Generating single microwave photons in a circuit

Abstract

Electromagnetic signals in circuits consist of discrete photons [1], though conventional voltage sources can only generate classical fields with a coherent superposition of many different photon numbers. While these classical signals can control and measure bits in a quantum computer (qubits), single photons can carry quantum information, enabling non-local quantum interactions, an important resource for scalable quantum computing [2]. Here, we demonstrate an on-chip single photon source in a circuit quantum electrodynamics (QED) architecture [3], with a microwave transmission line cavity that collects the spontaneous emission of a single superconducting qubit with high efficiency. The photon source is triggered by a qubit rotation, as a photon is generated only when the qubit is excited. Tomography of both qubit and fluorescence photon shows that arbitrary qubit states can be mapped onto the photon state, demonstrating an ability to convert a stationary qubit into a flying qubit. Both the average power and voltage of the photon source are characterized to verify performance of the system. This single photon source is an important addition to a rapidly growing toolbox for quantum optics on a chip.Numerous approaches to generating single photons, particularly optical photons, have been proposed and demonstrated in recent years [4]. The underlying principle for generating single photons from atoms or qubits is straightforward: an excited qubit can relax to its ground state by emitting a photon [5,6], A pulse that excites the qubit can therefore trigger a single photon emission, as long as the control and emission photons can be separated. Early experiments demonstrated this photon generation from single ions [7], atoms [8], molecules[9, 10, 11], nitrogen vacancies [12], and quantum dots [13,14], though radiation in all directions made efficient collection difficult. In a cavity QED source, the atom or qubit is coupled to a single photonic mode of a cavity, enhancing the rate of decay to that mode through the Purcell effect [15] and allowing a source where photons are emitted into a controlled channel. Atoms [16,17,18,19], ions [20], and quantum dots [14,21,22] have been used to generate optical photons efficiently in this manner.Here, we implement a cavity QED system in a circuit [23,24], where a superconducting qubit and transmission line cavity are coupled such that the dominant channel for relaxation of the qubit is to spontaneously emit a photon into the cavity. Each time the qubit is excited, the most likely outcome is the generation of one (and only one) photon at a random time, with the distribution of times characterized by the decay rate of the qubit. The challenge is to create a system where spontaneous emission dominates other relaxation channels. This spontaneous emission rate can be determined from the Hamiltonian of the system, the well-known JaynesCummings Hamiltonian, H =hω a σ z /2+hω r (a † a+1/2)+ * Authors with contributed equally to this work.h g(a † σ − + aσ + ). The first two terms represent...

Double Quantum Dot Floquet Gain Medium

“…Qubits separated by relatively large distances have been coherently coupled using a cavity bus [4,5]. Finally, cQED enables the generation of classical and nonclassical light [6][7][8][9].…”

“…Early work demonstrated single photon generation by applying a π pulse to a qubit to drive a transition from the ground state to the excited state. The qubit was then brought into resonance with the cavity for a short period of time, thereby transferring the excitation from the qubit to the cavity [6]. n-photon Fock states were generated by repeating this process many times [9].…”

Microwave quantum optics and electron transport through a metallic dot strongly coupled to a transmission line cavity

“…[8][9][10][11][12][13][14] A parallel development concerned the possibilities to perform fundamental quantum optics experiments with microwave photons in cavities. Experiments on microwave quantum optics range from arbitrary photon state preparation 15 and entanglement of cavity photons 16 to single photon generation, 17 microwave lasing 18 and fast tuning of cavity photon properties. 19,20 An important recent development is the efforts to reach the ultrastrong coupling regime, where the strength of the coupling between the qubit and the cavity becomes comparable to the frequency of the fundamental cavity mode.…”

Generation of Fock states in a superconducting quantum circuit

“…Hence the electric field of such pulses is completely uncertain, a fact which has recently been verified. [3] Hofheinz et al have made a tour-de-force advance by deterministically generating photon number Fock states containing up to N = 6 photons (N = 15 in recent unpublished work) using a superconducting qubit coupled to a resonator.The resonator supports discrete modes at integer multiples of the fundamental. Because the modes are widely spaced in frequency for short res-1…”

“…Single-photons-on-demand as well as coherent superpositions of 0 and 1 photons have been generated in a microwave resonator electrical circuit. [3] A classical signal generator produces a sine wave of constant amplitude, frequency and phase. The quantum equivalent (produced by a laser or a microwave signal generator) is a so-called coherent state.…”