Mediation of Entanglement between a Microscopic Two Level System and a Macroscopic Resonator Using a Superconducting Flux Qubit
Alexander Kemp, Shiro Saito, William John Munro*, and Kouichi Semba
Physical Science Laboratory, *Optical Science Laboratory
Circuit quantum electrodynamics in superconducting circuits has demonstrated the coupling of superconducting qubits to microwave photons in a superconducting resonator, paving a way for scaling up the qubit system . During these experiments, microscopic two level systems (TLS) coupled to the qubit were observed . Here we report on the study of an entangled state of a microscopic TLS and a macroscopic resonator by way of a superconducting flux qubit .
A spectrum of the flux qubit depicted in fig. 1(a) shows two anti-crossings at 6.17 GHz and 8.95 GHz. These represent the coupling of the qubit to the TLS and the resonator, respectively. The coupling strengths were 55 MHz and 154 MHz. The qubit energy can be controlled by an external magnetic flux through the qubit. Using a flux shift pulse, we can bring the qubit on resonance with the TLS (or resonator) and create any entangled state between the qubit and the TLS (or resonator). Utilizing such pulses we realized an entangled state between the TLS and the resonator.
Figure 1(b) shows a pulse sequence to make this entangled state. First, a microwave π-pulse prepares the qubit in the excited state |100> (where the state is represented by |qubit, TLS, resonator>). Next a pulse brings the qubit on resonance with the TLS creating the entangled state (|100>+|010>) / . Then a SWAP pulse between the qubit and the resonator realizes the entangled state (|001>+|010>) / . The state then acquires a relative phase φ = ΔEτ/ during a period of free evolution of a time τ . Here ΔE is the energy difference between the |001> and |010> levels. To observe this phase evolution we reverse our pulse sequence, we apply the pulse and then the SWAP pulse followed by the readout of the population of the qubits excited state Pex. Figure 2 shows that Pex as a function of τ oscillates with the frequency of ΔE/h. This indicates an entangled state between the TLS and the resonator is realized during the free evolution.
In the future we will replace the TLS by a controllable quantum memory and scale up our superconducting qubits with long-lived quantum memories.
This work was supported by KAKENHI.
 L. DiCarlo et al., Nature 467 (2010) 574.
 R. W. Simmond et al., Phys. Rev. Lett. 93 (2004) 077003.
 A. Kem et al., Phys. Rev. B 84 (2011) 104505.
Fig. 1. (a) Spectrum of a flux qubit.
(b) Pulse sequence for the experiment.
Fig. 2. (a) Switching probability Pex as a function of τ .
(b) Fourier spectrum of Pex.
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