our gate constitutes a promising alternative to present two-qubit gates and could have hardware scaling advantages in large-scale quantum processors as it neither requires additional drive lines nor tunable couplers. I. INTRODUCTION its so-called sweet spot frequency, at which the qubit is first-order insensitive to flux noise [19]. Superconducting circuits making use of the concepts of In the second class of approaches the qubits are fixed circuit quantum electrodynamics [1] constitute a promis- in frequency and two-qubit interactions are activated using platform for quantum computing. Recently, proces- ing a microwave tone [20–25]. The main advantage of sors containing several tens of superconducting qubits this approach is its potentially higher coherence when have been demonstrated [2–4]. While high-fidelity single- using fixed frequency qubits or frequency-tunable qubits qubit operations with error rates below 0.1% are rou- operated at their flux sweet spot. In addition, control tinely achieved, two-qubit gate errors are typically at the electronics and wiring requirements are somewhat lower percent level [5,6], with only a few recent experiments as no flux control lines are needed. Instead, one resorts achieving two-qubit gate errors of a few per mill [7–9]. to the same control and pulse shaping hardware as also Hence, two-qubit gates limit the performance of state-of- used for the realization of single-qubit gates. The main the-art quantum processors and a variety two-qubit gate disadvantage of all-microwave approaches is the typically schemes are currently explored. One typically distin- longer gate time [20–25]. guishes between two classes of approaches, flux-activated The cross-resonance gate [21, 26, 27], in particular, and microwave-activated gates. constitutes one of the most frequently used all-microwave The first class relies on the dynamic flux tunability of gates. However, for this gate to work, the detuning be- either the qubits or a separate coupling circuit. In this arXiv:2006.10639v1 [quant-ph] 18 Jun 2020 tween the two qubits has to be smaller than the an- class gates are activated by tuning the qubits in frequency harmonicity of the qubits. For multi-qubit devices this to fulfill certain resonance conditions between two-qubit condition imposes stringent requirements on fabrication states [10–14] or by parametrically modulating the qubit precision of Josephson junctions and leads to frequency transition frequency [15–17]. The main benefit are short crowding [28], eventually reducing gate speed and qubit gate times, which however come at the cost of degraded addressability due to a higher sensitivity to cross talk. coherence times or crossings with two-level-system de- Here, we present an all-microwave controlled-phase gate fects [18] when tuning the qubit frequency away from which allows for large detunings compared to the anharmonicity. Our gate is simple and resource-friendly as it requires only a single microwave drive tone applied to one of the qubits in contrast to two drive tones ∗ [email protected] [20, 25, 27], does not require re-focusing pulses during † S.K. and P.K. contributed equally to this work. the gate [29], nor does it make use of real photons in an ‡ andreas.wallraff@phys.ethz.ch additional resonator [20, 24, 25]. The only requirements 2 are a transverse coupling between the qubits and a strong (a) (b) microwave drive. Drive A Drive B II. SYSTEM AND SETUP The Hamiltonian describing two transversally coupled Qubit A Qubit B transmon qubits A and B in presence of a drive on qubit Readout A Readout B A reads In Out X αi H/ˆ = ω aˆ†aˆ + aˆ†aˆ†aˆ aˆ (c) ~ i i i 2 i i i i i=A,B f,g J + e,e † † † + + J aÂaˆB +a ÂaˆB + ΩA(t) aÂ +a Â , (1) A fgge gfgge with aˆ (aˆ†) the lowering (raising) operator of qubit i, i i e,g fgge A and ΩA(t) a microwave drive applied to qubit A. The superconducting device used in our experiment J g,e uses a frequency-tunable transmon qubit (qubit A) and A a fixed-frequency transmon qubit (qubit B). The first qubit is made tunable to provide more freedom in the g,g choice of operation frequencies, but could be at fixed frequency as well. The two qubits have frequencies FIG. 1. Device and gate scheme. (a) Micrograph and (b) ωA/2π = 6.496 GHz and ωB/2π = 4.996 GHz, energy circuit diagram of the core elements of the sample. Each of the relaxation times T1,A = 7 µs and T1,B = 20 µs, anhar- two capacitively coupled transmon qubits (red) is coupled to monicities αA/2π = −257 MHz and αB/2π = −271 MHz, an individual readout resonator (green) and drive line (blue). and are capacitively coupled with a coupling strength (c) Schematic of the energy-level diagram of qubit A with J/2π = 42(1) MHz, see Fig.1(a) and (b). We control the qubit B in the |gi state (left) and in the |ei state (right). state of each qubit using amplitude and phase modulated Virtual states are indicated by dashed lines. microwave pulses [30–32], which are generated by upcon- verting the signals from an arbitrary waveform generator and applied to the qubits through a dedicated drive line. III. GATE CONCEPT Prior to each experimental run we reset the qubits using the protocol introduced in [33], reducing the excited Our gate exploits a Raman transition between the two- state populations of qubit A and B to 0.6% and 0.8%, qubit states |f, gi and |g, ei. The transition is analo- respectively (see AppendixA for details). gous to the cavity-assisted Raman transition used re- For qubit readout, two resonators at frequencies cently for photon shaping and remote quantum commu- ωr,A/2π = 7.379 GHz and ωr,B/2π = 7.076 GHz are nication [38–41], qutrit reset [33, 42] and two-qubit gates dispersively coupled to qubit A and qubit B, respec- [25], with the distinction that here the cavity is replaced tively, with strength gA/2π = 52 MHz and gB/2π = by a second qubit. The coupling between |f, gi and 71 MHz. Both resonators are coupled to a common |g, ei is activated by a strong microwave tone ΩA(t) = feedline with coupling rates κA/2π = 0.67 MHz and Ωfgge cos(ωfgget) applied to the drive line of qubit A at a κB/2π = 0.63 MHz. We determine the |gi, |ei, and |fi frequency corresponding to the energy difference between state population of both qubits by applying two gated mi- the two states, i. e. at ωfgge,0/2π = (ωA + ∆ + αA)/2π ≈ crowave tones to the feedline of the readout resonators 7.739 GHz, with ∆ = ωA − ωB and the subscript ’0’ la- at frequencies and powers optimized for qutrit readout beling the unshifted transition frequency in absence of a [34]. The transmitted signal is amplified at 10 mK by a drive-induced ac-Stark shift on qubit A. The coupling traveling wave parametric amplifier [35] and at 4 K by is mediated by virtual states, which are coupled to |f, gi a high-electron-mobility transistor amplifier. At room and |g, ei via the drive Ωfgge and the direct qubit-qubit temperature the signal is further amplified, split into coupling J, see Fig.1(c). The two coupling paths be- two paths, which are separately down-converted using tween |f, gi and |g, ei indicated by the light blue arrows an I-Q mixer, digitized using an analog-to-digital con- interfere destructively and give rise to a total coupling verter, digitally down-converted and processed using a strength of field programmable gate array [36]. We extract the qutrit populations of each transmon using single-shot readout. ΩfggeJαA gfgge = √ . (2) We record each measurement trace 2000 (4000) times for 2∆(∆ + αA) all characterization (randomized benchmarking) experiments and account for readout errors [37, 38] (see Ap- Due to the large detuning between the qubits, a large pendixA).

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