Mutiqubit Phase Gate And Phase Measurement

Multiqubit phase gate is an important part of the quantum computation, and it is of greatsignificance to research quantum information processing. The interaction of the artificial atoms andthe field is the same as that of natural atoms. By using its controllability, we can achieve moreexcellent multiqubit phase gate. Quantum information processing will become more quickly andaccurately by using more advanced phase measurement. Therefore, the researching of multiqubitphase gate and phase measurement become more and more important.In this paper, we propose a scheme to realize a multiqubit phase gate by using four-levelsuperconducting artificial atoms in a cavity QED and we use a super-stable optical interferometerwith multiple passes for higher precision.First, we introduce two model of multiqubit phase gate, one is quantum qubitssimultaneously controlling the multiple target qubits and the other is multiple control qubits actingon one target qubit.Second, we describe the structure of the four level superconducting artificial atoms. Wepropose a scheme for realizing two-qubit quantum phase gates via an unconventional geometricphase shift using two four-level superconducting artificial atoms with cyclic population transfer ina cavity.Third, we use the non-traditional geometric phase gates to achieve the multiqubit phase gate.We describe the control mechanism and the operation in the multiqubit phase gates of one controlqubit control two target qubits, we compared our operation time to the six superconducting qubitssystem in standing-wave quasi-one-dimensional coplanar waveguide resonator.Finally, we propose a scheme to improve the measurement by using a super-stable opticalinterferometer with multiple passes.The result of this paper shows that, by using our scheme, the operation of the multiqubit phase gatebecome simple and the operating time is shorter. Under certain conditions, the artificial atom willdisentangled with the cavity mode field, the gate is insensitive to both the atomic spontaneous emission and the cavity decay. In our measurement scheme, under the same experimentalconditions, the standard quantum limit can be beaten more easily in our scheme.