17 research outputs found
A quantum causal discovery algorithm
Finding a causal model for a set of classical variables is now a
well-established task---but what about the quantum equivalent? Even the notion
of a quantum causal model is controversial. Here, we present a causal discovery
algorithm for quantum systems. The input to the algorithm is a process matrix
describing correlations between quantum events. Its output consists of
different levels of information about the underlying causal model. Our
algorithm determines whether the process is causally ordered by grouping the
events into causally-ordered non-signaling sets. It detects if all relevant
common causes are included in the process, which we label Markovian, or
alternatively if some causal relations are mediated through some external
memory. For a Markovian process, it outputs a causal model, namely the causal
relations and the corresponding mechanisms, represented as quantum states and
channels. Our algorithm provides a first step towards more general methods for
quantum causal discovery.Comment: 11 pages, 10 figures, revised to match published versio
Multipartite Causal Correlations: Polytopes and Inequalities
We consider the most general correlations that can be obtained by a group of
parties whose causal relations are well-defined, although possibly
probabilistic and dependent on past parties' operations. We show that, for any
fixed number of parties and inputs and outputs for each party, the set of such
correlations forms a convex polytope, whose vertices correspond to
deterministic strategies, and whose (nontrivial) facets define so-called causal
inequalities. We completely characterize the simplest tripartite polytope in
terms of its facet inequalities, propose generalizations of some inequalities
to scenarios with more parties, and show that our tripartite inequalities can
be violated within the process matrix formalism, where quantum mechanics is
locally valid but no global causal structure is assumed.Comment: 14 pages and 1 supplementary CDF fil
Time-resolved detection of relative intensity squeezed nanosecond pulses in a Rb87 vapor
We present theoretical and experimental results on the generation and
detection of pulsed, relative-intensity squeezed light in a hot Rb87 vapor. The
intensity noise correlations between a pulsed probe beam and its conjugate,
generated through nearly-degenerate four-wave mixing in a double-lambda system,
are studied numerically and measured experimentally via time-resolved balanced
detection. We predict and observe about -1 dB of time-resolved relative
intensity squeezing with 50 nanosecond pulses at 1 MHz repetition rate. (-1.34
dB corrected for loss).Comment: 11 pages, 9 figure
Semi-device-independent certification of quantum non-Markovianity using sequential Random Access Codes
The characterization of multi-time correlations in open quantum systems is of
fundamental importance. In this work, we investigate multi-time processes using
the process matrix formalism and show that the presence of a quantum
non-Markovian environment plays a significant role in enhancing the
communication capacity in sequential prepare-transform-measure Quantum Random
Access Codes (QRAC). The correlated environment enables a quantum advantage to
multiple parties, even with projective measurements. In particular, we show
that the Markovian and classical non-Markovian processes, i.e. quantum
processes with classical feedback from the environment, do not yield sequential
quantum advantage. In contrast, it is possible to achieve an advantage in the
presence of a quantum non-Markovian environment. Therefore this approach allows
a semi-device-independent certification of quantum non-Markovianity. As opposed
to entanglement-detection criteria which require the knowledge of the complete
process, this method allows to certify the presence of a quantum non-Markovian
environment from the observed measurement statistics. Moreover, quantum memory
ameliorates the unambiguous certifiable region of unsharp instruments in a
semi-device-independent manner.Comment: 16 Pages, 9 figure
Multi-time quantum process tomography of a superconducting qubit
Non-Markovian noise poses a formidable challenge to the scalability of
quantum devices, being both ubiquitous in current quantum hardware and
notoriously difficult to characterise. This challenge arises from the need for
a full reconstruction of a multi-time process, a task that has proven elusive
in previous efforts. In this work, we achieve the milestone of complete
tomographic characterisation of a multi-time quantum process on a
superconducting qubit by employing sequential measure-and-prepare operations
with an experimentally motivated post-processing technique, utilising both
in-house and cloud-based superconducting quantum processors. Employing the
process matrix formalism, we reveal intricate landscapes of non-Markovian noise
and provide evidence that components of the noise originate from quantum
sources. Our findings and techniques have significant implications for
advancing error-mitigation strategies and enhancing the scalability of quantum
devices.Comment: 9 pages, 5 figures, 4 table
Witnessing causal nonseparability
Our common understanding of the physical world deeply relies on the notion
that events are ordered with respect to some time parameter, with past events
serving as causes for future ones. Nonetheless, it was recently found that it
is possible to formulate quantum mechanics without any reference to a global
time or causal structure. The resulting framework includes new kinds of quantum
resources that allow performing tasks - in particular, the violation of causal
inequalities - which are impossible for events ordered according to a global
causal order. However, no physical implementation of such resources is known.
