Paper:
We study the control of symmetric linear dynamical systems with unknown dynamics and a hidden state. Using a recent spectral filtering technique for concisely representing such systems in a linear basis, we formulate optimal control in this setting as a convex program. This approach eliminates the need to solve the nonconvex problem of explicit identification of the system and its latent state, and allows for provable optimality guarantees for the control signal. We give the first efficient algorithm for finding the optimal control signal with an arbitrary time horizon , with sample complexity (number of training rollouts) polynomial only in log and other relevant parameters.

Review:
This paper proposes a new algorithm to generate the optimal control inputs for unknown linear dynamical systems (LDS) with known system dimensions.
The idea is exciting LDS by wave filter inputs and record the output and directly estimate the operator that maps the input to the output instead of estimating the hidden states. After obtaining this operator, this paper substitutes this operator to the optimal control problem and solve the optimal control problem to estimate the optimal control input, and show that the gap between the true optimal cost and the cost from applying estimated optimal control input is small with high probability.
I think estimating the operator from the input to the output is interesting, instead of constructing (A, B, C, D) matrices, but this idea and all the techniques are from Hazan et. el., 2017. After estimating this operator, it is straightforward to use this to generate the estimated optimal control input. So I think the idea is OK, but not a breakthrough.
Also I found the symmetric matrix assumption on A is quite limited. This limitation is from Hazan et. el., 2017, where the authors wants to predict the output. For prediction purposes, this restriction might be OK, but for control purposes, many interesting plants does not satisfy this assumption, even simple RL circuit. I agree with authors that this is an attempt to combine system identification with generating control inputs together, but I am not sure how to remove the restriction on A.
Dean et. el., 2017 also pursued this direction by combining system identification with robust controller synthesis to handle estimation errors in the system matrices (A, B) in the state-feedback case (LQR), and I can see that Dean et. el. could be extended to handle observer-feedback case (LQG) without any restriction.
Despite of this limitation I think the paper's idea is OK and the result is worth to be published but not in the current form. The paper is not clearly written and there are several areas need to be improved.
1. System identification.
Subspace identification (N4SID) won't take exponential time. I recommend the authors to perform either proper literature review or cite one or two papers on the time complexity and their weakness. Also note that subspace identification can estimate (A, B, C, D) matrices which is great for control purposes especially for the infinite horizon LQR.
2. Clarification on the unit ball constraints.
Optimal control inputs are restricted to be inside the unit ball and overall norm is bounded by L. Where is this restriction coming from? The standard LQG setup does not have this restriction.
3. Clarification on the assumption (3).
Where is this assumption coming from? I can see that this makes the analysis go through but is this a reasonable assumption?  Does most of system satisfy this constraint? Is there any? It's ok not to provide the answer if it's hard to analyze, but if that's the case the paper should provide some numerical case studies to show this bound either holds or the gap is negligible in the toy example.
4. Proof of theorem 3.3.
Theorem 3.3 is one of the key results in this paper, yet its proof is just "noted". The setup is slightly different from the original theorem in Hazan et. el., 2017 including the noise model, so I strongly recommend to include the original theorem in the appendix, and include the full proof in the appendix.
5. Proof of lemma 3.1.
I found it's hard to keep track of which one is inside the expectation. I recommend to follow the notation E[variable] the authors been using throughout the paper in the proof instead of dropping these brackets.
 
6. Minor typos
In theorem 2.4, ||Q||_op is used for defining rho, but in the text ||Q||_F is used. I think ||Q||_op is right.