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New Journal of Physics

Characterization of aπ-phase shift quantum gate for coherent-state qubits

Rémi Blandino, Franck Ferreyrol, Marco Barbieri, Philippe Grangier, Rosa Tualle-Brouri

Published Jan 11, 2012• 15 citations
Publisher

Abstract

We discuss the characterization of a π -phase shift quantum gate acting on a qubit encoded in superpositions of coherent states. We adopt a technique relying on some a priori knowledge about the physics underlying the functioning of the device. A parameter summarizing the global quality of the quantum gate is obtained by 'virtually' processing an entangled state. With such an approach, we can facilitate the characterization of our gate, focusing on the useful subspace rather than on the entire phase space.

DOI
10.1088/1367-2630/14/1/013017

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We discuss the characterization of a π -phase shift quantum gate acting on a qubit encoded in superpositions of coherent states. We adopt a technique relying on some a priori knowledge about the physics underlying the functioning of the device. A parameter summarizing the global quality of the quantum gate is obtained by 'virtually' processing an entangled state. With such an approach, we can facilitate the characterization of our gate, focusing on the useful subspace rather than on the entire phase space.
We discuss and implement experimentally a method for characterizing quantum gates operating on superpositions of coherent states. The peculiarity of this encoding of qubits is to work with a non-orthogonal basis, and therefore some technical limitations prevent us from using standard methods, such as process tomography. We adopt a different technique, that relies on some a-priori knowledge about the physics underlying the functioning of the device. A parameter characterizing the global quality of the quantum gate is obtained by \virtually" processing an entangled state.
We experimentally demonstrate a programmable single-qubit quantum gate. This device applies a unitary phase shift operation to a data qubit with the value of the phase shift being fully determined by the state of a program qubit. Our linear optical implementation is based on the encoding of qubits into polarization states of single photons, two-photon interference on a polarizing beam splitter, and measurement on the output program qubit. We fully characterize the programmable gate by quantum process tomography. The achieved average quantum process fidelity exceeding 97% illustrates very good performance of the gate for all values of the encoded phase shift. We also show that by using a different set of program states the device can operate as a programmable partial polarization filter.
Quantum information encoding is a fundamental challenge in quantum computing and communication. Traditional encoding methods often rely on physical manipulation of qubits, which can be susceptible to noise and decoherence. This paper explores a novel approach – encoding information using phase-shifted entanglement – that leverages the inherent properties of entangled states to achieve efficient and robust data transmission. We introduce a new entanglement encoding scheme where the phase shift directly correlates with the encoded data, simplifying the process and potentially leading to significant improvements in data transfer rates and reliability. This work presents a preliminary investigation into the feasibility and potential advantages of this approach, aiming to contribute to advancements in quantum information processing.
The field of quantum computing is growing at a rapid rate with the promise of dramatic improvements in the ability to solve critical computational problems. A leading approach for implementing fault-tolerant quantum computers is based on superconducting circuits containing Josephson junctions to form qubits. These qubits have many attractive qualities, but predicting the device performance requires detailed knowledge of the junction properties. Thus, precise characterization is crucial for realizing different qubit designs. The coherence of superconducting qubits is limited by several sources in the circuit en- vironment. Building a quantum computer requires effort to reduce gate errors caused by decoherence. This is typically achieved with quantum error correction. An alternative ap- proach involves designing qubits that are intrinsically protected from noise sources. One such design is the charge-parity qubit, which combines Josephson junctions in a novel regime. The capacitance of the junctions is a critical parameter for this qubit design that directly affects the engineered protection against decoherence. Beyond the conventional geometric capaci- tance, there is an excess contribution from virtual quasiparticle tunneling; we refer to this component as the electronic capacitance. This thesis focuses on the characterization of the capacitance of Josephson junctions, with the goal of isolating and measuring the electronic capacitance contribution, particularly for use in the charge-parity qubit. The thesis first reviews the initial implementation of the charge-parity qubit and the challenges that were encountered. Next, the theoretical origin is described and a framework for measuring the junction capacitance using superconducting resonant circuits is described. This technique is then applied to arrays of junctions fabricated with different parameters and the scaling of the electronic capacitance is presented. Finally, based on this knowledge of the junction capacitance, second-generation charge-parity qubits are implemented and the experimental results are presented.
We propose and analyze a quantum gate that transfers an arbitrary state of a qubit into a superposition of two quasiorthogonal coherent states of a cavity mode (qcMAP), with opposite phases. This qcMAP gate is based on conditional qubit and cavity operations exploiting the energy-level dispersive shifts in the regime where they are much stronger than the cavity and qubit linewidths. The generation of multicomponent superpositions of quasiorthogonal coherent states, nonlocal entangled states of two resonators, and multiqubit Greenberger-Horne-Zeilinger states can be efficiently achieved by this gate.
The Grover mixer quantum alternating operator ansatz (GM-QAOA) employs the Grover mixer to confine the quantum evolution to the feasible subspace defined by the problem. Its mixing unitary, however, requires a global multi-controlled phase-shift gate acting on all qubits, resulting in substantial circuit overhead on near-term quantum devices. In this work, we propose locally acting Grover mixers tailored to initial states that admit a product structure over disjoint qubit subsystems, which may be obtained by encoding only a subset of problem constraints into the initial state preparation. The proposed method preserves the search space defined by the initial state while significantly lowering implementation cost, as the global multi-controlled phase-shift gate is replaced with local operations on disjoint subsystems. Numerical simulations on the exact-cover problem and the traveling salesman problem (TSP) demonstrate that the proposed method achieves convergence behavior comparable to that of the original GM-QAOA, while using shallower circuits with fewer gates. We further compare two constraint encoding strategies for the TSP, encoding only a subset of constraints versus all constraints into the initial state preparation, and show that the former combined with the proposed mixer yields markedly more compact circuits at the point where comparable solution quality is achieved.