Session R08: Advances in Qubit Measurement I

Simultaneous Measurement of 53 Superconducting Transmons on the Sycamore Processor

Progress in low-noise amplifiers, device design, and device calibration has enabled high fidelity readout in transmon qubits. However, challenges arise when scaling up to large numbers of qubits, where deleterious effects such as crosstalk and unwanted interactions can degrade readout fidelity. In this talk, I will discuss how we overcame some of these challenges to achieve simultaneous readout of 53 qubits on Google’s Sycamore processor, enabling the demonstration of quantum supremacy. [1].

[1] Quantum supremacy using a programmable superconducting processor, Google AI Quantum and collaborators. Nature 574, 505-510 (2019).   

Single-shot readout and state preparation of a fluxonium qubit without the use of a parametric amplifier

High-fidelity qubit readout is an essential requirement for the implementation of quantum algorithms. A commonly used readout technique involves the dispersive interaction between a qubit and a resonator, which encodes the qubit state into the phase and amplitude of the microwave readout tone. In theory, the state discrimination can be substantially improved if the resonator is populated with high photon numbers [1]. In practice however, the optimal photon number, at which the best readout fidelity is obtained, is usually in the range of a few photons [2, 3], and parametric amplifiers with near quantum-limited noise are needed to overcome the noise of even the best commercial high electron mobility transistor amplifiers.
Using a fluxonium qubit with a granular aluminum superinductance [4], we demonstrate single shot qubit measurements without a parametric amplifier, at a readout power corresponding to hundreds of circulating photons. The dispersive shift decreases with photon number, and the qubit decay rate increases, resulting in a tradeoff between applied power and readout fidelity.

[1] A. Blais, et al. PRA 69, 062320, (2004)
[2] T. Walter, et al. PRA 75, 054020, (2017)
[3] U. Vool, et al. PRL. 113, 247001 (2014)
[4] L. Grünhaupt & M. Spiecker, et al. Nature Materials (2019)   

Measurement of quantum jumps of a fluxonium qubit using a Dimer Josephson Junction Array Amplifier operated at high power

Josephson parametric amplifiers have become an essential element in cQED dispersive readout measurement schemes, enabling single-shot qubit readout. Over the last decade there was significant progress in the increase of their saturation power [1,2,3,4], which now reaches several thousand photons per μs.
We demonstrate high saturation power in a Dimer Josephson Junction Array Amplifier [4] operating with a quantum efficiency of 60%. We use this parametric amplifier to perform dispersive readout of a fluxonium qubit with a superinductor made of granular aluminum [5]. We present quantum jump measurements acquired using up-to 100 circulating photons in the readout resonator, which allowed qubit state discrimination in tens of nanoseconds.

[1] C. Eichler and A. Wallraff, EPJ Quantum Technologies 1, 2 (2014)
[2] L. Planat et.al., PRA 11, 034014 (2019)
[3] V. V. Sivak et.al., arxiv:1909.08005
[4] P. Winkel & I. Takmakov et.al., arxiv:1909.08037
[5] L. Grünhaupt & M. Spiecker, et al. Nature Materials 18, 816–819 (2019)   

Towards autonomous digital feedback on a superconducting qubit

Quantum error correction requires conditional operations predicated on measurement results. Such feedback schemes in superconducting qubit systems, however, can be hardware intensive. They may require dedicated control hardware at ambient temperature and consume precious high-bandwidth connections between temperature stages. Here we present preliminary data on the state preparation and stabilization of a superconducting qubit where we keep the measurement information at the base temperature stage of the cryostat, and then use it to conditionally π pulse a qubit. This scheme avoids the aforementioned problems and can ease the development of stabilized quantum networks within a scalable architecture.   

