Quantum Inf Process (2017) 16:45
Overcoming experimental limitations in a nonlinear
two-qubit gate through postselection
Received: 23 August 2016 / Accepted: 23 November 2016 / Published online: 28 December 2016
© Springer Science+Business Media New York 2016
Abstract We introduce a modular-value two-qubit gate and explore its advantages in
experimentally limited situations. The gate is deﬁned such that the ﬁnal state of a qubit
is fully controlled by a pre- and postselection procedure in an ancillary qubit given
an (imperfect or technically limited) intermediate conditional qubit–qubit interaction.
As an example of the gate and its beneﬁt, we make the connection to a postselected
cross-phase modulation scenario when a undesired absorption is present. Probabilistic
ampliﬁcation of a small cross phase while mitigating relative absorption is possible,
and a complementary behavior between phase and amplitude emerges.
Keywords Quantum control · Postselection · Modular-value ampliﬁcation ·
Cross-phase modulation · Measurement-based quantum computation · Two-qubit
Weak-value ampliﬁcation (WVA) is a metrological precision technique for estimation
of small parameters that allows one to amplify a signal of interest above a technical-
noise ﬂoor. Such a response is possible with a unorthodox postselection procedure
in a system after weakly coupling it to a continuous degree of freedom or meter.
The technique’s advantages rely on the anomalous shift induced in the meter, which
is proportional to the weak value of the system’s observable. The weak value is a
unbounded complex quantity which emerges as the result of inducing a weak system–
Department of Physics and Astronomy and Center for Coherence and Quantum Optics, University
of Rochester, Rochester, NY 14627, USA