Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. A plethora of ways to achieve this has been proposed in the last decade, from active reset protocols to advances in materials and shielding. An instrumental tool to evaluate those methods and develop new ones is the ability to measure the population of excited states with high precision and in a short period of time. In this Letter, we propose a new technique of finding the excited state population of a qubit using correlations between two sequential measurements. We experimentally implement the proposed technique using a circuit QED platform and compare its performance with previously developed ones. Unlike other techniques, our method does not require high-fidelity readout and does not involve the excited levels of the system outside of the qubit subspace. We experimentally demonstrated measurement of the spurious qubit population with accuracy of up to 0.01%. This accuracy enabled us to perform "temperature spectroscopy" of the qubit, which helps to shed light on decoherence sources.
Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. A plethora of ways to achieve this has been proposed in the last decade, from active reset protocols to advances in materials and shielding. An instrumental tool to evaluate those methods and develop new ones is the ability to measure the population of excited states with high precision and in a short period of time. In this Letter, we propose a new technique of finding the excited state population of a qubit using correlations between two sequential measurements. We experimentally implement the proposed technique using a circuit QED platform and compare its performance with previously developed ones. Unlike other techniques, our method does not require high-fidelity readout and does not involve the excited levels of the system outside of the qubit subspace. We experimentally demonstrated measurement of the spurious qubit population with accuracy of up to 0.01%. This accuracy enabled us to perform "temperature spectroscopy" of the qubit, which helps to shed light on decoherence sources.
We demonstrate an active reset protocol in a superconducting quantum circuit. The thermal population on the excited state of a transmon qubit is reduced through driving the transitions between the qubit and an ancillary qubit. Furthermore, we investigate the efficiency of this approach at different temperatures. The result shows that population in the first excited state can be dropped from 7% to 2.55% in 27 ns at 30 mK. The efficiency improves as the temperature increases. Compared to other schemes, our proposal alleviates the requirements for measurement procedure and equipment. With the increase of qubit integration, the fast reset technique holds the promise of improving the fidelity of quantum control.
In the last decade, quantum information processing has transformed from a field of mostly academic research to an applied engineering subfield with many commercial companies announcing strategies to achieve quantum advantage and construct a useful universal quantum computer. Continuing efforts to improve qubit lifetime, control techniques, materials and fabrication methods together with exploring ways to scale up the architecture have culminated in the recent achievement of quantum supremacy using a programmable superconducting processor -- a major milestone in quantum computing en route to useful devices. Marking the point when for the first time a quantum processor can outperform the best classical supercomputer, it heralds a new era in computer science, technology and information processing.One of the key developments enabling this transition to happen is the ability to exert more precise control over quantum bits and the ability to detect and mitigate control errors and imperfections. In this thesis, ways to efficiently control superconducting qubits are explored from the experimental viewpoint.We introduce a state-of-the-art experimental machinery enabling one to perform one- and two-qubit gates focusing on the technical aspect and outlining some guidelines for its efficient operation. We describe the software stack from the time alignment of control pulses and triggers to the data processing organisation. We then bring in the standard qubit manipulation and readout methods and proceed to describe some of the more advanced optimal control and calibration techniques.The described toolbox together with some newly developed advances such as multi-level single-shot readout are applied to a practical problem of generating high-quality random numbers. Following the recent advances in the field, we certify our generator based on the Kochen-Specker theorem utilizing contextuality of quantum mechanics as an underlying resource of randomness. Our implementation based on the solid-state circuit quantum electrodynamics (QED) platform and the new certification scheme allows us to greatly enhance the bit rate of certified quantum random number generators (QRNGs) and approach the requirements for real-world applications.Addressing some of the control and state preparation errors, we develop a set of techniques to use a qubit as an in situ probe of distortions of its control lines and its effective temperature. Considering a qubit simultaneously driven by a longitudinal and a transverse MW-drives under certain resonant conditions, we show that in the rotating dressed-state picture (second rotating frame) one can get information about both the amplitude and the phase of the drive signals. This allows us to employ a qubit as an on-chip vector network analyzer and precisely measure the transfer functions of its control lines. As no other method with comparable frequency domain coverage exists, we verify our technique by cancelling out the distortions and increasing the fidelity of entangling two-qubit CPHASE gate.Finally, we show that the decay of the correlations between two sequential measurements of a quantum bit is a manifestation of its nonzero effective temperature. We use it to develop a precise method of evaluating the qubits population allowing for virtually unlimited suppression of absolute errors. We realize the method and demonstrate its applicability to the calibration of a quantum processor and evaluation of active reset techniques. As the actual mechanism of the increased excited-state population is currently unknown, we use our method's precision to show the qubit’s coupling to two separate independent thermal baths. We also introduce the ‘temperature spectroscopy’ as a tool to acquire more information about the qubits spurious excitation sources.
