Superconducting Quantum Interference Device Qubits Research Articles

Shortcuts to adiabatic for implementing controlled-not gate with superconducting quantum interference device qubits

We propose a protocol to implement controlled-not (C-NOT) gates of two superconducting quantum interference device (SQUID) qubits in a cavity. The Rabi frequencies of driven pulses are designed by means of the shortcuts to adiabaticity. The numerical simulations show that the fidelities of the C-NOT gates are insensitive to the deviations of control parameters. Besides, the C-NOT gates are robust against the decoherent factors, such as spontaneous emissions and dephasings of the SQUID qubits and the decay of the cavity mode. Therefore, the protocol may be useful in the fields of quantum computations.

Hybrid rf SQUID qubit based on high kinetic inductance

We report development and microwave characterization of rf SQUID (Superconducting QUantum Interference Device) qubits, consisting of an aluminium-based Josephson junction embedded in a superconducting loop patterned from a thin film of TiN with high kinetic inductance. Here we demonstrate that the systems can offer small physical size, high anharmonicity, and small scatter of device parameters. The work constitutes a non-tunable prototype realization of an rf SQUID qubit built on the kinetic inductance of a superconducting nanowire, proposed in Phys. Rev. Lett. 104, 027002 (2010). The hybrid devices can be utilized as tools to shed further light onto the origin of film dissipation and decoherence in phase-slip nanowire qubits, patterned entirely from disordered superconducting films.

Characterization of barrier-tunable radio-frequency-SQUID for Maxwell’s demon experiment**Project supported by the National Natural Science Foundation of China (Grant No. 11653001), the National Basic Research Program of China (Grant No. 2011CBA00304), the Tsinghua University Initiative Scientific Research Program, China (Grant No. 20131089314), and the Zhejiang Tianjingsheng Foundation, China, for Student Assistantships (Gang Li and Hao Li).

We present the design, fabrication, and characterization of a barrier-tunable superconducting quantum interference device (SQUID) qubit for the study of Maxwell’s demon experiment. In this work, a compound Josephson junction (CJJ) radio-frequency (RF)-SQUID qubit with an overdamped resistively shunted direct-current (DC)-SQUID magnetometer is used to continuously monitor the state of the qubit. The circuit is successfully fabricated with the standard Nb/Al-AlOx/Nb trilayer process of our laboratory and characterized in a low noise measurement system, which is capable of measuring coherent dynamics of superconducting qubits, in an Oxford dilution refrigerator. All circuit parameters are determined accurately by fitting experimental data to theoretical analysis and simulation, which allows us to make a quantitative comparison between the results of the experiment and theory.

Generation of three-qubit Greenberger–Horne–Zeilinger states of superconducting qubits by using dressed states

Combining the advantages of the dressed states and superconducting quantum interference device (SQUID) qubits, we propose an efficient scheme to generate Greenberger–Horne–Zeilinger (GHZ) states for three SQUID qubits. Firstly, we elaborate how to generate GHZ states of three SQUID qubits by choosing a set of dressed states suitably. Then, we compare the scheme by using dressed states with that via the adiabatic passage. Lastly, the influence of various decoherence factors, such as cavity decay, spontaneous emission and dephasing, is analyzed numerically. All of the results show that the GHZ state can be obtained fast and with high fidelity and that the present scheme is robust against the cavity decay and spontaneous emission. In addition, our scheme is more stable against the dephasing than the adiabatic scheme.

One-step implementation of a multiqubit controlled-phase gate with superconducting quantum interference devices coupled to a resonator

A scheme for one-step implementation of an n-qubit controlled-phase gate is proposed in a superconducting quantum interference device (SQUID) system. The distinguishing feature of this scheme is the simultaneous and nonidentical off-resonant Raman coupling of the n SQUID qubits to a single-mode resonator and the microwave pulses. The scheme is efficient and simple because it requires only one-step evolution, and no adjustments of the experimental parameters are needed during the operation.

Superconducting qubit manipulated by fast pulses: experimental observation of distinct decoherence regimes

A particular superconducting quantum interference device (SQUID) qubit, namely the double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses. In this system we observe coherent oscillations exhibiting non-exponential decay, indicating an unconventional decoherence mechanism. Moreover, via tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an ‘optimal’ point where the qubit is only weakly sensitive to intrinsic noise. We find that this behavior is in agreement with a model considering the decoherence caused essentially by low-frequency noise contributions, and we discuss the experimental results and possible issues.

Implementing Two-Qubit SWAP Gate with SQUID Qubits in a Microwave Cavity via Adiabatic Passage Evolution

We presented a scheme to implement SWAP gate in a microwave cavity. In our scheme, two superconducting quantum interference device (SQUID) qubits are coupled to a single-mode microwave cavity field by adiabatic passage method for their manipulation. This process of implementing SWAP gate is in the range of present experiments. The scheme can be easily obtained only by three steps, which does not require perform any operation. In the scheme, the operations only involve three lowest flux states of the SQUIDs, and the excited states would not be excited; therefore, the decoherence due to spontaneous emission of the SQUIDs’ levels would not affect the operations. In addition, during the whole procedure the cavity field is not necessary to be excited because it does not require transfer quantum information between the SQUID’s and the cavity field. Thus, the cavity decay is suppressed. Therefore our scheme may be realized in superconducting systems.