Isolating a qubit from amplifier backaction by coordinated switching

We demonstrate a cQED qubit measurement scheme in which the qubit is isolated from amplifier backaction without ferrite circulators. We swap a readout signal from the qubit cavity to an ancillary cavity by temporarily coupling them. The state of the ancillary cavity is then amplified by parametrically pumping past bifurcation into one of two large amplitude states that are correlated with the qubit state. Isolation and efficiency are comparable to readout using a ferrite circulator, but are achieved using a chip-scale device compatible with the integration of superconducting qubits. Because the measurement result is contained in the digital state of a superconducting bifurcation amplifier, it is possible to use this scheme to create autonomous feedback loops at the cryostat base temperature. The integrated nature of this readout suggests an avenue toward the scalable measurement of larger quantum networks.
  

On-chip single-pump interferometric Josephson Isolator

Nonreciprocal microwave devices, such as circulators and isolators, play several critical roles in superconducting quantum processors. They route readout signals in a directional manner, protect the quantum system against noise coming from the output chain, and enable reflection measurements by separating input from output. However, the reliance of these devices, on magnetic materials and strong magnets for breaking reciprocity, makes them disadvantageous in scalable superconducting architectures. In this work, we realize and measure an on-chip Josephson isolator, which is formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Isolation is created, in the active device, by operating the two mixers in frequency conversion mode using balanced, same-frequency microwave pumps, whose phase difference is \pi/2. The applied pumps are generated using an on-chip quadrature hybrid, which equally splits the single pump feeding the device and imposes the required phase difference between the split drives.   

Novel Measurements for Annealing Capable Flux Qubits

Quantum Flux Parametron (QFP) based measurements have demonstrated the ability to implement high fidelity z-basis readout for high coherence capacitively shunted flux qubits (CSFQs) at the end of an anneal. This talk discusses alternative QFP readout procedures including: (i) at early times in the anneal, (ii) for other non z-loop currents, and (iii) for Josephson phase slip qubits (JPSQs). It is shown how the POVM (positive-operator valued measure) that characterizes the qubit measurement in these novel regimes can be derived from the final QFP POVM by modeling the dynamics of the coupled QFP-qubit system.   

Realizing large, tunable dispersive shifts with parametric couplings

In recent years, parametric couplings are being deployed in a variety of applications, ranging from parametric gates, state preparation to quantum annealing. Unlike their traditional applications such as parametric amplification, these new functionalities require much stronger parametric coupling strengths that can dominate qubit decay and measurement rates. The theoretical framework to capture the effect of such strong time-dependent couplings, however, remains rudimentary. In this work, we analyze the primary building block of such a platform --- a single qubit parametrically coupled to a single-mode resonator. We establish that even this simple system can support a rich structure in dispersive shifts, that can be rendered large and tunable by choosing suitable parametric coupling drive amplitudes and frequencies. In addition, this platform can enable enhanced cavity squeezing by multi-tone parametric pumping. Our scheme can be realized with state-of-the-art circuit-QED architecture, and we discuss our early experimental efforts to support this approach.   

Remote entanglement via two mode squeezed light

Remotely entangling qubits which do not interact directly is very desirable for quantum information processing. One method for remote entanglement is to read out two qubits in parallel with their outputs entering the signal and idler mode of a phase-preserving amplifier. The amplifier squeezes away which-path information and can project the qubits into a Bell state. However, this measurement also produces an outcome-dependent phase kick on the qubits’ state. Thus, losses and inefficient amplification can prevent the generation of entangled states [1, 2]. However, we have recently demonstrated a new scheme which uses two-mode squeezed light (TMSL) in an interferometer formed by two phase-preserving amplifiers to measure a qubit. We find that the phase back-action of TMSL measurement alters the phase back-action relative to coherent readout, depending on the amplifiers’ relative phase. Here we present a two qubit, TMSL interferometric entangling readout. We will show how TMSL affects the back-action of the entangling readout and discuss the prospects for the use of TMSL to make the scheme more tolerant of hardware imperfections.