We report temperature measurements using a transmon qubit by detecting the population of its first three energy levels after applying a sequence of π pulses and performing projective dispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times τ 1 , τ 2 for three devices in the temperature range from 20 to 300 mK. We analyze the process of qubit thermalization to its effective environment consisting of multiple heat baths and support this with experimental data. The signal-to-noise ratio of the temperature measurement depends strongly on τ 1 , which drops at higher temperatures due to quasiparticle excitations, adversely affecting the measurements and setting an upper bound of the dynamic temperature range of the thermometer. The measurement relies on coherent dynamics of the qubit during the π pulses. The effective qubit temperature follows closely that of the cryostat in the range from 100 to 200 mK. We present a numerical model of the qubit population distribution and find hat it compares favorably with the experimental results. Finally, we compare our technique with previous work on qubit thermometry and discuss its application prospects.
Temperature is a deceptively simple concept that still raises deep questions at the forefront of quantum physics research. The observation of thermalization in completely isolated quantum systems, such as cold-atom quantum simulators, implies that a temperature can be assigned even to individual, pure quantum states. Here, we propose a scheme to measure the temperature of such pure states through quantum interference. Our proposal involves interferometry of an auxiliary qubit probe, which is prepared in a superposition state and subsequently decoheres due to weak coupling with a closed, thermalized many-body system. Using only a few basic assumptions about chaotic quantum systems, namely, the eigenstate thermalization hypothesis and the emergence of hydrodynamics at long times, we show that the qubit undergoes pure exponential decoherence at a rate that depends on the temperature of its surroundings. We verify our predictions by numerical experiments on a quantum spin chain that thermalizes after absorbing energy from a periodic drive. Our Letter provides a general method to measure the temperature of isolated, strongly interacting systems under minimal assumptions.
The realization of fault-tolerant quantum computing requires the execution of quantum error-correction (QEC) schemes, to mitigate the fragile nature of qubits. In this context, to ensure the success of QEC, a protocol capable of implementing both qubit reset and leakage reduction is highly desirable together with the ability to perform fast and high-fidelity readout of the qubit s’ quantum states. In this thesis, we tackle both these challenges in an architecture consisting of fixed-frequency transmon qubits pair-wise coupled via tunable couplers. We demonstrate a readout scheme that combines two microwave techniques: applying a shelving technique to the qubits that reduces the contribution of decay error during readout, and a two-tone excitation of the readout resonators to distinguish among qubits’ populations in higher energy levels. We perform single-shot frequency-multiplexed qubit readout, with a 140 ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout - without using a quantum-limited amplifier. We also demonstrate a reset scheme that is fast, unconditional, and achieves fidelities well above 99%, thus enabling fixed-frequency qubit architectures as future implementations of fault-tolerant quantum computers. Our reset protocol uses the tunable couplers to transfer any undesired qubits’ excitation to the readout resonators of the qubits, from which this excitation decays into the feedline. In total, the combination of qubit reset, leakage reduction, and coupler reset takes only 83 ns to complete. This reset protocol also provides a means to both reduce QEC cycle runtime and improve algorithmic fidelity on quantum processors.
A better understanding of the noise causing qubit decoherence is crucial for improving qubit performance. The noise spectrum affecting the qubit may be extracted by measuring dephasing under the application of pulse sequences but requires accurate qubit control and sufficiently long relaxation times, which are not always available. Here, we describe an alternative method to extract the spectrum from correlations of single-shot measurement outcomes of successive free induction decays. This method only requires qubit initialization and readout with a moderate fidelity and also allows independent tuning of both the overall sensitivity and the frequency region over which it is sensitive. Thus, it is possible to maintain a good detection contrast over a very wide frequency range. We discuss using our method for measuring both 1/f noise and the fluctuation spectrum of the nuclear bath of GaAs spin qubits.