Universal Quantum Cloning Machine in Circuit Quantum Electrodynamics

We propose a scheme for realizing the 1 → 2 universal quantum cloning machine (UQCM) with superconducting quantum interference device (SQUID) qubits in circuit quantum electrodynamics (circuit QED). In this scheme, in order to implement UQCM, we only need phase shift gate operation on SQUID qubits and the Raman transitions. The cavity number we need is only one. Thus our scheme is simple and has advantages in the experimental realization. Furthermore, both the cavity and the SQUID qubits are virtually excited, so the decoherence can be neglected.

One-step implementing three-qubit phase gate via manipulating rf SQUID qubits in the decoherence-free subspace with respect to cavity decay

We present a scheme for implementing a three-qubit phase gate via manipulating rf superconducting quantum interference device (SQUID) qubits in the decoherence-free subspace with respect to cavity decay. Through appropriate changes of the coupling constants between rf SQUIDs and cavity, the scheme can be realized only in one step. A high fidelity is obtained even in the presence of decoherence.

Unconventional Geometric Phase Gate with Superconducting Quantum Interference Device Qubits in Cavity QED

In the system with superconducting quantum interference devices (SQUID) in cavity, a scheme for constructing two-qubit quantum phase gate via a conventional geometric phase-shift is proposed by using a quantized cavity field and classical microwave pulses. In this scheme, the gate operation is realized in the subspace spanned by the two lower flux states of the SQUID system and the population operator of the excited state has no effect on it. Thus the effect of decoherence caused from the levels of the SQUID system is possible to minimize. Under cavity decay, our strictly numerical simulation shows that it is also possible to realize the unconventional geometric phase gate. The experimental feasibility is discussed in detail.

Scheme for the implementation of the optimal economical phase-covariant quantum cloning machine with SQUID qubits in cavity QED

Abstract We propose a scheme to realize the optimal economical 1 → 2 phase-covariant quantum cloning machine (QCM) in 2 dimension with superconducting quantum interference device (SQUID) qubits in a microwave cavity. In our scheme, two-SQUID qubits are fixed into a microwave cavity by adiabatic passage method for their manipulation. Using this method, we can realize the optimal phase-covariant QCM only by one step.

Realization of Greenberg–Horne–Zeilinger (GHZ) and W Entangled Stateswith Multiple Superconducting Quantum-Interference Device Qubits in Cavity QED

An alternative scheme is proposed for generating theGreenberg–Horne–Zeilinger (GHZ) and W types of the entangled stateswith multiple superconducting quantum-interference device (SQUID) qubitsin a single-mode microwave cavity field. In this scheme, there is notransfer of quantum information between the SQUIDs and the cavity, thecavity is always in the vacuum and thus the requirement on the qualityof cavity is greatly loosened. In addition, during the process of thegeneration of the W entangled state, the present method does not involvea real excitation of intermediate levels. Thus, decoherence due toenergy relaxation of intermediate levels is minimized.

Quantum logic gates operation using SQUID qubits in bimodal cavity

We present a scheme to realize the basic two-qubit logic gates such as the quantum phase gate and SWAP gate using a detuned microwave cavity interacting with three-level superconducting-quantum-interference-device (SQUID) qubit(s), by placing SQUID(s) in a two-mode microwave cavity and using adiabatic passage methods. In this scheme, the two logical states of the qubit are represented by the two lowest levels of the SQUID and the cavity fields are treated as quantized. Compared with the previous method, the complex procedures of adjusting the level spacing of the SQUID and applying the resonant microwave pulse to the SQUID to create transformation are not required. Based on superconducting device with relatively long decoherence time and simplified operation procedure, the gates operate at a high speed, which is important in view of decoherence.

Preparation of Greenberger-Horne-Zeilinger entangled states with multiple superconducting quantum-interference device qubits or atoms in cavity QED

A scheme is proposed for generating Greenberger-Horne-Zeilinger (GHZ) entangled states of multiple superconducting quantum-interference device (SQUID) qubits by the use of a microwave cavity. The scheme operates essentially by creating a single photon through an auxiliary SQUID built in the cavity and performing a joint multiqubit phase shift with assistance of the cavity photon. It is shown that entanglement can be generated using this method, deterministic and independent of the number of SQUID qubits. In addition, we show that the present method can be applied to preparing many atoms in a GHZ entangled state, with tolerance to energy relaxation during the operation.

Generation of Greenberger–Horne–Zeilinger entangled states with three SQUID qubits: a scheme with tolerance to non-uniform device parameters

We propose a scheme for creating Greenberger–Horne–Zeilinger (GHZ) type of entangled states with three superconducting quantum interference device (SQUID) qubits in a microwave cavity. The scheme operates essentially by an auxiliary SQUID built in the cavity as a microwave photon generator, and by performing a phase shift with the aid of the cavity photon. We show that the present scheme does not require identical cavity-SQUID coupling constants for each SQUID. Therefore, our scheme can be readily implemented since neither uniform device parameters nor exact placement of SQUIDs in the cavity is required. We note that the method can in principle be applied to prepare many SQUIDs in a GHZ state. In addition, how to prepare multiple SQUIDs in W-class entangled states is discussed.

Possible realization of entanglement, logical gates, and quantum-information transfer with superconducting-quantum-interference-device qubits in cavity QED

We present a scheme to achieve maximally entangled states, controlled phase-shift gate, and SWAP gate for two superconducting-quantum-interference-device (SQUID) qubits, by placing SQUIDs in a microwave cavity. We also show how to transfer quantum information from one SQUID qubit to another. In this scheme, no transfer of quantum information between the SQUIDs and the cavity is required, the cavity field is only virtually excited and thus the requirement on the quality factor of the cavity is greatly relaxed.