[1] Silveri et al, PRA 93, 062310 (2016)
[2] Zalys-Gellar et al, Arxiv: 1803.01275 (2018)   

Fast high fidelity quantum non-demolition superconducting qubit readout

The most common technique of qubit readout in cQED relies on the transverse dispersive coupling between a qubit and a microwave cavity. However, despite important progresses, implementing fast high fidelity and QND readout remains a major challenge. Indeed, inferring the qubit state is limited by the trade-off between speed and accuracy due to Purcell effect and unwanted transitions induced by readout photons in the cavity. To overcome this, we propose and experimentally demonstrate a new readout scheme based on a transmon molecule inserted inside a 3D-cavity_[1,2]. The full system presents a transmon qubit mode coupled to a readout mode through an original non-pertubative cross-Kerr coupling. The readout mode, called polariton mode, results from the hybridization between the microwave cavity and the transmon molecule circuit. The direct cross-Kerr coupling is a key point of our readout scheme since it protects the qubit from Purcell effect. This first implementation, though perfectible, already enables a very efficient single-shot QND readout of the qubit in only 50ns, with a QND-ness of 99% and a fidelity of 97.4%.
[1] I. Diniz et al, Phys. Rev. A 87 033837 (2013).
[2] R. Dassonneville et al, arxiv 1905.0027 (2019).
  

Frequency-domain pulse engineering for fast qubit readout and resonator reset

The state of a superconducting qubit is typically determined by measuring the dispersive shift χ of a superconducting resonator with coupling rate κ to a transmission line. Large χ/κ ratio is beneficial for high-fidelity readout. While small κ protects the qubit from the noisy environment, it has the big disadvantage of limiting the speed of measurement. In particular, photons linger in the resonator for a time of several 1/κ, causing decoherence of the qubit and blocking subsequent measurement. Here we present a microwave pulse scheme which, regardless of the state of the qubit, populates the readout resonator, measures the state of the qubit, and empties the resonator, all in a time much faster than 1/κ. The pulse is designed in the frequency domain by shaping its spectral content at the dressed-resonance frequencies. The pulse envelope is derived through inverse Fourier transform, rather than relying on brute-force numerical optimization. We evaluate the performance of the pulse scheme in different regimes of χ/κ through both simulation and experiment, and we comment on the theoretically achievable single-shot fidelity of the method.
  

High efficiency measurement of a superconducting qubit using a directional, phase-sensitive, parametric amplifier

The measurement of a superconducting quantum bit is often performed by encoding its state in a single quadrature of a microwave field. Ideal measurement efficiency of this observable could in principle be achieved by noiselessly amplifying the information-carrying quadrature, but in practice is limited by technical losses due to circulators, cables and connectors used in state-of-the-art amplification chains.
In this talk we will discuss how one can approach ideal measurement efficiency by directly connecting a 3D transmon to a non-reciprocal phase-sensitive amplifier [1]. After describing the setup and tuning of the amplifier, we will extract the measurement efficiency by comparing the qubit dephasing rate to the measurement rate.
[1] F. Lecocq. et al, ‘Microwave measurement beyond the quantum limit with a nonreciprocal amplifier’, ArXiv 1909.12964 (2019)
  

High fidelity dispersive qubit readout in circuit QED without using a Josephson Parametric Amplifier

In the cQED architecture, the Josephson parametric amplifier (JPA) enables high fidelity readout by providing minimal degradation of the signal-to-noise ratio (SNR) of the amplified readout signal. While the SNR can also be increased by increasing readout power and integration time, the fidelity typically gets limited due to unwanted transitions both within and outside the computational subspace. Here, we present and demonstrate an alternate design where qubit-cavity coupling does not rely on the dispersive approximation of the Jaynes-Cummings Hamiltonian. This multi-modal circuit is reminiscent of the quantronium qubit design but with two differences: the qubit is of the transmon type and the cavity is linear. The device is implemented in the 3D cQED architecture and we use a rectangular waveguide to couple the readout cavity to the measurement line. We achieve a readout fidelity of 97.8% for 800ns integration time with a histogram overlap error of only 0.3% without using a JPA. The best fidelity obtained with the JPA was 98.9% for a 300 ns integration time with an overlap error of less than 0.01%. We will conclude by discussing the variation of readout fidelity with measurement power and compare with a conventional transmon readout in cQED.