scholarly journals Improved Superconducting Qubit State Readout by Path Interference

2021 ◽  
Vol 38 (11) ◽  
pp. 110303
Author(s):  
Zhiling Wang ◽  
Zenghui Bao ◽  
Yukai Wu ◽  
Yan Li ◽  
Cheng Ma ◽  
...  
Keyword(s):  
Quantum Information Processing ◽  
Quantum Circuit ◽  
Qubit State ◽  
Microwave Cavity ◽  
Single Shot ◽  
Superconducting Qubit ◽  
Constructive Interference ◽  
State Discrimination ◽  
Convenient Approach ◽  
Photon States

High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting quantum circuit, the qubit state is usually determined by detecting the dispersive frequency shift of a microwave cavity from either transmission or reflection. We demonstrate the use of constructive interference between the transmitted and reflected signal to optimize the qubit state readout, with which we find a better resolved state discrimination and an improved qubit readout fidelity. As a simple and convenient approach, our scheme can be combined with other qubit readout methods based on the discrimination of cavity photon states to further improve the qubit state readout.

scholarly journals Critical slowing down in circuit quantum electrodynamics

2021 ◽  
Vol 7 (21) ◽  
pp. eabe9492
Author(s):  
Paul Brookes ◽  
Giovanna Tancredi ◽  
Andrew D. Patterson ◽  
Joseph Rahamim ◽  
Martina Esposito ◽  
...  
Keyword(s):  
Quantum Electrodynamics ◽  
Quantum Circuit ◽  
Critical Slowing Down ◽  
Microwave Cavity ◽  
Many Body ◽  
Circuit Quantum Electrodynamics ◽  
Slowing Down ◽  
The Rich ◽  
First Order Phase ◽  
Shed Light

Critical slowing down of the time it takes a system to reach equilibrium is a key signature of bistability in dissipative first-order phase transitions. Understanding and characterizing this process can shed light on the underlying many-body dynamics that occur close to such a transition. Here, we explore the rich quantum activation dynamics and the appearance of critical slowing down in an engineered superconducting quantum circuit. Specifically, we investigate the intermediate bistable regime of the generalized Jaynes-Cummings Hamiltonian (GJC), realized by a circuit quantum electrodynamics (cQED) system consisting of a transmon qubit coupled to a microwave cavity. We find a previously unidentified regime of quantum activation in which the critical slowing down reaches saturation and, by comparing our experimental results with a range of models, we shed light on the fundamental role played by the qubit in this regime.

Experimental demonstration of optimized quantum process tomography on the IBM quantum experience

2020 ◽  
pp. 2040004
Author(s):  
Akshay Gaikwad ◽  
Krishna Shende ◽  
Kavita Dorai
Keyword(s):  
Quantum Circuit ◽  
Quantum Gates ◽  
Experimental Demonstration ◽  
Superconducting Qubit ◽  
Quantum Process ◽  
Single Qubit ◽  
Quantum Process Tomography ◽  
Process Tomography ◽  
Quantum Processor ◽  
Least Squares Optimization

We experimentally performed complete and optimized quantum process tomography of quantum gates implemented on superconducting qubit-based IBM QX2 quantum processor via two constrained convex optimization (CCO) techniques: least squares optimization and compressed sensing optimization. We studied the performance of these methods by comparing the experimental complexity involved and the experimental fidelities obtained. We experimentally characterized several two-qubit quantum gates: identity gate, a controlled-NOT gate, and a SWAP gate. The general quantum circuit is efficient in the sense that the data needed to perform CCO-based process tomography can be directly acquired by measuring only a single qubit. The quantum circuit can be extended to higher dimensions and is also valid for other experimental platforms.

scholarly journals Logical measurement-based quantum computation in circuit-QED

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Jaewoo Joo ◽  
Chang-Woo Lee ◽  
Shingo Kono ◽  
Jaewan Kim
Keyword(s):  
Quantum Computation ◽  
Quantum Electrodynamics ◽  
Cluster State ◽  
Error Correcting Code ◽  
Microwave Cavity ◽  
Entangled State ◽  
Superconducting Qubit ◽  
Circuit Qed ◽  
Specific Protocol ◽  
Logical Qubit

Abstract We propose a new scheme of measurement-based quantum computation (MBQC) using an error-correcting code against photon-loss in circuit quantum electrodynamics. We describe a specific protocol of logical single-qubit gates given by sequential cavity measurements for logical MBQC and a generalised Schrödinger cat state is used for a continuous-variable (CV) logical qubit captured in a microwave cavity. To apply an error-correcting scheme on the logical qubit, we utilise a d-dimensional quantum system called a qudit. It is assumed that a three CV-qudit entangled state is initially prepared in three jointed cavities and the microwave qudit states are individually controlled, operated, and measured through a readout resonator coupled with an ancillary superconducting qubit. We then examine a practical approach of how to create the CV-qudit cluster state via a cross-Kerr interaction induced by intermediary superconducting qubits between neighbouring cavities under the Jaynes-Cummings Hamiltonian. This approach could be scalable for building 2D logical cluster states and therefore will pave a new pathway of logical MBQC in superconducting circuits toward fault-tolerant quantum computing.

