Interference at a beam splitter reveals both classical and quantum properties
of electromagnetic radiation. When two indistinguishable single photons impinge
at the two inputs of abeam splitter they coalesce into a pair of photons
appearing in either one of its two outputs. This effect is due to the bosonic
nature of photons and was first experimentally observed by Hong, Ou, and Mandel
(HOM) [1]. Here, we present the observation of the HOM effect with two
independent single-photon sources in the microwave frequency domain. We probe
the indistinguishability of single photons, created with a controllable delay,
in time-resolved second-order cross- and auto-correlation function
measurements. Using quadrature amplitude detection we are able to resolve
different photon numbers and detect coherence in and between the output arms.
This measurement scheme allows us to observe the HOM effect and, in addition,
to fully characterize the two-mode entanglement of the spatially separated beam
splitter output modes. Our experiments constitute a first step towards using
two-photon interference at microwave frequencies for quantum communication and
information processing, e.g. for distributing entanglement between nodes of a
quantum network [2, 3] and for linear optics quantum computation [4, 5].
Interference at a beam splitter reveals both classical and quantum properties of electromagnetic radiation. When two indistinguishable single photons impinge at the two inputs of abeam splitter they coalesce into a pair of photons appearing in either one of its two outputs. This effect is due to the bosonic nature of photons and was first experimentally observed by Hong, Ou, and Mandel (HOM) [1]. Here, we present the observation of the HOM effect with two independent single-photon sources in the microwave frequency domain. We probe the indistinguishability of single photons, created with a controllable delay, in time-resolved second-order cross- and auto-correlation function measurements. Using quadrature amplitude detection we are able to resolve different photon numbers and detect coherence in and between the output arms. This measurement scheme allows us to observe the HOM effect and, in addition, to fully characterize the two-mode entanglement of the spatially separated beam splitter output modes. Our experiments constitute a first step towards using two-photon interference at microwave frequencies for quantum communication and information processing, e.g. for distributing entanglement between nodes of a quantum network [2, 3] and for linear optics quantum computation [4, 5].
We measure the quantum fluctuations of a pumped nonlinear resonator, using a
superconducting artificial atom as an in-situ probe. The qubit excitation
spectrum gives access to the frequencyand temperature of the intracavity field
fluctuations. These are found to be in agreement with theoretical predictions;
in particular we experimentally observe the phenomenon of quantum heating.
We study bifurcation measurement of a multi-level superconducting qubit using
a nonlinear resonator biased in the straddling regime, where the resonator
frequency sits between two qubittransition frequencies. We find that
high-fidelity bifurcation measurements are possible because of the enhanced
qubit-state-dependent pull of the resonator frequency, the behavior of
qubit-induced nonlinearities and the reduced Purcell decay rate of the qubit
that can be realized in this regime. Numerical simulations find up to a
threefold improvement in qubit readout fidelity when operating in, rather than
outside of, the straddling regime. High-fidelity measurements can be obtained
at much smaller qubit-resonator couplings than current typical experimental
realizations, reducing spectral crowding and potentially simplifying the
implementation of multi-qubit devices.
We observe measurement-induced qubit state mixing in a transmon qubit
dispersively coupled to a planar readout cavity. Our results indicate that
dephasing noise at the qubit-readoutdetuning frequency is up-converted by
readout photons to cause spurious qubit state transitions, thus limiting the
nondemolition character of the readout. Furthermore, we use the qubit
transition rate as a tool to extract an equivalent flux noise spectral density
at f ~ 1 GHz and find agreement with values extrapolated from a $1/f^alpha$
fit to the measured flux noise spectral density below 1 Hz.
We provide a general method to find the Hamiltonian of a linear circuit in
the presence of a nonlinearity. Focussing on the case of a Josephson junction
embedded in a transmission-lineresonator, we solve for the normal modes of the
system by taking into account exactly the effect of the quadratic (i.e.
inductive) part of the Josephson potential. The nonlinearity is then found to
lead to self and cross-Kerr effect, as well as beam-splitter type interactions
between modes. By adjusting the parameters of the circuit, the Kerr coefficient
K can be made to reach values that are weak (K < kappa), strong (K > kappa)
or even very strong (K >> kappa) with respect to the photon-loss rate kappa.
In the latter case, the resonator+junction circuit corresponds to an in-line
version of the transmon. By replacing the single junction by a SQUID, the Kerr
coefficient can be tuned in-situ, allowing for example the fast generation of
Schr“odinger cat states of microwave light. Finally, we explore the maximal
strength of qubit-resonator coupling that can be reached in this setting.