One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. Although initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences--for instance, producing the Lamb shift of atomic spectra and modifying the magnetic moment of the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed whether it might be possible to more directly observe the virtual particles that compose the quantum vacuum. Forty years ago, it was suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. The phenomenon, later termed the dynamical Casimir effect, has not been demonstrated previously. Here we observe the dynamical Casimir effect in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length. The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (>10 gigahertz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process.
The study of light-matter interaction has led to many fundamental discoveries as well as numerous important technologies. Over the last decades, great strides have been made in increasing the strength of this interaction at the single-photon level, leading to a continual exploration of new physics and applications. Recently, a major achievement has been the demonstration of the so-called strong coupling regime [1, 2], a key advancement enabling great progress in quantum information science. Here, we demonstrate light-matter interaction over an order of magnitude stronger than previously reported, reaching the nonperturbative regime of ultrastrong coupling (USC). We achieve this using a superconducting artificial atom tunably coupled to the electromagnetic continuum of a one-dimensional waveguide. For the largest coupling, the spontaneous emission rate of the atom exceeds its transition frequency. In this USC regime, the description of atom and light as distinct entities breaks down, and a new description in terms of hybrid states is required [4, 8]. Our results open the door to a wealth of new physics and applications. Beyond light-matter interaction itself, the tunability of our system makes it a promising tool to study a number of important physical systems such as the well-known spin-boson [9] and Kondo models [12]. * These authors contributed equally to this work.
We have embedded an artificial atom, a superconducting transmon qubit, in an open transmission line and investigated the strong scattering of incident microwave photons (∼6 GHz). When an input coherent state, with an average photon number N≪1 is on resonance with the artificial atom, we observe extinction of up to 99.6% in the forward propagating field. We use two-tone spectroscopy to study scattering from excited states and we observe electromagnetically induced transparency (EIT). We then use EIT to make a single-photon router, where we can control to what output port an incoming signal is delivered. The maximum on-off ratio is around 99% with a rise and fall time on the order of nanoseconds, consistent with theoretical expectations. The router can easily be extended to have multiple output ports and it can be viewed as a rudimentary quantum node, an important step towards building quantum information networks.
We investigate the dynamical Casimir effect in a coplanar waveguide (CPW) terminated by a superconducting quantum interference device (SQUID). Changing the magnetic flux through the SQUID parametrically modulates the boundary condition of the CPW, and thereby, its effective length. Effective boundary velocities comparable to the speed of light in the CPW result in broadband photon generation which is identical to the one calculated in the dynamical Casimir effect for a single oscillating mirror. We estimate the power of the radiation for realistic parameters and show that it is experimentally feasible to directly detect this nonclassical broadband radiation.PACS numbers: 85.25. Cp, 42.50.Lc, 84.40.Az Two parallel mirrors in empty space are attracted to each other due to the vacuum fluctuations of the electromagnetic field, because of the different mode density inside compared to outside of the mirrors. This striking effect of quantum electrodynamics (QED) was predicted by Casimir in 1948 and since then it has also been verified experimentally (see, e.g., Ref.[1]).If the mirrors move, there is also a mismatch between vacuum modes at different instances of time. It was predicted [2] that this may result in the creation of real photons out of vacuum fluctuations. This dynamical Casimir effect (DCE) also holds for a single mirror subject to nonuniform acceleration in empty space [3]. Although receiving considerable interest [4,5] since its theoretical discovery, there is still no experimental verification of the DCE. This is mainly due to the fact that the rate of photon production is nonnegligible only when the mirror velocity approaches the speed of light, ruling out the use of massive mirrors [7]. Proposals for the experimental verification of the DCE have suggested rapidly changing the field boundary conditions in other ways, e.g., using lasers to modulate the reflectivity of a thin semiconductor film [6,7] or the resonance frequency of a superconducting stripline resonator [8].Here we show that a coplanar waveguide (CPW) terminated by a superconducting quantum interference device (SQUID), as shown in Fig. 1, is a very promising system for experimentally observing the DCE. The inductance of the SQUID can be controlled by the local magnetic flux threading the loop, giving a tunable boundary condition that is equivalent to that of a short-circuited transmission line with a tunable length. Because there is no massive mirror moving, the velocity of the effective boundary can approach the speed of light in the transmission line. The photon production from the vacuum can thus be made experimentally detectable, and its photon spectrum is identical to the one calculated in the DCE for a single oscillating mirror [9].Building on work on superconducting circuits for quan- The setup in (a) is equivalent to a short-circuited transmission line with a tunable length L eff , i.e., with a tunable "mirror". We analyze this system using the input/output formalism, which gives the spectrum of the scattered outgoing field, Φout,...
We have constructed a new type of amplifier whose primary purpose is the readout of superconducting quantum bits. It is based on the transition of an RF-driven Josephson junction between two distinct oscillation states near a dynamical bifurcation point. The main advantages of this new amplifier are speed, high-sensitivity, low back-action, and the absence of on-chip dissipation. Pulsed microwave reflection measurements on nanofabricated Al junctions show that actual devices attain the performance predicted by theory.Quantum measurements of solid-state systems, such as the readout of superconducting quantum bits [1,2,3,4,5,6,7], challenge conventional low-noise amplification techniques. Ideally, the amplifier for a quantum measurement should minimally perturb the measured system while maintaining sufficient sensitivity to overcome the noise of subsequent elements in the amplification chain. Additionally, the characteristic drift of materials properties in solid-state systems necessitates a fast acquisition rate to permit measurements in rapid succession. To meet these inherently conflicting requirements, we propose to harness the sensitivity of a dynamical system -a single RF-driven Josephson tunnel junction -tuned near a bifurcation point. The superconducting tunnel junction is the only electronic dipolar circuit element whose nonlinearity remains unchanged at arbitrary low temperatures. As the key component of present superconducting amplifiers [8,9,10], it is known to exhibit a high degree of stability. Moreover, all available degrees of freedom in the dynamical system participate in information transfer and none contribute to unnecessary dissipation resulting in excess noise. The operation of our Josephson bifurcation amplifier is represented schematically in Fig. 1. The central element is a Josephson junction whose critical current I 0 is modulated by the input signal using an application-specific coupling scheme (input port), such as a SQUID loop [4] or a SSET [2]. The junction is driven with an sinusoidal signal i RF sin(ωt) fed from a transmission line through a directional coupler (drive port). In the underdamped regime, when the drive frequency ω is detuned form the natural oscillation frequency ω p and when the drive current IB < i RF < I B ≪ I 0 , the system has two possible oscillation states which differ in amplitude and phase [11,12]. Starting in the lower amplitude state, at the bifurcation point i RF = I B the system becomes infinitely sensitive, in absence of thermal and quantum fluctuations, to variations in I 0 . The energy stored in the oscillation can always be made larger than thermal fluctuations by increasing the scale of I 0 , thus preserving sensitivity at finite temperature. The reflected component of the drive signal, measured through another transmission line connected to the coupler (output port), is a convenient signature of the junction oscillation state which carries with it information about the input signal. This arrangement minimizes the back-action of the amplifier since the...
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