scholarly journals Heralded orthogonalisation of coherent states and their conversion to discrete-variable superpositions

2017 ◽  
Vol 4 (1) ◽  
pp. 35-43 ◽  
Author(s):  
Regina Kruse ◽  
Christine Silberhorn ◽  
Tim Bartley
Keyword(s):  
Information Processing ◽  
Quantum Information ◽  
Coherent States ◽  
Quantum Information Processing ◽  
Fundamental Property ◽  
Discrete Variable ◽  
Successful Operation ◽  
State Discrimination ◽  
Unambiguous State Discrimination

Abstract The nonorthogonality of coherent states is a fundamental property which prevents them from being perfectly and deterministically discriminated. Here, we present an experimentally feasible protocol for the probabilistic orthogonalisation of a pair of coherent states, independent of their amplitude and phase. In contrast to unambiguous state discrimination, a successful operation of our protocol is heralded without measuring the states. As such, they remain suitable for further manipulation and the obtained orthogonal states serve as a discretevariable basis. Therefore, our protocol doubles as a simple continuous-to-discrete variable converter, which may find application in hybrid continuous-discrete quantum information processing protocols.

scholarly journals Generation of nonclassical photon states using a superconducting qubit in a microcavity

2004 ◽  
Vol 67 (6) ◽  
pp. 941-947 ◽  
Author(s):  
Yu-xi Liu ◽  
L. F Wei ◽  
Franco Nori
Keyword(s):  
Superconducting Qubit ◽  
Photon States

scholarly journals Harnessing temporal modes for multi-photon quantum information processing based on integrated optics

2017 ◽  
Vol 375 (2099) ◽  
pp. 20160244 ◽  
Author(s):  
G. Harder ◽  
V. Ansari ◽  
T. J. Bartley ◽  
B. Brecht ◽  
C. Silberhorn
Keyword(s):  
Integrated Optics ◽  
Frequency Conversion ◽  
Quantum Information Processing ◽  
Single Photon ◽  
Photon Number ◽  
Integrated Optics Devices ◽  
Sum Frequency ◽  
Temporal Modes ◽  
Quantum Technology ◽  
Photon States

In the last few decades, there has been much progress on low loss waveguides, very efficient photon-number detectors and nonlinear processes. Engineered sum-frequency conversion is now at a stage where it allows operation on arbitrary temporal broadband modes, thus making the spectral degree of freedom accessible for information coding. Hereby the information is often encoded into the temporal modes of a single photon. Here, we analyse the prospect of using multi-photon states or squeezed states in different temporal modes based on integrated optics devices. We describe an analogy between mode-selective sum-frequency conversion and a network of spatial beam splitters. Furthermore, we analyse the limits on the achievable squeezing in waveguides with current technology and the loss limits in the conversion process. This article is part of the themed issue ‘Quantum technology for the 21st century’.

scholarly journals Hybrid Quantum Circuit with a Superconducting Qubit Coupled to a Spin Ensemble

2011 ◽  
Vol 107 (22) ◽  
Author(s):  
Y. Kubo ◽  
C. Grezes ◽  
A. Dewes ◽  
T. Umeda ◽  
J. Isoya ◽  
...  
Keyword(s):  
Quantum Circuit ◽  
Superconducting Qubit

scholarly journals An analysis of reading out the state of a charge quantum bit

2003 ◽  
Vol 3 (2) ◽  
pp. 121-138
Author(s):  
H-S. Goan
Keyword(s):  
Density Matrix ◽  
Master Equation ◽  
The State ◽  
Qubit State ◽  
Reduced Density Matrix ◽  
Quantum Trajectory ◽  
Single Shot ◽  
Quantum Bit ◽  
Trajectory Approach ◽  
Reduced Density

We provide a unified picture for the master equation approach and the quantum trajectory approach to a measurement problem of a two-state quantum system (a qubit), an electron coherently tunneling between two coupled quantum dots (CQD's) measured by a low transparency point contact (PC) detector. We show that the master equation of ``partially'' reduced density matrix can be derived from the quantum trajectory equation (stochastic master equation) by simply taking a ``partial'' average over the all possible outcomes of the measurement. If a full ensemble average is taken, the traditional (unconditional) master equation of reduced density matrix is then obtained. This unified picture, in terms of averaging over (tracing out) different amount of detection records (detector states), for these seemingly different approaches reported in the literature is particularly easy to understand using our formalism. To further demonstrate this connection, we analyze an important ensemble quantity for an initial qubit state readout experiment, P(N,t), the probability distribution of finding N electron that have tunneled through the PC barrier(s) in time t. The simulation results of P(N,t) using 10000 quantum trajectories and corresponding measurement records are, as expected, in very good agreement with those obtained from the Fourier analysis of the ``partially'' reduced density matrix. However, the quantum trajectory approach provides more information and more physical insights into the ensemble and time averaged quantity P(N,t). Each quantum trajectory resembles a single history of the qubit state in a single run of the continuous measurement experiment. We finally discuss, in this approach, the possibility of reading out the state of the qubit system in a single-shot experiment.

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