Here we show that a recently demonstrated resource for quantum computation -
the quantum switch - is a genuine example of "indefinite causal order". We do
this by introducing a new tool - the causal witness - which can detect the
causal nonseparability of any quantum resource that is incompatible with a
definite causal order. We show however that the quantum switch does not violate
any causal nequality.Comment: 15 + 12 pages, 5 figures. Published versio
Causal and causally separable processes
The idea that events are equipped with a partial causal order is central to our understanding of physics in the tested regimes: given two pointlike events A and B, either A is in the causal past of B, B is in the causal past of A, or A and B are space-like separated. Operationally, the meaning of these order relations corresponds to constraints on the possible correlations between experiments performed in the vicinities of the respective events: if A is in the causal past of B, an experimenter at A could signal to an experimenter at B but not the other way around, while if A and B are space-like separated, no signaling is possible in either direction. In the context of a concrete physical theory, the correlations compatible with a given causal configuration may obey further constraints. For instance, space-like correlations in quantum mechanics arise from local measurements on joint quantum states, while time-like correlations are established via quantum channels. Similarly to other variables, however, the causal order of a set of events could be random, and little is understood about the constraints that causality implies in this case. A main difficulty concerns the fact that the order of events can now generally depend on the operations performed at the locations of these events, since, for instance, an operation at A could influence the order in which B and C occur in A's future. So far, no formal theory of causality compatible with such dynamical causal order has been developed. Apart from being of fundamental interest in the context of inferring causal relations, such a theory is imperative for understanding recent suggestions that the causal order of events in quantum mechanics can be indefinite. Here, we develop such a theory in the general multipartite case. Starting from a background-independent definition of causality, we derive an iteratively formulated canonical decomposition of multipartite causal correlations. For a fixed number of settings and outcomes for each party, these correlations form a polytope whose facets define causal inequalities. The case of quantum correlations in this paradigm is captured by the process matrix formalism. We investigate the link between causality and the closely related notion of causal separability of quantum processes, which we here define rigorously in analogy with the link between Bell locality and separability of quantum states. We show that causality and causal separability are not equivalent in general by giving an example of a physically admissible tripartite quantum process that is causal but not causally separable. We also show that there are causally separable quantum processes that become non-causal if extended by supplying the parties with entangled ancillas. This motivates the concepts of extensibly causal and extensibly causally separable (ECS) processes, for which the respective property remains invariant under extension. We characterize the class of ECS quantum processes in the tripartite case via simple conditions on the form of the process matrix. We show that the processes realizable by classically controlled quantum circuits are ECS and conjecture that the reverse also holds.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Time-resolved detection of relative intensity squeezed nanosecond pulses in a 87Rb vapor
We present experimental studies on the generation and detection of pulsed,
relative-intensity squeezed light in a warm rubidium vapor. The noise
correlations between a pulsed probe beam and its conjugate -generated through
nearly-degenerate four-wave mixing in a double-lambda system- are studied via
time-resolved balanced detection. We observe -0.96 dB of time-resolved relative
intensity squeezing with 50 nanosecond pulses at 1 MHz repetition rate. (-1.34
dB corrected for loss).Comment: This paper has been withdrawn by the authors. Major changes to
theoretical section and authorship order change. Will be resubmitted after
required changes are implemented to the manuscrip